Development of the RAQM2 aerosol chemical transport model and predictions of the Northeast Asian aerosol mass, size, chemistry, and mixing type

Abstract

A new aerosol chemical transport model, the Regional Air Quality Model 2 (RAQM2), was developed to simulate the Asian air quality. We implemented a simple version of a triple-moment modal aerosol dynamics model (MADMS) and achieved a completely dynamic (non-equilibrium) solution of a gas-to-particle mass transfer over a wide range of aerosol diameters from 1 nm to super-μm. To consider a variety of atmospheric aerosol properties, a category approach was utilized in which the aerosols were distributed into four categories: particles in the Aitken mode (ATK), soot-free particles in the accumulation mode (ACM), soot aggregates (AGR), and particles in the coarse mode (COR). The aerosol size distribution in each category is characterized by a single mode. The condensation, evaporation, and Brownian coagulations for each mode were solved dynamically. A regional-scale simulation (&Delta;<i>x</i> = 60 km) was performed for the entire year of 2006 covering the Northeast Asian region. The modeled PM₁/bulk ratios of the chemical components were consistent with observations, indicating that the simulated aerosol mixing types were consistent with those in nature. The non–sea-salt SO₄^2&minus; mixed with ATK + ACM was the largest at Hedo in summer, whereas the SOSO₄^2&minus; was substantially mixed with AGR in the cold seasons. Ninety-eight percent of the modeled NO₃^&minus; was mixed with sea salt at Hedo, whereas 53.7% of the NO₃^&minus; was mixed with sea salt at Gosan, which is located upwind toward the Asian continent. The condensation of HNO₃ onto sea salt particles during transport over the ocean accounts for the difference in the NO₃^&minus; mixing type at the two sites. Because the aerosol mixing type alters the optical properties and cloud condensation nuclei activity, its accurate prediction and evaluation are indispensable for aerosol-cloud-radiation interaction studies.