The Causation Of Spatial And Temporal Variations Of Black Carbon In The Guanzhong Region, China: Ambient Measurements And WRF-Chem Modeling

The Guan-Zhong(GZ) region has been undergoing severe air pollution in recent years, especially the pollution caused by the particulate matter with the diameter less than 2.5 μm(PM2.5), and its environmental problem leads to the region to be one of the “Three economic zones and ten urban clusters ” key regions that urgently need to improve the air quality. Black carbon(BC), as an important component of PM2.5, focuses on more attentions due to its influences on climate, environment and human health as well as its close association with anthropogenic activities and national energy utilizations. Therefore, it is quite necessary to investigate the spatial and temporal variations of BC and make clear what causes these variations. To achieve the above aims, a mesoscale regional dynamical and chemical model WRF-Chem(Weather Research and Forecasting with Chemistry) combined with ambient BC measurements in the GZ region is used. Firstly, we validate the model by comparing the simulated against observed BC concentrations. Secondly, we analyze the factors that lead to annual and seasonal variations of BC concentration and the impacts of the synoptic meteorology and local topography on BC concentrations in heavy air pollution during wintertime. The preliminary conclusions are summarized as follows:1. The evaluation of WRF-Chem model: the spatial and temporal distributions of the simulated BC concentration were consistent with the observed, i.e., the simulation reproduced the higher concentrations observed at urban sites than those at rural and remote sites, and the magnitude of BC concentration at urban sites were about 0.5-1.5 order higher than that at remote site; BC concentrations in winter were obviously the highest. However, there were some discrepancies of the magnitude between the simulations and the observations, i.e., the simulated BC concentrations at rural sites were obviously underestimated.2. The causation of annual and seasonal variations of BC concentrations in the GZ region: seasonal averaged wind speeds were significantly negative with BC concentrations which decreased from 20 μg/m3 to 7 μg/m3 when wind speeds increased from 1 m/s to 2 m/s. Seasonal averaged planetary boundary layer(PBL) heights were also negative with BC concentrations, but not as significantly as the wind speeds. When the PBL height was higher than 800 m, BC concentration was fluctuated around 10 μg/m3. Winter averaged BC concentration had a clear decline from 2003 to 2006, but little association with the wind speeds and PBL heights, and this mainly resulted from the reduction of BC emission during this season. Sensitive experiments by WRF-Chem model suggested seasonal change in local emission was the dominated factor controlling seasonal variation of BC concentration in the GZ region, and also local emission was the primary contributor to BC concentration in every season, especially in winter when 74.0% of BC particles were produced by local emission, and 56.1%, 38.1% and 54.4% in spring, summer and autumn, respectively. The second was regional transport, contributing in the range of 22.2%- 34.4%. In winter, the north and northwest regions in the GZ region had an obvious transport for BC particles while the northeast and east in the GZ region had a more obvious transport in other seasons. Local meteorology contributed 3.8%- 27.5% to BC particles in the GZ region in different seasons. In spring, a convergent wind was in favor of the converge of BC particles while the urban buildings in the center of the GZ region slowed down the wind speeds to be against the transport in summer, and the local winds in autumn were similar to the situation in spring but with low speeds, while gentle breezes or calm winds in winter easily led to BC accumulation.3. The impacts of meteorology and topography on BC concentrations in heavy air pollution during wintertime at Xi’an: the synoptic scale weather systems played an important role in BC concentration in the heavy air pollution during wintertime at Xi’an. BC concentrations rapidly decreased when strong cold high pressure passed through the city due to the fierce winds with the cold airflows. Then cold high pressure weakened and low pressure activated, BC concentrations gradually increased at the converged winds zone. With the retreat of low pressure, the city was absolutely controlled by the high pressure, and gentle breezes or calm winds dominated the city. Under such weather conditions, BC particles became to accumulate and rapidly reached to the maximum. Sensitive experiments by WRF-Chem model displayed local meteorological conditions transported a little bit of BC particles to the outside regions, and local winds indicated that the transport of BC particles was clearly affected by Qinling Mountains under different weather conditions. When north winds and northeast winds dominated, the mountains obviously blocked the airflows to result in BC accumulation in the north foothill where the Xi’an City locates. When the winds turned to the straight easterly, the airflows moved along the river valley and the narrow valley in the west speeded up the outflows and enhanced the transport of BC particles in the city to the downwind regions. When the gentle breezes or calm winds prevailed, local winds were too weak to transport BC particles, and strong mountain breezes trigged by the topographical effect moved BC particles to the northeast of the city, but the influence was limited within the city and BC concentrations in the city were still high. Therefore, the topographical blocking and mountain breeze effects of Qinling mountains were both adverse to the transport and diffusion of BC particles in the heavy air pollution in winter at Xi’an when the synoptic scale weather systems were relatively stable.At present, there are lack of scientific and efficient solutions and measures to prevent the air pollution and improve the air quality because of obscure knowledge about the source and causation of the pollutants. This study focuses on related analysis of BC aerosol, which is helpful to recognize how to control the primary pollutants, but the causation analysis and numerical simulation for other pollutants, especially secondary aerosols, need to be further enhanced.