The regional budget of O3 mass and concentration variations within the atmospheric boundary layer using the CMAQ model: An example from the Pearl River Delta, China
Authors: Qu Kun , Wang Xuesong , Cai Xuhui , Yan Yu , Jin Xipeng , Shen Jin , Xiao Teng , Zeng Limin , Zhang Yuanhang 
Published: March 2021

&lt;p&gt;Tropospheric O&lt;sub&gt;3&lt;/sub&gt; pollution notably contributes to the deterioration of air quality in many metropolitan regions, resulting in detrimental effects on human health and ecosystem. Due to the moderate atmospheric lifetimes of O&lt;sub&gt;3&lt;/sub&gt;, horizontal transport, exchange between atmospheric boundary layer (ABL) and free troposphere (FT), and chemical process within the ABL all potentially play important roles in regional O&lt;sub&gt;3&lt;/sub&gt; pollution. In this study, we developed a post-calculation tool to quantify the hourly contributions of these processes to the regional budget of O&lt;sub&gt;3&lt;/sub&gt; mass and concentration variations within the ABL based on the modelling results of the Community Multiscale Air Quality (CMAQ) model. The new features of this tool include: (1) the contributions of ABL-FT exchange on O&lt;sub&gt;3&lt;/sub&gt; pollution can be quantified; (2) horizontally, the targeted region can be freely defined by users and vertically, the volumes are non-fixed owing to the diurnal variations of ABL; and (3) the budgets of O&lt;sub&gt;3&lt;/sub&gt; mass and concentration variations are separately calculated and analysed. The Pearl River Delta (PRD) region, located in the South China and faced with severe O&lt;sub&gt;3&lt;/sub&gt; pollution, was selected as the target region in this study. Results show that the variations of total O&lt;sub&gt;3&lt;/sub&gt; mass within the ABL of the PRD were controlled by ABL-FT exchange, that is, the increase (decrease) of O&lt;sub&gt;3&lt;/sub&gt; mass in the morning (afternoon) was driven by O&lt;sub&gt;3&lt;/sub&gt; inflow (outflow) through ABL-FT exchange. By contrast, it was the chemical process that drove the variations of regional-mean O&lt;sub&gt;3&lt;/sub&gt; concentrations. Except that ABL-FT exchange contributed to the rise of O&lt;sub&gt;3&lt;/sub&gt; concentrations in several hours after sunrise, O&lt;sub&gt;3&lt;/sub&gt; transport did not lead to the notable variation of O&lt;sub&gt;3&lt;/sub&gt; concentration in the remaining hours of the day. Combining source apportionment methods, we found that outside O&lt;sub&gt;3&lt;/sub&gt; (including O&lt;sub&gt;3&lt;/sub&gt; produced by emissions within the East and Central China and background O&lt;sub&gt;3&lt;/sub&gt;) entered the PRD mainly through ABL-FT exchange. For chemical process, local sources played a major part, but the contributions of outside emissions cannot be neglected, suggesting the contributions of precursor transport. The effects of typhoon periphery, the weather system most related to O&lt;sub&gt;3&lt;/sub&gt; pollution in the PRD, were also examined by comparing the budget results on O&lt;sub&gt;3&lt;/sub&gt; pollution days with and without the occurrence of typhoons. The usage of this tool will help to comprehensively understand the influence of transport and chemical process in O&lt;sub&gt;3&lt;/sub&gt; pollution on the regional scale, which is crucial for effective and strategic O&lt;sub&gt;3&lt;/sub&gt; control.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;strong&gt;Acknowledgement.&lt;/strong&gt; This work is sponsored by the National Key Research and Development Program of China (Grant No. 2018YFC0213204, 2018YFC0213506) and the National Science and Technology Pillar Program of China (Grant No. 2014BAC21B01).&lt;/p&gt;