Reply on RC2

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Understanding the role of transport and photochemistry is essential to alleviate ambient ozone pollution. However, ozone budget and source apportionment studies often report conflicting conclusions &mdash; Local photochemistry is the main cause of ozone pollution based on the analyses of the former, while contrary, non-local ozone transported to the region accounts for the majority in the latter results. In order to explore its potential causes, we calculated the contributions of both processes to the variations of mean ozone concentration and total ozone mass (the corresponding budgets are noted as ozone concentration and mass budget, respectively) within the atmospheric boundary layer (ABL) of the Pearl River Delta (PRD), China, based on the modelling results of WRF-CMAQ. Quantified results show that photochemistry drives the rapid increase of ozone concentrations in the daytime, whereas transport, especially the vertical exchange near the ABL top, controls the ozone mass budget. The changes in transport contributions in ozone budgets indicate the influences of the ABL diurnal cycle and regional wind fields, including prevailing winds and local circulations (sea breezes), on regional ozone pollution. Though transport in our simulations had a relatively limited effect on ozone concentration, its high contribution to ozone mass increase in the morning determined that most ozone in the PRD emanated from the outer regions. Consequently, the role of transport and photochemistry in ozone pollution may differ, depending on which of the two budgets is concerned. For future studies targeting ozone and other pollutants with moderately long atmospheric lifetimes, we suggest that attention should be paid to budget-type selections.