Aerosol Optical Properties and Direct Radiative Effects over Central China
Central China is important for aerosols and climate because it is among the worst regions for air pollution in China. However, it is understudied due to a lag in establishing an atmospheric monitoring network. So we did a comprehensive analysis using multiple techniques to improve the understanding of aerosol optical properties and their radiative effect in this region. The results showed that high aerosol optical depth (AOD) was generally found in the northern and central parts, whereas low
... rts, whereas low values were observed in the southern and western parts. Most regions were predominantly loaded with small aerosol particles and a significant influence of long-distance transported dust was found in springtime. A strong and significantly decreasing trend was observed with a maximum decrease rate of −0.08 per year in the northern and western parts, related to the decreasing emission of aerosols and increasing rainfall. Aerosol optical properties and radiative effects were compared between an urban site, Wuhan, and a rural site, Dengfeng. The seasonal variations of AOD and Ångström exponent (AE) are similar for Wuhan and Dengfeng, but both values are larger in Wuhan than in Dengfeng. A greater dominance of coarse-mode and absorbing aerosols was found over Dengfeng. Annual averaged aerosol radiative effect (ARE) in shortwave spectrum (ARE SW ) and its efficiency (REE) are −48.01 W/m 2 and −51.38 W/m 2 , respectively, in Wuhan, −40.02 W/m 2 and −53.26 W/m 2 , respectively, in Dengfeng. The dependence of REE on aerosol absorptive and size properties was studied; the results showed that REE was strongly influenced by the aerosol absorptivity and size of fine-mode particles, but there was not a strong correlation between REE and AE. The percentage of ARE in visible spectrum (ARE VIS ) in ARE SW in Wuhan was 3% lower than in Dengfeng. The ARE VIS percentage depended largely on aerosol particle size, but was less influenced by aerosol absorptivity. Remote Sens. 2017, 9, 997 2 of 23 cycle [5,6]-for example, absorbing aerosols may be partly responsible for the increasing floods in southern China and drought in northern China in the past decades [5, 7] . Assessing aerosol effects on a climate system depends significantly on aerosol properties. The direct radiative effects of aerosols are governed by aerosol optical properties, which depend on particle composition and size  . Indirect effects of aerosol are also related to aerosol properties such as particle type, size, and concentration    . However, aerosols are extremely short-lived in the atmosphere (lifetime on the order of a week) [12, 13] and their sources, emissions, and removal mechanisms are very complex. Aerosols exist in various shapes, sizes, and structures; the composition of aerosols is dynamic and transforms as particles coagulate and absorb water [12, 14] . As a result, the spatial distribution and temporal variations of aerosol are highly inhomogeneous; the complicated aerosol properties further cause much uncertainty in the estimation of climate change [15, 16] . Thus, a thorough, region-oriented understanding of aerosol properties and their spatial and temporal distribution is necessary before the aerosol effects on the climate system can be evaluated accurately [17, 18] . To obtain aerosol properties and reduce uncertainty in estimating aerosol effects on the climate system, great efforts have been exerted to improve the aerosol observations    . Aerosol properties can be obtained through laboratory measurements, but such measurements can only characterize aerosols on the surface, and some sampling methods may change the natural status of ambient aerosol  . Satellite remote sensing measurements can provide information about the global distribution and dynamics of aerosol optical depth (AOD) and also an estimate of aerosol particle size. However, satellite observation cannot provide complete aerosol optical parameters for quantification of aerosol absorptive or scattering properties at present, although they are essential for assessing the direct radiative effects on the earth's surface and the top of the atmosphere . Ground-based remote sensing observations are generally characterized by high precision and temporal resolution but low spatial and temporal coverage. Ground-based measurements can provide relatively accurate inversions for aerosol optical and microphysical parameters, such as aerosol size distribution (ASD) and single scattering albedo (SSA). The integrated use of satellite and ground-based observations can provide a comprehensive understanding of the optical and microphysical properties of aerosols. Moreover, data from collocated instruments can reveal underlying aerosol-governing physics  . Based on the aerosol optical and microphysical properties derived from ground and satellite observations, the direct aerosol radiative effect (ARE) can be estimated to evaluate the scattering and absorption effect of aerosol on solar radiation. Most studies on ARE in the shortwave spectrum (SW) have focused on the entire SW spectrum or a certain spectral interval in the SW spectrum, such as visible (VIS) and ultraviolet (UV) spectra [25, 26] . Studies on ARE have found cooling effects of aerosols on the Earth's surface in the SW spectrum [27, 28] . However, knowledge of ARE on the surface in each spectral interval of the SW spectrum is still lacking. The seasonal variations of ARE in the UV, VIS, and near-infrared (NIR) intervals and their fractions in SW as well as the mechanism governing ARE variation need further investigation. Due to the complicated properties and inhomogeneous distributions of aerosol, region-oriented studies of aerosol properties and ARE have been conducted, especially in dust-affected regions such as the Middle East and heavily polluted areas such as the Yangtze River Delta in China [26, 29] . However, central China is relatively understudied compared to the coastal areas due to a lag in establishing an atmospheric monitoring network. It is important to study this area because it is a key transportation hub and among the most economically developed areas in China. Central China is located between 24 • N and 37 • N latitude and 108 • E and117 • E longitude (Figure 1 ), which covers the middle reach of the Yangtze River and the middle and lower reaches of the Yellow River. Fast-growing anthropogenic activities and an economic boom have resulted in increasing aerosol loading in this region, thereby causing serious air pollution and frequent haze  and making this area one of the most heavily populated regions in China. Limited knowledge is available on the radiative effect and spatial distribution of aerosol loading in this region, and studies on aerosol properties Remote Sens. 2017, 9, 997 3 of 23 focus on vast regions of countryside, mountains, and forests. Furthermore, information is lacking on the long-term variation trend of aerosol properties as a result of the short duration of ground-based observation. The serious air pollution in central China has drawn attention and some measures such as forbidding straw combustion and installing desulfurization and denitration equipment in thermal power plants have been conducted in the past few years. The question of whether air pollution has been suppressed requires an investigation of long-term, region-wide aerosol trends. Remote Sens. 2017, 9, 997 3 of 23 information is lacking on the long-term variation trend of aerosol properties as a result of the short duration of ground-based observation. The serious air pollution in central China has drawn attention and some measures such as forbidding straw combustion and installing desulfurization and denitration equipment in thermal power plants have been conducted in the past few years. The question of whether air pollution has been suppressed requires an investigation of long-term, regionwide aerosol trends. Figure 1. Elevation map of study area. JDP, PYP, and YRD represent the Jianghan and Dongting Plain, Poyang Plain, and Yangtze River Delta, respectively.