Comprehensive analyses of source sensitivities to and apportionments of PM 2.5 and ozone over Japan via multiple numerical techniques

Abstract. Source sensitivity and source apportionment are two major indicators representing source-receptor relationships, which serve as essential information when considering effective strategies to accomplish improved air quality. This study evaluated source sensitivities to and apportionments of ambient ozone and PM2.5 concentrations over Japan with multiple numerical techniques embedded in regional chemical transport models, including a brute forth method (BFM), a high-order decoupled direct method (HDDM), and an integrated source apportionment method (ISAM), to update the source-receptor relationships considering stringent emission controls recently implemented in Japan and surrounding countries. We also attempted to understand the differences among source sensitivities and apportionments calculated by multiple techniques. While domestic sources had certain source apportionments to ozone concentrations, transport from outside Japan dominated the source sensitivities. Although the PM2.5 concentrations and absolute magnitudes of their source sensitivities were significantly lower than those reported by previous studies, transport from outside Japan still has relatively large contributions to PM2.5 concentrations, implying that there has been a reduction in Japanese emissions, similar to surrounding countries including China, due to implementation of stringent emission controls. HDDM allowed us to effectively understand the importance of the nonlinear responses of PM2.5 concentrations to precursor emissions. Apportionments derived by ISAM were useful in distinguishing various direct and indirect influences on ozone and PM2.5 concentrations. It was suggested that that ozone transported from outside Japan plays a key role in exerting various indirect influences on the formation of ozone and secondary PM2.5 components. This study demonstrated that a combination of sensitivities and apportionments derived by the BFM, HDDM, and ISAM can provide critical information to identify key emission sources and processes in the atmosphere, which are vital for the development of effective strategies for improved air quality.