Rethinking the role of transport and photochemistry in regional ozone pollution: insights from ozone concentration and mass budgets

Abstract

Understanding the role of transport and photochemistry is essential to mitigate tropospheric ozone (O3) pollution within a region. In previous studies, the O3 concentration budget has been widely used to determine the contributions of two processes to the variations of O3 concentrations. These studies often conclude that local photochemistry is the main cause of regional O3 pollution; however, they fail to explain why O3 in a targeted region is often primarily derived from O3 and/or its precursors transported from the outside regions, as reported by many studies of O3 source apportionment. Here, we present a method to calculate the hourly contributions of O3-related processes to the variations of not only the mean O3 concentration but also the total O3 mass (the corresponding budgets are noted as the O3 concentration and mass budget, respectively) within the atmospheric boundary layer (ABL) of the concerned region. Based on the modelling results of WRF-CMAQ (Weather Research and Forecasting and Community Multiscale Air Quality), the two O3 budgets were applied to comprehensively understand the effects of transport and photochemistry on the O3 pollution over the Pearl River Delta (PRD) region in China. Quantified results demonstrate the different role of transport and photochemistry when comparing the two O3 budgets: photochemistry drives the rapid increase of O3 concentrations during the day, whereas transport, especially vertical exchange through the ABL top, controls both rapid O3 mass increase in the morning and decrease in the afternoon. The diurnal changes of the transport contributions in the two O3 budgets highlight the influences of the ABL diurnal cycle and regional wind fields on regional O3 pollution. Through high contributions to the O3 mass increase in the morning, transport determines that most O3 in the PRD originates from the global background and emissions outside the region. However, due to the simultaneous rapid increase of ABL volumes, this process only has a relatively limited effect on O3 concentration increase compared to photochemistry, and transport effect on the regional sources of O3 cannot be illustrated by the O3 concentration budget. For future studies targeting O3 and other secondary pollutants with moderately long atmospheric lifetimes (e.g. fine particulate matter and some of its components), insights from both concentration and mass budgets are required to fully understand the role of transport, chemistry and other related processes.