Abstract
Abstract. We have estimated the spatial changes in NO2 levels over different regions of India during the COVID-19 lockdown (25 March–3 May 2020) using the satellite-based tropospheric column NO2 observed by the Ozone Monitoring Instrument (OMI) and the Tropospheric Monitoring Instrument (TROPOMI), as well as surface NO2 concentrations obtained from the Central Pollution Control Board (CPCB) monitoring network. A substantial reduction in NO2 levels was observed across India during the lockdown compared to the same period during previous business-as-usual years, except for some regions that were influenced by anomalous fires in 2020. The reduction (negative change) over the urban agglomerations was substantial (∼ 20 %–40 %) and directly proportional to the urban size and population density. Rural regions across India also experienced lower NO2 values by ∼ 15 %–25 %. Localised enhancements in NO2 associated with isolated emission increase scattered across India were also detected. Observed percentage changes in satellite and surface observations were consistent across most regions and cities, but the surface observations were subject to larger variability depending on their proximity to the local emission sources. Observations also indicate NO2 enhancements of up to ∼ 25 % during the lockdown associated with fire emissions over the north-east of India and some parts of the central regions. In addition, the cities located near the large fire emission sources show much smaller NO2 reduction than other urban areas as the decrease at the surface was masked by enhancement in NO2 due to the transport of the fire emissions.
Highlights
Nitrogen oxides, NOx (NO + NO2), are one of the major air pollutants, as defined by various national environmental agencies across the world, due to their adverse impact on human health (Mills et al, 2015)
In order to link the observed reduction in NO2 levels with the traffic emissions over the urban areas, Fig. 10 shows the 7 d moving average of the daily percentage change observed by Ozone Monitoring Instrument (OMI), Tropospheric Monitoring Instrument (TROPOMI), and Central Pollution Control Board (CPCB) across urban India from 1 March to 31 May 2020 against the Google mobility percentage reduction for three mobility categories: transit stations, workplace, and residential
The changes in NO2 levels over India during the COVID19 lockdown (25 March–3 May 2020) have been studied using satellite-based VCDtrop NO2 observed by OMI and TROPOMI and surface NO2 concentrations obtained from CPCB
Summary
NOx (NO + NO2), are one of the major air pollutants, as defined by various national environmental agencies across the world, due to their adverse impact on human health (Mills et al, 2015). While recent studies have used either only satellite observations or only surface observations, this study goes further by adopting an integrated approach by combining both measurement types to investigate NO2 level changes over India in response to the COVID19 pandemic using OMI, TROPOMI, and surface observations over different regions. As both OMI and TROPOMI have similar local overpass times of approximately 13:30 (Penn and Holloway, 2020; van Geffen et al, 2020), diurnal influences on the retrievals of NO2 for both instruments are similar. This study provides an improved understanding of the spatial variations of tropospheric NO2 for future air quality management in India
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