Modeling comparison of precipitation schemes and implications on aerosol diameter treatment for better sulfate aerosol production in the early summer rainy season over Northeast Asia

In 2020, the Fine Particle Research Initiative in East Asia considering National Differences (FRIEND) project was launched to understand air quality issues over Northeast Asia better. In the FRIEND project, high-temporal-resolution measurements of gas and aerosol concentrations were taken simultaneously at five key sites in Northeast Asia. In this study, we used the dataset at Beijing in China, Seoul in Republic of Korea, and Noto in Japan. The second FRIEND campaign was conducted in early summer from June 1 to 30, 2021. Compared with the results of the first FRIEND campaign conducted in winter, it was revealed that the fraction of sulfate aerosol (SO₄²⁻) had dramatically increased in the upwind region of Northeast Asia (Beijing and Seoul). This period corresponds to the early rainy season in Northeast Asia; therefore, the role of the aqueous-phase oxidation process could be increased in SO₄²⁻ production. However, accurate modeling of precipitation is still challenging because of the parameterization in the meteorological model. Thus, we investigated the microphysics and cumulus schemes in the meteorological model and conducted 10 simulations. All schemes underestimated the precipitation amount and the cloud fraction. Hence, SO₄²⁻ concentration was underestimated with a lower conversion ratio from sulfur dioxide (SO₂) to SO₄²⁻ (F_S) at Beijing, Seoul, and Noto. At Seoul, the SO₄²⁻ concentration was underestimated with the aerosol ion monitor (AIM) measurements, corresponding to PM_2.5, but had an acceptable performance level. The SO₄²⁻ concentration at Seoul was sensitive to microphysics and cumulus schemes. However, the SO₄²⁻ concentration was compared with aerosol chemical speciation monitors (ACSMs), corresponding to PM_1.0, in Beijing and Noto, and showed greater underestimation. The sensitivities of SO₄²⁻ concentration to the precipitation schemes were small at Beijing and Noto. The simulated aerosol diameter shifted to a coarser range (1–2.5 μm) in the second campaign compared with the first campaign dataset with increasing temperature and relative humidity. The international measurement network in the FRIEND project demonstrates that the modeled aerosol diameter treatment must be revised carefully.