Integration of multi-phase chemistries into a 3D photochemical transport model to improve model performances

Abstract

In recent years, many studies have emphasized that the dangers of air pollutants such as O3 and particulate matter (PM) influencing premature mortality and adverse health effects. In response to these issues, governments have implemented emission control strategies based on the result of air quality models. However, such mitigating air pollutant policies proves to be highly ineffective due to uncertainties in model simulation. These findings suggest a critical need for further development focused on enhancing model performance and reliability. In this context, we developed updated Community Multiscale Air Quality (CMAQ) model that incorporates multiphase reactions and updated emissions associated with four representative processes: (i) HONO processes, (ii) halogen processes, (iii) sulfate processes, and (iv) HO₂ processes. In particular, these four processes were selected because of their close association with the formation of air pollutants. Based on the modeling framework, we evaluate the model performance against observations, then examined the impact of the four processes on atmospheric species. First, we incorporated newly HONO processes into our modeling system to simulate accurate key atmospheric species such as O3 and PM2.5. The HONO chemistry incorporated in this study were: (i) direct HONO emissions (such as soil, traffic and soil HONO emissions); (ii) gas-phase HONO reaction; (iii) heterogeneous reactions of NO2 on aerosol and ground surfaces; (iv) renoxifications. After establishing modeling framework, we explored the impact of HONO processes on oxidant capacity (OH, HO2, and O3), as well as particulate matter concentrations. As a result, the updated model showed improvements in the mixing ratios of both HONO and O3 simulated compared to observed mixing ratios with enhanced statistical metrics. Secondly, we investigated the influence of halogen radicals (Cl, Br, and I) on significant atmospheric species by incorporating halogen processes, including emissions of halogen compounds and 155 new gaseous-, 4 aqueous-, and 18 heterogeneous-halogen reactions, into the CMAQ model. We evaluate the updated model performance by utilizing the ClNO2 observations, and then examined the net Ox production rates (P(Ox)) in the presence of the halogen radicals. The results revealed that mixing ratios of ClNO2 simulated agreed well with observations, with the index of agreement (IOA) rising from 0.41 to 0.66. In addition, the P(Ox) increased in the land, while decreased over the ocean. Our findings highlight the critical role of halogen chemistry determining the levels of atmospheric species. Thirdly, in order to address underestimation in sulfate concentrations simulated, we developed a new parameterization of the SO2 uptake coefficient (γSO2) as a function of relative humidity, and NO2 and NH3 mixing ratios. We then evaluated the γSO2 used in this study, together with different parameterizations of γSO2 introduced in previous studies. Comparative analysis showed that our study has the best agreements with surface-observations. For example, sulfate concentrations simulated in our study increasing from 3.85 μg/m3 to 4.78 μg/m3, with the IOA increasing from 0.63 to 0.70. In addition, for extended 1-year simulations, our approach also significantly reproduced the sulfate concentrations during the spring and winter. It implied that the importance of accurate γSO2 for effective air quality management strategies. Finally, we attempted to provide a comprehensive insight into the HO2 processes in the atmosphere. From the findings in aircraft observations, we found that the mixing ratios of HO2 simulated may be strongly connected with NO distributions. In particular, for low-NO conditions (below 0.3 ppb), the mixing ratios of HO2 simulated overestimated by 6.0% due to missing removal pathways. In this context, we incorporated HO2 aerosol uptake, adopting an uptake coefficient of HO2 as 0.08, assuming no H2O2 production, which can lead to improved model performance. We also explored the impact of HO2 aerosol uptake on air pollutants depending on NO mixing ratios. As a result, this study emphasized complex interactions of HO₂ in atmospheric chemistry. In this study, we attempted to achieve three major objectives: (i) incorporate missing pathways into the CMAQ model using ‘state-of-the-art’ techniques; (ii) enhance the simulation accuracy for various substances; (iii) understand their interactions in the atmosphere. To establish the purpose, we investigated four representative atmospheric processes in terms of various perspectives. Based on the result, our findings are expected to provide a valuable opportunity to expand our knowledge on atmospheric chemistry and model development.