Spatial variability of trace gases (NO2, O3 and CO) over Indian region during 2020 and 2021 COVID-19 lockdowns.

COVID-19 lockdown has given us an opportunity to investigate the pollutant concentrations in response to the restricted anthropogenic activities. The atmospheric concentration levels of nitrogen dioxide (NO2), carbon monoxide (CO) and ozone (O3) have been analysed for the periods during the first wave of COVID-19 lockdown in 2020 (25th March-31st May 2020) and during the partial lockdowns due to second wave in 2021 (25th March-15th June 2021) across India. The trace gas measurements from Ozone Monitoring Instrument (OMI) and Atmosphere InfraRed Sounder (AIRS) satellites have been used. An overall decrease in the concentration of O3 (5-10%) and NO2 (20-40%) have been observed during the 2020 lockdown when compared with business as usual (BAU) period in 2019, 2018 and 2017. However, the CO concentration increased up to 10-25% especially in the central-west region. O3 and NO2 slightly increased or had no change in 2021 lockdown when compared with the BAU period, but CO showed a mixed variation prominently influenced by the biomass burning/forest fire activities. The changes in trace gas levels during 2020 lockdown have been predominantly due to the reduction in the anthropogenic activities, whereas in 2021, the changes have been mostly due to natural factors like meteorology and long-range transport, as the emission levels have been similar to that of BAU. Later phases of 2021 lockdown saw the dominant effect of rainfall events resulting in washout of pollutants. This study reveals that partial or local lockdowns have very less impact on reducing pollution levels on a regional scale as natural factors like atmospheric long-range transport and meteorology play deciding roles on their concentration levels.

Impact of regional transport on total OX (NO2+ O3) concentrations observed at a tropical rural location

In general, local emissions and long-range transport will dictate the pollution levels over a given place. The former source will be based on the location whether it is urban/rural but there are no boundaries to the latter source. During most of the time in a year, long-range transport will decide the pollution levels. Here we present the experimental evidence for regional (long-range) transport impacts on the total OX (NO2 + O3) concentration at a tropical rural location. Two major independent events of the year 2020 in India, the COVID-19 lockdown and Deep depression in Bay of Bengal (BoB2) have been used to demonstrate the known fact of mixing of aged and freshly emitted ozone precursors on the total OX concentration and their influence on observed total OX. COVID-19 lockdown event happened in the summer season whereas the BoB2 event happened during the post-monsoon period. Air mass transport during events at the observational site is completely different which brings the regional transport influence. Further, total OX observed has no local contribution and is controlled entirely by the regional contribution. These observations have been well supported by the CWT analysis and wind circulation patterns during the events. This study provides a clear demonstration of the regional pollution transport influence on the total OX at a rural environment (which control the oxidative capacity of the rural atmosphere). Thus, caution is advised in designating the location as rural/urban based on local emissions alone.

Impact of COVID-19 lockdown on the atmospheric boundary layer and instability process over Indian region.

The abrupt reduction in the human activities during the first lockdown of the COVID-19 pandemic created unprecedented changes in the background atmospheric conditions. Several studies reported the anthropogenic and air quality changes observed during the lockdown. However, no attempts are made to investigate the lockdown effects on the Atmospheric Boundary Layer (ABL) and background instability processes. In this study, we assess the lockdown impacts on the ABL altitude and instability parameters (Convective Available Potential Energy (CAPE) and Convective Inhibition Energy (CINE)) using WRF model simulations. Results showed a unique footprint of COVID-19 lockdown in all these parameters. Increase in the visibility, surface temperature and wind speed and decrease in relative humidity during the lockdown is noticed. However, these responses are not uniform throughout India and are significant in the inland compared to the coastal regions. The spatial variation of temperature (wind speed) and relative humidity shows an increase and decrease over the Indo Gangetic Plain (IGP) and central parts of India by 20% (100%) and 40%, respectively. Increase (80%) in the ABL altitude is larger over the IGP and central parts of India during lockdown of 2020 compared to similar time period in 2015-2019. This increase is attributed to the stronger insolation due to absence of anthropogenic activity and other background conditions. At the same time, CAPE decreased by 98% in the IGP and central parts of India, where it shows an increase in other parts of India. A prominent strengthening of CINE in the IGP and a weakening elsewhere is also noticed. These changes in CAPE and CINE are mainly attributed to the dearth of saturation in lower troposphere levels, which prevented the development of strong adiabatic ascent during the lockdown. These results provide a comprehensive observation and model-based insight for lockdown induced changes in the meteorological and thermo-dynamical parameters.

Influence of background dynamics on the vertical distribution of trace gases (CO/WV/O3) in the UTLS region during COVID-19 lockdown over India.

The COVID-19 pandemic lockdown has led to the significant reductions in the pollutant levels across the globe. Several studies have been carried out for examining and quantifying the improvement in the air quality due to the reduction of the pollution at the surface. Unlike most of the studies carried out earlier on COVID-19 lockdown, this study investigates the role of the dynamics on the vertical distribution of the trace gases (Carbonmonoxide (CO), Water Vapor (WV) and Ozone (O3)) over India in the Boundary Layer (BL), Middle Troposphere (MT) and Upper Troposphere (UT) during COVID-19 lockdown using satellite observations and re-analysis data products obtained during 2010-2020. Substantial differences in the time series and variability have been observed over different zones of India in different atmospheric layers. The changes observed in these species are large over Central India compared to South India and Indo-Gangetic plain regions. An enhancement in CO (~25-40%) and WV (50-60%) has been noticed over Central India in the UT at 147 hPa and 215 hPa, respectively, during lockdown. The strong updrafts before the lockdown and the extended weak zonal wind aloft over this region are found responsible for the observed enhancement in these trace gases in the UT. In spite of the non-availability of the anthropogenic pollution during the lockdown, this study highlights the transport of pollutants through long-range transport (always present even before lockdown) dominance over the Indian region not only near the surface but also aloft due to associated atmospheric dynamics.