Chunsheng Zhao

A possible positive feedback of reduction of precipitation and increase in aerosols over eastern central China

[1]xa0In 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.

Ozone photochemical production in urban Shanghai, China: Analysis based on ground level observations

Received 9 July 2008; revised 26 May 2009; accepted 3 June 2009; published 1 August 2009. [1] Ozone and its precursors were measured from 15 June 2006 to 14 June 2007 at an urban site in Shanghai and used to characterize photochemical oxidant production in this region. During the observation period, ozone displays a seasonal variation with a maximum in spring. Observed nitrogen oxides (NOx) and carbon monoxide (CO) reached a maximum in winter and a minimum in summer. NOx and CO has a similar double-peak diurnal cycle, implying that they are largely of motor vehicle origin. Total nonmethane organic compounds (NMOC) concentrations averaged over the morning, and the 24-hour periods have a large day-to-day variation with no apparent seasonal cycle. Aromatics play a dominant role in contributing to total NMOC reactivity and ozone-forming potential. Anthropogenic NMOC of diverse sources are major components of total NMOC and consist mainly of moderate and low reactivity species. In contrast, relatively low levels of biogenic NMOC concentrations were observed in urban Shanghai. The early morning NMOC/NOx ratios are typically below 8:1 with an average of around 4:1, indicating that the sampling location is situated in a NMOC-limited regime. Model simulations confirm that potential photochemical ozone production in Shanghai is NMOC-sensitive. It is presently difficult to predict the impact of future human activities, such as the increase of automobiles and vegetation-covered landscapes and the reduction of aerosol on ozone pollution in the fast developing megacities of China, and additional studies are needed to better understand the highly nonlinear ozone problem.

Aircraft study of Mountain Chimney Effect of Beijing, China

[1] In this paper, the three-dimensional distribution of air pollutants in the Beijing region using aircraft measurements is reported, and Mountain Chimney Effect (MCE) on the distribution of air pollutants in this region is studied. A remarkable two-pollution-layer structure was observed by aircraft measurement in Beijing on 18 August 2007. Gaseous and particle pollutants were well mixed with high concentrations in the planetary boundary layer. There was an elevated pollution layer (EPL) at the altitude of 2500–3500 m, and the concentrations of pollutants were high and comparable with that in the planetary boundary layer. Analysis of aircraft measurement indicates that pollutants in the two pollution layers originated from the same source. On the basis of analysis of the Weather Research and Forecasting (WRF)-TRACER model and wind profile data, the formation of EPL is discussed. The wind flow of Beijing region was dominated by mountain-valley breeze, which has MCE on the distribution of pollutants in this region. Air pollutants were injected from the planetary boundary layer into the free troposphere due to this effect. These pollutants were subsequently transported back over the city by the elevated northerly wind. Thus the structure of two pollution layers over Beijing is formed. Modeling results show that the persistence of a polluted layer over the boundary layer from the previous day has significant contribution to the surface concentrations of pollutants. When the mixing depth increases, the elevated pollutants are recaptured into planetary boundary layer and mixed downward. The rapid increase of surface concentrations of pollutants may be attributed to the vertical down-mixing of pollutants.

A review of atmospheric chemistry research in China: Photochemical smog, haze pollution, and gas-aerosol interactions

In this paper we present a review of atmospheric chemistry research in China over the period 2006–2010, focusing on tropospheric ozone, aerosol chemistry, and the interactions between trace gases and aerosols in the polluted areas of China. Over the past decade, China has suffered severe photochemical smog and haze pollution, especially in North China, the Yangtze River Delta, and the Pearl River Delta. Much scientific work on atmospheric chemistry and physics has been done to address this large-scale, complex environmental problem. Intensive field experiments, satellite data analyses, and model simulations have shown that air pollution is significantly changing the chemical and physical characters of the natural atmosphere over these parts of China. In addition to strong emissions of primary pollutants, photochemical and heterogeneous reactions play key roles in the formation of complex pollution. More in-depth research is recommended to reveal the formation mechanism of photochemical smog and haze pollution and their climatic effects at the urban, regional, and global scales.

Aircraft study of aerosol vertical distributions over Beijing and their optical properties

A set of 152 vertical profiles of aerosol number concentration and size distribution with diameter ranging from 0.12 to 3.0 μm observed by the airborne optical spectrometer probe in Beijing, China, between February 2005 and September 2006 is analysed and discussed. The statistic of aerosol number concentration (Na) reveals a high aerosol number density in this region with average surface level number concentration (N0) of about 6600 cm-3 (0.12–3.0 μm). The average vertical profile of Na approximately satisfies an exponential decline function with a scale height of 1419 m. The Na profiles are influenced by the structures of planetary boundary layer (PBL) significantly and two typical types of Na profile under different conditions of PBL are presented and parametrized in this study. The observations of aerosol size distribution show that, in most cases the aerosol size distributions are not very sensitive to altitude, with effective radii ranging from 0.16 to 0.28 μm. Comparison between aircraft-derived aerosol optical depth (AOD) and Moderate Resolution Imaging Spectroradiometer-derived AOD shows good agreement. The Mie model calculations suggest that the surface level number concentration, the PBL height and the structure of PBL can influence the AOD significantly.

Aircraft measurements of cloud droplet spectral dispersion and implications for indirect aerosol radiative forcing

[1]xa0Using a large amount of aircraft measurements of cloud droplet size distributions, the relationship between cloud spectral relative dispersion (ɛ) and cloud droplet number concentration (Nc) is studied. The results indicate that the value of ɛ varies between 0.2 to 0.8 when the cloud droplet number concentration is low (about 50 cm−3), and converges toward a narrow range of 0.4 to 0.5 when the cloud number concentration is higher. Because the distribution of the cloud droplet size is an important parameter in estimating the first indirect radiative effect of aerosols on the climate system, the uncertainty in the corresponding radiative forcing can be reduced by 10–40% (depending on cloud droplet number density) under high aerosol loading. This finding is important for improving climate change projections, especially for the regions where aerosol loading is high and continues to increase.

Relationship between climatic factors and dust storm frequency in Inner Mongolia of China

[1]xa0Meteorological dust storm frequency data from 37 stations of 40 years (1961–2000) throughout the Midwest Inner Mongolia of China are analyzed together with climatic factors of 40 years in this region. Dust storm frequency in this region has taken on a decline trend during the period of 1961–2000. The main climatic factors controlling dust storm frequency in this region are number of days with gale, intensity index of Asian polar vortex, and area index of the northern hemispheric polar vortex which are good representatives for large-scale cold air activities. The explanation for the decreasing trend is that it is mainly due to the variation in climatic factors determining dust storm frequency in Inner Mongolia of China.

Vertical profile and origin of wintertime tropospheric ozone over China during the PEACE‐A period

[1]xa0During the Pacific Exploration of Asian Continental Emission (PEACE) phase A mission in January 2002, we launched ozonesondes in subtropical southeast China at Hong Kong (114.17°E, 22.31°N), middle latitude northeast China at Beijing (116.47°E, 39.81°N), and northwest China at Xining (101.45°E, 36.43°N) in order to study long-range ozone (O3) transport from Eurasia, tropospheric O3 sources in China, and O3 outflow to the Pacific. Tropospheric O3 showed a complex vertical distribution with average tropospheric O3 columns of 39 ± 4, 23 ± 3, and 30 ± 6 DU in Hong Kong, Beijing, and Xining, respectively, which accounted for 17 ± 2%, 7 ± 1%, and 10 ± 1% of the total O3 column. The lower tropospheric and boundary layer (BL) O3 over Xining and especially Beijing exhibited low values, suggesting negligible O3 formation in middle latitudes of China during the winter season. The results also revealed frequent propagation of enhanced O3 layers from the lower stratosphere to the upper troposphere over Xining and especially Beijing, suggesting that stratospheric O3 is an important source of O3 in the upper troposphere of northern China. This “natural” O3 is transported downwind by the prevailing westerly wind and acts as a source of O3 to the east Asian coast and northwestern Pacific. We observed elevated O3, with a maximum mixing ratio up to 111 ppbv, at 1.5 km in the upper BL over Hong Kong. The elevated O3 was resulted from transport of pollutants from northwest-central or southwest China and regional O3 formation and accumulation in south China including the Pearl River Delta and Hong Kong. We also observed enhanced O3 (>95 ppbv) in the middle and upper troposphere over Hong Kong in air masses transported along the subtropical jet from tropical and subtropical East Africa, south Asia, and Southeast Asia. The O3 enhancements were most likely due to intrusion of stratospheric O3 into the troposphere in the Indo-Burmese region of tropical Southeast Asia, where substantial downward motion had been observed.

Changes in surface aerosol extinction trends over China during 1980–2013 inferred from quality‐controlled visibility data

Pollution in China has been attracting extensive attention both globally and regionally, especially due to the perceptually worsening “smog” condition in recent years. We use routine visibility measurements from 1980 to 2013 at 272 World Meteorological Organization stations in China to assess the temporal changes in the magnitude and the sign of pollution trends. A strict and comprehensive quality control procedure is enforced by considering several issues not typically addressed in previous studies. Two methods are used to independently estimate the trend and its significance level. Results show that, in general, a strong increase in aerosol extinction coefficient over the majority of China is observed in the 1980s, followed by a moderate decrease in the 1990s, another increase in the 2000s, and a shift to decrease since around 2006 for some regions. Seasonally, winter and fall trends appear to be the strongest, while summer has the lowest trend.

Deliquescent phenomena of ambient aerosols on the North China Plain

In this study, we report that the deliquescent phenomena of ambient aerosols on the North China Plain are frequently observed using a humidified nephelometer system. The deliquescence relative humidity (RH) primarily ranges from 73% to 81%, with an average of 76.8%. The observed deliquescent phenomena of ambient aerosols exhibit distinct diurnal patterns and are highly correlated with ammonium sulfate. The diurnal variations of ammonium and nitrate may play significant roles on occurrences of observed deliquescent phenomena. The frequently observed deliquescent phenomena of ambient aerosols in this paper imply that current parameterization schemes that describe the RH dependence of particle light scattering may result in a significant bias when estimating aerosol effects on climate.