| Summary: | We report the results of year-long PM<sub>2.5</sub> (particulate matter less than 2.5 µm in diameter) simulations over Northeast Asia for the base year of 2013 under the framework of the Long-range Transboundary Air Pollutants in Northeast Asia (LTP) project. LTP is a tripartite project launched by China, Japan, and Korea for cooperative monitoring and modeling of the long-range transport (LRT) of air pollutants. In the modeling aspect in the LTP project, each country’s modeling group employs its own original air quality model and options. The three regional air quality models employed by the modeling groups are WRF-CAMx, NHM-RAQM2, and WRF-CMAQ. PM<sub>2.5</sub> concentrations were simulated in remote exit-and-entrance areas associated with the LRT process over China, Japan, and Korea. The results showed apparent bias that remains unexplored due to a series of uncertainties from emission estimates and inherent model limitations. The simulated PM<sub>10</sub> levels at seven remote exit-and-entrance sites were underestimated with the normalized mean bias of 0.4 ± 0.2. Among the four chemical components of PM<sub>2.5</sub> (SO<sub>4</sub><sup>2−</sup>, NO<sub>3</sub><sup>−</sup>, organic carbon (OC), and elemental carbon (EC)), the largest inter-model variability was in OC, with the second largest discrepancy in NO<sub>3</sub><sup>−</sup>. Our simulation results also indicated that under considerable SO<sub>4</sub><sup>2−</sup> levels, favorable environments for ammonium nitrate formation were found in exit-and-entrance areas between China and Korea, and gas-aerosol partitioning for semi-volatile species of ammonium nitrate could be fully achieved prior to arrival at the entrance areas. Other chemical characteristics, including NO<sub>3</sub><sup>−</sup>/SO<sub>4</sub><sup>2−</sup> and OC/EC ratios, are discussed to diagnose the LRT characteristics of PM<sub>2.5</sub> in exit-and-entrance areas associated with transboundary transport over China, Japan, and Korea.
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