High ozone episodes at a semi-urban site in India: Photochemical generation and transport

Abstract

The present study focused on investigating the possible atmospheric conditions influencing the high ozone episodes over a semi-urban site (27.16° N, 78.08° E) in Agra, India during July 2014–June 2016. During the study period, average concentration of ozone was 32.3±22.7ppb, and concentrations of its precursors viz carbon monoxide (CO), nitrogen oxide (NO) and total nitrogen oxides (NOx⁎) were 527.3±482.7ppb, 6.5±8.2ppb, and 12.1±7.8ppb, respectively. The maximum monthly average ozone concentration was observed in June (48.2±31.0ppb) and it was nearly three times higher than the minimum levels observed in December (16.7±10.5ppb). Considering high ozone episodes, 78days exceed hourly ozone limit of 90 ppb specified by National Air Quality Standards (NAAQS, CPCB, 2009) for India while 75 days exceed the daily maximum 8 hourly ozone limit of 70ppb or above (NAAQS, EPA, 2015). Trajstat model was used for air-mass cluster analysis during episodic days; five clusters of air-masses were identified and among them, the cluster from northwest direction had the maximum frequency (35.4%). During the study period, four different types of high ozone episodes were identified and explained using prevailing meteorology. The episodes were attributed to local photochemistry and/or transport.