Effects of Mountains on Aerosols Determined by AERONET/DRAGON/J‐ALPS Measurements and Regional Model Simulations.

The NASA/AErosol RObotic NETwork field campaign Distributed Regional Aerosol Gridded Observation Networks/Joint work to the AerosoL Properties and Process Simulations was conducted from March 2020 to May 2021 in Nagano, Japan. Twelve sun photometers were installed around Nagano prefecture. The effects of topography on aerosols were studied using observations and simulations. In this study, a regional chemical transport model (SCALE‐Chem) was employed. Three numerical experiments were conducted: E1 (control experiment), E2 (E1 without topography), and E3 (E1 with removal of all anthropogenic emissions over Nagano prefecture). In E2, the terrain effect was not considered; the difference between E1 and E2 indicated the influence of mountains. The differences between E1 and E3 evaluate the local emission effect. In some cases, the mountainous terrain seemed to have suppressed aerosol inflow (i.e., reduced aerosol concentration), while in other cases, the mountains contributed to aerosol retention on days when aerosols tended to accumulate in mountain basins due to local emissions. Thus, while mountains prevent the inflow of aerosols from outside, they also contribute to increased aerosol concentration in the basin. Naturally, more significant effects are produced by meteorological conditions and the presence or absence of transboundary pollution from the outside. From observations and model simulations, we found that the aerosol concentration was not high around the J‐ALPS site because of the mountain effect that prevents advection from the outside, even when transboundary pollution was observed in Japan in March 2020. Plain Language Summary: Aerosol observations by the NASA/AErosol RObotic NETwork field campaign were conducted in the mountainous regions of Japan from March 2020 to May 2021. This field campaign is called Distributed Regional Aerosol Gridded Observation Network/Joint work to the AerosoL Properties and Process Simulation because the target area includes the mountains known as the Japanese Alps. In this study, we investigated how mountains affect aerosol distribution by using simulations with the regional chemical transport model SCALE‐Chem in conjunction with observational data in mountainous regions. To investigate the effect of mountains, simulations were conducted with and without mountains. In addition, to investigate the effect of local sources, we compared simulations with and without anthropogenic emissions in the target area. The simulation results showed that the mountains blocked aerosols and created a basin effect by increasing the aerosol concentration near the surface. Furthermore, averaged throughout March 2020, the effect of the mountains blocking aerosols was greater than the effect of the mountains increasing aerosol concentrations near the surface. This finding suggests that the blocking effect of the surrounding mountains prevented an increase in aerosol concentrations at the J‐ALPS site even on days when transboundary pollution from mainland China arrived in Japan. Key Points: To investigate the effect of mountains on aerosols, a regional model was implemented for field campaign over a Japanese mountain regionThis study found that, mountains have a blocking effect on advection aerosols, but also increase aerosol concentrations in basinsThe J‐ALPS sites showed that, the mountain effect prevented external advection, even on days with widespread transboundary pollution [ABSTRACT FROM AUTHOR]