Air Quality Assessment in Iran During 2016–2021: A Multi-Pollutant Analysis of PM_2.5, PM₁₀, NO₂, SO₂, CO, and Ozone

Abstract

Air pollution has emerged as one of the most critical public health challenges globally, with an astonishing 96% of the world’s population breathing air below the health standards. This study investigates the amount and distribution of six major air pollutants, PM₁₀, PM_2.5, O₃, SO₂, NO₂, and CO, at numerous air monitoring stations across Iran from 2016 to 2021. The primary objectives were to identify the cities with the highest pollution levels, and to assess the spatiotemporal evolution of air pollution across the country, aiming to provide a comprehensive overview and climatology of air quality. The results indicate that cities such as Zabol and Ahvaz consistently rank among the most polluted, with annual average PM₁₀ concentrations exceeding 190 µg m⁻³ and PM_2.5 reaching alarming levels up to 116.7 µg m⁻³. Furthermore, O₃ and SO₂ amounts were high in Zabol too, classifying it as the most polluted city in Iran. In addition, Tehran exhibits high NO₂, SO₂, and CO concentrations due to high industrial activity and vehicular emissions. Seasonal analysis reveals significant variations in pollutant levels, with PM concentrations peaking during specific months over various parts of the country, particularly driven by local and distant dust events. By integrating MERRA-2 reanalysis pollution data and ground measurements, this research provides a robust framework for understanding pollution dynamics, thereby facilitating more effective policy-making and public health interventions. The results underscore the necessity for immediate action to mitigate the adverse effects of air pollution on public health, particularly in areas prone to industrial activities (i.e., Tehran, Isfahan) and dust events (Zabol, Ahvaz).

1. Introduction

Air pollution is one of the most important issues responsible for millions of primary deaths every year around the world [1,2,3,4]. Nearly 96% of the world population breathes unhealthy air with high levels of particulate matter (PM) and potential toxic elements, mainly from anthropogenic emissions [5]. The main health concerns attributed to air pollution are respiratory diseases, cardiovascular issues, and lung cancer [6,7,8,9]. Air pollution compounds are divided into two main groups, gaseous pollutants and particulate matter (PM) which are divided in two main parts based on particle size: PM₁₀ (particles with diameter less than 10 µm), which mainly consists of dust particles, and PM_2.5 (diameter below 2.5 µm), which mainly originates from human and industrial activities [10,11,12,13]. PM_2.5 is more harmful than PM₁₀ for human health, since it can penetrate the respiratory system deeper than just into the lungs, even reaching alveoli and entering blood circulation, thus hurting human internal organs [14,15,16]. Therefore, inhalation of these particles may affect the heart and lungs and cause serious health effects [17,18,19,20]. PM_2.5 particles are mostly produced by the combustion of fossil fuels such as oil, gas, and coal; from the emissions of power plants and factories; as well as from forest fires and other natural processes. PM₁₀ sources mainly consist of desert dust and sea salt, but they also originate from construction activities, agricultural pollution, garbage dump, plant pollen, spores, and molds [21,22,23,24,25]. Especially in arid environments, the presence of both PM₁₀ and PM_2.5 increases sharply during dust events, while their ratio depends on the relative percentage of anthropogenic vs. natural sources [26,27,28]. In addition, both PM₁₀ and PM_2.5 have a great effect on visibility degradation in urban environments, being responsible for road accidents, cancelation of flights, etc. [29,30].

On the other hand, gaseous pollutants mainly include CO, NO₂, SO₂, O₃, and hydrocarbons, and are precursors of particulate pollution via atmospheric processes of gas-to-particle conversion and heterogeneous reactions (i.e., NOx to nitrate, SO₂ to sulfate, etc.) [31,32]. PM_2.5, along with four other pollutants (PM₁₀, O₃, NO₂, SO₂) is used to calculate the Air Quality Index (AQI), which is rated from 0 to 500 [33]. Although stratospheric ozone protects the human body against ultraviolet (UV) solar radiation, tropospheric ozone or surface ozone is a dangerous gas pollutant that is responsible for many deaths every year [34,35]. Surface ozone concentration usually exhibits an inverse relationship with NO₂, due to chemical reactions between sunlight and NOx compounds (NO, NO₂) for O₃ production and destruction [22,36,37]. Although NOx pollution is mostly related to the combustion of fossil fuels in automobiles and industries [38], there are two main pathways for the production of surface ozone: (i) stratospheric ozone intrusions in the lower troposphere and (ii) photochemical reactions between sunlight and NOx compounds. Many studies have indicated a large increase in surface O₃ concentrations in many polluted cities around the world such as Istanbul in Turkey [39], cities in South Korea [40], in Brazil [41], and in Tehran, Iran’s capital [42,43], due to photochemical reactions with NOx and volatile organic compounds (VOCs).

Several studies have analyzed air pollution levels, especially PM₁₀ and PM_2.5, in different parts of Iran. Jaafari et al. [44] investigated PM₁₀ and PM_2.5 concentrations in Tehran Province from 22 December 2016 to 6 July 2017. During this study period, daily PM₁₀ concentrations ranged from 27.2 to 244.9 μg m⁻³ and PM_2.5 from 8.4 to 77.9 μg m⁻³ in urban sites, and from 22.8 to 286.4 μg m⁻³ for PM₁₀ and 6 to 41.1 μg m⁻³ for PM_2.5 in rural sites. Furthermore, Barzeghar et al. [45] investigated the trend of ambient air PM₁₀ and PM_2.5 in Tabriz city, NW Iran, during 2006–2017. Their results indicated that in 59% of the cases for PM₁₀ and 48% for PM_2.5, the concentrations exceeded the WHO Air Quality Guidelines thresholds. Recently, Abadi et al. [30] investigated the spatiotemporal distribution of PM₁₀ and PM_2.5 concentrations in the three most polluted provinces of Iran, namely Khuzestan, Isfahan, and Tehran, during 2016–2021. Their analysis revealed different patterns and the seasonality of PM₁₀ and PM₁₀ concentrations between the provinces. More specifically, Khuzestan experienced the highest pollution levels in summer due to dust storms, while Isfahan and Tehran Provinces exhibited higher PM₁₀ and PM_2.5 levels in winter, associated with anthropogenic emissions. Furthermore, Isfahan and Tehran displayed increasing trends of PM₁₀ and PM_2.5 during the study period, but Khuzestan experienced decreasing trends in both pollutants [30]. However, a comprehensive spatial–temporal analysis of particulate and gaseous pollution throughout Iran is still missing and such an investigation will allow determining cities and regions suffering from specific air pollutants for targeted mitigation strategies, clean air policies and sustainable development.

In this respect, this study aims to investigate the presence of six major air pollution compounds namely PM₁₀, PM_2.5, O₃, SO₂, NO₂, and CO, at many air pollution monitoring stations all over Iran during the period 2016–2021. The primary objectives of this study are (i) to determine the most polluted cities and areas for each specific pollutant; (ii) to provide a comprehensive seasonality of the pollutants across Iran, determining the natural and anthropogenic contributing sources; and (iii) to establish a spatial air pollution map of Iran. Previous works have mostly focused on analyzing air pollution in specific cities or provinces and in shorter time intervals [4,7,30]. This is the first time that PM, CO, SO₂, NO₂, and O₃ pollutants have been analyzed both spatially and temporarily over the whole country. The results provide insights into the spatiotemporal evolution of specific pollutants across Iran which allows a better assessment of the pollution sources and seasonality in various parts of the country. The study also highlights the intricate relationship between meteorological conditions, human activities, and air quality, emphasizing the urgent need for targeted air quality management strategies tailored to the unique challenges faced in various regions of Iran.

2. Study Area

Iran (25° N–40° N, 44° E–63° E) is a country in the Middle East, with a total area of 1,648,195 ${\mathrm{k}\mathrm{m}}^2$, comprising 1% of Earth’s land mass. The Iranian landscape is characterized by a mountainous complex terrain with high mountains (highest point is Mount Damavand, at 5772 m), deep valleys (−28 m in the Caspian Sea coast), and extended deserts in the central and eastern plateaus, with an average height of 1200 m amsl (Figure 1). The country’s climate can be divided into four climatic zones: (i) a moderate and humid climate in the southern shores of the Caspian Sea, (ii) cold climate in mountains in the north and west of Iran, (iii) hot and dry climate in the central Iranian plateau, and (iv) hot and humid climate in the southern coast along the Persian Gulf and Oman Sea. The mean annual rainfall in Iran is about 260 mm, unevenly distributed across the country, from regions of high precipitation along the Caspian Sea to arid, dry landscapes (annual rainfall < 50 mm) in the southeast. Furthermore, the precipitation in the vast deserts such as Dasht-e Lut and Dasht-e Kavir is very low throughout the year. Due to its location in the dust belt, the whole country is highly affected by dust events originating from internal and external sources throughout the year [46,47,48,49,50]. Apart from the predominant dust [46,51,52,53,54,55,56], several petrochemical industries, power plants, and oil and gas refineries are major sources of anthropogenic aerosols and gaseous pollutants (NOx, SO₂, CO) in Iranian megacities (i.e., Tehran, Mashhad, Isfahan, Tabriz, etc.) [57,58,59,60]. Consequently, air quality in major urban areas and industrial zones has significantly deteriorated, especially during the last few decades [4,61,62]. Furthermore, improper management of water resources (rivers, lakes, aquifers) and the effect of climate change has turned the lakes in the Iranian interior (i.e., Urmia, Jazmourian, Hamouns) into dried lake beds, with an important contribution to the increase in dust emissions [21,63,64].

3. Data Set and Methodology

3.1. Ground Based Data

In this study, the hourly and daily concentrations of six main air pollutants (PM₁₀, PM_2.5, O₃, SO₂, NO₂, and CO) were obtained from numerous air monitoring stations in Iran, operated by the Department of Environment (Figure 1). Some of the stations (mostly in megacities) were established a long time ago but most of them were set up from 2016 onward. In addition, the number of stations that are used for the analysis of each pollutant is different, with 86 for PM₁₀, 77 for PM_2.5, 56 for O₃, 68 for SO₂, 69 for NO₂, and 54 for CO, due to data availability. The period of current analysis covers 6 years (2016–2021) and the pollutants are examined on an annual and seasonal basis, aiming to assess the spatiotemporal variability and distribution of pollution all over Iran. Data missing at each station is limited to certain hours or a few days and does not affect the pollutant time series and the monthly or seasonal values. The names of stations, characteristics, and measured variables are included in Supplementary Table S1. For measuring the air pollution in all monitoring stations in Iran, different instruments are operated from the Department of Environment Islamic Republic of Iran (DEIRI). Envirotec measuring instruments were used in 20 stations, Enviro S-A in 57 stations, Ecotec in 16 stations, and Horiba instruments in 20 stations. Air pollution measurements are obtained every hour from 00 to 23 local time and they are different from the meteorological stations’ reports, which are taken every 3 h, 8 times per day. The hourly air pollution data at each station were averaged on monthly, seasonal, and annual time scales and are compared between the stations, aiming to provide information on the spatial and temporal variability of air pollution across the country. Note that all pollution data are quality checked/quality assured (QC/QA) by the Ministry of Environment and parts of this database have been used in previous works [4,30,43]. PM₁₀ and PM_2.5 concentrations were measured using a portable PM monitor (GRIMM Aerosol Spectrometer, model 11E, Grimm Aerosol Technik GmbH, Ainring, Germany). Using this instrument, PM₁₀, PM_2.5, and PM₁ concentrations were measured simultaneously. This device automatically stores the values in its memory, capable of calculating averages over several time intervals. Operating on the principle of laser measurement, this device counts particles and measures their diameters using light scattering technology, in accordance with the USEPA and European Union standards. A Serinus 30 CO Analyser (Ecotech Pty Ltd., Knoxfield, Australia) was used for CO ambient air measurements (LDL < 40 ppb, range 0–200 ppm), while a Serinus^® 10 Ozone Analyzer (LDL < 0.25 ppb) was used for O₃ measurements. In addition, a Serinus^® 60 NO₂ analyzer (LDL < 0.04 ppb) provides continuous NO₂ measurements, while a Serinus 50 (Ecotech Pty Ltd.) was utilized for SO₂ recordings.

In addition, the World Meteorological Organization (WMO) dust related codes (06, 07, 08, 09, and 30 to 35) were also used in all the weather stations over Iran from 2016 to 2021. These codes were reported 8 times per day, every 3 h, from 00 UTC to 21 UTC. The total number of meteorological stations providing these data is 274. Code 06 presents widespread dust that is not raised by the wind at or near the station, so this code indicates non-local dust and low wind speed. In addition, code 07 is related to sand or dust raised near the station, thus indicating local dust associated with wind speed above 7 m/s. The codes 30–35 very rarely occurred in the Iranian meteorological stations and represent sand/dust storms (SDS) related to wind speed above 15 m/s and horizontal visibility below 1 km. In addition, code 08 is indicative of dust or sand whirl near the station during the preceding hour or at the time of observation, while code 09 shows SDS at the time of observation. These dust codes consider the presence and intensity of dust events, wind speed and horizontal visibility [4,47] and are used here as supporting data for justification the dust influence on PM₁₀ concentrations.

3.2. MERRA-2 Reanalysis

The MERRA-2 reanalysis model was developed by the National Aeronautics and Space Administration (NASA) and Global Modeling and Assimilation Office (GMAO) [65]. MERRA-2 updated the original MERRA reanalysis [66,67] by including a more comprehensive aerosol analysis [68]. In this study, the mean concentrations of PM₁₀, PM_2.5, CO, NO₂, O₃, and SO₂ were obtained from MERRA-2 reanalysis (0.5° × 0.625°) across Iran, using the GIOVANNI visualization tool [69] (https://giovanni.gsfc.nasa.gov/giovanni/ accessed on 10 September 2024) covering the period 2016–2021. These MERRA-2 data are qualitatively compared with the station’s air pollutants to assess the spatial distribution over Iran.

4. Results and Discussion

4.1. Monthly and Seasonal Variability of PM₁₀ and PM_2.5 Concentrations

The annual patterns of monthly mean PM₁₀ and PM_2.5 concentrations at stations located in the five most polluted provinces in Iran are presented in Figure 2 for the period 2016–2021. The monthly PM_2.5 and PM₁₀ patterns notably differ from Tehran and Isfahan Provinces to Khuzestan Province. At Khuzestan stations, the concentrations of both pollutants are maximized in summer (June to September). This area is mostly affected by Shamal wind and dust sources from the Mesopotamian plains in Iraq and Syria, which control the seasonality of PM₁₀ concentrations [21,70]. On the contrary, at selected stations in Tehran and Isfahan Provinces, the PM_2.5 and PM₁₀ concentrations were lower in spring (March to May) and higher in autumn/winter (November to January) due to an increase in combustion activities such as vehicular emissions and domestic heating under stagnant weather conditions [71,72,73].

The monthly mean PM₁₀ concentrations in Sistan present the highest values from May to October, reflecting the influence of intense Levar wind and frequent dust storms in this area [74,75]. Due to proximity to the dust source (Hamoun dry beds), the mean monthly PM₁₀ was much higher in Zabol with respect to other stations and reached 396.5 µg m⁻³ in September. However, in Iranshahr station, which is not directly affected by the dust storms originated from the Hamoun dry beds, PM₁₀ levels reached a maximum in March. On the other hand, the monthly PM_2.5 concentrations exhibited a different pattern in Sistan Province, since only two stations (Zabol and Iranshahr) display frequent data reports. In Zabol, the highest monthly PM_2.5 was found in June (106.1 µg m⁻³) and November (104.4 µg m⁻³), while in Iranshahr, the highest monthly PM_2.5 was in March (92.4 µg m⁻³) and April (79.4 µg m⁻³).

Moreover, the annual PM₁₀ and PM_2.5 patterns are notably different between the stations in Bushehr Province, lying in the northern coast of the Persian Gulf. In general, higher monthly PM₁₀ and PM_2.5 levels are shown in the warm months, while Genaveh port exhibited the highest concentrations, but without a distinct seasonality, reflecting high pollution levels throughout the year. In this industrial port area, the highest PM₁₀ concentration occurred in October (223.4 µg m⁻³), while the highest PM_2.5 was in April (117.8 µg m⁻³).

Figure 3 shows the average concentrations of PM₁₀, PM_2.5, O₃, SO₂, NO₂, and CO at the examined stations all over Iran during 2016–2021, aiming at spatial mapping of air pollution across the country. Due to remote, arid, and sparsely populated landscapes of east Iran, most of the air pollution monitoring stations are located in the western part of the country and along the southern coastal areas.

All stations reported annual mean PM₁₀ concentrations above 12 µg m⁻³, exceeding the WHO threshold of 15 µg m⁻³ (for an annual average) in 97% of the stations [76]. The lowest particulate pollution levels were detected at stations located in the NW and northern parts of the country, while at specific stations along the northern Persian Gulf and southeast Iran, the annual PM₁₀ levels were seen to exceed 125 µg m⁻³, indicating very unhealthy atmospheric conditions (Figure 3a). The highest annual PM₁₀ was detected in Zabol, with 190.8 µg m⁻³, due to intense and frequent dust storms that occurred mostly from mid-May until mid-September [74,77]. After Zabol, Ahvaz City in southwest Iran exhibited the highest annual mean PM₁₀ (182.3 µg m⁻³) during 2016–2021. Both cities rank among the most polluted in the world in terms of particulate pollution [78]. Furthermore, the coastal part of the Persian Gulf is mostly affected by local dust sources, although the contribution from dust events originated from the Mesopotamian marshes, the Iraqi Desert, and the eastern Arabian Peninsula is also high [79,80,81,82], thus contributing to enhanced PM₁₀ levels.

The station-wise spatial distribution of PM_2.5 concentrations over Iran presents some similarities with that of PM₁₀, with main difference being the increased PM_2.5 levels in polluted urban and industrialized areas in north Iran like Tehran, Ardebil, and Tabriz (Figure 3b). The minimum annual mean PM_2.5 concentration between the stations is 11 µg m⁻³, surpassing the WHO safe health threshold of 5 µg m⁻³ [83]. The maximum annual PM_2.5 is 116.7 µg m⁻³, detected at Baghershahr station near Tehran. The difference in spatial distribution patterns between PM₁₀ and PM_2.5 is attributed to the mostly anthropogenic origin of PM_2.5 (combustion activities related to traffic, industrial, and domestic sectors), against the natural sources of PM₁₀ (soil erosion, dust storms), although dust events exhibited a concurrent effect on both concentrations in several areas in SW, SE, and central Iran.

Furthermore, over the whole Iranian territory, there are 56 stations with continuous ozone measurements for the period 2016–2021. Tropospheric ozone is a secondary byproduct of heterogeneous reactions in the atmosphere that depends on NOx pollution and solar radiation [43,84]. According to a WHO report (2021) [85], safe annual ozone pollution levels should be less than 30 ppb [83], so the red and purple circles correspond to stations with significant exceedances in O₃ pollution across Iran (Figure 3c). The highest annual O₃ pollution levels were seen in Zabol (SE Iran) and Brojerd (in the west), with 58.2 ppb. Furthermore, four stations reported surface ozone levels higher than 40 ppb. Besides Zabol and Brojerd, one station is located in a commercial area (shopping marketplace) near the center of Tehran and the other near Isfahan (Central Iran).

On the other hand, SO₂, NO₂, and CO pollutants are mainly produced by the combustion of fossil fuels from vehicles, industries, central heating, and any kind of biomass burning [86,87]. Consequently, their highest levels are associated with anthropogenic pollution in big cities and industrialized areas. Based on a WHO report (2021) [85], the mean annual SO₂ pollution should be less than 15 ppb, so about half of the stations (33 out of 68) reported higher amounts of this pollutant. More specifically, eight stations reported SO₂ levels above 30 ppb, while an air monitoring station in central Isfahan reported 59.7 ppb, followed by Kerman (SE Iran), with 44.1 ppb. Regarding NO₂ pollution, a WHO report (2021) [85] considered annual mean NO₂ concentrations higher than 5 ppb as dangerous, so almost all the stations (68 out of 69) exceeded this threshold, with the highest concentration (111.7 ppb) recorded in Isfahan. Isfahan is a commercial and industrial city in the central Iranian Plateau containing many factories [4,88,89], so it is not surprising to be the most polluted city in terms of NO₂ and SO₂ pollutants. On the contrary, Iranian cities seem to face less challenges from CO pollution, since only 5 out of 54 stations reported CO levels higher than 4 ppm (Figure 3f). The highest amount of CO was detected in an industrialized port in the Persian Gulf (Asaluyeh), with 6.75 ppm. This port is famous due to the biggest oil refineries and petrochemical industrial sites in Iran (Pars Energy Special Economic Zone, established in 2008) being situated there and is highly polluted by heavy metals and potential toxic elements [90].

Supplementary Table S2 summarizes the maximum hourly PM₁₀, PM_2.5, O₃, SO₂, and NO₂ concentrations in all air monitoring stations in Iran during 2016–2021. A total of 43 stations reported maximum PM₁₀ levels above 1000 µg m⁻³, while Tabriz in northwest Iran (Urmia Lake Basin) reported the highest PM₁₀ of 7836.7 µg m⁻³ during a severe dust event. This city is usually affected by saline dust storms originating from the dried beds of Urmia Lake due to prevailing southwesterly winds [22,25]. At least seven stations reported higher PM₁₀ levels than that reported by Middleton [78] as the highest PM₁₀ (5619 µg m⁻³) during dust storms in the Middle East (occurring in July 2009 in Sanandaj, west Iran). Furthermore, Zabol, with 7198.8 µg m⁻³, ranks third in the highest hourly PM₁₀ concentrations during the study period, as it is affected by dust events approximately one third of the year [74]. The highest PM_2.5 hourly concentration was also reported in Tabriz City, as a combination of urban activities (combustion processes, vehicular exhausts, industries, domestic sector) and fine dust particles from the Dried Urmia lake bed.

Regarding the highest amounts of O₃, SO₂, and NO₂, concentrations larger than 100 ppb were observed in all the air monitoring stations in Iran. Surface ozone is produced by photochemical reactions between sunlight and NOx compounds and the highest O₃ hourly concentration was observed in Birjand, east Iran, with 376.2 ppb. The highest surface NO₂ (1328.1 ppb) was found in Sanandaj, west Iran. Furthermore, Zabol exhibited the highest SO₂ hourly concentrations of 581.3 ppb, while significant pollution levels were recorded at several sites within the Iranian territory.

Figure 4 shows the annual mean concentrations of PM₁₀ along with the number of dust events recorded in the weather stations across Iran from 2016 to 2021. Although there is an inconsistency in the number of weather stations (274) and PM₁₀ measurements (86), the spatial distribution shows that there is a considerable agreement between PM₁₀ and dust events across the country, suggesting that stations with the highest PM₁₀ levels are frequently impacted by dust events of various intensities [30]. Dust events mostly occur along the northern coast of the Persian Gulf, in the western part of Iran, and in the Sistan Basin, east Iran. The highest number of dust events (3167 in 2016–2021) was reported in Dayyer port, located in the Persian Gulf coast, followed by Zabol in Sistan Basin, with 3020 dust reports; Arak, in the northern part of Iran (2777); and Zahak in the Sistan Basin (2683 dust reports). On the other hand, the highest annual mean PM₁₀ measurements were in Zabol (190.8 µg m⁻³), Ahvaz (182.4 µg m⁻³), and Genaveh port, and along the northern coast of the Persian Gulf (170.8 µg m⁻³). However, the inconsistency in the number and spatial distribution between the weather–dust stations and PM₁₀ stations does not allow a comprehensive and statistically robust correlation analysis between the two groups of data.

The monthly mean PM₁₀ concentrations at the air monitoring stations across Iran during 2016–2021 are shown in Figure 5 and could be used as the basis for the seasonality of particulate pollution in the Middle East. On a monthly and seasonal basis, higher PM₁₀ concentrations were observed in the southwest part of Iran that is usually affected by dust storms raised from desert areas in Syria and Iraq [47]. Furthermore, high PM₁₀ concentrations were observed in the northern shores of the Persian Gulf, in the Sistan Basin and in stations in the central Iranian Plateau, affected by local, regional, and distant dust sources. Significantly lower PM₁₀ levels could be seen at stations in NW Iran from December to April, while the dust-prone stations exhibited larger PM₁₀ concentrations during the summer months due to increased dust activity. In general, there is a rather weak seasonality in PM₁₀ concentrations across the country, since high PM₁₀ levels can be seen throughout the year at several stations due to multiple anthropogenic and natural sources. Khorshiddoust et al. [91] investigated PM₁₀ concentrations in Tabriz metropolitan area from 2005 to 2012, revealing summer maximum and winter minimum levels in all the examined stations.

Figure 6 shows the spatial distribution of the monthly mean PM_2.5 concentrations in 2016–2021. The highest amounts of PM_2.5 are mostly seen over megacities like Tehran due to vehicular exhausts, the burning of fossil fuels, and other anthropogenic activities, as well as industrialized cities and sites majorly affected by dust storms (like Zabol). A main finding is the uneven seasonality of PM_2.5 between the examined stations, since in areas prone to dust storms in eastern Iran, higher PM_2.5 levels are seen during the warm period of the year, while in Tehran, PM_2.5 levels are higher during the cold season. This is attributed to enhanced emissions for domestic heating, cold and stagnant weather conditions, and the frequent inversions related to the Tehran’s topography [92,93,94]. Another station that is highlighted for its high monthly PM_2.5 levels is Ganaveh port, in the northern coast of the Persian Gulf, especially in the warm season, when the enhanced pollution levels are attributed to increased shipping emissions and port activities. Furthermore, enhanced monthly PM_2.5 is shown in the western stations from May to November, as well as in eastern stations from March to August, related to maximized dust activity in these areas.

4.2. Gaseous Pollutants

Figure 7 shows the monthly O₃ concentrations across the stations, with a spatial distribution of this pollutant over the whole Iranian territory. As expected, maximum surface ozone concentrations are shown in the warm months due to chemical reactions between NOx and solar radiation [95,96,97]. Therefore, O₃ pollution increases significantly from March to September, with most stations presenting a decreasing trend after October. Although the number of stations with availability of O₃ measurements is lower (56) than other air pollutants, the spatiotemporal distribution across the country is quite notable. The surface O₃ levels in Zabol are high in certain months, exhibiting an uneven seasonality that may be related to the activities of cement and concrete factories near the city. As expected, O₃ concentrations are high in the whole Tehran Province due to high population (~14 million) and enhanced NOx and VOCs (volatile organic compounds) emissions from many vehicles and industries [98,99]. Furthermore, high levels of O₃ pollution are seen in Bandar Abbas, a major port in the Persian Gulf, from March to July. This port is especially important for the Iranian economy in terms of commercial shipping and logistics, thus resulting in high pollution emissions throughout the year. The highest monthly mean O₃ concentration was shown in July (55.3 ppb) and August (49.6 ppb) in Khoram Abad (Lorestan Province in west Iran), attributed to increased industrial emissions and the specific topography, with valley-like characteristics that allow the accumulation of pollutants and high O₃ formation during summer.

Figure 8 shows the monthly mean spatial distribution of NO₂ across the air pollution stations during 2016–2021. This pollutant is harmful for human health in large quantities and is mainly produced by fossil fuel combustion with a short atmospheric lifetime [100,101,102]. Therefore, NO₂ concentrations are high in Tehran Province, especially in autumn and winter months, due to higher emission rates and accumulation of pollutants due to thermal inversions, rendering NO₂ pollution a serious problem in Tehran and in the major Iranian cities and industrialized areas [92,103,104,105,106]. In this respect, Mashhad in NE Iran, Tabriz (NW Iran), and Yazd (central Iranian Plateau) exhibited high NO₂ levels, especially in the winter period. In addition, Khoram Abad (Lorestan Province) exhibits high NO₂ levels due to several steel and petrochemical industries, while the high NO₂ pollution in Genaveh port in August and September is associated with intense port activities and the high density of commercial shipping. On the contrary, Zabol exhibits moderate NO₂ concentrations throughout the year due to less industrial activity and fossil fuel emissions in this city [90]. In general, NO₂ exhibits a distinct seasonality across the stations with highest levels in the cold period, since in the warm season intense solar radiation facilitates the photochemical reactions leading to NO₂ destruction and the formation of ozone [43,107].

Figure 9 shows the monthly mean concentrations of SO₂ at 60 stations across the Iranian territory. The spatial distribution and levels show that Iranian cities are mostly impacted by PM₁₀, PM_2.5, O₃, and NO₂ rather than SO₂ pollution. In general, there is an uneven spatial distribution of SO₂ across Iran, also associated with a station-dependent seasonality.

The monthly mean SO₂ concentrations in Zabol were high in four months, i.e., from January to March, and especially in August (40.5 ppb), likely related with activities in cement and concrete factories near the city, although there is lack of data in some months like July and October–December. In addition, high monthly mean SO₂ concentrations are seen in Kerman, southeastern Iran, from April to December, and especially, in August with 54.7 ppb. This city has a population of more than half million, while several mines are located near the city including copper, iron, coal, and titanium. Furthermore, nearby industries such as automobile and rubber factories have a direct effect on enhanced SO₂ levels [108,109,110]. Another station that presents high monthly SO₂ levels, especially from September (41.5 ppb) to December (44.2 ppb), is Khoy, located at NW site of the Urmia Basin, northwest Iran. The population of the city is more than 200 thousand people (second most populated city in West Azerbaijan Province) and is located near several factories, including ones for food products, wire and cable manufacturing, soap and cleaning product production, pharmaceuticals, and herbal medicinal products, which greatly contribute to SO₂ pollution in this area.

Figure 10 shows the spatial distribution of the monthly mean CO concentrations in 2016–2021. High levels of CO are mostly produced in areas with heavy traffic, industrial activity, and combustion emissions. CO levels reach a maximum at stations along the Persian Gulf coast, as well as in Tehran, Kerman, and Yazd. Furthermore, there is no distinct seasonality, with these stations exhibiting high CO levels throughout the year [111,112]. Ship emissions are responsible for the high CO levels in the coastal stations, while the industrial zones near to Kerman and Yazd contribute to enhanced CO pollution in these cities. Domestic heating during winter, due to harsh weather conditions, is a major factor of CO pollution in Tehran, along with vehicular emissions and industrial activity [113]. Yazd is one of the largest centers of heavy industries in Iran. The city was ranked first in Iran in the production of steel, tiles, and textile products, causing the deterioration of the atmospheric environment, with high CO emissions throughout the year.

Several previous studies have analyzed air pollution status in Iran, focusing mostly on major urban and industrialized centers, contrary to the current analysis that attempted the first nation-wide measurement of pollution levels. Ansari et al. [114] investigated PM₁₀ and PM_2.5 concentrations in 19 air monitoring stations in Tehran. The analysis showed higher PM₁₀ levels in summer, while PM_2.5 reached its maximum in winter in all the examined stations, with both pollutants being lower in spring. Almost all populated cities in Iran exhibited significant air pollution levels and for several days per year, the air quality was at unhealthy levels [115]. In Tehran, more than 70% of annual deaths were due to cardiovascular or respiratory diseases related to unhealthy air conditions [115]. In addition, Mehmood et al. [116] showed that PM_2.5 levels increased drastically in large Iranian cities like Tehran, Shiraz, Isfahan, Mashhad, and Karaj during 2000–2018. Asadi et al. [117] showed maximum ozone concentrations in Tehran in July and August and NO₂ maxima in November and December. Mohammadi et al. [118] reported that the air pollution in Mashhad City (NE Iran) consisted mostly of PM_2.5 and NO₂ in 2012–2014. Furthermore, Safavy et al. [119] found that the main components of air pollution in Tabriz were PM_2.5 and CO, which maximized in the cold period, while in Mashhad (NE Iran) the highest PM_2.5 occurred in autumn [120]. Heidari et al. [121] investigated CO, NO₂, SO₂, PM₁₀, PM_2.5, and O₃ concentrations in Kerman (SE Iran) from April 2015 to March 2016, concluding that PM_2.5 is the major pollution issue in the city.

4.3. Reanalysis Observations of Air Pollution in Iran

This section shows the spatial distribution of the examined air pollutants over Iran, using MERRA-2 reanalysis data, aiming to present the model-assimilated distribution of air pollution (Figure 11). The MERRA-2 outputs generally confirmed the measured results (see Figure 5) indicating higher PM_2.5 concentrations over SW Iran compared with central and northern parts of the country (Figure 11a). MERRA-2 also exhibited increased PM_2.5 levels in the Sistan Basin in SE Iran, while higher PM_2.5 is also shown along the Persian Gulf coastline, in general agreement with ground measurements. Tehran and Isfahan cities presented high PM_2.5 pollution levels, which are not so notable in MERRA-2 outputs, thus contributing to discrepancies in spatial distribution of the pollution between ground measurements and satellite remote sensing.

Figure 11b shows the spatial distribution of CO obtained from MERRA-2 over Iran during 2016–2021. MERRA-2 presented low CO levels in the eastern part of Iran, in general consistency with measured data, although there is a limited number of CO monitoring stations in this region. In addition, station data and MERRA-2 show high CO concentrations along the Persian Gulf coast, likely reflecting the shipping contribution to combustion sources and CO levels. Furthermore, note the consistency between the high MERRA-2 and stations’ CO data in Isfahan (central Iran) and Tehran, reflecting the high levels of combustion activities and air pollution in these cities.

In addition, MERRA-2 simulated high levels of SO₂ in the northern parts of the Persian Gulf during 2016–2021 (Figure 11c), while the measured data generally confirmed the high amounts of SO₂ pollution in this area. Also, MERRA-2 effectively represented the high SO₂ levels in hotspot regions like Tehran and Kerman, which is confirmed by the station’s data. However, MERRA-2 failed to effectively reproduce the high SO₂ levels detected by the stations’ data over northwest Iran and Zabol, as well as in some stations in the Iranian interior.

The spatial distribution of MERRA-2 PM₁₀ over Iran during 2016–2021 is shown in Figure 11d, reflecting highest concentrations over Khuzestan plain in SW Iran. Moreover, the results show high PM₁₀ levels along the northern shores of the Persian Gulf and the Oman Sea, in general consistency with the ground-based pollutants. Furthermore, high MERRA-2 PM₁₀ levels are detected over the Sistan Basin, which is one of the major dust hotspots in Iran [122,123,124,125,126], while the moderate PM₁₀ levels in the central Iranian Plateau were not in good agreement with the station’s data, which showed greater levels of PM₁₀ over this region.

The total columnar O₃ amount, as well as the spatial distribution, obtained from MERRA-2 (Figure 11e), are notably different compared to the stations’ data, since the MERRA-2 amount of O₃ presents a distinct latitudinal dependence, as it contains columnar and not near-surface pollution data. Therefore, the higher O₃ amounts in northern latitudes are inconsistent with the data for the highest O₃ levels from the stations, observed in the Sistan Basin and in specific stations in the central Iranian Plateau.

Table 1. Correlation coefficient (r) values between all air monitoring stations and MERRA-2 outputs in Iran during 2016–2021.

	PM2.5	CO	SO2	PM10	O3
r	0.22	0.37	0.12	0.03	0.13

shows the correlation coefficient (r) values obtained from the comparison between pollution data from all the air monitoring stations and daily MERRA-2 outputs (averaged over the stations) in 2016–2021. The correlation coefficients between the station data and MERRA-2 are weak for all pollutants, and especially for PM₁₀ (0.03), while the highest consistency occurred for CO (0.37). Nevertheless, MERRA-2 effectively represents the highest amounts of pollutants at specific stations, mostly in the large Iranian cities and along the Persian Gulf shoreline. The large inconsistency between MERRA-2 and ground-based PM₁₀ is likely attributed to the increased uncertainties in reproducing PM from satellite data and assimilation techniques used in reanalysis products [66,68], the rather coarse temporal evolution of MERRA-2 to determine the exact PM₁₀ value over a specific station, and the high spatiotemporal variability of PM₁₀, due to changes in concentrations of natural and urban resuspended dust. The correlation between MERRA-2 and ground-based PM_2.5 is higher at r = 0.22, possibly due to less influence of dust variability on PM_2.5 concentrations in urban areas.

This study analyzed the spatial–temporal evolution of air pollution across Iran based on a dense (at least in the western part of the country) network of air pollution stations. Current results could constitute the basis for further studies at local or regional scales dealing with air pollution status in Iran, also emphasizing specific cases of severe pollution (either natural or anthropogenic). In recent works, Abadi et al. [30] analyzed the dust-event frequency across Iran over a long period, while Abadi et al. [4] focused on the PM₁₀ and PM_2.5 pollution status of the three most polluted Iranian Provinces, namely Tehran, Khuzestan, and Isfahan, which, combined with current results, provide a comprehensive overview of air pollution over the country. On the other hand, satellite observations provide a unique mapping of the spatial–temporal distribution of pollutants (i.e., O₃, NO₂, CO, SO₂) that support localized, high-quality station data. High-resolution satellite observations may support ground-based measurements, especially in largely urbanized and polluted areas, like Tehran metropolis. However, satellite remote sensing captures mostly columnar measurements that are difficult to extrapolate at ground level; despite this, these data are very helpful for assessments of air pollution status in rural and remote areas due to the high spatial resolution and temporal regularity of satellite remote sensing.

5. Conclusions

In this study, a detailed analysis of the spatiotemporal variation of six air pollutants, i.e., PM_2.5, PM₁₀, SO₂, NO₂, CO, and O₃, was performed across Iran, based on data from the national air pollution network during the period 2016–2021. All stations reported annual-mean PM₁₀ concentrations above 12 µg m⁻³, exceeding the WHO threshold, while the highest annual PM₁₀ was observed in Zabol, with 190.8 µg m⁻³, attributed to frequent dust storms driven by Levar wind, followed by Ahvaz (182.4 µg m⁻³). The minimum annual-mean PM_2.5 concentration between the stations was 11 µg m⁻³, above the WHO safe health threshold of 5 µg m⁻³, while the maximum PM_2.5 was 116.7 µg m⁻³, detected at Baghershahr station near Tehran. The highest annual O₃ pollution was seen in Zabol (SE Iran) and Brojerd (west Iran) with 58.2 ppb. Furthermore, four stations reported surface ozone pollution levels above 40 ppb. Based on the guidance of the WHO, the mean annual SO₂ pollution should be less than 15 ppb, while about half of the stations (33 out of 68) reported higher concentrations and eight stations reported pollution levels above 30 ppb. Regarding NO₂ pollution, almost all the stations (68 out of 69) presented annual NO₂ concentrations significantly above 5 ppb (the WHO threshold), with the highest amount (111.7 ppb) recorded in Isfahan City in central Iran. On the contrary, it seems that the Iranian cities face less challenges with CO pollution, since only 5 out of 54 stations reported CO levels higher than 4 ppm. The highest CO concentration was detected at a highly industrialized port in the Persian Gulf (Asaluyeh), with 6.75 ppb.

In general, higher PM₁₀ concentrations were observed in the west part of Iran, in the northern shores of the Persian Gulf, in SW Iran, and in the Sistan Basin. The highest amounts of PM_2.5 are mostly seen over megacities like Tehran, due to vehicular emissions, the burning of fossil fuels, and other anthropogenic activities, as well as in industrialized cities and sites majorly affected by dust storms (like Zabol). The spatial and seasonal distributions of PM₁₀ and partly of PM_2.5 are closely associated with those of dust events, which were most frequent in the northern parts of the Persian Gulf, SW Iran, and in the Sistan Basin, east Iran. The highest number of dust events was detected in Dayyer port, with 3167 cases during 2016–2021, followed by Zabol (3020 cases), Arak (2777 cases), and Zahak (2683 cases).

O₃ concentrations were high in the whole Tehran Province due to high population (~14 million) and enhanced NOx and VOCs (volatile organic compounds) emissions from many vehicles and industries, while the highest hourly O₃ concentration was observed in Birjand, east Iran, with 376.2 ppb. NO₂ concentrations were also high in Tehran, especially in the autumn and winter months due to higher emission rates and the accumulation of pollutants due to thermal inversions, making NO₂ pollution a serious problem. The highest surface NO₂ was found in Sanandaj, west Iran, with 1328.1 ppb. The monthly SO₂ concentration in Zabol was high in four months, i.e., January, February, March, and especially in August. In addition, high monthly mean SO₂ concentrations are seen in Kerman, SE Iran, from April to December due to emissions from heavy industries. Another station that presented high monthly SO₂ levels is Khoy, located at NW part of the Urmia Basin, especially from September (41.5 ppb) to December.

This study provided valuable insights into the temporal and spatial distribution of air pollutants in Iran and underscores the urgent need for immediate and effective policy interventions. Implementing targeted air quality management measures is crucial to mitigate the adverse health effects associated with air pollution, particularly in the most affected regions. Moreover, the health implications of prolonged exposure to high levels of PM_2.5, PM₁₀, and O₃ are profound, contributing to respiratory diseases, cardiovascular conditions, and even premature mortality. Vulnerable populations, including children, the elderly, and those with pre-existing health conditions, are at higher risk. Therefore, it is imperative for policymakers to adopt a multi-faceted approach that includes stricter emission regulations, public awareness campaigns, and investment in clean energy technologies. Collaborative efforts among governmental agencies, environmental organizations, and the public are essential to foster a healthier environment and improve overall air quality standards in Iran. By prioritizing air quality management and investing in sustainable practices, Iran cannot only protect public health but also enhance the quality of life for its citizens, paving the way for a more sustainable future. The findings of this study serve as a critical call to action for all stakeholders involved in environmental health and policy-making to address this urgent issue effectively.