Elevated PM 2.5 concentrations frequently cause severe air pollution events in Delhi. Till recently, the effect of crop residue burning on the air quality in Delhi has not been fully quantified and the approaches to control the impact of fire emissions have not been effective. In this study, for the first time, we quantified the statewise contribution of post-monsoon crop residue burning in the northwestern states of India to surface PM 2.5 concentrations in Delhi using several sensitivity experiments with the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) and FINNv1.5 fire emission inventory. Results were evaluated with ground-based observations in Delhi (21 stations), Punjab, and Haryana (14 stations). On average, ∼20% of PM 2.5 concentration in Delhi during the post-monsoon season (October−November) was found to be contributed by nonlocal fire emissions. However, on typical air pollution events, fire emissions contributed as high as 50−75% (80−120 μg/m 3 ) to PM 2.5 in Delhi, highlighting the importance of both external transport and local emissions to PM 2.5 pollution in Delhi.

Dust is the dominant aerosol species by mass in the Earth's atmosphere, and emissions from the deserts and arid regions across the globe range between 1,000 and 4,000 Tg yr −1 annually (e.g., Huneeus et al., 2011; Urban et al., 2018). Dust aerosols play an important role in the Earth's climate dynamics (e.g., Kok et al., 2018; Sokolik et al., 2001), affecting the Earth's radiative balance, and also rainfall (Shepherd et al., 2016). Dust also enriches oceanic biological productivity by supplying micronutrients, which in turn helps the oceans to act as carbon sinks by reducing atmospheric carbon dioxide concentrations (e.g.,

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The lockdown measures that were taken to combat the COVID-19 pandemic minimized anthropogenic activities and created natural laboratory conditions for studying air quality. Both observations and WRF-Chem simulations show a 20–50% reduction (compared to pre-lockdown and same period of previous year) in the concentrations of most aerosols and trace gases over Northwest India, the Indo Gangetic Plain (IGP), and the Northeast Indian regions. It is shown that this was mainly due to a 70–80% increase in the height of the boundary layer and the low emissions during lockdown. However, a 60–70% increase in the pollutants levels was observed over Central and South India including the Arabian sea and Bay of Bengal during this period, which is attributed to natural processes. Elevated (dust) aerosol layers are transported from the Middle East and Africa via long-range transport, and a decrease in the wind speed (20–40%) caused these aerosols to stagnate, enhancing the aerosol levels over Central and Southern India. A 40–60% increase in relative humidity further amplified aerosol concentrations. The results of this study suggest that besides emissions, natural processes including background meteorology and dynamics, play a crucial role in the pollution concentrations over the Indian sub-continent.

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