Changes in ambient concentrations of monoterpenes during the winter-summer transition period in an urban site of India

In the earth’s atmosphere, volatile organic compounds (VOCs) are emitted from natural (biogenic) and anthropogenic sources. VOCs are important components of photochemical processes with strong significance to atmospheric chemistry and climate change through the formation of ozone and organic aerosols. Despite their large biogenic emissions and strong photochemical cycling under the tropical conditions, the speciated measurements of biogenic-VOCs (BVOCs) over the South Asia region are extremely rare. Recently, a project “Network of Volatile Organic Compounds (VOCs) Measurements in India: Biosphere-Atmosphere Exchange” has been implemented for the measurements of BVOCs over different environments of India and surrounding oceanic regions. We have conducted ambient air measurements of C6-C12 compounds at an urban site of Ahmedabad in western part of India during January-May 2020. The well time resolved continuous measurements provided excellent dataset to characterize the diurnal, day-to-day, and seasonal variations of VOCs originated from both biogenic and anthropogenic sources. The mains scientific focus of this study is to characterize the ambient air variations of α- and β-pinene, which are the main representatives of the monoterpene group. Unlike the large reductions in concentrations of anthropogenic VOCs during summer also coinciding with COVID-19 lockdown, the mixing ratios of α- and β-pinene showed a strong increasing trend from winter to summer. The monoterpenes showed clear diurnal patterns with higher night-time and daytime concentrations during winter and summer season, respectively. The monthly mean mixing ratios of α-pinene and β-pinene varied n the ranges of 10-22 and 3-16 pptv, respectively. Despite minimum anthropogenic influences and intense photo-oxidation loss in summer of 2020, the huge enhancements of monoterpenes in ambient air indicate the strong biogenic emissions from local vegetation. Our analysis indicate the combined effect of the northwest wind flow and higher air temperatures leading to high emissions of BVOCs from local vegetation.