Column-Integrated Aerosol Optical Properties during Summer and Autumn of 2012 in Xi'an, China

Column-integrated aerosol optical properties were derived systematically from measurements made in Xi&#39;an, which is located in Guanzhong Plain of central China with a ground-based CIMEL sun photometer from May to November 2012. Aerosol optical depths (AODs), Angstrom exponents, water vapor contents, and aerosol optical and micro-physical properties, including aerosol volume size distribution, complex refractive indices and single scattering albedo (SSA), were determined. Daily variations in AODs at 440 nm (tau(440)) generally followed those of the 24-hr PM2.5 mass concentrations, but there were differences in the relationships in summer and autumn. August showed the highest monthly tau(440) (1.13) while the largest monthly Angstrom exponent (alpha(440-870) = 1.30) and water vapor content (C-w = 4.28) both occurred in July. Monthly averages of the aerosol size distributions showed the dominance of coarse mode aerosols, except in July and August, when the contribution of the accumulation and coarse modes were fairly comparable. Monthly changes in the complex refractive index (including both real and imaginary parts) and SSA were also studied, including their wavelength dependences; these analyses implied changes in the abundances of the aerosol types. Finally, an episode involving urban and dust aerosols was analyzed using sun photometer aerosol retrievals; MODIS images captured by Aqua satellite and average wind vectors from the NCEP operational global analyses were also considered in the case study.</p

Seasonal variation and four-year trend of black carbon in the Midwest China: The analysis of the ambient measurement and WRF-Chemmodeling

In-situ measurement of black carbon (BC) concentration from September 2003 to August 2007 in the Xi'an City at the Guanzhong Basin located in the mid-western China (the Guanzhong Basin) was analyzed. A regional dynamics and aerosol model (WRF-Chem) was used to quantify the impacts of local emission, meteorological conditions, and regional atmospheric transport on seasonal variation of BC concentration at the Guanzhong Basin. The results show that the regional prevailing winds at the Guanzhong Basin were unfavorable for the horizontal transport. The mean wind speeds ranged from 1.0 m/s to 1.9 m/s. During winter, the wind at the Guanzhong Basin was very weak (∼1.0 m/s). During spring and autumn, there was a wind convergent zone at the Guanzhong Basin, constraining the BC concentrations inside the Guanzhong Basin. As a result, the BC concentrations were persistently high at the Guanzhong Basin. In addition to the high background concentrations, there was a strong seasonal variation, with a maximum in winter (winter maximum) and a minimum in summer (summer minimum), with the maximum of the mean concentration of 30 μg m−3 in 2003–2004 winter, and the minimum of 5 μg m−3 in 2004 summer. The model sensitivity study shows that the seasonal variation of BC concentration was largely due to the seasonal variation of BC emission, especially during winter with the maximum of BC emission. A strong annual decrease trend of the BC concentration was found from 2004 to 2007. It is interesting to note that the decrease of the BC concentration only occurred in winter. For example, the winter maximum was 20 μg m−3 in 2003, and reduced to 11 μg m−3 in 2006, with about 50% decrease. In contrast, the summer minimum was 10 μg m−3 in 2004 and 9 μg m−3 in 2007, with only 10% decrease. This study suggests that the rapid decrease in the winter maximum was mainly due to the reduction of the BC emission in winter, implying the effective winter emission control at the Guanzhong Basin

Physicochemical characteristics of black carbonaerosol and its radiative impact in a pollutedurban area of China

Black carbon (BC) aerosol plays an important role in the Earth&rsquo;s radiative balance. An intensive measurement campaign was conducted at Xi&rsquo;an, China, from December 2012 to January 2013 to investigate the sources and physicochemical characteristics of refractory BC (rBC) and its direct radiative forcing at the surface. The overall average rBC concentration for the campaign was 8.0 &plusmn; 7.1 &mu;g m 3. Source apportionment based on positive matrix factorization showed that traffic was the dominant rBC source (46.0%), followed by coal burning (33.9%) and biomass burning (20.1%). The rBC mass size distributions were monomodal and lognormal with larger mass median diameters for coal burning source (215 nm) compared with the traffic source (189 nm). Coal burning rBC was more strongly associated with sulfate than traffic rBC, suggesting a higher cloud condensation nuclei activity. The slope of a robust linear regression between rBC and carbon monoxide (CO) for all samples was 5.9 &mu;g m 3 ppm 1, and the slope for the coal burning source (4.5 &mu;g m 3 ppm 1) was larger than that for the traffic source (2.7 &mu;g m 3 ppm 1). The net rBC emission during winter of 2009 was estimated to be 4.5 Gg based on the relationship between rBC and CO. A Tropospheric Ultraviolet and Visible radiation model showed that the average daytime value for the clear-sky direct radiative forcing due to rBC from 23 December 2012 to 31 January 2013 was 47.7 &plusmn; 28.9 W m 2, which amounted to an average of 45.7% of the total surface atmospheric aerosol forcing.</p

The Optical Properties of Urban Aerosol in Northern China: A Case Study at Xi’an

Simultaneous measurements of particle scattering coefficient (Bscat) and absorption coefficient (Babs) were conducted at Xi\u27an from mid-August to mid-October 2012 to estimate the particle single scattering albedo (SSA) and the Ångström coefficients in highly polluted urban air. The hourly averaged Bscat was 272 Mm− 1 at 532 nm and 82 Mm− 1 at 870 nm, while hourly averaged Babs was 31 Mm− 1 at 532 nm and 19 Mm− 1 at 870 nm. Similar diurnal variations for Bscat and Babs were observed between the two wavelengths. The averaged SSA was 0.88 at 532 nm and 0.78 at 870 nm. Based on the Ångström coefficients, anthropogenic fine particles show dominant contribution during the sampling period, accompanied by occasional dust events. Moreover, the major contributors to aerosol optical properties are attributed to the mixture of black carbon (BC) and brown carbon (BrC) with non-absorbing components over urban area in northern China. The findings provide useful insights into the factors affecting the visibility in northern Chinese cities and therefore essential knowledge for improving the air quality