Observation And Simulation Study Of Aerosol Features And Their Direct Radiative Forcing Over Northwestern China

Northwestern China is regarded as one of the major dust aerosol source regions in Asia. Aerosols over this region, which have complicated type, rich concentration, dramatic spatial and temporal variations, have a significant impact on regional environment and climate change. Due to lack of the detailed and accurate data sets of aerosol optical and absorbing properties, the studies of aerosol radiative forcing on a regional scale have been little progress, most of which are intensive period observed data and without representatives of the regional scale and long-term climate change significance. Based on the SACOL platform, our team has continually observed the optical, physical, chemical, and radiative properties of aerosol over Loess Plateau region since2006. In addition, several intensive field campaigns on dust aerosols were conducted at Jingtai farmland, Zhangye Gobi area, Minqin near by the Badain Jaran Desert, and Dunhuang farmland (located in the Gobi Desert region and nearby the Kumtag Desert), respectively, during the spring of2008,2010, and2012. We can obtain much invaluable datasets of the aerosol optical, absorptive and physicochemical features in different typical locations over northwest China from these field experiments. These datasets of aerosol characteristics are vital to understand in depth the relative contributions of the aerosol particles impacting on the radiation energy balance and the regional climate change. We analyzed the multi-year aerosol characteristics at different sites over northwest China, and the major conclusions are summarized as followings.(1) The normal Langley plot method are performed excellently at Dunhuang site due to the very clear and clean sky, low background aerosol optical depth (AOD) level, stable atmospheric conditions and invariant aerosol concentrations; While the improved Langley plot methods are more suitable for the other sites of northwest China. The AOD values derived from the CE-318sun photometer, POM-01sky radiometer, and MS-700grating high-spectroradiometer measurements, agreed well with each other. The volume size distributions (dV/dlnr) of coarse-mode paritcles retrieved from the sky radiometer are consistent with the sun photometer, whereas the dV/dlnr of fine-mode paritcles derived from sky radiometer are somewhat smaller than that from sun photomer. Furthermore, the single-scattering albedo (SSA) values from the sky raidometer are much larger than that of sun photometer.(2) The aerosol optical features over different sites of northwest China exhibited dramatically diurnal variations, monthly variations and seasonal variations, and were obviously affected by the intrusion of dust particles during the spring and the emission of soot and air pollutants during the winter time. The aerosol concentrations in autumn period could represent the background levels at different locations attributed to the low aerosol emissions and the wet deposition removal by the prevalent rainfall process. The values of Angstrom exponent presented the opposite variation tendency with AOD at each site. The relationships between the AOD versus water vapor content (WVC) and relative humidity (RH) showed no significant correlation. The coarse-mode particles were dominant during the whole year. The concentrations of coarse-mode particle were significantly high, and the concentrations of fine-mode particle in the summer and autumn were comparable to the coarse-mode particle. The annual mean dV/dlnr showed the consistent variation features. The spectral dependences of single-scattering albedo are different between the dusty and non-dusty conditions. In the presence of dust, the SSAs increased with the wavelength. When the dust was not a major component, the corresponding values decreased with wavelength.(3) The aerosols at Lanzhou city site, Jingtai farmland, and Zhangye site had got moderately to strongly absorption of solar radiation in the visible wavelength during the period. The main reasons were explained in this study. Lanzhou city site was influenced by the emissions of soot and air pollutants during the winter coal-burning heating period. The Jingtai farmland site was affected by the emissions of carbon materials due to the prevalent straw combustion in spring. And the Zhangye site might be effected by the marked anthropogenic and vehicle emissions. Except for the three sites, the aerosol particles at other sites had not strong absorption, especially the weakest absorption was observed at the dust source sites (e.g., Minqin and Dunahung).(4) We performed a very excellent radiative closure experiment at Minqin site using a combination of high-quality aerosol optical properties and ground-based radiation fluxes and SBDART radiative transfer model. Our results showed that the aerosol shortwave direct radiative forcing (ARF) over northwest China exhibited remarkable diurnal, seasonal, and yearly variations. On a regional scale, our results indicated that the aerosol particles over northwest China had small cooling at the top of the atmosphere (TOA), moderately cooling at the surface (SFC), and moderately warming in the atmosphere (ATM) during the period. The absolute values of ARFTOA, ARFSFC and ARFATM all significantly increased with the increase of AOD at each site, that is, the AOD values directly determined the order of magnitude of ARF. The SSA or imaginary part (RI) of complex refractive index decided the sign of ARFTOA value (negative or positive). Namely, for a fixed underlying surface, when SSA500<0.85or RI500>0.015, the ARFTOA values were positive. The ARFTOA at Lanzhou city, Jingtai farmland and Zhangye sites showed small positive values ascribed to the moderately absorption of aerosol, and the ARFTOA at other sites all presented small negative values.(5) SPRINTARS radiative transfer regional model could not well simulated the average diurnal variations of aerosol optical properties and ARFs over the Loess Plateau region of northwest China. The simulations of AOD500, Angstrom exponent, and ARFs from regional model were more reasonable with the ground-based observations in summer and autumn periods, while the differences between the simulations and observations became much bigger in Spring and Winter periods. This suggested that the inventory emissions datasets of aerosol particles and air pollutants and the dynamic process in the regional model were more reliable over the Loess Plateau during the summer and autumn time. The overall mean of ARFTOA, ARFSFC and ARFATM derived from the SBDART model were about1.1,2.5and4.2times, respectively, than that from SPRINTARS regional model. Our results indicated that the simulated accuracy of SPRINTARS regional model should be improved over the Loess Plateau region using the more accurate and representative emissions inventory datasets of aerosol characteristics and air pollutants. In order to achieve this scientific goal, we have to conduct and carry out long-term and continuous observations of aerosol optical, physicochemical properties and the air pollutants over this region.