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Heterogeneous chemistry and aerosol-radiation interaction (ARI) are considered two of the key mechanisms affecting haze formation and their reasonable parameterizations in atmospherically chemical models play an important role in accurate haze or air quality prediction. However, until the present, their relative contributions to severe haze have rarely been evaluated. Using the double-way atmospherically chemical model GRAPES_Meso5.1/CUACE CW, the relative contributions of heterogeneous chemistry and ARI to PM 2.5 are investigated under different pollution levels in Middle-Eastern China (MEC) during December 2016. Study results show that the combined effects of heterogeneous chemistry and ARI generate 29.7% of total PM 2.5 mass in MEC of December 2016, of which ARI contributes 19.7%, much higher than heterogeneous chemistry (6.1%). However, heterogeneous chemistry is important for reasonable reproduction of the proportion of secondary inorganic aerosols (SIA) of PM 2.5 , especially during heavy haze. Both the contributions of heterogeneous chemistry and ARI to PM 2.5 increase with the haze pollution aggravation but to different degrees. The contribution of heterogeneous chemistry to modeled PM 2.5 concentration rises from 4.0% on clean days to 6.6% under moderate pollution and 7.8% under heavy haze. And that of ARI increases rapidly from 9.1% on clean days to 21.0% under moderate pollution and 29.1% during heavy pollution. It is worth noticing that there is an additional contribution to PM 2.5 caused by the combination of heterogeneous chemistry and ARI except for their respective contributions, which reaches up to 7.3% of PM 2.5 mass and 18.5% of SIA generation under heavily polluted conditions. This is because ARI further promotes heterogeneous oxidation of SO 2 and NO x by increasing the aerosol surface area concentration (SAC), causing additional SIA production via heterogeneous pathways. This work indicates the contribution of ARI to severe haze formation not only by altering meteorological conditions but also by reforming the chemical processes in the model. (a) The interaction mechanism between heterogeneous chemistry and ARI in atmospheric chemistry model GRAPES_Meso5.1/CUACE; (b) Percentage contributions of the combined effects of heterogeneous chemistry and ARI (Δ1), heterogeneous chemistry alone (Δ2), ARI alone (Δ3), and the interaction between the two (Δ4) to PM 2.5 mass concentration in MEC. [Display omitted] • Relative contributions of heterogeneous chemistry and ARI to PM 2.5 are evaluated under different pollution levels. • ARI contributes much more to extreme PM 2.5 during severe haze than heterogeneous chemistry. • Their combined effects exceed the sum of the two because ARI further promotes the heterogeneous oxidation of SO 2 and NO x. [ABSTRACT FROM AUTHOR]

The aerosol-radiation interaction mechanism (ARI) is important for air quality modeling, and its absence in atmospheric chemistry models may cause considerable uncertainties. In this study, the real-time calculated aerosol optical parameters, i.e., mass extinction coefficient (K ext), single scattering albedo (ω), and asymmetry factor (g) from the simulated aerosol concentrations are introduced into the Global/Regional Assimilation and Prediction System 5.1 version coupled with the Chinese Unified Atmospheric Chemistry Environment model (GRAPES_Meso5.1/CUACE) to establish the ARI mechanism for the two-way feedback between aerosols and weather processes. Focusing on Beijing-Tianjin-Hebei (BTH), China, in January 2017, ARI impacts on meteorology and haze-fog are simulated by using this atmospheric chemistry model. The results show that during the severe pollution, ARI can significantly alter the meteorology, especially for the heavily polluted central and southern BTH: The surface solar radiation (SSR), the temperature at 2 m (T2), the vertical diffusion coefficient (VDC), and the planetary boundary layer height (PBLH) are reduced by up to 50 W m−2, 2.5°C, 4 m2 s−1, and 100 m, respectively. In addition to surface meteorology, ARI affects the vertical structure of the planetary boundary layer (PBL), by cooling and humidifying the lower air and heating and drying the higher air. These modifications stabilize the PBL, resulting in an increase of 8% in relative humidity at 2 m (RH2), a key factor influencing secondary aerosol generation and aerosol hygroscopic growth. Stabilized PBL and increased RH2 ultimately result in a 100 μg m−3 increase in PM 2.5 concentrations and a 1.5 km reduction in atmospheric visibility (VIS). By considering the ARI, the model generally improves numerical weather prediction (NWP) and haze-fog prediction. For the entire BTH region, the mean biases for T2 and RH2 are reduced by up to 2°C and 4% respectively. This more accurate NWP will lead to an overall improvement in haze-fog prediction, particularly in a heavily polluted city like Shijiazhuang (SJZ), the biases for PM 2.5 and VIS are reduced by 28% and 125% respectively. The results show the positive feedback between meteorology and haze-fog, and demonstrate the importance of ARI in NWP and haze-fog prediction. • Aerosol-radiation interaction (ARI) has a critical influence on the outbreak of severe haze-fog. • The ARI mechanism is achieved in GRAPES_Meso5.1/CUACE. • The two-way coupled model considering ARI significantly improves the prediction of both meteorology and haze-fog. [ABSTRACT FROM AUTHOR]