Impact of anthropogenic and biogenic sources on ambient air isoprene during winter and summer seasons at an urban site in western India

Volatile organic compounds (VOCs) are emitted from various 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. Time-resolved continuous measurements of ambient isoprene mixing ratios at an urban location in western India were conducted from January to May 2020. The measurement period represents the gradual changes in meteorological parameters from winter to summer, as well as the reductions in anthropogenic emissions from the pre-lockdown phase of COVID-19 to the lockdown period. The day-to-day variations between 0.78-3.25 ppb during January-March and 1.07-2.25 ppb during April-May were associated mainly with the variabilities in night and day data, respectively. Diurnal patterns with higher evening-early morning and daytime concentrations in winter and summer months resemble the features of predominant anthropogenic and biogenic emissions, respectively. The analysis of the ratios of isoprene to aromatic compounds revealed the influence of biogenic sources on diurnal and seasonal variations. The afternoon isoprene/aromatic ratios increased exponentially at higher temperatures (25-42 oC), leading to increasing trends of biogenic contribution during the winter-to-summer transition period. Despite predominant biogenic contributions, reductions in anthropogenic emissions due to the COVID-19 lockdowns could also be a factor for very enhancements of isoprene/xylenes (23.0-30.5 ppb ppb-1), isoprene/ethylbenzene (28.7-37.2 ppb ppb-1), and isoprene/benzene (5.1-9.6 ppb ppb-1) ratios than in winter. The present study shows that there are no significant differences in isoprene mixing ratios between winter and summer seasons. However, tracer-based analysis shows a significant seasonality in the relative apportionment between anthropogenic and biogenic contributions. In addition to relative changes in anthropogenic and biogenic contributions, the trend of the isoprene mixing ratio also reflects the impact of meteorological factors influencing photo-oxidation and dilution.