Change in monoterpene concentrations during winter-to-summer transition period and impact of COVID-19 lockdown at an urban site in India

Abstract

The significance of terpenoid emissions from traffic-related and biogenic sources in urban areas of the developing world is largely unknown, mainly due to the lack of measurements. This study is based on well time-resolved continuous measurements of ambient monoterpene concentration at an urban site in western India during January–May 2020, coinciding with the winter-to-summer and COVID-19 pre-lockdown to lockdown transitions. Despite large day-to-day variations of α-pinene (7–76 ppt) and β-pinene (6–55 ppt) their levels in winter were slightly higher than in summer. The trends in monoterpenes concentrations do not directly reflect the impact of enhanced biogenic contributions in summer due to counterbalancing effects of increased rates of oxidation and dilution. However, the increase of ∑α+β-pinene/∑BTEX ratios from 5 to 13 in winter to 61–87 ppt ppb⁻¹ in summer highlights the impact of increased biogenic contributions at higher temperatures. The very high ∑α+β-pinene/∑BTEX ratios during the strict COVID-19 lockdown phases than the pre-lockdown period could be additionally attributed to the reductions of anthropogenic emissions. The estimated relative daytime biogenic contributions to α-pinene increased from 66 ± 10% in January to 88 ± 13% in May, while that of β-pinene from 56 ± 8% to 70 ± 19%. Overall, depending on the season, the complex interplay between the variability in anthropogenic activities, biogenic emissions, and photochemical/meteorological factors controls the ambient air variability of monoterpenes. The study provides insights into seasonal changes in anthropogenic and biogenic contributions of atmospheric monoterpenes. The knowledge of monoterpene sources is critical to assess secondary organic aerosol (SOA) formation, contributing to regional climate change.