Seasonal Evolution of Sulfur Dioxide Over the Indian Subcontinent Lakhima Chutia, Narendra Ojha, Imran A. Girach, Binita Pathak, Lokesh K. Sahu, and Pradip K. Bhuyan Abstract – Reanalysis, which combines the chemical-transport model with remote-sensing mea- surements, has shown potential to fill data gaps over observationally sparse regions of the globe. Here, the seasonal distribution of sulfur dioxide (SO 2 ) over the Indian region was analyzed for 2005–2015 period using Copernicus Atmosphere Monitoring Service (CAMS) reanalysis. CAMS reproduced general features and seasonality observed in surface SO 2 over this region. Elevated levels were revealed across the Indo-Gangetic Plain (IGP) and over eastern and central India. SO 2 shows a prominent seasonality over India, with a maximum typically during winter and a minimum during the summer monsoon. The winter maximum is attributed to weaker chemical sink, stagnant meteoro- logical conditions, and elevated emissions, whereas wet scavenging, inflow of marine air, and stronger sulfate formation efficiency cause lower SO 2 during the monsoon. SO 2 levels exhibited an enhancement over IGP and central and eastern India, with rates in the range of 0.5 ppbv to 4 ppbv per decade. Like the distribution, trends also showed seasonal dependence, with weaker trends during the summer monsoon and stronger trends during the winter and postmonsoon. In situ observations of SO 2 and related species especially over the identified hot spots are recommended to validate satellite and model data sets and evaluate air- quality and climate implications. 1. Introduction Sulfur dioxide (SO 2 ) is an air pollutant with adverse health effects, mostly produced by the com- bustion of fossil fuels [1]. As a key air pollutant and aerosol precursor, SO 2 affects air quality, tropospheric chemistry, and the climate [2]. The chemistry of SO 2 in the troposphere is linked with gas-to-particle conversion processes—e.g., SO 2 oxidation into sulfate aerosols—as well as formation from reduced sulfur compounds [3]. Sulfate aerosols substantially influence the earth’s radiation budget by scattering solar radiation and acting as cloud condensation nuclei [4, 5]. The dominant pathway of sulfate formation is through aqueous-phase oxidation of SO 2 by hydrogen peroxide and ozone [3]. In the gas phase, SO 2 is oxidized by hydroxyl radicals (OH) to sulfur trioxide, which further leads to the formation of sulfuric acid [3]. The power-generation sector is the largest con- tributor (~46%) to SO 2 emissions in India, followed by the industrial and residential sectors [6]. Due to its relatively shorter lifetime and diverse emission sources, SO 2 distribution shows strong spatial heterogeneity as reported from different environments in the Indian region [7–9]. The distribution is also affected by chemistry, emissions, and seasonal changes in meteo- rological conditions. Due to sparse in situ measure- ments, an accurate understanding of seasonal changes in SO 2 distribution and its chemical transformations is still an intriguing challenge over the Indian region. Limited studies based on satellite observations revealed en- hancements in SO 2 loading over the Indian region, particularly over the states of Chhattisgarh and Odisha, during the 2005–2015 period [10]. Model reanalysis, combining the chemical-transport model and remote- sensing measurements, has shown the potential to fill data gaps over observationally sparse regions of the globe [11, 12]. However, studies evaluating model reanalysis data sets against observations, seasonality in SO 2 levels, and its conversion into sulfate using both observations as well as model reanalysis have been still lacking over the Indian region. Here we consider a study period of 10 years (2005–2015) to investigate seasonal variability as well as seasonal trends in SO 2 over the Indian region by analyzing Copernicus Atmosphere Monitoring Service (CAMS; https://ads.atmosphere.copernicus.eu/cdsapp# !/dataset/cams-global-reanalysis-eac4-monthly?tab= form) model reanalysis and Ozone Monitoring Instru- ment (OMI, https://acdisc.gesdisc.eosdis.nasa.gov/data/ Aura_OMI_Level3/OMSO2e.003/) satellite observa- tions. We have estimated the efficiency of sulfate formation for different seasons. 2. Data and Methodology 2.1 CAMS Model CAMS reanalysis provides consistent gridded fields of atmospheric composition based on an integrat- Manuscript received 29 August 2020. Lakhima Chutia and Pradip K. Bhuyan are with the Centre for Atmospheric Studies, Dibrugarh University, NH 37, Dibrugarh, Assam 786004, India; e-mail: chutialakhima.tsk@gmail.com, pkbhuyan@gmail.com. Narendra Ojha and Lokesh K. Sahu are with the Space and Atmospheric Sciences Division, Physical Research Laboratory, Ahmedabad 380 009, India; e-mail: ojha@prl.res.in, lokesh@prl.res.in. Imran A. Girach is with the Space Physics Laboratory, Vikram Sarabhai Space Centre, Veli, Thiruvananthapuram, Kerala 695021, India; e-mail: imran.girach@gmail.com. Binita Pathak is with the Department of Physics and Centre for Atmospheric Studies, Dibrugarh University, NH 37, Dibrugarh, Assam 786004, India; e-mail: binita@dibru.ac.in. URSI RADIO SCIENCE LETTERS, VOL. 2, 2020 DOI: 10.46620/20-0046 1