Abstract. The online coupled Weather Research and Forecasting-Chemistry (WRF-Chem) model was applied to simulate a haze event that happened in January 2010 in the North China Plain (NCP), and was validated against various types of measurements. The evaluations indicate that WRF-Chem provides reliable simulations for the 2010 haze event in the NCP. This haze event was mainly caused by high emissions of air pollutants in the NCP and stable weather conditions in winter. Secondary inorganic aerosols also played an important role and cloud chemistry had important contributions. Air pollutants outside Beijing contributed about 64.5 % to the PM2.5 levels in Beijing during this haze event, and most of them are from south Hebei, Tianjin city, Shandong and Henan provinces. In addition, aerosol feedback has important impacts on surface temperature, relative humidity (RH) and wind speeds, and these meteorological variables affect aerosol distribution and formation in turn. In Shijiazhuang, Planetary Boundary Layer (PBL) decreased about 278.2 m and PM2.5 increased more than 20 µg m−3 due to aerosol feedback. It was also shown that black carbon (BC) absorption has significant impacts on meteorology and air quality changes, indicating more attention should be paid to BC from both air pollution control and climate change perspectives.

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Abstract. The online coupled Weather Research and Forecasting-Chemistry (WRF-Chem) model was applied to simulate a haze event that happened in January 2010 in the North China Plain (NCP), and was validated against various types of measurements. The evaluations indicate that WRF-Chem provides reliable simulations for the 2010 haze event in the NCP. This haze event is mainly caused by high emissions of air pollutants in the NCP and stable weather conditions in winter. Secondary inorganic aerosols also played an important role and cloud chemistry had important contributions. Air pollutants outside Beijing contributed about 47.8 % to the PM2.5 levels in Beijing during this haze event, and most of them are from south Hebei, Shandong and Henan provinces. In addition, aerosol feedback has important impacts on surface temperature, Relative Humidity (RH) and wind speeds, and these meteorological variables affect aerosol distribution and formation in turn. In Shijiazhuang, Planetary Boundary Layer (PBL) decreased about 300 m and PM2.5 increased more than 20 μg m-3 due to aerosol feedback. Feedbacks associated to Black Carbon (BC) account for about 50 % of the PM2.5 increases and 50 % of the PBL decreases in Shijiazhuang, indicating more attention should be paid to BC from both air pollution control and climate change perspectives.

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Particles containing black carbon (BC), a strong absorbing substance, exert a rather uncertain direct and indirect radiative forcing in the atmosphere. To investigate the mass concentration and absorption properties of BC particles over central Europe, the model WRF-Chem was used at a resolution of 12 km in conjunction with a high-resolution BC emission inventory (EUCAARI 42-Pan-European Carbonaceous Aerosol Inventory; 1/8 degrees x 1/16 degrees). The model simulation was evaluated using measurements of equivalent soot carbon, absorption coefficients and particle number concentrations at seven sites within the German Ultrafine Aerosol Network, PM10 mass concentrations from the dense measurement network of the German Federal Environmental Agency at 392 monitoring stations, and aerosol optical depth from MODIS and AERONET. A distinct time period (25 March to 10 April 2009) was chosen, during which the clean marine air mass prevailed in the first week and afterwards the polluted continental air mass mainly from the southeast dominated with elevated daily average BC concentration of up to 4 mu g m(-3). The simulated PM10 mass concentration, aerosol number concentration and optical depth were in good agreement with the observations, while the modelled BC mass concentrations were found to be a factor of 2 lower than the observations. Together with back trajectories, detailed model bias analyses suggested that the current BC emission in countries to the east and south of Germany might be underestimated by a factor of 5, at least for the simulation period. Running the model with upscaled BC emissions in these regions led to a smaller model bias and a better correlation between model and measurement. In contrast, the particle absorption coefficient was positively biased by about 20% even when the BC mass concentration was underestimated by around 50 %. This indicates that the internal mixture treatment of BC in the WRF-Chem optical calculation is unrealistic in our case, which overamplifies the light absorption by BC-containing particles. By adjusting the modelled mass absorption cross-section towards the measured values, the simulation of particle light absorption of BC was improved as well Finally, the positive direct radiative forcing of BC particles at the top of the atmosphere was estimated to be in the range of 0 to +4 W m(-2) over Germany for the model mn with improved BC mass concentration and adjusted BC light absorption cross-section. This adjustment lowered the positive forcing of BC by up to 70 %, compared with the internal mixing treatment of BC in the model simulation

The winter haze is a growing problem in North China, but the causes are not well understood. The chemistry version of the Weather Research and Forecasting model (WRF-Chem) was applied in North China to examine how PM 2.5 concentrations change in response to changes in emissions (sulfur dioxide (SO 2 ), black carbon (BC), organic carbon (OC), ammonia (NH 3 ), and nitrogen oxides (NO x )), as well as meteorology (temperature, relative humidity (RH), and wind speeds) changes in winter. From 1960 to 2010, the dramatic changes in emissions lead to +260 % increases in sulfate, +320 % increases in nitrate, +300 % increases in ammonium, +160 % increases in BC, and +50 % increases in OC. The responses of PM 2.5 to individual emission species indicate that the simultaneous increases in SO 2 , NH 3 , and NO x emissions dominated the increases in PM 2.5 concentrations. PM 2.5 shows more notable increases in response to changes in SO 2 and NH 3 as compared to increases in response to changes in NO x emissions. In addition, OC also accounts for a large fraction in PM 2.5 changes. These results provide some implications for haze pollution control. The responses of PM 2.5 concentrations to temperature increases are dominated by changes in wind fields and mixing heights. PM 2.5 shows relatively smaller changes in response to temperature increases and RH decreases compared to changes in response to changes in wind speed and aerosol feedbacks. From 1960 to 2010, aerosol feedbacks have been significantly enhanced due to higher aerosol loadings. The discussions in this study indicate that dramatic changes in emissions are the main cause of increasing haze events in North China, and long-term trends in atmospheric circulations may be another important cause since PM 2.5 is shown to be substantially affected by wind speed and aerosol feedbacks. More studies are necessary to get a better understanding of the aerosol-circulation interactions.

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