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. PM2.5, generated via both direct emission and secondary formation,can have varying environmental impacts due to different physical andchemical properties of its components. However, traditional methods to quantify different PM2.5 components are often based on online or offline observations and numerical models, which are generally high economic cost- or labor-intensive. In this study, we develop a new method,named Multi-Tracer Estimation Algorithm (MTEA), to identify the primary andsecondary components from routine observation of PM2.5. By comparingwith long-term and short-term measurements of aerosol chemicalcomponents in China and the United States, it is proven that MTEA cansuccessfully capture the magnitude and variation of the primary PM2.5(PPM) and secondary PM2.5 (SPM). Applying MTEA to the China National Air Quality Network, we findthat (1) SPM accounted for 63.5 % of the PM2.5 in cities in southern Chinaon average during 2014–2018, while the proportion dropped to 57.1 % in the north of China,and at the same time the secondary proportion in regional background regionswas ∼ 19 % higher than that in populous regions; (2) thesummertime secondary PM2.5 proportion presented a slight but consistentincreasing trend (from 58.5 % to 59.2 %) in most populous cities, mainlybecause of the recent increase in O3 pollution in China; (3) thesecondary PM2.5 proportion in Beijing significantly increased by 34 %during the COVID-19 lockdown, which might be the main reason for the observedunexpected PM pollution in this special period; and finally, (4) SPM andO3 showed similar positive correlations in the Beijing-Tianjin-Hebei (BTH) and Yangtze River Delta (YRD) regions, butthe correlations between total PM2.5 and O3 in these two regions, as determined from PPM levels,were quite different. In general, MTEA is a promisingtool for efficiently estimating PPM and SPM, and has huge potential forfuture PM mitigation.

Estimation of secondary PM&lt;sub&gt;2.5&lt;/sub&gt; in China and the United States using a multi-tracer approach

Estimation of secondary PM<sub>2.5</sub> in China and the United States using a multi-tracer approach