Quantifying mutual transport contributions of ozone and its precursors among adjacent county-level cities during typical pollution episodes

As the demand for continuous improvement in air quality in China grows, county-level cities face growing pressure to mitigate ozone (O₃) pollution, highlighting the urgent need to understand its sources. This study focused on the source apportionment of O₃ in Yucheng and Qihe, two neighboring county-level cities in Dezhou, Shandong Province, which experienced the most severe O₃ pollution during a typical pollution episode from June 22 to 28, 2021. The Community Multiscale Air Quality model, coupled with the Integrated Source Apportionment Method, was employed to quantify the O₃ transport contributions. The results indicate that Qihe's mean contributions to Yucheng's volatile organic compounds (VOCs), nitrogen oxides (NO_x), and Maximum Daily 8-h Average O₃ concentration (O₃_MDA8) were approximately 5.6, 3.3, and 1.9 %, respectively. Their maximum hourly contributions reached approximately 53 ppb, 14, and 21 μg/m³—each higher than the reverse contributions from Yucheng to Qihe. For Yucheng, joint emission control measures with Qihe and Lingcheng have the potential to reduce hourly O₃ concentrations by approximately 56 μg/m³. For Qihe, collaboration with Yucheng and Lingcheng has the potential to reduce hourly O₃ concentrations by approximately 33 μg/m³. In addition, Lingcheng, a neighboring county-level city, contributed significantly to O₃ and its precursors in both Qihe and Yucheng during the pollution period, with concentrations and percentages much higher than those during the clean period. Despite the challenges of joint emission control with distant cities, prioritizing emission reduction strategies among neighboring county-level cities is a feasible approach that can yield significant O₃ reduction potential.