In situ ozone production is highly sensitive to volatile organic compounds in Delhi, India

Beth S Nelson,Lisa K Whalley,Sam Cox,Will S Drysdale,James M Cash,Andrew R Rickard,Jacqueline F Hamilton,Eloise Slater,Bhola R Gurjar,Alastair C Lewis,Rachel E Dunmore,Ranu Gadi,Roberto Sommariva,Mohammed S Alam,Leigh R Crilley,Mike J Newland,Eiko Nemitz,Gareth J Stewart,William J Bloss,James R Hopkins,B Langford ,David C S Beddows ,W Joe F Acton ,D E Heard ,P M Edwards ,Shivani ,Adam Vaughan ,C N Hewitt ,Ülkü Alver Şahin ,James Lee

doi:10.5194/acp-21-13609-2021

Abstract

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 and agricultural crop losses, and adversely affect human health. During October 2018, concentrations of speciated non-methane hydrocarbon 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 Emissions Database for Global Atmospheric Research (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.

Highlights

The majority of the world’s population lives in urban areas
A detailed chemical box model constrained to an extensive observational dataset of 86 volatile organic compounds (VOCs), 34 photolysis rates, NO, NO2, carbon monoxide (CO), SO2, HONO, temperature, pressure, and relative humidity was used to explore the sensitivity of photochemical O3 production, P (O3), to VOCs and nitrogen oxides (NOx) in the Indian megacity of Delhi
Our analysis examined the sensitivity of VOC classes to mean morning P (O3), and the aromatic VOC class was identified as being the most important, with a 50 % reduction in ambient concentrations leading to a reduction in modelled morning P (O3) of 15.6 %, followed by monoterpenes and alkenes (13.1 % and 12.9 %, respectively)

Summary

Introduction

The majority of the world’s population lives in urban areas This is projected to increase from 55 % in 2018 to 68 % of the global population by 2030, with 90 % of this growth occurring in Asia and Africa (Molina, 2021; United Nations, Department of Economic and Social Affairs, Population Division, 2019, 2018). Increasing industrialisation and urbanisation, coinciding with fast population growth, have led to worsening air quality in many of these densely populated regions. This is driven by the increasing emissions of nitrogen oxides (NOx = NO + NO2), largely associated with transport, and volatile organic compounds (VOCs), released from a diverse range of sources. O3 exposure has been linked to both acute and chronic pulmonary and cardiovascular health outcomes through both animal toxicological and human clinical studies, with one study showing statistically significant decreases in the lung function of adults on an average exposure of 70 ppbV of O3 across five 6.6 h windows (WHO, 2005, 2013; Schelegle et al, 2009; EPA, 2020; Fleming et al, 2018)

Results

Discussion

Conclusion