Improved estimation of new particle formation rate for air quality model in a polluted region of South Korea

Abstract

Ambient atmospheric aerosol particles consist of primary particles that are directly emitted and secondary particles that are formed through nucleation or chemical reactions. The number concentration of aerosol particles in the atmosphere, which has a significant impact on air quality and climate, is largely influenced by secondary particles formed through nucleation. Therefore, a precise understanding of nucleation is essential. Homogeneous nucleation is a mechanism in which new condensed-phase substances, aerosol particles, are generated through the phase transition of vapor under conditions of vapor supersaturation. Several theories have been proposed to explain nucleation processes, including H₂SO₄-H₂O binary homogeneous nucleation, H₂SO₄-H₂O-NH₃ ternary homogeneous nucleation, and ion-mediated nucleation. In practice, many air quality models including Community Multiscale Air Quality Modeling system (CMAQ) still use only H₂SO₄-H₂O binary homogeneous nucleation module to calculate nucleation rates. The comparison and validation of different nucleation theories in the polluted regions of South Korea has never been conducted. In this study, we used CMAQv5.2 to simulate the aerosol number concentration at Seoul Olympic Park during the KORUS-AQ campaign. The results revealed that the model underestimated the aerosol number concentration by approximate a factor of 10 compared to observations. This is primarily due to nucleation rates, which raises the need for an alternative module with higher accuracy. To simulate nucleation rates and aerosol number concentrations, we applied five different nucleation modules to a box model Korea Air quality observation-based Box model (KAB) as well as a 3-dimensional chemical transport model CMAQ. Based on the KAB modeling results, the ion-mediated H₂SO₄-H₂O binary nucleation module resulted in a significantly higher nucleation rate compared to the calculations from the existing modules (0.06 cm⁻³s⁻¹ vs. 1.16 cm⁻³s⁻¹), providing values closer to the observed nucleation rates. And according to the CMAQ modeling results, ternary H₂SO₄-H₂O-NH₃ homogeneous nucleation module gave the calculated aerosol number concentration the closest to the observed value. Such comparisons between modules are expected to be useful for selecting the most suitable nucleation module for the atmospheric conditions in South Korea.