Environ Sci Pollut Res DOI 10.1007/s11356-013-2387-1
RESEARCH ARTICLE
The local and regional atmospheric oxidants at Athens (Greece) C. A. Varotsos & J. M. Ondov & M. N. Efstathiou & A. P. Cracknell
Received: 29 August 2013 / Accepted: 19 November 2013 # Springer-Verlag Berlin Heidelberg 2013
Abstract In the present study, the investigation of the levels of the local and regional oxidants concentration at Athens, Greece, is attempted by analyzing the observations obtained at an urban and a rural station, during 2001–2011 and 2007– 2011, respectively. A progressive increase of the daytime and nighttime average of [NO2]/[Ox] versus [NOx] is observed showing a larger proportion of Ox in the form of NO2 when the level of NOx increases. Similar results are observed when studying the variation of mean values of [NO2]/[NOx] versus [NOx]. The results obtained when compared with those that have earlier detected elsewhere, revealed similarities and discrepancies that are discussed in detail. The parameterized curves that are presented for the first time in this paper may be used by the air quality planners to track the trends in other cities also, and to understand what is or was driving them.
(NO), and nitrogen dioxide (NO2) as a function of nitrogen oxides (NOx =NO+NO2), using monitoring data from the UK Automatic Urban and Rural Network. They attempted to establish how the level of “oxidant” (Ox =O3 +NO2) varies with the level of NOx, in order to contribute to the knowledge of the atmospheric sources of Ox, particularly at polluted urban locations (see also Kley et al. 1994). The concepts of NOx and Ox are useful, because they define sets of species that are readily interconverted by chemical processes in the atmosphere, namely, NO þ O3 ¼ NO2 þ O2
ð1Þ
NO2 þ lightðþO2 Þ ¼ NO þ O3
ð2Þ
Keywords Air pollution . Oxidation
Introduction Clapp and Jenkin (2001) studied the relationships between the ambient concentration of ozone (O3), nitrogen monoxide Responsible editor: Gerhard Lammel C. A. Varotsos : M. N. Efstathiou (*) Climate Research Group, Division of Environmental Physics and Meteorology, Faculty of Physics, University of Athens, University Campus Bldg. Phys. V, Athens 15784, Greece e-mail: [email protected] J. M. Ondov Department of Atmospheric and Oceanic Science, University of Maryland, College Park, MD 20742, USA A. P. Cracknell Division of Electronic Engineering and Physics, University of Dundee, Dundee DD1 4HN, Scotland, UK
As a result, the ambient levels of NOx and Ox are unaffected by this chemistry, allowing the sources to be identified and quantified more easily. In this regard, Clapp and Jenkin (2001) suggested that Ox concentration consists of the NOx-independent “regional” contribution (corresponding to the regional background O3 level) and the NOx-dependent “local” contribution (linearly correlated with the level of local pollution). Furthermore, they have proposed that the local oxidant source has plausible contributions from direct NO2 emissions, from the thermal reaction of NO with oxygen at high NOx and from commonsource emission of species that promote NO to NO2 conversion. Clapp and Jenkin (2001) provided relationships, which describe the possible variation of the annual mean NO2 versus NOx taking into account the possible changes in the regional background of O3. Three years later, Jenkin (2004) examined the daytime and seasonal dependence of sources of oxidant, and their origins, by using hourly mean concentration data for NO, NO2, and O3
Environ Sci Pollut Res
at Marylebone Road (a city-center curbside site in London). He studied the concentrations of oxidant ([Ox]) as a function of the sum of an NOx-independent regional contribution and a linearly NOx-dependent local contribution. According to the findings of Jenkin (2004), the regional [Ox] displays a significant seasonal variation, with a pronounced peak in April, giving similar results to those reported for background ozone at low altitude sites in northwest Europe. On the other hand, the local Ox contribution seemed to obey a strong daytime variation, throughout the year, with maximum values at the daytime hours. Han et al. (2011) investigated the relationship between the O3 distribution and its association with ambient concentrations of NO, NO2, and NOx, using continuous measurement of NO, NO2, NOx, and O3 in Tianjin, China (during the period 9 September to 15 October 2006). The hourly concentrations of the studied pollutants seemed to be maximized in succession in the daytime, whereas the ground-level ozone concentration peaked at mid-day. Han et al. (2011) also proposed a linear relationship between [NO2] and [NOx], as well as between [NO] and [NOx], and a polynomial relationship between [O3] and [NO2]/[NO]. In the simple first order model, ozone concentration is generally proportional to the ratio of [NO2]/[NO]. This is because formation of ozone requires atomic oxygen, which in the polluted troposphere will be predominately made from photolysis of NO2 with light in the visible range. The other major source of atomic O is photolysis of ozone made from lightning. This makes O1D, which can react with water molecules to make OH, which in turn attacks hydrocarbons, making peroxy radicals that can oxidize NO to NO2. Thus, ozone made naturally can jump-start urban smog formation. Clearly ozone made upwind by any means could jump-start ozone formation downwind but there are generally other sources of OH radical (e.g., HONO, which rapidly photolyzes to NO+OH). HONO concentrations build at night, when it is dark, and the OH produced when the sun comes up is an important factor in initiating HC oxidation, which, as just mentioned, produces peroxy radicals that oxidize NO to NO2. Note also, that ozone and NO react so quickly that they cannot coexist. One titrates the other quantitatively until only the reagent in molar excess remains. NO2 +NO is fairly constant except for HNO3 and HONO formed from the former by reaction with OH. Nitric acid will stick to particles, be absorbed by cloud droplets, and also be removed by reaction with ammonia. Thus, changes in NOx occur when sources contribute more NO or NOx (note that NO is typically by far the major emission from combustion sources), and by conversion/removal as HNO3 occurs as emissions age. HONO photolyzes very quickly, so its formation simply reduces the OH and NO concentrations (and hence O3) by acting as a reservoir species. Only at night is the reservoir concentration of HONO very high (compared with daytime).
Han et al. (2011) confirmed previous studies, showing that the concentration of Ox at a given location consists of two parts: one, independent of, and the other dependent on NO2 concentration, while the independent part (∼20 ppb in Tianjin) was considered as a regional contribution. They also noticed a significant difference in NO, NOx, and O3 concentrations between weekdays and weekends and finally studied the daytime O3 variation in different meteorological conditions (Mazzeo et al. 2005). Notario et al. (2012) studied the concentrations of O3, NO, NO2, NOx, and Ox in the southwest of the Iberian Peninsula (Seville) during 2004; this is an area with frequent photochemical pollution events, mainly in the warm season. They used observation data obtained at an urban traffic station and a suburban one. Notario et al. (2012) considered the monthly dependence of regional and local [Ox] variation, at both stations, along with the annual variation of the daily mean NOx and Ox. For the daytime in the summer months, they found that at the suburban station the maximum levels of [Ox]> 190 μg m−3 are accompanied with [NOx]
RESEARCH ARTICLE
The local and regional atmospheric oxidants at Athens (Greece) C. A. Varotsos & J. M. Ondov & M. N. Efstathiou & A. P. Cracknell
Received: 29 August 2013 / Accepted: 19 November 2013 # Springer-Verlag Berlin Heidelberg 2013
Abstract In the present study, the investigation of the levels of the local and regional oxidants concentration at Athens, Greece, is attempted by analyzing the observations obtained at an urban and a rural station, during 2001–2011 and 2007– 2011, respectively. A progressive increase of the daytime and nighttime average of [NO2]/[Ox] versus [NOx] is observed showing a larger proportion of Ox in the form of NO2 when the level of NOx increases. Similar results are observed when studying the variation of mean values of [NO2]/[NOx] versus [NOx]. The results obtained when compared with those that have earlier detected elsewhere, revealed similarities and discrepancies that are discussed in detail. The parameterized curves that are presented for the first time in this paper may be used by the air quality planners to track the trends in other cities also, and to understand what is or was driving them.
(NO), and nitrogen dioxide (NO2) as a function of nitrogen oxides (NOx =NO+NO2), using monitoring data from the UK Automatic Urban and Rural Network. They attempted to establish how the level of “oxidant” (Ox =O3 +NO2) varies with the level of NOx, in order to contribute to the knowledge of the atmospheric sources of Ox, particularly at polluted urban locations (see also Kley et al. 1994). The concepts of NOx and Ox are useful, because they define sets of species that are readily interconverted by chemical processes in the atmosphere, namely, NO þ O3 ¼ NO2 þ O2
ð1Þ
NO2 þ lightðþO2 Þ ¼ NO þ O3
ð2Þ
Keywords Air pollution . Oxidation
Introduction Clapp and Jenkin (2001) studied the relationships between the ambient concentration of ozone (O3), nitrogen monoxide Responsible editor: Gerhard Lammel C. A. Varotsos : M. N. Efstathiou (*) Climate Research Group, Division of Environmental Physics and Meteorology, Faculty of Physics, University of Athens, University Campus Bldg. Phys. V, Athens 15784, Greece e-mail: [email protected] J. M. Ondov Department of Atmospheric and Oceanic Science, University of Maryland, College Park, MD 20742, USA A. P. Cracknell Division of Electronic Engineering and Physics, University of Dundee, Dundee DD1 4HN, Scotland, UK
As a result, the ambient levels of NOx and Ox are unaffected by this chemistry, allowing the sources to be identified and quantified more easily. In this regard, Clapp and Jenkin (2001) suggested that Ox concentration consists of the NOx-independent “regional” contribution (corresponding to the regional background O3 level) and the NOx-dependent “local” contribution (linearly correlated with the level of local pollution). Furthermore, they have proposed that the local oxidant source has plausible contributions from direct NO2 emissions, from the thermal reaction of NO with oxygen at high NOx and from commonsource emission of species that promote NO to NO2 conversion. Clapp and Jenkin (2001) provided relationships, which describe the possible variation of the annual mean NO2 versus NOx taking into account the possible changes in the regional background of O3. Three years later, Jenkin (2004) examined the daytime and seasonal dependence of sources of oxidant, and their origins, by using hourly mean concentration data for NO, NO2, and O3
Environ Sci Pollut Res
at Marylebone Road (a city-center curbside site in London). He studied the concentrations of oxidant ([Ox]) as a function of the sum of an NOx-independent regional contribution and a linearly NOx-dependent local contribution. According to the findings of Jenkin (2004), the regional [Ox] displays a significant seasonal variation, with a pronounced peak in April, giving similar results to those reported for background ozone at low altitude sites in northwest Europe. On the other hand, the local Ox contribution seemed to obey a strong daytime variation, throughout the year, with maximum values at the daytime hours. Han et al. (2011) investigated the relationship between the O3 distribution and its association with ambient concentrations of NO, NO2, and NOx, using continuous measurement of NO, NO2, NOx, and O3 in Tianjin, China (during the period 9 September to 15 October 2006). The hourly concentrations of the studied pollutants seemed to be maximized in succession in the daytime, whereas the ground-level ozone concentration peaked at mid-day. Han et al. (2011) also proposed a linear relationship between [NO2] and [NOx], as well as between [NO] and [NOx], and a polynomial relationship between [O3] and [NO2]/[NO]. In the simple first order model, ozone concentration is generally proportional to the ratio of [NO2]/[NO]. This is because formation of ozone requires atomic oxygen, which in the polluted troposphere will be predominately made from photolysis of NO2 with light in the visible range. The other major source of atomic O is photolysis of ozone made from lightning. This makes O1D, which can react with water molecules to make OH, which in turn attacks hydrocarbons, making peroxy radicals that can oxidize NO to NO2. Thus, ozone made naturally can jump-start urban smog formation. Clearly ozone made upwind by any means could jump-start ozone formation downwind but there are generally other sources of OH radical (e.g., HONO, which rapidly photolyzes to NO+OH). HONO concentrations build at night, when it is dark, and the OH produced when the sun comes up is an important factor in initiating HC oxidation, which, as just mentioned, produces peroxy radicals that oxidize NO to NO2. Note also, that ozone and NO react so quickly that they cannot coexist. One titrates the other quantitatively until only the reagent in molar excess remains. NO2 +NO is fairly constant except for HNO3 and HONO formed from the former by reaction with OH. Nitric acid will stick to particles, be absorbed by cloud droplets, and also be removed by reaction with ammonia. Thus, changes in NOx occur when sources contribute more NO or NOx (note that NO is typically by far the major emission from combustion sources), and by conversion/removal as HNO3 occurs as emissions age. HONO photolyzes very quickly, so its formation simply reduces the OH and NO concentrations (and hence O3) by acting as a reservoir species. Only at night is the reservoir concentration of HONO very high (compared with daytime).
Han et al. (2011) confirmed previous studies, showing that the concentration of Ox at a given location consists of two parts: one, independent of, and the other dependent on NO2 concentration, while the independent part (∼20 ppb in Tianjin) was considered as a regional contribution. They also noticed a significant difference in NO, NOx, and O3 concentrations between weekdays and weekends and finally studied the daytime O3 variation in different meteorological conditions (Mazzeo et al. 2005). Notario et al. (2012) studied the concentrations of O3, NO, NO2, NOx, and Ox in the southwest of the Iberian Peninsula (Seville) during 2004; this is an area with frequent photochemical pollution events, mainly in the warm season. They used observation data obtained at an urban traffic station and a suburban one. Notario et al. (2012) considered the monthly dependence of regional and local [Ox] variation, at both stations, along with the annual variation of the daily mean NOx and Ox. For the daytime in the summer months, they found that at the suburban station the maximum levels of [Ox]> 190 μg m−3 are accompanied with [NOx]
