Insights into aerosol vertical distribution, subtype, and secondary particle formation in central Himalayas: A COVID-19 lockdown perspective

Abstract

Pristine Himalayan environment is being increasingly affected by the exogenous pollutants which are often observed over mountains. This study, for the first time, elucidates upon the impact of COVID-19 lockdown on the concentrations of primary and secondary aerosols along with associated dynamics, across the boundaries of two regions with very different geographical features. PM_2.5 measurements from northern Indian atmosphere during three distinct phases (P-I, P-II, P-III) of 2020, were investigated against previous three year's mean (2017–2019) over Delhi/NCR (DN) in the Indo-Gangetic Plain (IGP) and Nainital (NT) in the Central Himalaya (CH) using integrated ground based, spaceborne and reanalysis datasets. Strict lockdown measures led to a substantial reduction in PM_2.5 levels in north India, with ∼62% decrease in DN and a minor ∼8% in NT region, for a couple of weeks, along with decrease in other primary absorbing aerosols (BC, OC and dust), conversely, an enhancement was observed in the scattering aerosol (SO₄²⁻). Vertical profiles of Extinction Coefficient from CALIPSO satellite revealed substantial reductions (∼60%) in aerosol content across 0–6 km, over study region with a larger decline in the foothills. Particulate depolarization ratio (PDR) and Particulate color ratio (PCR) declined with altitude by 44% and 31% respectively due to prevalence of fine particles over CH and irregular shapes near the surface (DN). In P-II, aerosol subtype occurrences changed significantly above boundary layer (2–4 km) and reduction in primary aerosols did not contribute to reducing PM_2.5 concentrations. P-III exhibited enhanced secondary particle formation exacerbated by Stratosphere-Troposphere Exchange (STE) events of O₃ that increased the oxidizing capacity in atmosphere. Overall, an upsurge in SO₄²⁻ and HNO₃, was observed in CH and DN region respectively. These findings highlight the significance of secondary aerosols in reduced primary emissions and need of comprehensive case study employing box and regional chemistry models.