Reduction in Anthropogenic Emissions Suppressed New Particle Formation and Growth: Insights From the COVID-19 Lockdown

Abstract

To control the spread of the novel 2019 Coronavirus (COVID-19), the Government of India enforced nationwide social and transportation restrictions (lockdown) in three phases from the evening of 24 March to 31 May 2020, which resulted in a significant reduction of primary emissions. Here, we performed the analyses of particle number size distribution measurements in the particle size range of 1.2–3 nm and 10–514 nm carried out from 15 April to 31 May 2020 (lockdown, LCD) and compared with measurements from the previous year during the same time period (15 April to 31 May 2019, business-as-usual, BAU) at University of Hyderabad in Hyderabad, India. The number concentrations of sub-3 nm particles were comparable between LCD and BAU, but the number concentrations of particles greater than 10 nm diameter were lower by about 85% during LCD than BAU. It indicates that the reduction in primary anthropogenic emissions did not inhibit the formation of sub-3 nm particles. But the frequency of occurrence of the new particle formation and growth (NPF&G) events was three-fold lower during LCD than BAU. The ratio of formaldehyde to nitrogen dioxide indicated that India falls in a NOx-limited regime, which reduces ambient ozone concentrations (lower condensable vapors via ozone oxidation of volatile organic compounds). Besides, the lower temperature (lower hydroxyl radical concentration) and lower wind speed during LCD may have contributed to the suppression of NPF&G events. Therefore, we emphasize the need to account for processes and interactions related to NPF&G in formulating particulate pollution mitigation policies in urban environments.