Implementation and initial application of new chemistry-aerosol options in WRF/Chem for simulating secondary organic aerosols and aerosol indirect effects for regional air quality
Atmospheric aerosols play important roles in affecting regional meteorology and air quality through aerosol direct and indirect effects. Two new chemistry-aerosol options have been developed in WRF/Chem v3.4.1 by incorporating the 2005 Carbon Bond (CB05) mechanism and coupling it with the existing aerosol module MADE with SORGAM and VBS modules for simulating secondary organic aerosol (SOA), aqueous-phase chemistry in both large scale and convective clouds, and aerosol feedback processes (hereafter CB05-MADE/SORGAM and CB05-MADE/VBS). As part of the Air Quality Model Evaluation International Initiative (AQMEII) Phase II model intercomparison that focuses on online-coupled meteorology and chemistry models, WRF/Chem with the two new options is applied to an area over North America for July 2006 episode. The simulations with both options can reproduce reasonably well most of the observed meteorological variables, chemical concentrations, and aerosol/cloud properties. Compared to CB05-MADE/SORGAM, CB05-MADE/VBS greatly improves the model performance for organic carbon (OC) and PM2.5, reducing NMBs from −81.2% to −13.1% and from −26.1% to −15.6%, respectively. Sensitivity simulations show that the aerosol indirect effects (including aqueous-phase chemistry) can reduce the net surface solar radiation by up to 53 W m−2 with a domainwide mean of 12 W m−2 through affecting cloud formation and radiation scattering and reflection by increasing cloud cover, which in turn reduce the surface temperature, NO2 photolytic rate, and planetary boundary layer height by up to 0.3 °C, 3.7 min−1, and 64 m, respectively. The changes of those meteorological variables further impact the air quality through the complex chemistry-aerosol-cloud-radiation interactions by reducing O3 mixing ratios by up to 5.0 ppb. The results of this work demonstrate the importance of aerosol indirect effects on the regional climate and air quality. For comparison, the impacts of aerosol direct effects on both regional meteorology and air quality are much lower with the reduction on net surface solar radiation only by up to 17 W m−2 and O3 only by up to 1.4 ppb, which indicates the importance and necessity to accurately represent the aerosol indirect effects in the online-couple regional models.