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<p><span id="page7654"/>Understanding the role of transport and photochemistry is essential to mitigate tropospheric ozone (O<span class="inline-formula">₃</span>) pollution within a region. In previous studies, the O<span class="inline-formula">₃</span> concentration budget has been widely used to determine the contributions of two processes to the variations of O<span class="inline-formula">₃</span> concentrations. These studies often conclude that local photochemistry is the main cause of regional O<span class="inline-formula">₃</span> pollution; however, they fail to explain why O<span class="inline-formula">₃</span> in a targeted region is often primarily derived from O<span class="inline-formula">₃</span> and/or its precursors transported from the outside regions, as reported by many studies of O<span class="inline-formula">₃</span> source apportionment. Here, we present a method to calculate the hourly contributions of O<span class="inline-formula">₃</span>-related processes to the variations of not only the mean O<span class="inline-formula">₃</span> concentration but also the total O<span class="inline-formula">₃</span> mass (the corresponding budgets are noted as the O<span class="inline-formula">₃</span> 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<span class="inline-formula">₃</span> budgets were applied to comprehensively understand the effects of transport and photochemistry on the O<span class="inline-formula">₃</span> 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<span class="inline-formula">₃</span> budgets: photochemistry drives the rapid increase of O<span class="inline-formula">₃</span> concentrations during the day, whereas transport, especially vertical exchange through the ABL top, controls both rapid O<span class="inline-formula">₃</span> mass increase in the morning and decrease in the afternoon. The diurnal changes of the transport contributions in the two O<span class="inline-formula">₃</span> budgets highlight the influences of the ABL diurnal cycle and regional wind fields on regional O<span class="inline-formula">₃</span> pollution. Through high contributions to the O<span class="inline-formula">₃</span> mass increase in the morning, transport determines that most O<span class="inline-formula">₃</span> 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<span class="inline-formula">₃</span> concentration increase compared to photochemistry, and transport effect on the regional sources of O<span class="inline-formula">₃</span> cannot be illustrated by the O<span class="inline-formula">₃</span> concentration budget. For future studies targeting O<span class="inline-formula">₃</span> 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.</p>