The assessment of return probability of maximum ozone concentrations in an urban environment of Delhi: A Generalized Extreme Value analysis approach

Abstract

High ozone episodes have become a serious issue specially in megacities of developing countries. In this study an attempt has been made to understand the changes in the extreme ozone concentration due to different precursor compounds and meteorological variables in a given time period by applying Generalized Extreme Value (GEV) theory. The return probabilities of the extreme ozone concentrations were estimated for both classical stationary assumption as well as nonstationary assumption given the fact that ozone time series have both trend and multiple periodicities. Under stationary case, the distribution parameters were allowed to remain stationary, but in nonstationary case, distribution parameters were allowed to vary as a function of precursor compounds (Benzene, Toluene, mp-Xylene, NO, NO2) and meteorological variables (temperature and relative humidity). Daily maximum ozone concentrations were found to follow heavy tailed Fréchet distribution in both stationary and nonstationary conditions. The inclusion of covariate into the classical model explains the dynamic nature of ozone depending on its precursor variables. Principal components were also used as covariates in nonstationary GEV distribution model. The estimated return levels of ozone from the stationary model, were found to be 97.89, 144.34, 188.44, 232.10, 310.33, 366.47 and 429.97 μg/m3 for 3, 7, 15, 30, 90, 180 and 365-day return period, respectively. In case of non-stationary GEV model the estimated return levels for 3, 7, 15, 30, 90, 180 and 365-day period were in the range 38.38–140.96, 84.31–184.35, 115.92–225.26, 166.61–337.21, 189.46–388.30 and 214.98–445 μg/m3, respectively. The highest median return level of O3 for 3-day return period (100.35 μg/m3) was observed due to temperature as covariate, for 7-day (146.04 μg/m3) and 15- day (189.85 μg/m3) return period, it was observed due to Benzene as a covariate and for rest of the 4 different return periods, NO was found responsible for the highest median return level of O3 (233.52, 312.40, 369.02 and 433.06 μg/m3 for 30, 90, 180 and 365-day respectively). Seasonal analysis finds O3 extremes to be high in monsoon and premonsoon seasons and low in winter period. The impact of nonstationary condition is exemplified by the fact that the 365 day return level of maximum ozone concentration was found to exceed within just 20 days for a lower concentration of precursor Benzene (i.e. 214.48 μg/m3). Results underline the role of the principal components of the precursor compounds in governing the maximum ozone concentration in any city.