Intra-urban variability of ozone in a tropical city—characterization of local and regional sources and major influencing factors

Abstract

Tropospheric ozone (O3), a secondary air pollutant, was assessed to evaluate the intra-urban variability of O3, its local and distant sources, and the influence of O3 precursor gases and meteorological variables on seasonal and temporal trends of O3 from 2013 to 2017 in a tropical urban city located in the middle Indo-Gangetic Plains of India. Passive samplers and data from real-time air quality monitoring station were considered. Trajectory statistical models, multivariate statistical methods, and geographic information system were further used to identify spatial-temporal variability and source apportionment. O3 concentrations showed significant intra-urban variability with higher concentrations in suburban or background regions of the city, which were mostly attributed to the transport of O3 at high wind speed from distant and rural areas away from the city, while lower concentrations in traffic and commercial areas were due to titration of O3 by nitric oxide (NO). Variations in meteorological variables (temperature, relative humidity, wind speed) and planetary boundary layer height were responsible for seasonal variations in O3 concentrations. Long-range and regional transport of O3 and its precursors such as volatile organic compounds and NOx from the northwestern and eastern directions of the city significantly influenced O3 variability with distinct seasonal patterns. A negative trend in O3 levels was recorded during the study period which may be due to increase in NOx emission in the city. The observed outcomes suggest significant intra-urban variability of O3 in the city which is influenced by traffic as well as by distant and local sources.