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

Understanding the role of transport and photochemistry is essential to mitigate tropospheric ozone (O-3) pollution within a region. In previous studies, the O-3 concentration budget has been widely used to determine the contributions of two processes to the variations of O-3 concentrations. These studies often conclude that local photochemistry is the main cause of regional O-3 pollution; however, they fail to explain why O-3 in a targeted region is often primarily derived from O-3 and/or its precursors transported from the outside regions, as reported by many studies of O-3 source apportionment. Here, we present a method to calculate the hourly contributions of O-3-related processes to the variations of not only the mean O-3 concentration but also the total O-3 mass (the corresponding budgets are noted as the O-3 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 O-3 budgets were applied to comprehensively understand the effects of transport and photochemistry on the O-3 pollution over the Pearl River Delta (PRD) region in China. Quantified results demonstrate the different role of transport and photochemistry when comparing the two O-3 budgets: photochemistry drives the rapid increase of O-3 concentrations during the day, whereas transport, especially vertical exchange through the ABL top, controls both rapid O-3 mass increase in the morning and decrease in the afternoon. The diurnal changes of the transport contributions in the two O-3 budgets highlight the influences of the ABL diurnal cycle and regional wind fields on regional O-3 pollution. Through high contributions to the O-3 mass increase in the morning, transport determines that most O-3 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 O-3 concentration increase compared to photochemistry, and transport effect on the regional sources of O-3 cannot be illustrated by the O-3 concentration budget. For future studies targeting O-3 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.