Aerosol Optical Properties Observed At An Urban Aera Of Beijing

Aerosol plays an important role in the global and regional climate change, not only influence the radiation balance of earth-atmosphere system and the formation and distribution of precipitation, but also has a significant environmental effect as a part of atmospheric pollutants in urban areas. With the rapid development of economic and urbanization, the regional air pollution problems of the areas around the perimeter of Bohai Bay has been serious gradually, and seriously affects the living environment of mankind. To research the optical properties of aerosol in the regional composite pollution and provide part of scientific basic data of environment for visibility improving, air pollution control regulation formulating and the sustainable development of economic, some long-period observation of aerosol is necessary, which is increasing important for the assessment of aerosol radiative forcing of this region or even East Asia, and the understanding of regional climate change.A continuous observation of aerosol optical and micro physical properties including the scattering coefficient (σs), the absorption coefficient (σa), optical depth (AOD) and aerosol particle size distribution was carried out from Oct. to Dec.2010in Beijing. To reveal the host factor which impacts the aerosol SSA of Beijing and the optical properties under pollution weather, the daily variation and diurnal variations of the optical parameters, such as σs, σa, AOD and single scattering albedo (SSA), the relationship between the aerosol optical parameters under different wind directions, the Angstrom index(a)-AOD relation under different control conditions, and the optical characteristics of aerosol from different sources, are discussed. The main conclusions are summarized as follows:During the observation period, the aerosol optical parameters fluctuated wildly, and average values for σs, σa and SSA were438.2±507.3Mm-1,128.6±124.1Mm-1and0.72±0.08respectively, and the average values were0.5±0.69,0.95±0.33and0.24±0.31for AOD, a and turbidity. Also, the diurnal variation of σs was not very obvious, with the higher scattering coefficient at night; the peak of σa appeared during21:00to22:00, with a minimum around15:00pm. The ratio of back scattering coefficient to total scattering coefficient (ratio) appeared a bimodal distribution with the higher value in the daytime; SSA peak value appeared at15:00. AOD changed slightly with the peak at15:00, and there were relatively high content of fine particles in the atmosphere during the non-polluted days, while during the polluted days, AOD fluctuated wildly with no significant diurnal variation, with the main component are fine particles.σs appeared a similar trend with σa during the observation period, but there was neither simple relationship nor any dominant controlling factor among other aerosol optical parameters, especially in the southwest wind direction(SW); scattering coefficient Angstrom index (αn) increased with the σs decreasing, and Ratio was consistent with αn changing; σs increased larger than σa increased with the wind direction of east-northeast(EEN) and the northwest(NW); the scattering aerosol increased mainly in heavy-pollution weather conditions; when the dominant wind direction was SW, the SSA with absorption coefficient change was slightly different with the ones under EEN and NW.During the non-polluted days, a smaller than0.5is opposite with AOD, but a had the same trend with AOD when a is above1.0. There was no clear correlation between a and AOD during the polluted days, while a lager than1.0showed a decreasing trend with increasing AOD. When Vmax is smaller than lm/s or Vmax is smaller than4m/s but larger than lm/s, the correlations between a and AOD were similar, it indicated that there are not any strong sources around the observation site; When Vmax is above4m/s, both a and AOD are very small with a contrary trend. Wind direction is another crucial factor impact the relation between a and AOD, and they show a positive correlation especially the wind direction is ENE. Although the background aerosols or dust is not hygroscopic, when a is larger than0.5, AOD showed a positive correlation with relative humidity (RH).The relationship between σs and wind speed(WS) and direction(WD) was as same as the one between σa and WS and WD, and the high concentration of pollutants appeared in the small WS at any WD; strong diffusion of pollutants occurred in the SW sector because of high winds; the Ratio above0.12regardless of any dominant WD, and the fine particle pollution plays an important role in Beijing; there were a large number of lower SSA values in small WS area, and a lot of absorbent aerosols in local sources. The AOD was smaller, and the smaller a appeared in the SW sector at the larger wind speed; the lager AOD occurred in the NE sector of the smaller WS.SSA increased when the extinction coefficient increasing; the increased of scattering aerosol was the main cause of high pollution conditions’occurrence, while the absorbent aerosol content was relatively high at the background. coal-burning source aerosol had a lower SSA, while the SSA of urban aerosol main of scattering aerosol increased with the increasing of the extinction coefficient; but for dust aerosol, SSA does not change more with the extinction coefficient increasing; mixed aerosol accounting for the dominant position, the SSA with high value showed that the scattering aerosol is the main pollutants, however, when the extinction coefficient is very high, the SSA reduces. Decreasing SSA with increasing αn indicates that the most absorbent particles are fine particles. SSA of coal burning particles with αN higher than1.0is less than0.8, while SSA of dust aerosol is higher, and aN is greater than1.0, indicating that the near-surface SSA was influenced little by this dust process. The maximum absorption coefficient Angstrom Index (αp) of dust aerosol was1.4with a large change of aN, while aerosols from coal burning has a higher ap and aN greater than1.0and its absorption properties is quite different with black carbon.The scattering coefficient and AOD showed two different trends with the change of the wavelength for dust; the AOD of biomass burning aerosol curve was very steep with wavelength shift, and most aerosols were fine particles. Mixed aerosol, coal burning aerosol, and dust aerosol showed a strong absorption in the short-wave band, while absorption coefficient changes a little with the wavelength increasing of the urban aerosol. SSA of dust increases with wavelength increasing; SSA of mixed aerosol and coal burning aerosol decreased with wavelength increasing, but the trend was not obvious.