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In this study, we developed a data assimilation (DA) system for chemical transport model (CTM) simulations using an ensemble Kalman filter (EnKF) technique. This DA technique is easy to implement in an existing system without seriously modifying the original CTM and can provide flow-dependent corrections based on error covariance by short-term ensemble propagations. First, the PM2.5 observations at ground stations were assimilated in this DA system every 6 h over South Korea for the period of the KORUS–AQ campaign from 1 May to 12 June 2016. The DA performances with the EnKF were then compared to a control run (CTR) without DA and a run with three-dimensional variational (3D-Var) DA. Consistent improvements owing to the initial conditions (ICs) assimilated with the EnKF were found in the DA experiments at a 6 h interval compared to the CTR run and to the run with 3D-Var. In addition, we attempted to assimilate the ground observations from China to examine the impacts of improved boundary conditions (BCs) on the PM2.5 predictability over South Korea. The contributions of the ICs and BCs to improvements in the PM2.5 predictability were also quantified. For example, the relative reductions in terms of the normalized mean bias (NMB) were found to be approximately 27.2 % for the 6 h reanalysis run. A series of 24 h PM2.5 predictions were additionally conducted each day at 00:00 UTC with the optimized ICs. The relative reduction of the NMB was 17.3 % for the 24 h prediction run when the updated ICs were applied at 00:00 UTC. This means that after the application of the updated BCs, an additional 9.0 % reduction in the NMB was achieved for 24 h PM2.5 predictions in South Korea.

To better understand seasonal variations in the components of PM_〈2.5〉 and related trace gases, and the impacts of long-range transport from the Asian continent to Japan, we analyzed long-term, simultaneous observations of fineand coarse-mode nitrate, related aerosol components (SO_4^〈2-〉, NH_4^+, BC, OC), and trace gases (CO, NH_3, HNO_3) around the Chikushi Campus of Kyushu University, Fukuoka (33.52°N, 130.47°E), from August 2014 to October 2015. Daily and monthly mean aerosol concentrations were analyzed to assess the main seasonal variation in pollution. We found that fine aerosols (fSO_4^〈2-〉, fNO_3^-, and fNH_4^+) explained most of the seasonal change in PM_〈2.5〉. Polarization optical particle counter measurements were useful for detecting the onset of dust transport from the Asian continent, and we observed high concentrations of coarse-mode NO_3^- (cNO_3^-) during dust period, which indicated the formation of dust nitrate during transport. We also analyzed diurnal variation in the basic aerosol components for different seasons, weekday/weekend effects (for CO, BC, and NH3), and sensitivity on days with and without precipitation (for CO, BC, NH_3, PM_〈2.5〉, and PMc). Observational data for HNO_3, fNO_3^-, and cNO_3^- demonstrated that cNO_3^- accounted for the largest proportion of total nitrate (defined as the sum of fNO_3^-, cNO_3^-, and HNO_3), constituting 40–55% of the total nitrate during the winter, whereas HNO_3 gas constituted approximately 40% of the total nitrate in summer and fNO_3^- peaked during the winter.

Atmospheric ammonia (NH_3) plays an important role in the formation and understanding of secondary inorganic aerosols, but has not been examined using chemical transport models (CTMs) and atmospheric levels have not been compared with satellite observations. Here, we analyzed the seasonal variation in atmospheric NH_3 using long-term hourly NH_3/NH_4^+ measurements made at the Chikushi Campus of Kyushu University in Fukuoka, nationwide filter pack (FP) network observations over Japan, GEOS-Chem CTM results, and Infrared Atmospheric Sounding Interferometer (IASI) satellite retrievals over East Asia. We found that the IASI NH_3 retrieval over central and north China had a strong correlation with the CTM results (R = 0.74). There was a similar correlation over south China, but the IASI NH_3 retrieval was only 48% of the CTM value. CTM results over Japan reproduced the major seasonal variations compared with surface observations, but IASI observations over Japan were below or near the detection limit and it was difficult to obtain a reasonable correlation with measurements.

Heterogeneous processes play an important role in changing the chemical, physical, optical, and radiative characters of dust particles at regional and global scales, especially in East Asia, where both the emissions of mineral dust and anthropogenic pollutants are huge. To investigate the chemical and physical impacts of heterogeneous reactions on dust particles, three dust and/or air pollution episodes were observed and simulated in Beijing, China and Fukuoka, Japan during 27 March and 2 April, 2015. The results confirmed that heterogeneous reactions were the major mechanisms producing coarse mode nitrate reaching 19 μg/m^3 in Beijing and 4 μg/m^3 in Fukuoka. We also found that heterogeneous processes and subsequent hygroscopic growth significantly changed the mixing state and size of dust particles. As a result of the internal mixing of nitrate, sulfate, and aerosol liquid water, the volume concentration of dust doubled when the relative humidity (RH) was relatively high (>80%), and the dust particles tended to be spherical when the volume fraction of dust coatings reached 20%.

Mass concentrations of chemical compounds in both fine (particle diameter, Dp < 2.5 μm) and coarse mode (2.5 < Dp < 10 μm) were measured in urban sites of the Fukuoka area using a continuous dichotomous Aerosol Chemical Speciation Analyzer (ACSA-12) at a time-resolution of 1 h throughout 2014. We found that mass concentrations of particulate matter (PM), sulfate, and water-soluble organic compounds in fine mode showed a predominant peak during the spring, indicative of long-range transport due to the prevailing westerly wind. Nitrate mass in fine mode showed an obvious warm-season-low and cold-season-high pattern; thus, it was difficult to accurately quantify the cross-boundary transport of nitrate because of the gas-aerosol phase equilibrium process, particularly under high temperature conditions. Mineral dust and sea salt were the two most important medians carrying coarse mode nitrates. This study found that the transported anthropogenic pollutants contributed to ambient PM mass in Fukuoka city, although urban emission was not negligible.