key: cord-0914238-yilhi0qv
authors: Borhani, Faezeh; Shafiepour Motlagh, Majid; Stohl, Andreas; Rashidi, Yousef; Ehsani, Amir Houshang
title: Tropospheric Ozone in Tehran, Iran, during the last 20 years
date: 2021-10-18
journal: Environ Geochem Health
DOI: 10.1007/s10653-021-01117-4
sha: 38b4b3eba46a90744f2c8e4f54af59ed2265aad8
doc_id: 914238
cord_uid: yilhi0qv

Air pollution and its effects on human health and the environment are one of the main concerns in urban areas. This study focuses on the distribution and changes in the concentrations of ozone and its precursors (i.e., NO, NO(2) and CO) in Tehran for the 20-year period from 2001 to 2020. The effects of precursors and meteorological conditions (temperature, wind speed, dew point, humidity and rainfall) on ozone were investigated using data from 22 stations of the Air Quality Control Company (AQCC) and meteorological stations. Regression models were applied to evaluate the dependence of ozone concentration on its precursors and meteorological parameters based on monthly average values. Finally, the monthly and annual levels of surface ozone and total column ozone were compared during the study period. The results show that the average ozone concentration in Tehran varied substantially between 2001 and 2008, and decreased after 2008 when stringent air quality control measures were implemented. The highest average concentration of ozone occurred in the southwest of Tehran. Although mobile and resident sources play an important role in the release of precursors, the results also indicate a significant effect of meteorological conditions on the changes in ozone concentration. This study is an effective step toward a better understanding of ozone changes in Tehran under the changing influence of precursors and meteorological conditions.

Air pollution is one of the main problems in industrial cities. Reducing the damage caused by air pollution requires careful evaluation of the pollutant concentrations and influencing parameters. In addition to pollutant emitters, meteorological parameters have a significant role in the amount of pollution and consequently the air quality of the urban environment (Javanbakht Amiri & Khatami, 2012; Kayes et al., 2019) . The city of Tehran, the capital of Iran, is facing an air pollution crisis in recent years due to population growth, increase in the number of vehicles, as well as the intense concentration and accumulation of industries (Sotoudeheian et al., 2014) . Tehran is one of the most polluted cities in the world (EEA, 2018) . Therefore, it is very important to study the air pollution trends in the city.

Tropospheric ozone (O 3 ) is a very strong oxidant that is an indicator of photochemical oxidants. Ozone is a summer pollutant and an important short-lived climate pollutant (Faridi et al., 2018; Liu et al., 2019) . Tropospheric ozone plays an important role in atmospheric chemistry, air quality, and climate change and is harmful to human health and agricultural production (Agathokleous et al., 2020; Monks et al., 2015; WHO, 2003; Zeng et al., 2008) . In recent years, high levels of ozone (O 3 ) and its effect on air quality have become a global challenge Paoletti et al., 2014) . It has been suggested that for every 10 lg/m 3 increase in ozone concentration, the daily human death rate increases by about 0.3% and the death rate of heart patients by about 0.4% (Colls & Tiwary, 2009 ). Ozone has an associated radiative forcing of 0.35 W/m 2 , but its maximum concentrations on the ground rarely last more than two to three hours (Cross & Pierson, 2013) . The half-life of tropospheric ozone at 20 degrees Celsius is about three days (Shindell et al., 2012) . After carbon dioxide and methane, tropospheric ozone is the third leading cause of global warming (Fowler et al., 2008; Stevenson et al., 2006) . Tropospheric ozone is produced by chemical reactions between Volatile Organic Compounds (VOCs) and carbon monoxide (CO) and hydroxyl radicals (OH) in the presence of nitrogen oxides (NOx) in the atmosphere. Therefore, nitrogen oxides (NO X ) and VOCs (as well as CO) are precursors of tropospheric ozone (Crutzen, 1974; Retama et al., 2015) . In other words, ozone is formed when pollutants react chemically in the presence of sunlight (Cao et al., 2019; Sudo & Akimoto, 2007) . In addition to natural sources, anthropogenic emissions of ozone precursors play a dominant role, especially in industrial cities such as Tehran. The main sources of carbon monoxide (CO), as one of the most important precursors of ozone, are the combustion of fossil fuels in vehicles, power plants and the burning of biomass (Brook et al., 2004; Godish et al., 2003) .

Ozone concentrations are mostly controlled by the photochemical production from the precursors NO X , VOCs and CO, which in turn is influenced by the meteorological conditions (Calvert et al., 2015; Ibarra-Berastegi et al., 2008; Lu et al., 2019) . Generally, photochemical production of ozone in the troposphere is the main factor in increasing the concentration of this pollutant in the troposphere (Archibald et al., 2020; Liu et al., 1987; Murao et al., 1990; Penkett et al., 1986; Seinfeld et al., 1986) . The main reaction that produces ozone is:

where NO, O, O 2 , M, NO 2 , RO, RO 2 and hv are nitrogen monoxide, oxygen atoms, oxygen molecules, reaction energy adsorbent molecules, nitrogen dioxides, organic alkoxy radicals, organic peroxy radicals (where R is any organic group) and sunlight radiation, respectively. Ozone molecules absorb UV rays. This adsorption process occurs when UV light breaks down an ozone molecule into an atom and an oxygen molecule. The oxygen atom then immediately combines with the next oxygen molecule to form another ozone molecule, and the energy is released into space as thermal (kinetic) energy. During these reactions, ROG (reactive organic gases) is an active organic gas that contains the active part R and hydrogen H.

The available studies indicate that meteorological parameters such as wind direction, wind speed, temperature and relative humidity have a significant effect on changes in surface ozone concentration and its precursors in different months and seasons of the year (Elminir, 2005; Guicherit & Van Dop, 1977; Kovac-Andric et al., 2009; Pawlak & Jaroslawski, 2015; Szulejko et al., 2018) . Duenas et al. (2002) investigated the effect of meteorological parameters on ozone concentration in the Malaga region. Climate change can lead to a significant increase in surface ozone, especially in polluted areas (Fiore et al., 2015; Kirtman et al., 2013) . Mulumba et al. (2015) presented a model for evaluating surface ozone based on meteorological parameters such as temperature and relative humidity. Sousa et al. (2007) studied the effect of wind speed, relative humidity, temperature, nitrogen dioxide concentration and nitrogen oxide on ozone concentration. A study of tropospheric ozone precursors has shown serious pollution in Guangzhou, Shanghai and Lanzhou (Xue et al., 2014) . O 3 production in Shanghai and Guangzhou was limited by the availability of VOCs, whereas in Lanzhou it was limited by NO x . Zhang et al. (2020) measured concentration of tropospheric ozone and its precursors as well as some meteorological parameters in the area near Shanghai, China, and reported one of the reasons for the high concentration of ozone (O 3 ) in this area as high air temperature. Akbar and Sharee (2007) studied the changes in surface ozone in relation to meteorological conditions at the synoptic station of the Geophysical Institute in Tehran during 2002. Their results show average ozone levels of about 72 ppb in the afternoon and about 26 ppb in the morning. Also, the highest concentration of ozone (about 97 ppb) was recorded in spring and the lowest (about 14 ppb) in winter (Akbar & Sharee, 2007) . Ghiasodin and Sourati (2014) studied the concentration of ozone at one of the monitoring stations in Tehran and found an increase in the concentration of ozone due to sunlight and consequently an increase in photochemical activity. It has been suggested that surface stability increases the stability of photochemical processes in the atmosphere and thus increases the concentration of tropospheric ozone (Karimi, 2014) .

Disentangling the relative roles of precursor emissions and meteorological conditions for tropospheric ozone formation in Tehran requires a long-term study. For this study, meteorological parameters (including temperature, T, wind speed, WS, dew point, DWP, relative humidity, RH, and rainfall, PP) and the concentrations of ozone precursors (NO, NO 2 , NO X and CO) in Tehran were collected from 2001 to 2020. The spatial distribution of 20-year average concentration of ozone precursors in air quality monitoring stations in Tehran was analyzed using ArcGIS program. Regression models of ozone concentration were developed based on ozone precursor concentrations and meteorological parameters, and resulting correlations were investigated. Finally, the monthly and annual average values of surface ozone and total ozone concentration were compared over the study period.

Tehran is one of the most populous cities in the world and the capital of Iran. The city with an area of 751 km 2 is located at 35 68092 00 N and 51 38090E. The city of Tehran is limited to the north by the Alborz Mountains (at an altitude of about 1980 m above sea level) and to the south by the Central Desert (at an altitude of about 1200 m above sea level). The mean annual rainfall is 230 mm, the maximum, mean and minimum temperatures are 43, 17 and -15°C, respectively, and the mean relative humidity is 40%. 70% of wind speeds in Tehran are less than 3 m per second (Keyhani, 2010) . Air pollution in Tehran is generally due to vehicles and industrial centers. Limited air exchange due to surrounding mountains, dry conditions and high emissions of ozone precursors create conditions that are very conducive to photochemical formation of ozone.

In the current research, air pollution data measured by Tehran Air Quality Control Company (AQCC) were used. NO, NO 2 , CO and O 3 concentration data from 2001 to 2020 were collected from 22 monitoring stations (i.e., Aqdasiyeh, Sharif University, Ray, District 21, Punak, Golbarg, Masoudieh, Tarbiat Modares University, District 10, Fath Sq., Setad Bohran, District 19, Shad Abad, Mahallati, Region 22, Sadr, Piroozi, District 2, District 11, Rose Park, District 4, Darous) of Tehran Air Quality Control Company (AQCC) and Environmental Protection Organization (DOE). Figure 1 shows the locations of these stations. They can be classified into two groups: Urban Stations and Traffic Stations (Table 1) . WHO criteria were used to validate the data. Distorted information, including incorrect data (zero and negative data) and the data that were very inconsistent with other data due to local phenomena such as environmental issues (i.e., household waste, construction activities or fire and etc.) in the vicinity of the station, was removed from the database. The exact amount of ozone concentration in June 2006 was not recorded, and this data gap was filled with the average value of May and July 2006. Only stations with more than 75% of hourly concentration data were used. The resulting data were converted to standard concentrations based on clean air standards provided by the US Environmental Protection Agency (USEPA, 1997). In Fig. 1 Locations of the air quality monitoring stations and the meteorological station in Tehran city, a Iran, b Tehran Province, c) Tehran City this standard, a maximum concentration of eight hours was used for carbon monoxide, a maximum concentration of one hour and eight hours for ozone and a maximum concentration of one hour for nitrogen dioxide.

At the stations of the air quality control company, ozone was measured with an analyzer O 3 42 model (Environnement S.A., Poissy, France). This analyzer is a continuous ozone analyzer and measures ozone concentration with advanced UV (ultraviolet) technologies. The Beer-Lambert law was used to determine the O 3 concentration:

where I and I 0 are the transfer and production flux densities, l is substrate analysis coefficient, X is the path lengths through the substrate, a is the adsorption cross section and C is the ozone concentration.

The chemiluminescence technique was used to measure the concentrations of nitrogen oxides (instruments APNA-370 of Horiba, Japan; AC 32 M of Environment SA, France; and EC 9841 of Ecotech, Australia). CO concentrations were measured using non-dispersive infrared (NDIR) analyzers (model Teledyne API model 300E, San Diego, CA).

In air quality control stations in Tehran, inorganic gaseous pollutants NO, NO 2 , CO and O 3 are analyzed according to international air quality standards in Europe 2008/50/EC and revised standard 2015/1480/ EC. Periodic services and multiple calibrations of the analyzers are performed based on a specific schedule and at specified intervals according to international standards in the sampling process so that the quality of the results is in accordance with the Quality Assurance program. Satellite data from the Total Ozone Mapping Spectrometer were measured by EP/TOMS (built by TRW Space & Electronics Group, Redondo Beach, CA) from 2001 to 2004 and the Ozone Monitoring E (see Fig. 1 ), in the vicinity of Mehrabad Airport, inside the city.

Monthly, Seasonal and annual trends of ozone and its precursors Table 1 presents the average annual concentration of ozone and its precursors (NO, NO 2 , NO X and CO) at 22 air quality monitoring stations in Tehran. Figure 2 shows the spatial distribution of the 20 year average (Table A) . The maximum annual average concentrations of NO, NO 2 and NO X were 179 ppb, 113 ppb and 290 ppb, respectively, at the District 10 station (Figs. 2a, b, c and Table 1 ). These values are higher than the EPA limit (i.e., 100 ppb). The minimum average concentrations of NO and NO X were recorded in Masoudieh station, equal to 30 ppb and 72 ppb, respectively (Table 1 ). The minimum average concentration of NO 2 was recorded in Shadabad station, equal to 35 ppb (Table 1) . Investigation of carbon monoxide (CO) changes shows that the maximum and minimum average annual concentrations were 5.92 ppm in 2005 and 1.84 ppm in 2020, respectively ( Table A) . The maximum and minimum average daily concentrations during recent 20 years have been recorded at Sadr station, a traffic station, and District 22 station, an urban station, respectively (Table 1 and Fig. 2d ). An examination of the trend of changes in carbon monoxide concentrations in Tehran indicates that the CO concentration was decreasing from 2001 to 2002, increasing from 2002 to 2005, followed by a substantial decrease to the minimum value in 2020. It should be noted that the CO concentration recorded in these years was less than the allowable EPA value (i.e., 9.4 ppm). The main reason for the low 2020 value was the spread of the Corona virus and the related restrictions that caused a reduction in industrial and traffic activities in Tehran (Borhani et al., 2021) . Worden et al. (2013) also investigated pollutants in the eastern USA and Europe and parts of Asia, showing that the carbon monoxide concentration decreased from 2000 to 2011. The relatively declining trend of carbon monoxide concentrations in Tehran from 2001 to 2020 shows that the implementation of policies such as gasification of cars, use of traffic plans, technical inspection of cars, replacement of worn-out cars and Metro development has played a vital role in reducing CO concentrations. NO 2 , NO x and CO show similar seasonal variability, with high levels recorded in late autumn and winter and low levels recorded in spring and summer (Table 2 ). This is different from the seasonality of O 3 (Figs. 3a, b, c and 4d), which increases from January and reaches its maximum value in June, the driest month. This finding is in agreement with the results of previous studies (Jo & Park, 2005; Raddatz & Cummine, 2001; Saito et al., 2002) . Then, with increasing relative humidity and rainfall, the surface ozone concentration decreases until December. The low concentration of surface ozone is significant in cold seasons (USEPA, 2011). Table 2 presents the average seasonal variation in total column ozone. The maximum ozone concentration in March and April is 315.92 DU and 314.70 DU, respectively, and the minimum in October and November is 265.03 DU and 269.74 DU units, respectively (Fig. 5) . Figure 6 shows the changes in total ozone and surface ozone from 2001 to 2020 using (Table 3) . Surface ozone is part of the total atmosphere ozone. Total atmospheric ozone is the sum of tropospheric and stratospheric ozone in a vertical column with a cross-sectional area of one square centimeter from the atmosphere boundary to the ground (Danielsen, 1968; Fishman et al., 2003; Junge, 1962) . Investigation of the trend of annual changes in total ozone indicates that the greatest change occurs in winter and spring. Relative decrease in average surface ozone concentration in winter (20.57 ppb) despite high levels of total ozone (309.16 DU) can be due to increased rainfall (35.15 mm) and decreased temperature (8.40°C) and subsequently attenuation of overnight temperature inversion and reduction in high concentrations of pollutants (Fig. 5 ). Table 3 presents the average Pearson correlation coefficient (R) (Ozbay, 2012) between ozone, ozone precursors and meteorological parameters. The value of P \ 0.05 was considered statistically significant. There is a negative correlation between ozone and NO, NO 2 , NO x and CO. Nitrogen monoxide (NO) has the highest negative correlation (Fig. 3a) because it leads to titration of ozone; with the decrease in NOxtitration effects, the ozone concentration increases significantly (Li et al., 2013; Sillman, 1999) . The temperature (T) and dew point (DWP) have the highest positive correlation with surface ozone (Fig. 4a and d) . This can be attributed to the predominant role of temperature in photochemical reactions in ozone formation (Chelani, 2009; Shan, 2009 ). In addition, on an annual time scale, a significant correlation was observed between temperature and dew point (r = 0.966). Relative humidity and rainfall rate are negatively correlated with surface ozone (Fig. 4c and e) . Han et al. (2020) , examining the effects of meteorological parameters on ozone concentration in East China reported that higher humidity is associated with cloud formation and slows ozone production because of a reduction in solar insolation. Camalier et al. (2007) show that high humidity may be partially associated with atmospheric instability and cause surface ozone dispersion.

Ozone concentrations increased with increasing wind speed (P [ 0.05) (Fig. 4b) . Wind can transport air that contains ozone or other pollutants from their sources. Sometimes the combined effect of two or more parallel or inverted parameters is more pronounced in the amount of surface ozone concentration. The combined effect of rainfall and relative humidity 

The changes in ozone concentration due to changes in its precursors (NO, NO 2 , NO X and CO) as well as meteorological conditions (temperature, humidity, wind speed, dew point and rainfall) in the last two decades in Tehran were investigated. Ozone and precursor data were collected from 22 air quality control stations in Tehran, including urban and traffic stations. Meteorological data were also gathered. The spatial distribution of the average concentration of ozone and its precursors was determined according to the geographical location of the stations. Investigating the changes in pollutants from 2001 to 2020 in the city of Tehran indicated that the concentrations in traffic stations are higher than urban stations. According to distribution of annual average concentration in air quality monitoring stations, the highest average concentration of ozone is recorded in the southwest of Tehran. The concentration of ozone in the outskirts of Tehran is higher than its central region. The study of changes in ozone concentration in the last two decades in Tehran shows a 25% decrease in the second decade management methods, improvement in public transportation such as metro, prevention of expansion and continuation of polluting industries in urban areas, application of traffic restrictions may be considered the main factors in reducing the amount of ozone concentration in Tehran during the second decade. The average annual ozone concentration in 2020 compared to 2001 has increased by 23.77%. This is due to changes in meteorological conditions and climate change.

The seasonal variation in ozone shows low concentrations in late autumn and winter and high concentrations in late spring and early summer. The seasonal changes in ozone precursors are different the pattern of seasonal changes in ozone. In general, increasing precursors of tropospheric ozone concentrations the length of the day is related to human activities and traffic. Correlations of O 3 with its precursors as well as meteorological parameters were investigated using regression analysis. Pearson correlation was used to show the effect of each parameter on average ozone concentration. The results showed that O 3 had a positive and significant correlation with temperature, wind speed and dew point and a negative correlation with relative humidity and rainfall rate. The concentration of ozone precursors has a significant positive correlation with moisture and rainfall and a significant negative correlation with temperature, wind speed and dew point. In general, temperature and relative humidity have the highest significant correlation with tropospheric ozone concentration. Maximum tropospheric ozone concentrations occur at high temperatures and low humidity. The results show that seasonal changes in O 3 concentration and its precursors are not fully consistent with seasonal changes in meteorological parameters. This three-atom oxygen combination is a dangerous air pollutant due to natural and human factors affecting air quality such as stratospheric air infiltration and increasing concentration of ozone precursors as the result of industrial activities, vehicle traffic and pollutants in Tehran. The correlation of tropospheric ozone data with the climatic data, especially temperature in Tehran, as well as the analysis of O 3 variations during the last two decades, indicates that the cybernetic climate system is one of the main controllers of the tropospheric ozone concentration.

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Acknowledgements Special thanks are extended to Tehran Air Quality Control Company (AQCC), Institute of Geophysics of the University of Tehran (IGUT) and Department of Environment (DOE) for efficient helps in providing the database of this study.

Author contribution Faezeh Borhani took part in conception or design of the work, data collection, data analysis and interpretation, drafting the article and final approval of the version to be published, Majid Shafiepour Motlagh involved in conception or design of the work, critical revision of the article and final approval of the version to be published, Andreas Stohl participated in data analysis and interpretation, critical revision of the article and final approval of the version to be published, Yousef Rashidi took part in data collection and final approval of the version to be published, and Amir Houshang Ehsani participated in data collection and final approval of the version to be published.

The authors did not receive support from any organization for the submitted work.Data availability The database analyzed during the present study are available from the corresponding author on reasonable request.

Conflict of interest The authors declare that they have no conflict of interest.Consent to participate Informed consent was obtained from all individual participants included in the study.

See Table 4 .