Seasonal behavior and long-term trends of tropospheric ozone, its precursors and chemical conditions over Iran: A view from space

To identify spatial and temporal variations over the Iranian region, this study analyzed tropospheric formaldehyde (HCHO) and nitrogen dioxide (NO2) columns from Ozone Monitoring Instrument (OMI), carbon monoxide (CO) columns from the Measurement of Pollution in the Troposphere (MOPITT), and tropospheric column O-3 (TCO) from OMI/MLS (Microwave Limb Sounder) satellites from 2005 to 2012. The study discovered high levels of HCHO (similar to 12 x 10(15) molec./cm(2)) from plant isoprene emissions in the air above parts of the northern forest of Iran during the summer and from the oxidation of HCHO precursors emitted from petrochemical industrial facilities and biomass burning in South West Iran. This study showed that maximum NO2 levels (similar to 18 x 10(15) molec./cm(2)) were concentrated in urban cities, indicating the predominance of anthropogenic sources. The results indicate that maximum concentrations were found in the winter, mainly because of weaker local winds and higher heating fuel consumption, in addition to lower hydroxyl radicals (OH). The high CO concentrations (similar to 2 x 10(18) molec./crn(2)) in the early spring were inferred to mainly originate from a strong continental air mass from anthropogenic CO hotspots including regions around Caspian Sea, Europe, and North America, although the external sources of CO were partly suppressed by the Arabian anticyclone and topographic barriers. Variations in the TCO were seen to peak during the summer (similar to 40 DU), due to intensive solar radiation and stratospheric sources. This study also examined long-term trends in TCO and its precursors over a period of eight years in five urban cities in Iran. To perform the analysis, we estimated seasonal changes and inter-seasonal variations using least-squares harmonic estimation (LS-HE), which reduced uncertainty in the trend by 5-15%. The results showed significant increases in the levels of HCHO (similar to 0.08 +/- 0.06 x 10(15) molec./cm(2) yr(-1)), NO2 (similar to 0.08 +/- 0.02 x 10(15) molec./cm(2) yr(-1)), and peak annual TCO (similar to 0.59 +/- 0.56 DU yr(-1)) but decreases in minimum annual TCO (similar to-0.42 +/- 0.60 DU yr(-1)) caused by an increase in NO2 species and annual CO (similar to 0.95 +/- 0.41 x 10(16) molec./cm(2) yr(-1)) partly resulting from the transport of reduced CO. The time series of the HCHO/NO2 column ratio (a proxy for the chemical conditions) indicated that during the last decade, the cities of Tehran, Ahvaz, and Isfahan exhibited steady chemical conditions while Tabriz and Mashhad exhibited a change from NOx-saturated/mixed to more NOx-sensitive chemical conditions. (C) 2015 Elsevier Ltd. All rights reserved.