Seasonal estimates of ozone and secondary organic aerosol formation from volatile organic compounds in a rural atmosphere of India

Abstract

Volatile Organic Compounds (VOCs) serve as precursors for tropospheric ozone (O3) and Secondary Organic Aerosol (SOA) formation. The formation of O3 and SOA are the indicators of the oxidative capacity specific to a given chemical environment. The current study investigates the oxidative capacity of the relatively less explored tropical rural atmosphere. This study is accomplished by measuring the concentrations of various VOCs and combining them with OH loss rates to estimate the potentials for O3 and SOA formation (OFP and SOAP, respectively). Continuous diel VOC measurement data from Gadanki (13.5°N, 79.2°E), Peninsular India, encompassing four distinct seasons and comprising over 4000 samples, have been utilized to estimate OFP and SOAP and their variations across different seasons. Additionally, efforts have been made to comprehend the contribution of different VOC sources to O3 and SOA formation. The results indicate that, 1, 3, 6-trimethyl benzene (20.09 %) among the VOCs and aromatics (44.37%) among the VOC groups exhibit the highest OFP at the observational site. Among seasons, the post-monsoon period exhibits the highest OFP (31.94%). The increased presence of biogenic VOCs, such as ethylene, propylene, and 1-butene during monsoon, likely due to the increased vegetation cover can be attributed for the elevated OFP. Similarly, n-dodecane (43.22%) and the VOC group of alkanes (50.79%) show the highest SOAP. The summer season has the highest SOAP (29.7%), owing to the enhanced concentrations and photochemistry initiated by OH radicals. Within the PMF-modelled sources, biomass-burning VOCs make a substantial contribution to both OFP and SOAP, distinguishing the rural atmosphere from its urban counterpart, where traffic emissions predominantly influence OFP and SOAP.