In situ Ozone Production is highly sensitive to Volatile Organic Compounds in the Indian Megacity of Delhi

Nelson, B and Stewart, Gareth and Drysdale, W.S. and Newland, Mike and Vaughan, Adam and Dunmore, Rachel and Edwards, Pete and Lewis, Alastair C. and Hamilton, Jacqueline F. and Acton, Joe and Hewitt, C N and Crilley, Leigh and Alam, Mohammed Salim and Sahin, Ulku and Beddows, David and Bloss, William and Slater, Eloise and Whalley, Lisa and Heard, Dwayne and Cash, James and Langford, Ben and Nemitz, Eiko and Sommariva, Roberto and Cox, Sam and Shivani and Gadi, Ranu and Gurjar, Bhola and Hopkins, James R. and Rickard, Andrew and Lee, James (2021) In situ Ozone Production is highly sensitive to Volatile Organic Compounds in the Indian Megacity of Delhi. Atmospheric Chemistry and Physics, 21 (17). 13609–13630. ISSN 1680-7316

Abstract

The Indian megacity of Delhi suffers from some of the poorest air quality in the world. While ambient NO2 and particulate matter (PM) concentrations have received considerable attention in the city, high ground level ozone (O3) concentrations are an often overlooked component of pollution. O3 can lead to significant ecosystem damage, agricultural crop losses, and adversely affect human health. During October 2018, concentrations of speciated non-methane hydrocarbons volatile organic compounds (C2 – C13), oxygenated volatile organic compounds (o-VOCs), NO, NO2, HONO, CO, SO2, O3, and photolysis rates, were continuously measured at an urban site in Old Delhi. These observations were used to constrain a detailed chemical box model utilising the Master Chemical Mechanism v3.3.1. VOCs and NOx (NO + NO2) were varied in the model to test their impact on local O3 production rates, P(O3), which revealed a VOC-limited chemical regime. When only NOx concentrations were reduced, a significant increase in P(O3) was observed, thus VOC co-reduction approaches must also be considered in pollution abatement strategies. Of the VOCs examined in this work, mean morning P(O3) rates were most sensitive to monoaromatic compounds, followed by monoterpenes and alkenes, where halving their concentrations in the model led to a 15.6 %, 13.1 % and 12.9 % reduction in P(O3), respectively. P(O3) was not sensitive to direct changes in aerosol surface area but was very sensitive to changes in photolysis rates, which may be influenced by future changes in PM concentrations. VOC and NOx concentrations were divided into emission source sectors, as described by the EDGAR v5.0 Global Air Pollutant Emissions and EDGAR v4.3.2_VOC_spec inventories, allowing for the impact of individual emission sources on P(O3) to be investigated. Reducing road transport emissions only, a common strategy in air pollution abatement strategies worldwide, was found to increase P(O3), even when the source was removed in its entirety. Effective reduction in P(O3) was achieved by reducing road transport along with emissions from combustion for manufacturing and process emissions. Modelled P(O3) reduced by ~20 ppb h−1 when these combined sources were halved. This study highlights the importance of reducing VOCs in parallel with NOx and PM in future pollution abatement strategies in Delhi.