Understanding the effect of seasonal variability of VOCs and NOx on the ozone budget and its photochemical processing over a rural atmosphere.

Abstract

Ozone (O₃) in the troposphere is critically important because it serves as a significant oxidant, playing a vital role in atmospheric chemistry and influencing various environmental and health impacts. Several factors impact O₃ formation which include precursor concentrations, meteorological conditions, and transport. In the present study, a rural observational site located in Gadanki (13.5°N, 79.2°E), India, has been identified as a NO_x-limited regime, which is a common photochemical processing regime for rural atmospheres, using the O₃ production regime indicator (Θ) value. The Θ value approached 0.01 during the daytime in all seasons, indicating a different O₃ production regime compared to urban atmospheres, which are generally Volatile Organic Compounds (VOC)-limited. Subsequently, efforts have been made to understand the role of VOC and NO_x concentrations, along with meteorological conditions and transport, in quantifying the seasonal variation of O₃ formation in terms of VOC/NO_x sensitivity at the observational site using the photochemical model O₃ Isopleth Plotting Package (OZIPR). The VOC cross-over points obtained from the O₃ isopleth diagrams using the OZIPR model have shown a significant correlation with O₃ Formation Potential (OFP) and Propylene Equivalent Concentration (PEC) values, supporting the variability in O₃ formation in most of the seasons at the observational site. Additionally, the highest O₃ concentration measured in the summer season is also well reproduced by the VOC cross-over correlations with the OFP and PEC values. Biomass burning VOCs have been found to be the highest contributors to the O₃ formation due to their higher emission in the summer season and higher contribution to both OFP and PEC.