Episode analysis of regional contributions to tropospheric ozone in Beijing using a regional air quality model

Abstract

Tropospheric ozone (O3) is a major photochemical pollutant during the summer in Beijing and an analysis of its sources and their regional contributions is important in the formulation of O3 control strategies. A typical heavy O3 pollution event occurred in Beijing from July 1 to July 10, 2015. The first two days were defined as clean days with a prevailing northerly wind and low O3 concentrations. The following period from July 3 to July 10 was regarded as polluted days with a prevailing southerly wind and high O3 concentrations. The Integrated Source Apportionment Method (ISAM) implemented in the Regional Atmospheric Modeling System Community Multiscale Air Quality (RAMS-CMAQ) model was applied to quantify the regional contributions to O3 concentrations in Beijing during this pollution episode. The model evaluation shows that the model reproduced the spatiotemporal variations of meteorological conditions (temperature, relative humidity and wind vector) and concentration field (O3 and NO2) well. The diurnal variations of O3 and NOx in both urban and suburban areas are unimodal. The concentration of O3 decreased rapidly in the urban areas after reaching a peak value, but only slowly in the suburbs. The peaks of NOx concentration in suburban areas appear around 05:00 a.m., while those in urban areas appear at 08:00 a.m. With the change in wind direction and enhanced local emissions, the O3 in Beijing was mainly affected by the boundary conditions on clean days and increased by contributions from Tianjin, Hebei, Shandong, and local emissions on polluted days. By contrast, the O3 precursor NO2 was mostly sourced from local and nearby emissions and was only slightly influenced by the meteorological conditions.