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

page7654Understanding the role of transport and photochemistry is essential to mitigate tropospheric ozone (Oinline-formula₃) pollution within a region. In previous studies, the Oinline-formula₃ concentration budget has been widely used to determine the contributions of two processes to the variations of Oinline-formula₃ concentrations. These studies often conclude that local photochemistry is the main cause of regional Oinline-formula₃ pollution; however, they fail to explain why Oinline-formula₃ in a targeted region is often primarily derived from Oinline-formula₃ and/or its precursors transported from the outside regions, as reported by many studies of Oinline-formula₃ source apportionment. Here, we present a method to calculate the hourly contributions of Oinline-formula₃-related processes to the variations of not only the mean Oinline-formula₃ concentration but also the total Oinline-formula₃ mass (the corresponding budgets are noted as the Oinline-formula₃ 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 Oinline-formula₃ budgets were applied to comprehensively understand the effects of transport and photochemistry on the Oinline-formula₃ pollution over the Pearl River Delta (PRD) region in China. Quantified results demonstrate the different role of transport and photochemistry when comparing the two Oinline-formula₃ budgets: photochemistry drives the rapid increase of Oinline-formula₃ concentrations during the day, whereas transport, especially vertical exchange through the ABL top, controls both rapid Oinline-formula₃ mass increase in the morning and decrease in the afternoon. The diurnal changes of the transport contributions in the two Oinline-formula₃ budgets highlight the influences of the ABL diurnal cycle and regional wind fields on regional Oinline-formula₃ pollution. Through high contributions to the Oinline-formula₃ mass increase in the morning, transport determines that most Oinline-formula₃ 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 Oinline-formula₃ concentration increase compared to photochemistry, and transport effect on the regional sources of Oinline-formula₃ cannot be illustrated by the Oinline-formula₃ concentration budget. For future studies targeting Oinline-formula₃ 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.