Measurements Of Aerosol Radiative Properties Over Semi-arid Region Of The Loess Plateau

Atmospheric aerosol, as a crucial composition of atmosphere affects regional and global climate change through its direct, indirect, and semi-direct radiative effects modulation of the surface and atmospheric radiation budgets.It is a key factor of aerosol radiative forces as working on climate change, and also for climate change's uncertainty. The largest uncertainty of aerosol radiative effect has also been lack of fully understanding its space and temporal distribution, physical, and optical characteristics. The semi-arid region of the Loess Plateau with especial underlying surface is a transition area from arid to semi-wet region, and its climate is extremely vulnerable. However, only a few observatories are operating in the region, resulting in a lack of measurement and analysis of aerosol. The long continuous measurement of aerosol from the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) can provide significant scientific support for understanding the space and temporal distribution, physical, and optical characteristics of aerosol in this region, and also provide basic theoretical proof for deeply knowing aerosol radiative effect and globle climate change.The vertical distribution and temporal evolution of aerosol over the semi-arid region of the Loess Plateau are analyzed by using the data of the Mie scattering lidar (CE-370-2) and the dual-wavelength polarization lidar (L2S-SM II) from SACOL. Then the properties of concentration and size of aerosol, PM10concentration, airflow path, and the relationship between PM10concentration and meteorological elements are presented based on the measurements of particle sizer (APS-3321)and PM10particulate monitor (TEOM RP1400a) combining with the HYbrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT-4) model and National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis project data (2.5°×2.5°).Finally, using Mie theory in combination with the data of the number concentration size distribution of aerosol and the measurement of the nephelometer, the distributions of aerosol scattering coefficient with particle size in dust process and coal combustion period are calculated and analysed. Some valuable results are retrieved. The main results are as follows.(1)Using the observational data of the lidar (CE-370-2) since2006to2011,the results from the retrieval show that aerosol optical depth (AOD) during March to May and November to December is larger, and AOD during June to October is small. AOD in springtime is the largest with the value of0.42, and AODs in winter and fall are larger with the values of0.36and0.30respectively. AOD in summer is the least with the value of0.21.The analysis of frequency shows that AOD mostly ranges0.3around, and has significant seasonal differences. Aerosol in this region mostly distributes below2km, and has absolute dominance below1km. In daytime, aerosol extinction coefficient within the lowest troposphere (below2.0km) is large, and that in nighttime is small. The vertical distribution height of aerosol in summer is the highest, next in fall, and the height in winter is very low. In dust processes, aerosol mostly distributes below3km, and in strong dust events, aerosol can penetrate up to6km or more than6km.The temporal evolution trend of dust AOD is similar to that of PM10mass concentration, and their correlation coefficient is0.75. AOD (ranging from0.2to0.5) in coal combustion period is lager than that of background (ranging from0.08to0.15), and its diurnal variation presents dual-peaks structure, aerosol mostly distributing below1km.(2) The polarization lidar system can distinctly distinguish aerosol and cloud, and roughly identify ice cloud, water cloud and mixed cloud, based on the measurement of dual-wavelength polarization lidar (L2S-SM II). The depolarization ratio (532nm change) of aerosol in clean condition is0.10, and0.30for dust condition while0.13for coal combustion condition respectively. Dust aerosol contains more irregular coarse particles while aerosol in coal combustion condition includes more finer spherical particles.(3)Using the data of the ground lidar (L2S-SM II) and the space lidar (CALIOP), the comparison between the measurements shows that it is an approximate result in no cloud and dust condition which the relative error is below25%above2km, and that of error is larger below2km caused by the inhomogeneous horizontal distribution of aerosol within boundary layer; in dust condition, the two lidar systems both can observe dust layer, notwithstanding the intensity and height of dust layer have differences which the relative error is below35%above3km while exceeding50%below3km.(4) Based on the measurement of Aerodynamic Particle Sizer Spectrometer (APS-3321)since2007to2009, the results demonstrate that the maximum mass concentration (MC) of aerosol appears in May while the minimum appears in June; number concentration (NC) of aerosol in January is the maximal, and NC in April is the minimum; in spring MC is large, and NC is low which is caused by dust weather; in winter MC and NC are both larger; in summer and fall, MC and NC are both low; the primary composition of aerosol in this region is finer particle which accounts for98%;mass size distribution (MSD) of aerosol presents dual-peaks structure which the values of dual-peaks appear at0.72and5.05μm, respectively; number size distribution (NSD) shows typical one peaks appearing at0.72μm; in dust condition, MSD and NSD both present one peak, and the number of coarse particle (2.5μm<r≤10.0μm) increases markedly; in coal combustion period, MSD and NSD both present similar distribution with that of background, but the values are much larger.(5) The distribution characteristics of PM10mass concentration and airflow path over SACOL (semi-arid region) and Zhangye (arid region) are carried out using the data of2008China-U.S. joint dust storm experiment. The results show:in the two regions, the maximum mass concentration of PM10appears in May, MC in April is bit large, and MC in June is low; the diurnal variation trends of PM10over SACOL and Zhangye is almost consistent, and the day by day variation trends of that present much undulation; in April, the dominant path of airflow (DPA) at0.5km height over SACOL is west and southeast while that over Zhangye is west and southwest; in May the DPA over SACOL is west and north while that is west over Zhangye; in June the DPA is north and south while that is northwest over Zhangye; in the two region, the dominant path of airflow at4km is northwest. The mass concentration of PM10has compact relation with wind speed and direction, and has negative relation with vertical wind and precipitation; the inversion layer of temperature appearing at22:00is a major factor to PM10high concentration.(6) The distribution of aerosol scattering coefficient (ASC) with particle size is analyzed based on Mie theory combination the measurement data. It shows a Gaussian distribution of dust ASC against effective diameter, and the contribution percentages of PM2.5and PM10are respectively20.95%and83.88%, meaning that the coarse particle (2.5μm<r≤10.0μm) is a vital factor as dust aerosol scattering properties while that in coal combustion condition presents approximate Gaussian distribution, and the contribution percentages of PM2.5and PM10are respectively66.23%and91.93%,meaning that the finer particle is a crucial factor for ASC.