Reply on RC2

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Fine particulate matter (PM_2.5) pollution is a severe problem in China. Research on the sources of Chinese PM_2.5 pollution has focused on the contributions of China&rsquo;s domestic emissions. However, the impact of foreign anthropogenic emissions has typically been simplified or neglected, partly due to the perception that the short lifetime of PM_2.5 (a few days) does not allow long-distance transport. Here we explore the role of foreign anthropogenic emissions in Chinese PM_2.5 pollution in 2015 using the GEOS-Chem chemical transport model. We validate the model simulations with a comprehensive set of observations of PM_2.5 and its compositions, including sulfate, nitrate, ammonium, black carbon and primary organic aerosols, over China and its surrounding regions. We find that 8 % of PM_2.5 (5 &micro;g m^-3) and 19 % of nitrate (2.6 &micro;g m^-3) over eastern China in 2015 was contributed by foreign anthropogenic emissions. The contributions were the highest in January (6.9 &micro;g m^-3 PM_2.5, with 68 % nitrate) and the lowest in July (2.7 &micro;g m^-3 PM_2.5, with 11 % nitrate). Yet, only 30 % of such foreign contributions in January was through direct atmospheric transport. The majority (70 %) was instead through chemical interactions between foreign-transported aerosol precursors and China&rsquo;s domestic emissions of pollutants. Specifically, the transport of non-methane volatile organic compounds (NMVOCs) from foreign countries enhanced the atmospheric oxidizing capacity and facilitated the oxidation of Chinese nitrogen oxides (NO_x) to form nitric acid (HNO₃) over the eastern China. The abundance of Chinese ammonia (NH₃) further partitioned nearly all HNO₃ gas to particulate nitrate, leading to the considerable foreign contributions of nitrate and PM_2.5 to the eastern China. Over southwestern China, foreign anthropogenic emissions contributed 4.9 &micro;g m^-3 PM_2.5 concentrations (18 % of total PM_2.5 mass) to Yunnan province, with 37 % as organics and 27 % as sulfate. Our findings suggest that foreign anthropogenic emissions play an important role in Chinese PM_2.5 pollution, because of direct aerosol transport and, more importantly, chemical interactions between transported pollutants and China&rsquo;s local emissions. Thus, foreign emission reductions will be very beneficial for improving Chinese air quality.