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

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.

Characteristics of VOC Composition at Urban and Suburban Sites of New Delhi, India in Winter

“…Whereas, O 3 production rate decreases to 1.1 ppbv hr −1 in +50% NO x case. Even though O 3 levels are lower at the suburban site, the result that O 3 level is enhanced by VOCs (but saturated by NO x ) is in agreement with another station in the Delhi region (Nelson et al., 2021). The model consists of important isoprene reactions with for example, NO 3 , O 3 , and OH (Calvert et al., 2008).…”

Trends in Summer-Time Tropospheric Ozone during COVID-19 Lockdown in Indian Cities Might Forecast a Higher Future Risk

“…However, the majority of those reports pointed to an unusual higher concentration of O 3 during the same lockdown period in a significant number of places throughout the India [33,36]. It was generally found that the increase/decrease phenomena of photochemically processed tropospheric O 3 are complicated because of its non-linear relationship with its precursors (NO 2 or NO X and VOCs) and meteorological variables; and the reduction of precursor does not necessarily lead to a reduction in tropospheric O 3 [21]. Keeping these in mind, the present study only focuses on the tropospheric O 3 profile of 30 major cities throughout the India during the lockdown of 2020, with reference to the pre-lockdown of 2020, and the same time span of the previous years 2018 and 2019; and, with the aim to understand if the changes in O 3 profile are regional/seasonal precursor and/or meteorological variables dependent/limited or not.…”

“…In VOCs limited environment, the higher O 3 levels mostly depend on increasing VOCs and rarely on NO X ; but in NO X limited environment, the O 3 generation pattern and precursor dependence become vice-versa. It is important to determine which environment prevails before developing any O 3 management strategies [21].…”

“…Being a secondary pollutant, the generation as well as mitigation of tropospheric O 3 largely depend on multiple factors, such as the dynamics of emission sources for precursor molecules, the ratio and saturation levels of the available precursor molecules, alteration of meteorological variables, and others [19][20][21]. Based on the source and sink mechanism, the precursors such as nitrogen oxides (NO X : NO and NO 2 ), volatile organic compounds (VOCs) including benzene, toluene, ethylbenzene, xylene (BTEX), the proportion of NO X to VOCs [19][20][21], along with the meteorological variables like solar radiation intensity, temperature, relative humidity, wind speed, and rainfall [22][23][24][25], play a key role in the synthesis of O 3 in the troposphere. Reshmi et al [26], while discussing the relations of precursor molecules with O 3 formation in the Kannur city, India, pointed towards a negative correlation with NO X, but positive correlation with carbon monoxide (CO).…”

Spaceborne evidence for significant anthropogenic VOC trends in Asian cities over 2005–2019

“…The satellite-based urban HCHO trends are discussed and compared to the estimated AVOC emission trends in bottom-up inventories and to the respective OMI NO 2 trends calculated over the same cities. VOC and NO x are the key ingredients in the formation of tropospheric ozone (Calvert et al 2015), of which abundances have been generally increasing over East and South Asia in the last decades (Nelson et al 2021). Although road transport and other sectors contribute significantly to both AVOC and NO x emissions, the AVOC/NO x emission ratio is sectordependent and some of the major sources of NO x and AVOC (power generation and solvent use, respectively) are markedly different (Li et al 2017).…”