Biogenic and anthropogenic sources of isoprene and monoterpenes and their secondary organic aerosol in Delhi, India

Abstract

Abstract. Isoprene and monoterpene emissions to the atmosphere are generally dominated by biogenic sources. The oxidation of these compounds can lead to the production of secondary organic aerosol; however the impact of this chemistry in polluted urban settings has been poorly studied. Isoprene and monoterpenes can form secondary organic aerosol (SOA) heterogeneously via anthropogenic–biogenic interactions, resulting in the formation of organosulfate (OS) and nitrooxy-organosulfate (NOS) species. Delhi, India, is one of the most polluted cities in the world, but little is known about the emissions of biogenic volatile organic compounds (VOCs) or the sources of SOA. As part of the DELHI-FLUX project, gas-phase mixing ratios of isoprene and speciated monoterpenes were measured during pre- and post-monsoon measurement campaigns in central Delhi. Nocturnal mixing ratios of the VOCs were substantially higher during the post-monsoon (isoprene: (0.65±0.43) ppbv; limonene: (0.59±0.11) ppbv; α-pinene: (0.13±0.12) ppbv) than the pre-monsoon (isoprene: (0.13±0.18) ppbv; limonene: 0.011±0.025 (ppbv); α-pinene: 0.033±0.009) period. At night, isoprene and monoterpene concentrations correlated strongly with CO during the post-monsoon period. Filter samples of particulate matter less than 2.5 µm in diameter (PM2.5) were collected and the OS and NOS content analysed using ultra-high-performance liquid chromatography tandem mass spectrometry (UHPLC-MS2). Inorganic sulfate was shown to facilitate the formation of isoprene OS species across both campaigns. Sulfate contained within OS and NOS species was shown to contribute significantly to the sulfate signal measured via AMS. Strong nocturnal enhancements of NOS species were observed across both campaigns. The total concentration of OS and NOS species contributed an average of (2.0±0.9) % and (1.8±1.4) % to the total oxidized organic aerosol and up to a maximum of 4.2 % and 6.6 % across the pre- and post-monsoon periods, respectively. Overall, this study provides the first molecular-level measurements of SOA derived from isoprene and monoterpene in Delhi and demonstrates that both biogenic and anthropogenic sources of these compounds can be important in urban areas.

Ubiquity of Anthropogenic Terpenoids in Cities Worldwide: Emission Ratios, Emission Quantification and Implications for Urban Atmospheric Chemistry

“…The re‐analysis of different VOC observations collected over the last decade has shown the systematic presence of anthropogenic isoprene and monoterpenes in cities worldwide and also recently demonstrated in Delhi, India (Bryant et al., 2023). Here we show that monoterpenes, as well as isoprene, are potentially emitted at the exhaust of fossil fuel vehicles by re‐examining the DATAbASE data set and the literature and by collecting new observations in the developing world (Vietnam).…”

Important Roles and Formation of Atmospheric Organosulfates in Marine Organic Aerosols: Influence of Phytoplankton Emissions and Anthropogenic Pollutants

“…Under the severe anthropogenic impacts, organic aerosols from anthropogenic sources (e.g., coal combustion, biomass burning) would result in an obvious elevation of the total OC concentrations, and biogenic VOC-derived OSs would occupy a relatively low fraction among organic aerosols. In Figure , the locations of (OSs + NOSs)/OM versus OC for some wintertime samples over the Bohai Sea overlapped with the region for the samples in polluted atmospheres (region C) in urban Beijing and Shanghai, China, or Delhi, India. ,− The sulfur content of the quantified OSs and NOSs elevated to >2% of those in nss-SO 4 2– in winter (Figure S8), which was mainly attributed to the increased contribution of monoterpene-NOSs (Figure S4). The influence of anthropogenic outflows on the atmospheric sulfur cycle over marginal seas in winter should be considered with organic sulfur involved.…”

Extreme Concentrations of Nitric Oxide Control Daytime Oxidation and Quench Nocturnal Oxidation Chemistry in Delhi during Highly Polluted Episodes

“…Our findings build on the work by Nelson et al and Bryant et al by focusing on the role NO plays in regulating OH and NO 3 radical concentrations within Delhi. 19,24 The unique chemistry investigated in this study deviates from our typical understanding of nocturnal radical oxidation chemistry and has important policy implications, highlighting the need to reduce nighttime NO x emissions alongside VOC emissions, to not further exacerbate PM and O 3 concentrations through secondary formation.…”

“…The nature of radical chemistry at night, when pollution is high, may have significant consequences on the subsequent daytime oxidation chemistry and secondary pollution formation. A recent study by Bryant et al presented evidence of the removal of nighttime NO 3 radicals by NO in Delhi via a decrease in the concentration of isoprene-derived nitrooxy-organosulfate (NOSi) species produced via NO 3 oxidation chemistry . In addition, a study by Nelson et al, corroborated by Chen et al, found central Delhi to be VOC-limited with respect to O 3 production and significant titration of O 3 , owing to very high NO concentrations at night during the winter periods. , The ratio of NO x to non-methane VOCs has also been shown to impact rates of secondary organic aerosol formation .…”