In this study, observed precipitation, MODIS data and meteorological sounding data over eastern central China were analyzed. The result shows that the precipitation in this region is significantly reduced during the last 40 years and this reduction of precipitation is strongly correlated to the high concentrations of aerosols. Meteorological sounding data indicates that the atmospheric stability in the troposphere has been increasing during the last 17 years. It is speculated that the aerosol layer in the lower troposphere affects the radiative processes, which lead to changes in atmospheric stability. The enhancement in the atmospheric stability tends to depress upward motion and precipitation in this region. Because precipitation plays a major role for the sink for aerosol particles through washout process, less precipitation will lead to an increase in aerosol particles. This possible positive feed back cycle (more aerosols → less precipitation → more aerosols) will induce an acceleration process for the reduction of precipitation in eastern central China.
 In recent years, the rapid growth of industrialization and urbanization in China has led to a significant increase in fossil fuel usage in the region. The usage of fossil fuel (coal and crude oil) is 823 Mtons (million tons) in 2002 compared to 263 Mtons in 1973 [International Energy Agency, 2004].This rapid increase in fossil fuel usage has resulted in significant enhancement in aerosol particles (such as both soot and sulfate) in China, especially in eastern central China. Satellite observations have shown that the concentrations of aerosols and pollutants related to fossil fuel usage are among the highest levels of any major country, and are generally getting worse [Richter et al., 2005; Penner et al., 1992]. Tie et al.  show that the aerosol concentrations are much higher in eastern China than in the eastern US. These particles not only represent a health hazard to the people living in the region, but they can also have a significant impact on the region's hydrological cycle and climate [Menon et al., 2002]. Study on interactions between air pollution and precipitation over this region is an important issue regarding regional climate response to the rapidly increase in air pollutants.
Twomey  suggests that aerosols play important role to affect cloud properties. Aerosols in the atmosphere linked to natural and anthropogenic emissions influence the Earth-Atmosphere system in several distinct ways [Zhang et al., 2004]. Aerosols produced by anthropogenic air pollution are recognized as sources of large concentrations of small cloud condensation nuclei (CCN), which lead to the formation of a high concentration of small cloud droplets and therefore suppressed precipitation. Evidences of pollution modulate cloud properties are presented by satellite data of ship tracks [Ferek et al., 2000] and pollution tracks [Rosenfeld, 2000]. Rosenfeld  shows that substantially reduced precipitation is found from satellite observations in the downwind of pollution source. Most of these studies focused on the role of aerosol as CCN in cloud microphysics. However, cloud formation is strongly controlled by meteorological conditions, such as temperature and atmospheric convection. Therefore, the atmospheric instability plays important roles in controlling the rate of precipitation.
 In this study, 40-year's observation of precipitation, satellite aerosol data and meteorological sounding data were analyzed, and the relationship between precipitation trend and aerosol pollutions over eastern central China was investigated.
2. Data and Method
 Three data sets are used in this study, including precipitation data, MODIS (MODerate resolution Imaging Spectroradiometer) aerosol optical depth (AOD) data [Chu et al., 2002] and meteorological sounding data. The observed daily precipitation data set used here are from Chinese National Meteorological Center with the period from 1961–2000 and 521 stations. To calculate the atmospheric stability, we collected meteorological sounding data from 1986 to 2004 in eastern central China. There are 4 sounding stations which record a long term data in this region, and they are located at Taiyuan (112.5E, 37.9N), Beijing (116.3E, 39.9N), Xingtai (114.4E, 37N), and Zhangjiakou (114.9E, 40.8N), respectively. Aerosol optical depths are retrieved at 0.55 μm by MODIS data, and fine (aerosol radius less than 0.5 μm) and coarse (aerosol radius greater than 0.5 μm) aerosol mode fractions are derived [Remer et al., 2002].
 Trend of atmospheric instability is derived from the parameters of Convective INhibition (CIN) and Convective Available Potential Energy (CAPE) calculated from the sounding data. Unstable day frequency is defined as the ratio of days when the difference of (CAPE—CIN) is positive to the total sounding days of a year. Trend of unstable day frequency reflects the variation of atmospheric stability of a sound station. CIN is the energy needed to pseudoadiabatically lift an air parcel from its originating level to its level of free convection. CAPE is the maximum energy available to an ascending parcel. They are expressed as follows:
where pi is the pressure at the level at which the parcel originates, pf is the pressure at the level of free convection, Rd is the specific gas constant for dry air, Tvp is the virtual temperature of the lifted parcel, and Tve is the virtual temperature of the environment, αe is the environmental specific volume profile, αp is the specific volume of a parcel moving upward moist-adiabatically from the level of free convection, and pn is the pressure at the level of neutral buoyancy.
3. Results and Discussions
 Surface precipitation measurements from 1961 to 2000 over 521 meteorological stations over China are calculated to analyze the trend of precipitation. Figure 1a shows the result of the trend of precipitation during this period. There is clear evidence that over eastern central China, marked by the black circle below, the precipitation is significantly reduced. The decrease trend in some stations is larger than 0.4% per year. Compared to the distribution of aerosol, this large reduction of precipitation is closely correlated to the magnitude of aerosol optical depth observed by satellite instrument MODIS (see Figure 1b). MODIS image indicates that fine AOD concentrations are high (with maximum of 0.9) over eastern central China. Most of this air pollution comes from industrial emissions, automobile exhaust, and other human induced emission. The aerosol over this region is rich in sulfates, nitrates, organic and black carbon [Tie et al., 2005; Chin et al., 2002].
 Although there are only 6 years measurements of AOD from MODIS, there is a clear indication that the aerosol loading in eastern central China rapidly increases. Figure 2 shows that the averaged AOD in this region is approximately 0.55 in 2000 and enhanced to 0.72 in 2005, suggesting approximately 30% enhancement in aerosol loading during this period.
 The high coherence between the reduction of precipitation and the high aerosol concentrations in eastern central China suggests that the high aerosol concentrations may be responsible for the reduction of precipitation. Studies show that aerosol strongly affects the formation of clouds and precipitation [Twomey, 1991; Charlson et al., 1992; Rosenfeld, 2000].
 In eastern central China, the aerosol particles are mainly comprised of sulfate, black carbon (soot), mineral dust, and organic carbon [Tie et al., 2005; Chin et al., 2002; Sun et al., 2004]. Among these particles, soot particle is of special interest because it absorbs sunlight and heats air, which results in the changes in the air temperature and atmospheric stability [Chameides and Bergin, 2002; Menon et al., 2002; Ramanathan et al., 2005]. If the atmosphere becomes more stable, the upward motions are depressed, and cloud formation is reduced, resulting in reduction of precipitation.
 To illustrate the changes in atmospheric stability, we calculate the unstable day frequency using sounding data collected from 1986 to 2004 in eastern central China. The unstable day frequency represents the percent of days in a year when the atmosphere is in unstable condition, in which the air is rapidly uplifted from the lower troposphere to the upper troposphere. Under this unstable situation coupled with certain favorable moisture conditions, the regional precipitation often occurs. The detailed method of the calculation for the unstable days is given in Section 2. The averaged unstable day frequency at the 4 stations during 1986 to 2003 is shown in Figure 3. The result clearly indicates that the averaged unstable day is significantly decreased during the last 17 years. For example, the unstable day frequency in eastern central China is approximately 34% per year during the late 1980s, and reduced to about 17% per year in 2000s, which is about 50% reduction during this period, suggesting that the atmosphere becomes more stable in this region. The stable atmosphere has a tendency to prevent the formation of clouds and precipitation, especially for the small scale convective precipitation, which is often the popular type of the precipitation during summer in this region. This result is consistent with several previous studies which suggest that soot particles play important roles to affect precipitation [Rosenfeld, 2000; Ackerman et al., 2000; Koren et al., 2004; Kaiser, 1998].
 It should be noted that there is also a microphysical impacts of soot particles on cloud and precipitation formation through aerosol-cloud microphysical processes. Freshly emitted soot particles tend to be hydrophobic, and aged soot particles become hydrophilic after the soot particles are coagulated with sulfate, water, and other particles. The hydrophilic soot particles undergo hygroscopic growth to become internally mixed aerosol particles. Studies show that the heating rate of the air due to the internally mixed soot particles is significantly enhanced compared to the values resulted from pure soot particles [Jacobson, 2000, 2001].
 The reduction of precipitation could lead to many important impacts on economical and social problems in this region. One of the important consequences is that the aerosol loading in this region will be enhanced as resulted from the reduction of the precipitation. Because a major sink of aerosol is due to wet deposition [Chin et al., 2000], the reduction of precipitation tends to increase the lifetime of aerosol particles in the atmosphere. As a result, the aerosol loading increases due to the reduction of precipitation. This relationship between aerosol loading and precipitation leads a positive feedback cycle of more aerosols loading → less precipitation → more aerosol loading.
 In this paper, we suggest that the anthropogenic aerosols might contribute significantly to the observed reduction of precipitation over eastern central China and provided a possible feedback cycle of aerosol loading and precipitation. This feedback cycle produces considerable harmful impacts on air quality, hydrological cycle, crop, and other environmental problems. Under the effect of this cycle, a small perturbation of the increase of aerosol emission will lead to a large perturbation in the enhancement of aerosol loading. As a result, the aerosol concentrations are amplified with this positive feedback cycle. On the other hand, the reduction of aerosol emissions will accelerate the decrease of the aerosol loading in Eastern central China, showing that a clean air action is urgently needed in this region.
 We thank Alan Chu for providing the MODIS data. This research is supported by the National Natural Science Foundation of China (NSFC) under Grant 40318001, 40475003 and National Key Research Project on Acid Rain in China (2005CB422200). The National Center for Atmospheric Research is sponsored by the National Science Foundation and operated by UCAR.