On the processes governing the variability of PTR-MS based VOCs and OVOCs in different seasons of a year over hillocky mega city of India

Abstract

The rapidly increasing industrialization and urbanization, in combination with under representation of in-situ measurements, especially Volatile Organic Compounds (VOCs), has posed serious challenges in understanding air quality and atmospheric chemistry over India. Here we present yearlong measurements of VOCs, carried out during March 2014–February 2015 using a Proton Transfer Reaction Mass Spectrometer (PTR-MS) from an urban site Pune (18.58oN, 73.91°E, 559 m amsl) in western tropical India. The averaged diurnal variation in VOCs at Pune exhibited a characteristic bimodal pattern, with highly pronounced peak during morning hours and secondary enhancement during late evening/night, except for Isoprene. Isoprene mixing ratios were observed to be higher (1.30 ± 0.69 ppbv) during the daytime (0900–1800 h) and lower (0.89 ± 0.65 ppbv) during the nighttime (after 1900 h and before 0900 h). While, Methanol, Acetonitrile, and Acetaldehyde are observed to be the highest (14.3 ± 5.9, 0.9 ± 0.4, and 6.1 ± 2.6 ppbv respectively) during spring, Benzene and Toluene were highest (1.6 ± 1.1, 2.6 ± 2.2 ppbv respectively) during winter. Isoprene mixing ratios showed broader maxima during autumn and winter (1.3 ppbv). Nominally, all the VOCs were observed to be lowest during the monsoon season. The two peaks in the seasonal cycle of acetonitrile along with VIIRS retrieved fire counts indicated impact of the regional biomass burning during spring and winter. Analysis of air-mass residence time further revealed potential influences from the highly polluted Indo-Gangetic Plain during the autumn and winter, whereas local pollution supplemented with regional biomass burning found to be the dominant process during spring. These yearlong measurements are a step forward in filling a major gap of observational data of VOCs over the tropical Indian region, which would be invaluable for evaluation of chemistry transport models and understanding of regional tropospheric chemistry.