Enhancement of biogenic emissions of VOCs in the semi-arid region of India during winter to summer transition period: Role of meteorological conditions

Abstract. Emissions of biogenic volatile organic compounds (BVOCs) play important roles in ecophysiology and atmospheric chemistry at large spatial and temporal scales. Tropical regions are a main global source of BVOCs and magnitude and chemical compositions are highly variable. This study is based on the measurements of monoterpenes using proton transfer reaction-time of flight-mass spectrometer (PTR-TOF-MS) at a semi-arid site in western India during the winter-to-summer transition. Mixing ratio of monoterpenes showed strong diurnal variation with elevated values from evening till midnight and lowest in the afternoon. The daily data does not show clear trends with monthly means of ~ 0.35 ppbv during each month. Exceptionally high levels of 3–6 ppbv were measured during the sporadic biomass burning and bonfire event during Holi festival. The daytime data of monoterpenes do not clearly reflect the impact of biogenic emission due to the competing influences of mixing and OH-reaction loss. In the afternoon, the monoterpenes/benzene ratio of 0.43 ppbv ppbv⁻¹ in second half of March was ~ 3 times higher than that in first half of February. It showed strong response with temperature as it increased from 0.27 ppbv ppbv⁻¹ (< 30 °C) to 0.50 ppbv ppbv⁻¹ (> 30 °C). The dependence with wind speed is represented by exponential decay but rate of decline in February was ~ 2 times greater than that in March. The ratios of monoterpenes/isoprene in the night were significant higher than those during the day indicating light independent but temperature dependent emissions of monoterpenes. The nighttime MTs/isoprene ratio increased from 0.25 ppbv ppbv⁻¹ in the first half of February to 0.43 ppbv ppbv⁻¹ in the second half of March. Overall, the ratios of monoterpenes/isoprene agree with the values reported for a topical forest region in SE Asia. The estimated contribution from local biogenic sources to ambient monoterpenes increased from 31 % in first half of February to 67 % in second half of March. This trend suggests the increasing biogenic contribution from February to March. The NW winds and higher ambient temperatures in March favored the local emissions and regional transport of BVOCs.