Science of the Total Environment 502 (2015) 641–649
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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
The impacts of summer monsoons on the ozone budget of the atmospheric boundary layer of the Asia-Pacific region Xuewei Hou a,b, Bin Zhu a,b,⁎, Dongdong Fei a, Dongdong Wang a,b a b
Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science & Technology, Nanjing 210044, China Key Laboratory for Aerosol-Cloud-Precipitation of China Meteorological Administration, Nanjing University of Information Science and Technology, Nanjing 210044, China
H I G H L I G H T S • The Asia-Pacific monsoon greatly affects O3 seasonal and inter-annual variations. • The differences of emissions and zonal winds lead to pollutants transition zone. • Advection plays a key role in the monsoon impact on O3 inter-annual variation.
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Article history: Received 31 May 2014 Received in revised form 10 September 2014 Accepted 22 September 2014 Available online xxxx Editor: Xuexi Tie Keywords: Tropospheric ozone Transition zone Asia-Pacific monsoon
a b s t r a c t The seasonal and inter-annual variations of ozone (O3) in the atmospheric boundary layer of the Asia-Pacific Ocean were investigated using model simulations (2001–2007) from the Model of Ozone and Related chemical Tracers, version 4 (MOZART-4). The simulated O3 and diagnostic precipitation are in good agreement with the observations. Model results suggest that the Asia-Pacific monsoon significantly influences the seasonal and inter-annual variations of ozone. The differences of anthropogenic emissions and zonal winds in meridional directions cause a pollutants' transition zone at approximately 20°–30°N. The onset of summer monsoons with a northward migration of the rain belt leads the transition zone to drift north, eventually causing a summer minimum of ozone to the north of 30°N. In years with an early onset of summer monsoons, strong inflows of clean oceanic air lead to low ozone at polluted oceanic sites near the continent, while strong outflows from the continent exist, resulting in high levels of O3 over remote portions of the Asia-Pacific Ocean. The reverse is true in years when the summer monsoon onset is late. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Tropospheric ozone is an important greenhouse gas, pollutant, and source of OH radicals. Its contribution to global warming from the preindustrial era to the present is regarded as the third most important, following those of carbon dioxide and methane (Solomon et al., 2007). The level of tropospheric ozone also affects human health and natural ecosystems. Elucidation of the processes determining spatial and temporal variations in tropospheric ozone is important for evaluating the effect of ozone on regional air quality and climate change. Monsoons are a seasonal variation of wind, particular of wind directions, which result from the variations of meridional differences in solar radiation and the thermal difference between the land and sea. It is an important element of the global climate system. Asian monsoons are ⁎ Corresponding author at: Key Laboratory for Aerosol-Cloud-Precipitation of China Meteorological Administration, Nanjing University of Information Science and Technology, Nanjing 210044, China. Tel.: +86 2558699785. E-mail address: [email protected] (B. Zhu).
http://dx.doi.org/10.1016/j.scitotenv.2014.09.075 0048-9697/© 2014 Elsevier B.V. All rights reserved.
composed of three sub-systems, which are the tropical Indian monsoon, the tropical Western North Pacific (WNP) monsoon, and the subtropical East Asian (EA) monsoon (Zhu et al., 1986; Wang and Lin, 2002). Over the Asia-Pacific region, low-level winds seasonally reverse from winter easterlies to summer westerlies for the WNP monsoon and from winter northerlies to southerlies for the EA monsoon. The EA and WNP monsoons are considered together as the Asia-Pacific monsoon hereafter in this study. Whether considering the EA monsoon or the WNP monsoon, the transition of the winter monsoon to the summer monsoon results in a series of changes in weather, such as wind shifts, convection, precipitation, and air temperature. Together these changes significantly affect the transport paths and photochemical production of pollutants. The observed summer minimum of surface O3 over the Asia-Pacific region was attributed to the incursion of the monsoon, which transports oceanic air with less background O3 to the region, causing lower O3 concentrations (Chan et al., 1998; Pochanart et al., 2002; Wang et al., 2006; Yamaji et al., 2006; Zbinden et al., 2006; He et al., 2008). Tanimoto et al. (2005) and He et al. (2008) identified a relationship between monsoons and the O3 spring maximum at the surface. He et al. (2008) found that
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the basic common features of O3 seasonal behaviors over the monsoon region are the pre- and post-monsoon peaks with a summer trough; these bimodal seasonal patterns become weaker or even disappear outside of the monsoon region. Kurokawa et al. (2009) investigated the influence of meteorological variability on the interannual variation of the springtime boundary layer ozone over Japan, and found some correlation between spring ozone over Japan and the El Niño-Southern Oscillation. Zhu (2012) determined that ozone seasonal cycle features were mainly controlled by the seasonal transitions of Asia-Pacific monsoon circulations by using monthly surface ozone data and related wind field and precipitation data. This paper addresses the influence of the Asia-Pacific monsoon on seasonal and inter-annual variations of boundary layer ozone over the Asia-Pacific region, based on an analysis of the ozone budget using the chemical transport model (CTM), Model of Ozone and Related chemical Tracers, version 4 (MOZART-4). A fixed emission experiment was performed to identify the influence of early/late summer monsoon onset on the O3 budget. For the Asia-Pacific monsoon region, various indices based on considerations of thermodynamics and dynamics from different aspects have already been defined. Each monsoon index pays attention to some specific physical processes and represents its own meanings, independent of others. In this study, meridional wind (Lau and Li, 1984; Chen et al., 1991) was used to reveal the seasonal march of the summer monsoon, and a dynamical normalized seasonality monsoon index developed by Li and Zeng (2003) was used to distinguish the early and late transition periods of the Asia-Pacific summer monsoon onset. The importance of the impact on the ozone levels by advection, convection, diffusion, and photochemistry over the Asia-Pacific monsoon region was evaluated. 2. Data and methods 2.1. Observed data Observations of daily O3 at seven regional stations were taken from the Acid Deposition Monitoring Network in East Asia (EANET): Rishiri, Tappi, Sado-Seki, Oki, Yusuhara, Hedo, and Ogasawara. Descriptions of these sites can be found at http://www.eanet.asia/site/index.html. Observations from two additional sites, Yonagunijima and Minamitorishima, were obtained from the WMO-World Data Centre for Greenhouse Gases (WDCGG) (http://ds.data.jma.go.jp/gmd/wdcgg/). The site location information including latitude, longitude and elevation height above sea level for all sites is provided in Fig. 1. Daily precipitation data collected from 2001 to 2007 are taken from the Global Precipitation Climatology Project (GPCP) with a resolution of 1° × 1° (http://precip.gsfc.nasa.gov/). The GPCP dataset reflects the spatial and temporal distributions of precipitation very well, as a combination of GPCC (Global Precipitation Climatology Center) ground observations from precipitation gauges and a precipitation inversion from satellite remote sensing observations. 2.2. Model setup A detailed description and evaluation of the standard version of MOZART-4, and the upgrade over MOZART-2 (Horowitz et al., 2003), is given by Emmons et al. (2010). MOZART-4 includes an updated chemical scheme of hydrocarbon and bulk aerosols (Tie et al., 2001, 2005), and improved emissions compared to MOZART-2. It does not include explicit stratospheric chemistry, but constrains the climatological mixing ratios of ozone and other species in the stratosphere. The Synoz (synthetic ozone) scheme (McLinden et al., 2000) is used as a flux upper boundary condition for ozone in the stratosphere and yields a cross-tropopause ozone flux of 500 Tg/yr. More details about the physical and chemical mechanisms were discussed in Emmons et al. (2010). In this study, MOZART-4 is run with the standard chemical mechanism [see Emmons et al., 2010 for details], with online calculation of dry deposition. It is driven by meteorological parameters from the
NCAR reanalysis of the National Centers for Environmental Prediction (NCEP) forecasts. The output from the model run is available at a temporal resolution of 6 h, a horizontal spatial resolution of approximately 2.8° × 2.8°, and 28 hybrid levels in the vertical. The top model level is located at approximately 2 hPa. The initial condition and emissions are based on the NCAR Community Data Portal (http://cdp.ucar.edu/). The model was run in time steps of 20 min from June 2000 to December 2007, with the first seven months used to spin up. The experiments used fixed emissions from 2001 and the meteorological parameters were varied for each year over the simulations, with the first seven months as spin-up time. Note that the modeled results are based on the mean values in the atmospheric boundary layer (the six lowermost layers in the model, surface to ~ 2 km) except for the model validations in Section 3.1 and the special definitions in Section 3.2. 2.3. Monsoon index and ozone budget Previous studies showed that the East Asian monsoon is characterized as a seasonal reversal of the lower-troposphere meridional wind direction (Lau and Li, 1984; Chen et al., 1991), and the 850 hPa wind is used to reflect the low-level atmospheric circulation (Lu and Chan, 1999). Therefore, the daily meridional wind at 850 hPa is used to show the seasonal march of the Asia-Pacific monsoon in this study. A pre-defined monsoon index (MI) can provide a useful insight into quantitatively examining the strength and variation of monsoon circulation in monsoon regions. For the Asia-Pacific monsoon region, many studies have already defined various indices based on considerations of dynamics and thermodynamics from different aspects, such as wind field, precipitation, difference of ocean–land temperature, and so on. Each MI is designed to capture specific physical processes. In this study, we used a dynamically normalized seasonality MI developed by Li and Zeng (2003) to examine the wind field to investigate the influence of the Asia-Pacific monsoon on the inter-annual variability of ozone. The factors controlling ozone levels in the atmospheric boundary layer (transport, net photochemical production, and deposition) are discussed in this study. Each term affecting the ozone budget is evaluated quantitatively at 850 hPa in Section 3.2 and in a column from the surface to approximately 2 km (the atmospheric boundary layer) in Section 3.3. The rate of change of ozone can be expressed as: dO3 ¼ Chem þ Adv þ Con þ Dif −Dep dt where Chem represents the net chemical production; Adv, Con, and Dif are the transport fluxes associated with advection, convection and diffusion, respectively; Dep is the dry deposition rate. Fig. 4 shows each of these components of the ozone budget for the study region: net chemistry, net transport flux (advection, convection, and diffusion), and O3S. O3S is an O3 tracer that tags O3 transported from the stratosphere (Sudo and Akimoto, 2007). 3. Results and discussion 3.1. Validation of simulated O3 and precipitation The modeling system described in Section 2.2 has previously been used for analyzing tropospheric O3 over the northern hemisphere (Horowitz et al., 2003; Liu et al., 2005; Pfister et al., 2008a,b; Emmons et al., 2010), and, in these studies, the simulated results showed good agreement with observations. Stevenson et al. (2006) suggested that the tropospheric O3 budget in MOZART-4 is in good agreement with the mean of 26 models, whereas the stratospheric input value has been determined to be realistic by Wild (2007). In this section, we further evaluate the general performance of our modeling system for O3 over the Asia-Pacific region.
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Fig. 1. Comparison of modeled O3 (ppbv, black line) to observation (red line) at selected sites over the Asia-Pacific rim region between 2001 and 2007. All of the data were calculated based on the mean of seven years' data (similarly hereinafter). R is the correlation coefficient. The degrees of freedom are 363, and p values in the test are 0.05. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 1 shows a comparison of modeled daily O3 with observations over the Asia-Pacific region. Seasonal variations with a spring maximum and a summer minimum are fairly well reproduced by the model. Simulated results of the spring maximum at middle latitude sites (Rishiri, Tappi, and Sado-Seki) are underestimated relative to observations, while summer minima at all sites are overestimated. These discrepancies may result from the coarse horizontal model resolution in meteorology and pollutant sources due to sub-grid-scale processes that cannot be accurately simulated. Significant effects of cloud and wet processes may also be partly responsible for the discrepancies in summer ozone, as discussed below. The correlation-coefficient (R) between simulated and observed daily results ranges from approximately 0.47 to 0.87. They all passed the t test under the 95% confidence level. The relatively strong correlations (an R of approximately 0.8) at four oceanic sites (Ogasawara, Hedo, Yonagunijima, and Minamitorishima) are primarily attributed to their remote locations with less impact from continental emissions. Other sites at mid-latitude can be affected both by local to regional anthropogenic emissions and complex topography, which might account for the less than satisfactory model performance at these locations. The increase of precipitation intensity and the northward migration of the rain belt are key features of the summer monsoon onset. Although direct precipitation scavenging of ozone is relatively unimportant due to its low solubility, precipitation can affect ozone chemical production by altering solar radiation and through wet removal of soluble ozone precursors. Consequently, it is important to evaluate the simulated precipitation. Fig. 2 indicates that the diagnosis precipitation closely represents the GPCP data in January. In April, July, and October, strong precipitation over Japan and its northern regions in the GPCP data is underestimated by the model. Because MOZART-4 is an offline model, the accuracy of simulated precipitation depends largely on the accuracy of the meteorological fields, which may have some errors in mid- and high-latitudes that lead to an underestimation of precipitation. On the other hand, the coarse horizontal resolution in the global model may inaccurately diagnose sub-grid-scale processes, leading to these discrepancies. At relatively low latitudes, the diagnosis result represents the GPCP precipitation very well. 3.2. The impact of the Asia-Pacific monsoon march on O3 seasonal variation Wang and Lin (2002) demonstrated the onset time of the Asia-Pacific summer monsoon varies across different regions based on the
precipitation analysis, with the 30th pentad (150th Julian day) reaching 20°–30°N and the 40th pentad (200th Julian day) being north of 30°N. Accordingly, it is necessary to divide the Asia-Pacific monsoon region to show the onset period. In this study, the monsoon regions covering 120°–140°E longitude and 20°–30°N and 30°–50°N latitude were chosen, considering the spatial extent of the region affected by the monsoon and the distribution of sites available for estimation. The meridional wind at 850 hPa can be used to show the seasonal march of the Asia-Pacific monsoon, where positive meridional winds represent the prevailing summer monsoon (Lau and Li, 1984; Chen et al., 1991; Lu and Chan, 1999). Fig. 3 shows that the sub-tropical summer monsoon onset is in March at 20°–30°N and is in early April to late May in 30°–50°N. The retreat of the summer monsoon occurs earlier at 30°–50°N than at 20°– 30°N. The onset and retreat of the Asia-Pacific summer monsoon match the results of Wang and Lin (2002) very well. As shown in Fig. 4a and b, there is a transition zone of pollutants near 20°N in the winter. South of 20°N, pollutant concentrations of O3, CO, NO are relatively low with a lesser impact of continental emissions. North of 30°N, pollutant concentrations are relatively high with a strong influence from anthropogenic emissions or transport. The difference in anthropogenic emissions plays a key role in the formation of a transition zone. In addition, the difference in prevailing winds is also largely responsible for the formation of a transition zone. South of 20°N, where the tropical monsoon resides, easterly winds prevail in the winter. North of 20°N, where the subtropical monsoon is located, northwesterly winds prevail in the winter. Because of the differences in zonal wind and the weak meridional wind, a shear line is located at approximately 20°N, which limits the air masses' transport and exchange between north and south. With the onset of summer monsoons in late March at 20°N, southerly winds prevail, causing the transition zone to move northward. In summer, the transition zone continuously moves northward with the advance of the summer monsoon. In autumn, the summer monsoon retreats. Southerly winds become weak, and the zonal wind component increases. The North–south differences in the zonal wind component decrease, causing the transition zone to retreat back to 20°N. Eventually, a transition zone of pollutants moves back and forth in a south to north direction with the advance of the summer monsoon. Tropospheric ozone, as a secondary pollutant, is affected by solar radiation and the emission of precursor species. The transition from the winter monsoon to the summer monsoon results in a series of changes
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Fig. 2. Comparison of monthly mean precipitation (shaded) to GPCP data (contour) over the Asia-Pacific rim region in January, April, July and October during 2001–2007, unit: mm/d. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
in weather, such as atmospheric circulations and precipitation, which significantly affect the pollutants' transport paths and ozone photochemical production. South of 20°N, the precursors of ozone are very low, especially NO, which is as low as 4 pptv, resulting in a net photochemical loss of ozone (see Fig. 4b and c). Precipitation increases south of 20°N in early May (Fig. 4c), slowing down the photochemical reactions there. In summer, southerly winds prevail and transport low latitude clean air deeply towards higher latitudes at approximately 45°N. Therefore, the O3 seasonal pattern with maximum pollutant levels in the winter or early spring and a broad minimum in the summer is present south of 20°N.
Fig. 3. The mean meridional wind at 850 hPa (blue points) between 120° and 140°E and different latitude regions (a, 30°–50°N; b, 20°–30°N). The solid black line represents the wind speed at 0 m/s. The solid red line is a Gaussian fitting curve. The green dashed line represents the 95% prediction bands. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
In comparison with that of south 20°N, the concentration of NO is high in the winter over north of 30°N, but the net chemical production of ozone is low under the winter weak solar radiation. In spring, net ozone chemical production increases with the enhancement of solar radiation, leading to a maximum ozone concentration at 40°N in May. Due to the later onset of the summer monsoons in the regions between 30° and 50°N, westerly winds prevail until late June. High precursor concentrations, transported from upwind regions or locally emitted, lead to the accumulation of ozone through strong chemical production under strong solar radiation. In summer, cloudiness and precipitation are enhanced at 30°–50°N. The concentrations of ozone precursors decrease, resulting from the inflow of clean marine air masses by prevailing southerly winds and the wet removal by the rain belt. Net ozone production is reduced by decreased solar radiation and ozone precursors. The net ozone transport flux during this time is outflow, but the outflow is weaker than in the spring. The overall result of the factors discussed above is a decrease in net chemical production, eventually leading to an ozone minimum in summer. The direct transport of ozone by wind has a relatively weak impact on the summer minimum due to the weak outflow or inflow. The shift of south winds to westerly winds in early September reflects the relatively early retreat of summer monsoons at 30°–50°N. High concentrations of precursors enhance the net ozone chemical production. Similarly as in spring, the ozone concentration is higher in autumn than in summer. The contribution of stratospheric ozone (Fig. 4d) includes a winter maximum and a summer minimum. Its seasonal variation has an influence on ozone concentrations, especially south of 20°N where the contribution of stratospheric ozone is approximately 8 ppbv in winter, or approximately 30%.
3.3. The impact of the Asia-Pacific summer monsoon on inter-annual variation of O3 This section focuses on the influence of the early/late onset of the Asia-Pacific sub-tropical summer monsoon on the budget of O3 in the
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(a)
(b)
(c)
(d)
Fig. 4. The zonal mean ozone mixing ratio (ppbv, shaded), wind (m/s, vector), zonal wind speed (m/s, contour) (a); CO (ppbv, shaded), NO (pptv, contour) and wind (m/s, vector) (b); precipitation (mm/d, contour) and the O3 net chemical production (ppbv/d, shaded) (c); the contribution of stratospheric O3 (ppbv, contour) and the net O3 transport flux (ppbv/d, shaded) (d) at 850 hPa between 1° and 65°N. The black solid line is the 40 ppbv of O3 in (a). The dashed lines are the latitudes of 20°N, 30°N, and 50°N. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
atmospheric boundary layer over the low-latitude oceanic stations, where model results were well-correlated with observations (see Section 3.1). The correlation coefficients of simulated and observed O3 anomalies are 0.56, 0.36, 0.5, and 0.52 at Hedo, Ogasawara, Yonagunijima, and Minamitorishima, respectively. As shown in Fig. 5, most of the simulated and observed anomalies of O3 concentration are significant at the 95% confidence level, and they all pass the test at the 95% confidence level. The model results can be used to analyze the
inter-annual variation of ozone in the atmospheric boundary layer. Ogasawara is located in an area with a strong gradient of wind speeds and direction, a region that is most sensitive to the monsoon transition (seen Fig. 8). It is difficult to capture the ozone inter-annual variations accurately in the global model, leading to the relatively low correlation coefficient of O3 at Ogasawara. We calculated the dynamically normalized seasonality monsoon index defined by Li and Zeng (2003) over the region 20–50°N and
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Fig. 5. Scatter plots showing the relationship between observed and simulated daily surface ozone anomalies at Hedo, Ogasawara, Yonagunijima and Minamitorishima from 2001 to 2007 (OB and MZ4 represent observations and simulations, respectively). The simulated ozone anomaly is the simulation for the fixed emissions. The solid red line is the linear regression equation. The green dashed line is the 95% confidence level. N is the number of paired samples. R is the correlation coefficient. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
120–140°E using NCEP reanalysis data, to obtain an Asia-Pacific monsoon index (AMI) and its anomaly (shown in Fig. 6). From 2001 to 2007, AMI is negative in the winter and positive in the summer, reasonably representing the winter and summer monsoons. In March and April, a negative AMI value implies that the monsoon is still in its winter phase. In May, it has transitioned to its summer phase, on average. The AMI and its anomaly are positive in May 2003, 2004 and 2006, indicating the early onset of Asia-Pacific summer monsoon in these years. In April 2003, the AMI is near zero and has a high positive anomaly implying a much earlier transition of the winter monsoon into the summer monsoon. To unify the transition period, we chose May in 2004 and 2006 as the early transition periods of the winter monsoon to the summer monsoon (strong monsoon period) and May in 2002 and 2007 as the late transition periods for the negative anomalies (weak monsoon period) to discuss the influence of the summer monsoon on O3 over the Asia-Pacific Ocean. Budget analysis from Fig. 7 shows the net chemistry is − 0.03 ppbv/d in Hedo and 0.32 ppbv/d in Yonagunijima, while it is − 2.1 ppbv/d in Ogasawara and − 2.8 ppbv/d in Minamitorishima. The absolute value of net chemistry in polluted oceanic sites (Hedo
Fig. 6. The Asia monsoon index (AMI, blue line) and the anomaly of AMI (red bar). The bar marked with the green line is the anomaly of AMI in March, April, and May, every year. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
and Yonagunijima) is lower than in remote oceanic sites (Ogasawara and Minamitorishima) due to stronger chemical production in polluted oceanic regions. In addition, according to the division of AsiaPacific monsoon regions by Wang and Lin (2002), Hedo and Yonagunijima are located in the subtropical East Asia (EA) monsoon region, Ogasawara is at the boundary of the EA and western North Pacific (WNP) monsoon regions, and Minamitorishima is in the tropical WNP monsoon region. Fig. 7 shows O3 during early transition periods is lower than average in Yonagunijima and Hedo, and higher than the mean at Ogasawara and Minamitorishima. In late transition periods the effect is opposite, except in Ogasawara due to its location on the boundary of the EA and WNP monsoon regions (Wang and Lin, 2002) with sharp gradients of wind speed and direction. As the star reveals in Fig. 7, the contribution of stratospheric O3 is 3.5 ppbv at Hedo in early transition periods, lower by 0.1 ppbv than the mean value, which cannot account for the decreased 3 ppbv of O3 from the average. In late transition periods, the contribution is higher by 1.1 ppbv than the mean at Hedo, which does not lead to an O3 increase of 4 ppbv. We can also find similar facts at Ogasawara, Yonagunijima, and Minamitorishima. Accordingly, ozone intruded from the stratosphere has an influence on the inter-annual variation of the boundary layer O3 in the spring, but it is not the only responsible factor. The controlling sources of ozone in polluted oceanic sites are different from that in remote oceanic sites. As the mean values show in Fig. 7, controlling sources are convection and diffusion in Hedo, chemistry and diffusion in Yonagunijima, and convection and advection in Ogasawara and Minamitorishima. Chemistry and diffusion have relatively low correlation with the variation of the monsoon transition period, as well as dry deposition. The anomalies of convection are relatively large in polluted oceanic sites. They are correlated with O3 anomalies in Yonagunijima while they are anti-correlated in Hedo. Therefore, convection may have some contribution to the O3 interannual variability. The anomalies of advection are relatively large, and positively correlated with O3 anomalies in all oceanic sites. In polluted oceanic sites,
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Fig. 7. Boundary layer O3 mixing ratios (unit: ppbv), O3S (unit: ppbv) and the daily rates of change in O3 mixing ratios (unit: ppbv/d) at four oceanic sites in May in the multi-year average (mean), early transition period (early), and late transition period (late). Black dots are O3. Stars are O3S. The values marked on the color bars are the daily rates of change in O3 from individual processes (values lower than 0.1 are not marked). The value on the black box indicates the influence of the stratospheric tracer (O3S). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
advection is O3 outflow from the regions, −0.4 ppbv/d. The outflow is strong in the early transition period, about − 1 ppbv/d, and weak in the late transition period, − 0.2 ppbv/d. Advection is O3 inflow in remote oceanic sites, approximately 1.2 ppbv/d. It is also strong in the early transition period, approximately 2 ppbv/d, and weak in the late transition period, approximately 1 ppbv/d. We find that advection clearly plays a key role in the impact of the summer monsoon on the inter-annual variation of the O3 boundary layer in May (spring). Net chemistry, convection, diffusion, and dry deposition may disturb the impact. Winds and their anomalies were analyzed to investigate O3 advection (export and import) in Fig. 8. As shown in Fig. 8a, the average prevailing wind is southwesterly in Yonagunijima and Hedo and is southeasterly in Minamitorishima. This suggests that Yonagunijima and Hedo are influenced by continental outflow of O3 and O3 precursors, and Minamitorishima is influenced by the transport of pollutants around the Pacific Ocean under the effect of a subtropical high. In the early transition period, the mean wind field (Fig. 8a) and wind anomalies (Fig. 8b) in Yonagunijima and Hedo show that the westerly components and southerly components are both stronger than the corresponding components of the mean wind field, resulting in strong net inflows of clean oceanic air in Yonagunijima and Hedo. In Minamitorishima, the northeasterly wind anomalies indicate the weak southerly components and strong easterly components, making the transport around the Pacific Ocean strong. Thus the inflow of O3 in remote oceanic sites is strong. There are negative O3 anomalies in polluted oceanic sites, and positive O3 anomalies in remote oceanic sites. In late transition periods, as shown in Fig. 8c, there is a weak westerly component of winds on the coast of East China and Japan, due to the northeastern wind anomalies, causing a weak inflow of clean oceanic air through advection, leading to the high O3 value in Yonagunijima and Hedo. However, polluted air masses from the continent tend to be transported to higher latitudes by the strong southerly component of the winds east of 160°E and north of 30°N (Fig. 8c). As a
result, O3 inflow is weak in Minamitorishima, leading to low O3 values in the late transition period. Ogasawara is located at the boundary of the wind anomaly transition, making the effects of the monsoon transition on local ozone concentrations apparent. 4. Conclusions This study investigated the seasonal and inter-annual variations of O3 in the atmospheric boundary layer over the Asia-Pacific region between 2001 and 2007 using multi-year simulations by MOZART-4. The influence of the Asia-Pacific monsoon march on ozone seasonal variations was introduced by an analysis of the ozone budget. The relationship between the Asia-Pacific summer monsoon onset and the inter-annual variation of O3 was also examined by a seven-year numerical experiment with fixed emissions from 2001 and varied meteorology (from NCEP data) for 2001–2007 over the Asia-Pacific Ocean. The daily meridional wind at 850 hPa was used to show the seasonal march of the Asia-Pacific summer monsoon. A dynamically normalized seasonality MI developed by Li and Zeng (2003) on the basis of wind field calculations was used to distinguish the early and late transition periods of the summer monsoon onset. The main findings of the study are summarized as follows: 1. The model simulation reasonably reproduced both seasonal and interannual variations of O3, particularly at relatively low latitude oceanic sites where correlation coefficients between modeled and observed values reached 0.87. The modeled results also represented GPCP precipitation patterns for the region. 2. The Asia-Pacific monsoon has a great influence on the seasonal and inter-annual variations of O3 over the Asia-Pacific Ocean. Differences of zonal wind in the meridional direction create a transition zone near 20°N between high and low pollutant regions in the winter. The onset of the summer monsoon with the northward migration of the rain belt helps move the transition zone northward.
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(a) May
(b) early
(c) late
Fig. 8. The multi-year average of boundary layer (BL) O3 mixing ratios (ppbv) and wind filed (m/s) in May from 2001 to 2007 (a), and the anomalies of O3 (ppbv) and wind (m/s) in the early transition period (b) and in the late transition period (c). The large arrows overlaid on the figure are the wind direction, showing the large scale circulation over the north Pacific.
3. North of 30°N, the transport of clean marine air masses by prevailing southerly winds and the northward migration of the rain belt upon the onset of the summer monsoon result in low concentrations of ozone precursors in summer. This makes net O3 chemical production weak and results in a summer ozone minimum. The direct transport of ozone by wind has a relatively weak impact on the summer minimum. 4. In the early transition of the Asia-Pacific winter monsoon to the summer monsoon phase, strong inflows of clean oceanic air tend to lead to lower levels of O3 than average at polluted oceanic sites near the continent. At the same time, stronger outflows of continental air masses result in higher levels of O3 at remote oceanic sites. The reverse is true in years when the summer monsoon onset is late. Advection appears to play a key role in the impact of the summer monsoon onset on the inter-annual variation of O3 in spring.
Acknowledgments This work was supported by the National Natural Science Foundation of China (40875078), the European Union Seventh Framework Program ([FP7/2007–2013]) under grant agreement no. 606719 (PANDA project), the Natural Science Foundation of the Higher Education Institutions of Jiangsu Province, China (12KJA170003), the project of Jiangsu provincial “333” and Six Talent Peaks, and the Program for Postgraduates Research Innovation of Jiangsu Higher Education Institutions (grant no. CXZZ13_0509). We would like to acknowledge the entire staff of the EANET and the WDCGG. Special thanks go to Stacy Walters and Louisa Emmons of NCAR who provided MOZART-4 source codes and helped us install and run it. We thank Jeff Collett at Colorado State University in America and Tianliang Zhao at Nanjing University of Information science & Technology in China for their valuable suggestions on improving the manuscript.
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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
The impacts of summer monsoons on the ozone budget of the atmospheric boundary layer of the Asia-Pacific region Xuewei Hou a,b, Bin Zhu a,b,⁎, Dongdong Fei a, Dongdong Wang a,b a b
Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science & Technology, Nanjing 210044, China Key Laboratory for Aerosol-Cloud-Precipitation of China Meteorological Administration, Nanjing University of Information Science and Technology, Nanjing 210044, China
H I G H L I G H T S • The Asia-Pacific monsoon greatly affects O3 seasonal and inter-annual variations. • The differences of emissions and zonal winds lead to pollutants transition zone. • Advection plays a key role in the monsoon impact on O3 inter-annual variation.
a r t i c l e
i n f o
Article history: Received 31 May 2014 Received in revised form 10 September 2014 Accepted 22 September 2014 Available online xxxx Editor: Xuexi Tie Keywords: Tropospheric ozone Transition zone Asia-Pacific monsoon
a b s t r a c t The seasonal and inter-annual variations of ozone (O3) in the atmospheric boundary layer of the Asia-Pacific Ocean were investigated using model simulations (2001–2007) from the Model of Ozone and Related chemical Tracers, version 4 (MOZART-4). The simulated O3 and diagnostic precipitation are in good agreement with the observations. Model results suggest that the Asia-Pacific monsoon significantly influences the seasonal and inter-annual variations of ozone. The differences of anthropogenic emissions and zonal winds in meridional directions cause a pollutants' transition zone at approximately 20°–30°N. The onset of summer monsoons with a northward migration of the rain belt leads the transition zone to drift north, eventually causing a summer minimum of ozone to the north of 30°N. In years with an early onset of summer monsoons, strong inflows of clean oceanic air lead to low ozone at polluted oceanic sites near the continent, while strong outflows from the continent exist, resulting in high levels of O3 over remote portions of the Asia-Pacific Ocean. The reverse is true in years when the summer monsoon onset is late. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Tropospheric ozone is an important greenhouse gas, pollutant, and source of OH radicals. Its contribution to global warming from the preindustrial era to the present is regarded as the third most important, following those of carbon dioxide and methane (Solomon et al., 2007). The level of tropospheric ozone also affects human health and natural ecosystems. Elucidation of the processes determining spatial and temporal variations in tropospheric ozone is important for evaluating the effect of ozone on regional air quality and climate change. Monsoons are a seasonal variation of wind, particular of wind directions, which result from the variations of meridional differences in solar radiation and the thermal difference between the land and sea. It is an important element of the global climate system. Asian monsoons are ⁎ Corresponding author at: Key Laboratory for Aerosol-Cloud-Precipitation of China Meteorological Administration, Nanjing University of Information Science and Technology, Nanjing 210044, China. Tel.: +86 2558699785. E-mail address: [email protected] (B. Zhu).
http://dx.doi.org/10.1016/j.scitotenv.2014.09.075 0048-9697/© 2014 Elsevier B.V. All rights reserved.
composed of three sub-systems, which are the tropical Indian monsoon, the tropical Western North Pacific (WNP) monsoon, and the subtropical East Asian (EA) monsoon (Zhu et al., 1986; Wang and Lin, 2002). Over the Asia-Pacific region, low-level winds seasonally reverse from winter easterlies to summer westerlies for the WNP monsoon and from winter northerlies to southerlies for the EA monsoon. The EA and WNP monsoons are considered together as the Asia-Pacific monsoon hereafter in this study. Whether considering the EA monsoon or the WNP monsoon, the transition of the winter monsoon to the summer monsoon results in a series of changes in weather, such as wind shifts, convection, precipitation, and air temperature. Together these changes significantly affect the transport paths and photochemical production of pollutants. The observed summer minimum of surface O3 over the Asia-Pacific region was attributed to the incursion of the monsoon, which transports oceanic air with less background O3 to the region, causing lower O3 concentrations (Chan et al., 1998; Pochanart et al., 2002; Wang et al., 2006; Yamaji et al., 2006; Zbinden et al., 2006; He et al., 2008). Tanimoto et al. (2005) and He et al. (2008) identified a relationship between monsoons and the O3 spring maximum at the surface. He et al. (2008) found that
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the basic common features of O3 seasonal behaviors over the monsoon region are the pre- and post-monsoon peaks with a summer trough; these bimodal seasonal patterns become weaker or even disappear outside of the monsoon region. Kurokawa et al. (2009) investigated the influence of meteorological variability on the interannual variation of the springtime boundary layer ozone over Japan, and found some correlation between spring ozone over Japan and the El Niño-Southern Oscillation. Zhu (2012) determined that ozone seasonal cycle features were mainly controlled by the seasonal transitions of Asia-Pacific monsoon circulations by using monthly surface ozone data and related wind field and precipitation data. This paper addresses the influence of the Asia-Pacific monsoon on seasonal and inter-annual variations of boundary layer ozone over the Asia-Pacific region, based on an analysis of the ozone budget using the chemical transport model (CTM), Model of Ozone and Related chemical Tracers, version 4 (MOZART-4). A fixed emission experiment was performed to identify the influence of early/late summer monsoon onset on the O3 budget. For the Asia-Pacific monsoon region, various indices based on considerations of thermodynamics and dynamics from different aspects have already been defined. Each monsoon index pays attention to some specific physical processes and represents its own meanings, independent of others. In this study, meridional wind (Lau and Li, 1984; Chen et al., 1991) was used to reveal the seasonal march of the summer monsoon, and a dynamical normalized seasonality monsoon index developed by Li and Zeng (2003) was used to distinguish the early and late transition periods of the Asia-Pacific summer monsoon onset. The importance of the impact on the ozone levels by advection, convection, diffusion, and photochemistry over the Asia-Pacific monsoon region was evaluated. 2. Data and methods 2.1. Observed data Observations of daily O3 at seven regional stations were taken from the Acid Deposition Monitoring Network in East Asia (EANET): Rishiri, Tappi, Sado-Seki, Oki, Yusuhara, Hedo, and Ogasawara. Descriptions of these sites can be found at http://www.eanet.asia/site/index.html. Observations from two additional sites, Yonagunijima and Minamitorishima, were obtained from the WMO-World Data Centre for Greenhouse Gases (WDCGG) (http://ds.data.jma.go.jp/gmd/wdcgg/). The site location information including latitude, longitude and elevation height above sea level for all sites is provided in Fig. 1. Daily precipitation data collected from 2001 to 2007 are taken from the Global Precipitation Climatology Project (GPCP) with a resolution of 1° × 1° (http://precip.gsfc.nasa.gov/). The GPCP dataset reflects the spatial and temporal distributions of precipitation very well, as a combination of GPCC (Global Precipitation Climatology Center) ground observations from precipitation gauges and a precipitation inversion from satellite remote sensing observations. 2.2. Model setup A detailed description and evaluation of the standard version of MOZART-4, and the upgrade over MOZART-2 (Horowitz et al., 2003), is given by Emmons et al. (2010). MOZART-4 includes an updated chemical scheme of hydrocarbon and bulk aerosols (Tie et al., 2001, 2005), and improved emissions compared to MOZART-2. It does not include explicit stratospheric chemistry, but constrains the climatological mixing ratios of ozone and other species in the stratosphere. The Synoz (synthetic ozone) scheme (McLinden et al., 2000) is used as a flux upper boundary condition for ozone in the stratosphere and yields a cross-tropopause ozone flux of 500 Tg/yr. More details about the physical and chemical mechanisms were discussed in Emmons et al. (2010). In this study, MOZART-4 is run with the standard chemical mechanism [see Emmons et al., 2010 for details], with online calculation of dry deposition. It is driven by meteorological parameters from the
NCAR reanalysis of the National Centers for Environmental Prediction (NCEP) forecasts. The output from the model run is available at a temporal resolution of 6 h, a horizontal spatial resolution of approximately 2.8° × 2.8°, and 28 hybrid levels in the vertical. The top model level is located at approximately 2 hPa. The initial condition and emissions are based on the NCAR Community Data Portal (http://cdp.ucar.edu/). The model was run in time steps of 20 min from June 2000 to December 2007, with the first seven months used to spin up. The experiments used fixed emissions from 2001 and the meteorological parameters were varied for each year over the simulations, with the first seven months as spin-up time. Note that the modeled results are based on the mean values in the atmospheric boundary layer (the six lowermost layers in the model, surface to ~ 2 km) except for the model validations in Section 3.1 and the special definitions in Section 3.2. 2.3. Monsoon index and ozone budget Previous studies showed that the East Asian monsoon is characterized as a seasonal reversal of the lower-troposphere meridional wind direction (Lau and Li, 1984; Chen et al., 1991), and the 850 hPa wind is used to reflect the low-level atmospheric circulation (Lu and Chan, 1999). Therefore, the daily meridional wind at 850 hPa is used to show the seasonal march of the Asia-Pacific monsoon in this study. A pre-defined monsoon index (MI) can provide a useful insight into quantitatively examining the strength and variation of monsoon circulation in monsoon regions. For the Asia-Pacific monsoon region, many studies have already defined various indices based on considerations of dynamics and thermodynamics from different aspects, such as wind field, precipitation, difference of ocean–land temperature, and so on. Each MI is designed to capture specific physical processes. In this study, we used a dynamically normalized seasonality MI developed by Li and Zeng (2003) to examine the wind field to investigate the influence of the Asia-Pacific monsoon on the inter-annual variability of ozone. The factors controlling ozone levels in the atmospheric boundary layer (transport, net photochemical production, and deposition) are discussed in this study. Each term affecting the ozone budget is evaluated quantitatively at 850 hPa in Section 3.2 and in a column from the surface to approximately 2 km (the atmospheric boundary layer) in Section 3.3. The rate of change of ozone can be expressed as: dO3 ¼ Chem þ Adv þ Con þ Dif −Dep dt where Chem represents the net chemical production; Adv, Con, and Dif are the transport fluxes associated with advection, convection and diffusion, respectively; Dep is the dry deposition rate. Fig. 4 shows each of these components of the ozone budget for the study region: net chemistry, net transport flux (advection, convection, and diffusion), and O3S. O3S is an O3 tracer that tags O3 transported from the stratosphere (Sudo and Akimoto, 2007). 3. Results and discussion 3.1. Validation of simulated O3 and precipitation The modeling system described in Section 2.2 has previously been used for analyzing tropospheric O3 over the northern hemisphere (Horowitz et al., 2003; Liu et al., 2005; Pfister et al., 2008a,b; Emmons et al., 2010), and, in these studies, the simulated results showed good agreement with observations. Stevenson et al. (2006) suggested that the tropospheric O3 budget in MOZART-4 is in good agreement with the mean of 26 models, whereas the stratospheric input value has been determined to be realistic by Wild (2007). In this section, we further evaluate the general performance of our modeling system for O3 over the Asia-Pacific region.
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Fig. 1. Comparison of modeled O3 (ppbv, black line) to observation (red line) at selected sites over the Asia-Pacific rim region between 2001 and 2007. All of the data were calculated based on the mean of seven years' data (similarly hereinafter). R is the correlation coefficient. The degrees of freedom are 363, and p values in the test are 0.05. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 1 shows a comparison of modeled daily O3 with observations over the Asia-Pacific region. Seasonal variations with a spring maximum and a summer minimum are fairly well reproduced by the model. Simulated results of the spring maximum at middle latitude sites (Rishiri, Tappi, and Sado-Seki) are underestimated relative to observations, while summer minima at all sites are overestimated. These discrepancies may result from the coarse horizontal model resolution in meteorology and pollutant sources due to sub-grid-scale processes that cannot be accurately simulated. Significant effects of cloud and wet processes may also be partly responsible for the discrepancies in summer ozone, as discussed below. The correlation-coefficient (R) between simulated and observed daily results ranges from approximately 0.47 to 0.87. They all passed the t test under the 95% confidence level. The relatively strong correlations (an R of approximately 0.8) at four oceanic sites (Ogasawara, Hedo, Yonagunijima, and Minamitorishima) are primarily attributed to their remote locations with less impact from continental emissions. Other sites at mid-latitude can be affected both by local to regional anthropogenic emissions and complex topography, which might account for the less than satisfactory model performance at these locations. The increase of precipitation intensity and the northward migration of the rain belt are key features of the summer monsoon onset. Although direct precipitation scavenging of ozone is relatively unimportant due to its low solubility, precipitation can affect ozone chemical production by altering solar radiation and through wet removal of soluble ozone precursors. Consequently, it is important to evaluate the simulated precipitation. Fig. 2 indicates that the diagnosis precipitation closely represents the GPCP data in January. In April, July, and October, strong precipitation over Japan and its northern regions in the GPCP data is underestimated by the model. Because MOZART-4 is an offline model, the accuracy of simulated precipitation depends largely on the accuracy of the meteorological fields, which may have some errors in mid- and high-latitudes that lead to an underestimation of precipitation. On the other hand, the coarse horizontal resolution in the global model may inaccurately diagnose sub-grid-scale processes, leading to these discrepancies. At relatively low latitudes, the diagnosis result represents the GPCP precipitation very well. 3.2. The impact of the Asia-Pacific monsoon march on O3 seasonal variation Wang and Lin (2002) demonstrated the onset time of the Asia-Pacific summer monsoon varies across different regions based on the
precipitation analysis, with the 30th pentad (150th Julian day) reaching 20°–30°N and the 40th pentad (200th Julian day) being north of 30°N. Accordingly, it is necessary to divide the Asia-Pacific monsoon region to show the onset period. In this study, the monsoon regions covering 120°–140°E longitude and 20°–30°N and 30°–50°N latitude were chosen, considering the spatial extent of the region affected by the monsoon and the distribution of sites available for estimation. The meridional wind at 850 hPa can be used to show the seasonal march of the Asia-Pacific monsoon, where positive meridional winds represent the prevailing summer monsoon (Lau and Li, 1984; Chen et al., 1991; Lu and Chan, 1999). Fig. 3 shows that the sub-tropical summer monsoon onset is in March at 20°–30°N and is in early April to late May in 30°–50°N. The retreat of the summer monsoon occurs earlier at 30°–50°N than at 20°– 30°N. The onset and retreat of the Asia-Pacific summer monsoon match the results of Wang and Lin (2002) very well. As shown in Fig. 4a and b, there is a transition zone of pollutants near 20°N in the winter. South of 20°N, pollutant concentrations of O3, CO, NO are relatively low with a lesser impact of continental emissions. North of 30°N, pollutant concentrations are relatively high with a strong influence from anthropogenic emissions or transport. The difference in anthropogenic emissions plays a key role in the formation of a transition zone. In addition, the difference in prevailing winds is also largely responsible for the formation of a transition zone. South of 20°N, where the tropical monsoon resides, easterly winds prevail in the winter. North of 20°N, where the subtropical monsoon is located, northwesterly winds prevail in the winter. Because of the differences in zonal wind and the weak meridional wind, a shear line is located at approximately 20°N, which limits the air masses' transport and exchange between north and south. With the onset of summer monsoons in late March at 20°N, southerly winds prevail, causing the transition zone to move northward. In summer, the transition zone continuously moves northward with the advance of the summer monsoon. In autumn, the summer monsoon retreats. Southerly winds become weak, and the zonal wind component increases. The North–south differences in the zonal wind component decrease, causing the transition zone to retreat back to 20°N. Eventually, a transition zone of pollutants moves back and forth in a south to north direction with the advance of the summer monsoon. Tropospheric ozone, as a secondary pollutant, is affected by solar radiation and the emission of precursor species. The transition from the winter monsoon to the summer monsoon results in a series of changes
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Fig. 2. Comparison of monthly mean precipitation (shaded) to GPCP data (contour) over the Asia-Pacific rim region in January, April, July and October during 2001–2007, unit: mm/d. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
in weather, such as atmospheric circulations and precipitation, which significantly affect the pollutants' transport paths and ozone photochemical production. South of 20°N, the precursors of ozone are very low, especially NO, which is as low as 4 pptv, resulting in a net photochemical loss of ozone (see Fig. 4b and c). Precipitation increases south of 20°N in early May (Fig. 4c), slowing down the photochemical reactions there. In summer, southerly winds prevail and transport low latitude clean air deeply towards higher latitudes at approximately 45°N. Therefore, the O3 seasonal pattern with maximum pollutant levels in the winter or early spring and a broad minimum in the summer is present south of 20°N.
Fig. 3. The mean meridional wind at 850 hPa (blue points) between 120° and 140°E and different latitude regions (a, 30°–50°N; b, 20°–30°N). The solid black line represents the wind speed at 0 m/s. The solid red line is a Gaussian fitting curve. The green dashed line represents the 95% prediction bands. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
In comparison with that of south 20°N, the concentration of NO is high in the winter over north of 30°N, but the net chemical production of ozone is low under the winter weak solar radiation. In spring, net ozone chemical production increases with the enhancement of solar radiation, leading to a maximum ozone concentration at 40°N in May. Due to the later onset of the summer monsoons in the regions between 30° and 50°N, westerly winds prevail until late June. High precursor concentrations, transported from upwind regions or locally emitted, lead to the accumulation of ozone through strong chemical production under strong solar radiation. In summer, cloudiness and precipitation are enhanced at 30°–50°N. The concentrations of ozone precursors decrease, resulting from the inflow of clean marine air masses by prevailing southerly winds and the wet removal by the rain belt. Net ozone production is reduced by decreased solar radiation and ozone precursors. The net ozone transport flux during this time is outflow, but the outflow is weaker than in the spring. The overall result of the factors discussed above is a decrease in net chemical production, eventually leading to an ozone minimum in summer. The direct transport of ozone by wind has a relatively weak impact on the summer minimum due to the weak outflow or inflow. The shift of south winds to westerly winds in early September reflects the relatively early retreat of summer monsoons at 30°–50°N. High concentrations of precursors enhance the net ozone chemical production. Similarly as in spring, the ozone concentration is higher in autumn than in summer. The contribution of stratospheric ozone (Fig. 4d) includes a winter maximum and a summer minimum. Its seasonal variation has an influence on ozone concentrations, especially south of 20°N where the contribution of stratospheric ozone is approximately 8 ppbv in winter, or approximately 30%.
3.3. The impact of the Asia-Pacific summer monsoon on inter-annual variation of O3 This section focuses on the influence of the early/late onset of the Asia-Pacific sub-tropical summer monsoon on the budget of O3 in the
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(a)
(b)
(c)
(d)
Fig. 4. The zonal mean ozone mixing ratio (ppbv, shaded), wind (m/s, vector), zonal wind speed (m/s, contour) (a); CO (ppbv, shaded), NO (pptv, contour) and wind (m/s, vector) (b); precipitation (mm/d, contour) and the O3 net chemical production (ppbv/d, shaded) (c); the contribution of stratospheric O3 (ppbv, contour) and the net O3 transport flux (ppbv/d, shaded) (d) at 850 hPa between 1° and 65°N. The black solid line is the 40 ppbv of O3 in (a). The dashed lines are the latitudes of 20°N, 30°N, and 50°N. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
atmospheric boundary layer over the low-latitude oceanic stations, where model results were well-correlated with observations (see Section 3.1). The correlation coefficients of simulated and observed O3 anomalies are 0.56, 0.36, 0.5, and 0.52 at Hedo, Ogasawara, Yonagunijima, and Minamitorishima, respectively. As shown in Fig. 5, most of the simulated and observed anomalies of O3 concentration are significant at the 95% confidence level, and they all pass the test at the 95% confidence level. The model results can be used to analyze the
inter-annual variation of ozone in the atmospheric boundary layer. Ogasawara is located in an area with a strong gradient of wind speeds and direction, a region that is most sensitive to the monsoon transition (seen Fig. 8). It is difficult to capture the ozone inter-annual variations accurately in the global model, leading to the relatively low correlation coefficient of O3 at Ogasawara. We calculated the dynamically normalized seasonality monsoon index defined by Li and Zeng (2003) over the region 20–50°N and
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Fig. 5. Scatter plots showing the relationship between observed and simulated daily surface ozone anomalies at Hedo, Ogasawara, Yonagunijima and Minamitorishima from 2001 to 2007 (OB and MZ4 represent observations and simulations, respectively). The simulated ozone anomaly is the simulation for the fixed emissions. The solid red line is the linear regression equation. The green dashed line is the 95% confidence level. N is the number of paired samples. R is the correlation coefficient. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
120–140°E using NCEP reanalysis data, to obtain an Asia-Pacific monsoon index (AMI) and its anomaly (shown in Fig. 6). From 2001 to 2007, AMI is negative in the winter and positive in the summer, reasonably representing the winter and summer monsoons. In March and April, a negative AMI value implies that the monsoon is still in its winter phase. In May, it has transitioned to its summer phase, on average. The AMI and its anomaly are positive in May 2003, 2004 and 2006, indicating the early onset of Asia-Pacific summer monsoon in these years. In April 2003, the AMI is near zero and has a high positive anomaly implying a much earlier transition of the winter monsoon into the summer monsoon. To unify the transition period, we chose May in 2004 and 2006 as the early transition periods of the winter monsoon to the summer monsoon (strong monsoon period) and May in 2002 and 2007 as the late transition periods for the negative anomalies (weak monsoon period) to discuss the influence of the summer monsoon on O3 over the Asia-Pacific Ocean. Budget analysis from Fig. 7 shows the net chemistry is − 0.03 ppbv/d in Hedo and 0.32 ppbv/d in Yonagunijima, while it is − 2.1 ppbv/d in Ogasawara and − 2.8 ppbv/d in Minamitorishima. The absolute value of net chemistry in polluted oceanic sites (Hedo
Fig. 6. The Asia monsoon index (AMI, blue line) and the anomaly of AMI (red bar). The bar marked with the green line is the anomaly of AMI in March, April, and May, every year. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
and Yonagunijima) is lower than in remote oceanic sites (Ogasawara and Minamitorishima) due to stronger chemical production in polluted oceanic regions. In addition, according to the division of AsiaPacific monsoon regions by Wang and Lin (2002), Hedo and Yonagunijima are located in the subtropical East Asia (EA) monsoon region, Ogasawara is at the boundary of the EA and western North Pacific (WNP) monsoon regions, and Minamitorishima is in the tropical WNP monsoon region. Fig. 7 shows O3 during early transition periods is lower than average in Yonagunijima and Hedo, and higher than the mean at Ogasawara and Minamitorishima. In late transition periods the effect is opposite, except in Ogasawara due to its location on the boundary of the EA and WNP monsoon regions (Wang and Lin, 2002) with sharp gradients of wind speed and direction. As the star reveals in Fig. 7, the contribution of stratospheric O3 is 3.5 ppbv at Hedo in early transition periods, lower by 0.1 ppbv than the mean value, which cannot account for the decreased 3 ppbv of O3 from the average. In late transition periods, the contribution is higher by 1.1 ppbv than the mean at Hedo, which does not lead to an O3 increase of 4 ppbv. We can also find similar facts at Ogasawara, Yonagunijima, and Minamitorishima. Accordingly, ozone intruded from the stratosphere has an influence on the inter-annual variation of the boundary layer O3 in the spring, but it is not the only responsible factor. The controlling sources of ozone in polluted oceanic sites are different from that in remote oceanic sites. As the mean values show in Fig. 7, controlling sources are convection and diffusion in Hedo, chemistry and diffusion in Yonagunijima, and convection and advection in Ogasawara and Minamitorishima. Chemistry and diffusion have relatively low correlation with the variation of the monsoon transition period, as well as dry deposition. The anomalies of convection are relatively large in polluted oceanic sites. They are correlated with O3 anomalies in Yonagunijima while they are anti-correlated in Hedo. Therefore, convection may have some contribution to the O3 interannual variability. The anomalies of advection are relatively large, and positively correlated with O3 anomalies in all oceanic sites. In polluted oceanic sites,
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Fig. 7. Boundary layer O3 mixing ratios (unit: ppbv), O3S (unit: ppbv) and the daily rates of change in O3 mixing ratios (unit: ppbv/d) at four oceanic sites in May in the multi-year average (mean), early transition period (early), and late transition period (late). Black dots are O3. Stars are O3S. The values marked on the color bars are the daily rates of change in O3 from individual processes (values lower than 0.1 are not marked). The value on the black box indicates the influence of the stratospheric tracer (O3S). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
advection is O3 outflow from the regions, −0.4 ppbv/d. The outflow is strong in the early transition period, about − 1 ppbv/d, and weak in the late transition period, − 0.2 ppbv/d. Advection is O3 inflow in remote oceanic sites, approximately 1.2 ppbv/d. It is also strong in the early transition period, approximately 2 ppbv/d, and weak in the late transition period, approximately 1 ppbv/d. We find that advection clearly plays a key role in the impact of the summer monsoon on the inter-annual variation of the O3 boundary layer in May (spring). Net chemistry, convection, diffusion, and dry deposition may disturb the impact. Winds and their anomalies were analyzed to investigate O3 advection (export and import) in Fig. 8. As shown in Fig. 8a, the average prevailing wind is southwesterly in Yonagunijima and Hedo and is southeasterly in Minamitorishima. This suggests that Yonagunijima and Hedo are influenced by continental outflow of O3 and O3 precursors, and Minamitorishima is influenced by the transport of pollutants around the Pacific Ocean under the effect of a subtropical high. In the early transition period, the mean wind field (Fig. 8a) and wind anomalies (Fig. 8b) in Yonagunijima and Hedo show that the westerly components and southerly components are both stronger than the corresponding components of the mean wind field, resulting in strong net inflows of clean oceanic air in Yonagunijima and Hedo. In Minamitorishima, the northeasterly wind anomalies indicate the weak southerly components and strong easterly components, making the transport around the Pacific Ocean strong. Thus the inflow of O3 in remote oceanic sites is strong. There are negative O3 anomalies in polluted oceanic sites, and positive O3 anomalies in remote oceanic sites. In late transition periods, as shown in Fig. 8c, there is a weak westerly component of winds on the coast of East China and Japan, due to the northeastern wind anomalies, causing a weak inflow of clean oceanic air through advection, leading to the high O3 value in Yonagunijima and Hedo. However, polluted air masses from the continent tend to be transported to higher latitudes by the strong southerly component of the winds east of 160°E and north of 30°N (Fig. 8c). As a
result, O3 inflow is weak in Minamitorishima, leading to low O3 values in the late transition period. Ogasawara is located at the boundary of the wind anomaly transition, making the effects of the monsoon transition on local ozone concentrations apparent. 4. Conclusions This study investigated the seasonal and inter-annual variations of O3 in the atmospheric boundary layer over the Asia-Pacific region between 2001 and 2007 using multi-year simulations by MOZART-4. The influence of the Asia-Pacific monsoon march on ozone seasonal variations was introduced by an analysis of the ozone budget. The relationship between the Asia-Pacific summer monsoon onset and the inter-annual variation of O3 was also examined by a seven-year numerical experiment with fixed emissions from 2001 and varied meteorology (from NCEP data) for 2001–2007 over the Asia-Pacific Ocean. The daily meridional wind at 850 hPa was used to show the seasonal march of the Asia-Pacific summer monsoon. A dynamically normalized seasonality MI developed by Li and Zeng (2003) on the basis of wind field calculations was used to distinguish the early and late transition periods of the summer monsoon onset. The main findings of the study are summarized as follows: 1. The model simulation reasonably reproduced both seasonal and interannual variations of O3, particularly at relatively low latitude oceanic sites where correlation coefficients between modeled and observed values reached 0.87. The modeled results also represented GPCP precipitation patterns for the region. 2. The Asia-Pacific monsoon has a great influence on the seasonal and inter-annual variations of O3 over the Asia-Pacific Ocean. Differences of zonal wind in the meridional direction create a transition zone near 20°N between high and low pollutant regions in the winter. The onset of the summer monsoon with the northward migration of the rain belt helps move the transition zone northward.
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(a) May
(b) early
(c) late
Fig. 8. The multi-year average of boundary layer (BL) O3 mixing ratios (ppbv) and wind filed (m/s) in May from 2001 to 2007 (a), and the anomalies of O3 (ppbv) and wind (m/s) in the early transition period (b) and in the late transition period (c). The large arrows overlaid on the figure are the wind direction, showing the large scale circulation over the north Pacific.
3. North of 30°N, the transport of clean marine air masses by prevailing southerly winds and the northward migration of the rain belt upon the onset of the summer monsoon result in low concentrations of ozone precursors in summer. This makes net O3 chemical production weak and results in a summer ozone minimum. The direct transport of ozone by wind has a relatively weak impact on the summer minimum. 4. In the early transition of the Asia-Pacific winter monsoon to the summer monsoon phase, strong inflows of clean oceanic air tend to lead to lower levels of O3 than average at polluted oceanic sites near the continent. At the same time, stronger outflows of continental air masses result in higher levels of O3 at remote oceanic sites. The reverse is true in years when the summer monsoon onset is late. Advection appears to play a key role in the impact of the summer monsoon onset on the inter-annual variation of O3 in spring.
Acknowledgments This work was supported by the National Natural Science Foundation of China (40875078), the European Union Seventh Framework Program ([FP7/2007–2013]) under grant agreement no. 606719 (PANDA project), the Natural Science Foundation of the Higher Education Institutions of Jiangsu Province, China (12KJA170003), the project of Jiangsu provincial “333” and Six Talent Peaks, and the Program for Postgraduates Research Innovation of Jiangsu Higher Education Institutions (grant no. CXZZ13_0509). We would like to acknowledge the entire staff of the EANET and the WDCGG. Special thanks go to Stacy Walters and Louisa Emmons of NCAR who provided MOZART-4 source codes and helped us install and run it. We thank Jeff Collett at Colorado State University in America and Tianliang Zhao at Nanjing University of Information science & Technology in China for their valuable suggestions on improving the manuscript.
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