Sources, sinks and chemistry of stabilized criegee intermediates, hydrocarbons and halocarbons in the Indo-Gangetic Plain

Abstract

The Indo-Gangetic Plain (IGP) is a complex atmospheric environment due to its high population density and mix of urban, agricultural and rural emission sources. The ambient speciation of hydrocarbons, measurements of halocarbons and role of nocturnal oxidants like stabilized criegee intermediates (SCI) remain poorly understood over the region. My PhD research addresses these critical gaps by providing the first world-wide investigation of the chemistry of stabilized criegee intermediates (SCI), ambient abundance of non-methane hydrocarbons (NMHCs) and halocarbons in both clean and polluted periods, including the first emission inventory for five halocarbons, from open waste burning over India. As part of the thesis work, field studies were carried out in the megacity of Delhi and the much smaller city of Mohali to assess similarities and differences in the shared Indo-Gangetic Plain air-shed. Methods were developed and optimized to quantify 53 speciated hydrocarbons and 7 halocarbons, using a thermal-desorption gas –chromatograph flame ionization detector and electron capture detector (TD-GC-FID/ECD). In the first process-based investigation, I analyzed the nighttime oxidation chemistry in periods affected by agricultural waste burning when night-time ozone and alkenes are high, providing favorable conditions for high SCI formation. Ethene, propene, 1-butene, cis-2-butene, trans-2 butene, 1-pentene, and 1-hexene were found to drive SCI formation, resulting in average SCI concentrations of 4.4 (±3.6) × 10³ molecules cm⁻³. Z-CH₃CHOO was identified as the dominant SCI species, contributing 55% to the total SCI pool, followed by Z-RCHOO and PINOO (α pinene-derived SCI). The study revealed that SCIs were most abundant during the evening, with their production rates peaking during pollution episodes caused by crop residue burning. Such events increased SCI production by more than two-fold, reaching 7.4 (±2.5) × 10⁵ molecules cm⁻³ s⁻¹. Importantly, SCIs were found to play a dominant role in the nighttime oxidation of sulfur dioxide, with reaction rates reaching 1.4 (±1.1) × 10⁴ molecules cm⁻³ s⁻¹. This pathway could contribute substantially to fine sulfate aerosol formation (PM2.5), particularly during summertime pollution episodes. These findings emphasize the critical role of SCIs in the atmospheric chemistry of the IGP, especially under conditions influenced by biomass burning, which had hitherto been neglected prior to my thesis work. NMHCs are important precursors for ozone and secondary organic aerosols (SOA), the latter of which constitutes a significant fraction of ambient PM2.5. In my second study, I addressed the question of which NMHCs contribute the highest to ambient abundance and reactivity, and therefore need to be prioritized for systematic monitoring and regulation. I measured 53 NMHCs simultaneously during 2022 at two sites: Delhi, a megacity with a population exceeding 20 million, and Mohali, a smaller city located 300 km north of Delhi. The study period covered the “clean” monsoon and the “polluted” post-monsoon seasons, the latter of which is strongly influenced by agricultural waste burning of paddy stubble. Results showed that the ten most abundant NMHCs were propane, n-butane, ethane, ethene, propene, i-butane, i-pentane, acetylene, toluene and benzene. These were the same across sites and seasons, and collectively made up over 60% of the total NMHC concentrations. However, absolute levels varied markedly, with post-monsoon concentrations in Delhi four times higher relative to monsoon season levels and reaching 361.4 (±53.6) μg m⁻³. In Mohali, post-monsoon NMHC levels were 110.2 (±16.5) μg m⁻³, about 1.4 times higher than the monsoon levels. During the post-monsoon season, increases in OH reactivity, ozone formation potential, and SOA formation potential, occurred by factors of ~2.3, ~2.9, and ~4.0, respectively in Delhi, and ~1.2, ~1.4, and ~1.4, in Mohali. This indicated that secondary formation of pollutants got enhanced in addition to primary emissions, during the post-monsoon season. Traffic emissions and fugitive evaporative fuel emissions were identified as dominant sources for these NMHCs at both sites. This work not only provided the first dataset for several NMHCs during the post-monsoon season but also identified and quantified compounds that should be prioritized for future air quality monitoring and management in the Indo-Gangetic Plain air shed. The third study of my thesis addressed a significant knowledge gap regarding halocarbon emissions in India. Halocarbons affect global atmospheric chemistry and climate due to their potential impact on stratospheric ozone, yet ground-based measurements from India have been unavailable. This study provides the first ambient dataset for seven halocarbons, namely vinyl chloride (C2H3Cl), methyl iodide (CH3I), dichloromethane (CH2Cl2), chloroform (CHCl3), carbon tetrachloride (CCl4), 1,2-dichloroethane (C2H4Cl2), and perchloroethylene (C2Cl4) over Delhi and Mohali. Chloroform was the most abundant halocarbon in Delhi, with an average mixing ratio of 225 ppt, significantly higher than global levels. In Mohali, dichloromethane was the most abundant, with a mean mixing ratio of 230 ppt. Using measured emission factors (g emitted per kg fuel burnt) and available activity data, I also compiled an India-wide national emission inventory for vinyl chloride, dichloromethane, chloroform, 1,2-dichloroethane and methyl iodide from open waste fires. Total reactive chlorine emissions from these compounds were 3.2 Gg y⁻¹, with over 40% of emissions for some species originating from mixed wet waste fires. 70% of methyl iodide emissions could be attributed to a single source, namely wheat residue burning. These results highlight the significant contribution of anthropogenic and biomass-burning activities to halocarbon emissions in the IGP, with implications for regional air quality and global climate. Collectively, these three studies provide new understanding of atmospheric processes and emissions in the IGP, and insights gained herein will aid in designing effective air quality management strategies, based on evidence- based understanding of basic atmospheric chemistry processes at play in the Indo-Gangetic Plain.