Characteristics And Sources Apportionment Of PM_2.5 And Ozone Pollution In The Pearl River Delta Based On The Weather Classification

The Pearl River Delta（PRD）,a leading region in China’s reform and opening-up,has received significant attention due to its atmospheric environmental issues.In order to explore the potential for the control of PRD air pollution,this research uses a weather classification tool to objectively classify the weather patterns in the PRD from 2016 to 2020 based on the European Centre for Medium-Range Weather Forecasts（ECMWF）global atmospheric reanalysis data,and analyzes the causes of air pollution by combining meteorological,pollution source,and atmospheric chemistry elements with ground-level air quality and meteorological data.Moreover,the source apportionment of PM_2.5 and Volatile Organic Compounds（VOCs）using Positive Matrix Factorization（PMF）model is conducted based on the composition of the atmospheric composite monitoring site,and the sources and causes of atmospheric pollution under different weather types are quantified.Furthermore,the differences in the sources and causes of air pollution before and after the COVID-19 pandemic are explored.Results show that the PM_2.5 concentration in PRD has regular temporal and spatial distribution characteristics based on the weather patterns.Five cold air weather patterns have significantly higher PM_2.5 concentrations than other patterns,and the PM_2.5 pollution is most severe under the eastward cold air and weak cold air weather patterns,with the PM_2.5 pollution under the former primarily driven by low wind speed and high humidity,while the high probability of vertical pollutant transport under the large day-night temperature difference of the latter may be the main cause of its frequent nighttime PM_2.5 pollution.Analysis of the PM_2.5 components under cold air weather patterns reveals that with the increase of PM_2.5concentration,the proportion of nitrate gradually increases,while the proportions of sulfate and organic carbon decrease.As the proportion of carbon increases,the value of OC/EC also increases,with the mean OC/EC ratio under the eastward cold air weather pattern being the lowest,mostly due to local emission pollution.For the westward cold air and cold front weather patterns,they are mostly in an ammonia-rich state,which promotes the secondary generation of PM_2.5 and may be the main reason for the pollution exceeding the standards under these weather patterns.The PMF results show that there are some differences in the contributions of different sources of pollution under different weather patterns,with secondary sources（32%-48%）,vehicle emissions（21%-24%）,coal combustion（16%-18%）,industrial sources（6%-14%）,biomass burning（2%-6%）,and other sources（2%-8%）being the primary contributing factors.Ozone pollution in the PRD is mainly affected by three types of subtropical high systems and two types of typhoon systems,with higher levels of pollution being observed under the outer ring of typhoon weather patterns,and the southwest region experiencing the highest levels of pollution under this type of weather pattern.Under the outer ring of typhoon weather patterns with the addition of the cold high pressure ridge weather pattern,widespread pollution is observed,with the concentration of pollutants being significantly higher in the southeast region than in the northwest region.Under the subtropical high system,large-scale outbreaks of pollution are not easily observed,and ozone concentrations are significantly reduced when affected by the monsoon,showing a strong correlation with temperature,humidity,and precursors.Among these,there is a significant negative correlation with humidity and precursor concentrations,and a significant positive correlation with temperature.Large-scale heavy pollution events are more likely to occur at wind speeds of around 10 m/s.The PMF source analysis results show that mobile sources（24%-52%）,solvent usage sources（16%-37%）,natural sources（4%-10%）,combustion sources（4%-9%）,and other sources（14%-29%）are the major contributors of ozone pollution.The OBM results indicate that the period of high ozone pollution in PRD is mostly under VOCs control,but under the outer ring of typhoon weather patterns,it is under NOx control.There are some differences in the optimal VOCs and NOx reduction ratios under different weather patterns,but when NOx emissions are reduced by over 40%,most weather patterns can achieve efficient reductions in ozone concentrations.Therefore,short-to medium-term VOCs reduction and deep NOx emissions reduction are important measures to improve ozone pollution in the PRD.