Youfei Zheng

Preface to special section on East Asian Studies of Tropospheric Aerosols: An International Regional Experiment (EAST‐AIRE)

daily mean surface solar radiation by � 30–40 W m � 2 , but barely changed solar reflection at the top of the atmosphere. Aerosol loading, particle size and composition vary considerably with location and season. The MODIS AOD data from Collection 5 (C5) agree much better with ground data than earlier releases, but considerable discrepancies still exist because of treatments of aerosol SSA and surface albedo. Four methods are proposed/adopted to derive the SSA by means of remote sensing and in situ observation,

Seasonal variations of aerosol optical properties, vertical distribution and associated radiative effects in the Yangtze Delta region of China

[1]xa0Four years of columnar aerosol optical properties and a one-year vertical profiles of aerosol particle extinction coefficient at 527 nm are analyzed at Taihu in the central Yangtze River Delta region in eastern China. Seasonal variations of aerosol optical properties, vertical distribution, and influence on shortwave radiation and heating rates were investigated. Multiyear variations of aerosol optical depths (AOD), Angstrom exponents, single scattering albedo (SSA) and asymmetry factor (ASY) are analyzed, together with the vertical profile of aerosol extinction. AOD is largest in summer and smallest in winter. SSAs exhibit weak seasonal variation with the smallest values occurring during winter and the largest during summer. The vast majority of aerosol particles are below 2 km, and about 62%, 67%, 67% and 83% are confined to below 1 km in spring, summer, autumn and winter, respectively. Five-day back trajectory analyses show that the some aerosols aloft are traced back to northern/northwestern China, as far as Mongolia and Siberia, in spring, autumn and winter. The presence of dust aerosols were identified based on the linear depolarization measurements together with other information (i.e., back trajectory, precipitation, aerosol index). Dust strongly impacts the vertical particle distribution in spring and autumn, with much smaller effects in winter. The annual mean aerosol direct shortwave radiative forcing (efficiency) at the bottom, top and within the atmosphere are −34.8 ± 9.1 (−54.4 ± 5.3), −8.2 ± 4.8 (−13.1 ± 1.5) and 26.7 ± 9.4 (41.3 ± 4.6) W/m2 (Wm−2 τ−1), respectively. The mean reduction in direct and diffuse radiation reaching surface amount to 109.2 ± 49.4 and 66.8 ± 33.3 W/m2, respectively. Aerosols significantly alter the vertical profile of solar heating, with great implications for atmospheric stability and dynamics within the lower troposphere.

SO2 over central China: Measurements, numerical simulations and the tropospheric sulfur budget

[1]xa0SO2in central China was measured in situ from an aircraft and remotely using the Ozone Monitoring Instrument (OMI) from the Aura satellite; results were used to develop a numerical tool for evaluating the tropospheric sulfur budget - sources, sinks, transformation and transport. In April 2008, measured ambient SO2 concentrations decreased from ∼7 ppbv near the surface to ∼1 ppbv at 1800 m altitude (an effective scale height of ∼800 m), but distinct SO2 plumes were observed between 1800 and 4500 m, the aircrafts ceiling. These free tropospheric plumes play a major role in the export of SO2 and in the accuracy of OMI retrievals. The mean SO2 column contents from aircraft measurements (0.73 DU, Dobson Units) and operational OMI SO2 products (0.63 ± 0.26 DU) were close. The OMI retrievals were well correlated with in situ measurements (r = 0.84), but showed low bias (slope = 0.54). A new OMI retrieval algorithm was tested and showed improved agreement and bias (r = 0.87, slope = 0.86). The Community Multiscale Air Quality (CMAQ) model was used to simulate sulfur chemistry, exhibiting reasonable agreement (r = 0.62, slope = 1.33) with in situ SO2 columns. The mean CMAQ SO2 loading over central and eastern China was 54 kT, ∼30% more than the estimate from OMI SO2 products, 42 kT. These numerical simulations, constrained by observations, indicate that ∼50% (35 to 61%) of the anthropogenic sulfur emissions were transported downwind, and the overall lifetime of tropospheric SO2 was 38 ± 7 h.

Aerosol optical depth measurements in eastern China and a new calibration method

[1]xa0We present a new calibration method to derive aerosol optical depth (AOD) from the MultiFilter Rotating Shadowband Radiometer (MFRSR) under extremely hazy atmospheric conditions during the East Asian Study of Tropospheric Aerosols: an International Regional Experiment (EAST-AIRE) and the Atmospheric Radiation Measurement (ARM) Mobile Facility (AMF) deployment in China. MFRSR measurements have been made at Xianghe since September 2004 and at Taihu and Shouxian since March and May 2008, respectively. Aerosol property retrievals from CIMEL Electonique, Paris, Sun and sky radiometers located at each site show that aerosol loading is substantial and highly variable during a given year (averaged daily AOD550 = 0.80 ± 0.14). The conventional application of the Langley method to calibrate the MFRSR is not possible at these sites because there is a dearth of stable atmospheric and low-AOD conditions. To overcome this limitation of the traditional Langley plot method, highest irradiance values at a given air mass during a given period are used here. These highest values can represent the clear-sky and minimum aerosol loading conditions. A scatterplot of the AOD estimated by this method with the CIMEL Sun and sky radiometer AOD shows very good agreement: correlation coefficients are on the order of 0.98–0.99, slopes range from 0.93 to 0.97, and offsets are less than 0.02 for the three sites. AOD and Angstrom exponents were derived from application of the method to all MFRSR data acquired at the three sites. AOD values at 500 nm are τ500 = 0.99 ± 0.71 (α500–870 = 1.45 ± 0.59) at Xianghe, 0.87 ± 0.54 (1.14 ± 0.31) at Taihu, and 0.84 ± 0.43 (1.15 ± 0.28) at Shouxian. Anthropogenic aerosols appear to dominate in the study region with significant contributions from large dust particles and influence of hydroscopic growth.

Analysis of cloud condensation nuclei properties at a polluted site in southeastern China during the AMF‐China Campaign

[1] Cloud condensation nuclei (CCN) measurements are essential to understanding cloud processes but CCN measurements are scarce. This study analyzes CCN measurements acquired at Shouxian, a polluted site in southeastern China, from August 1–October 31, 2008 during the deployment of the U.S. Department of Energy’s (DOE) Atmospheric Radiation Measurement (ARM) Mobile Facility (AMF). The ranges of daily mean condensation nuclei concentrations (NCN) were approximately 3100–12000, 2300–7400, and 4260–15500 cm −3 in August, September, and October, respectively; the corresponding ranges of CCN concentrations (NCCN) at 0.49% supersaturation were about 1960–5670, 1770–3530, and 1500–5700 cm −3 . The average ratio of NCCN/NCN was 0.04, 0.12, 0.35, 0.53, 0.65, 0.69 and 0.72 for supersaturation values of 0.08%, 0.20%, 0.34%, 0.49%, 0.78%, 1.07% and 1.37%, respectively. NCN and NCCN peaked in the early morning and late afternoon, when human activities were most intense. CCN were more abundant in air masses influenced by anthropogenic pollution from densely populated areas. NCCN was proportional to NCN, but NCCN/NCN decreased with increasing NCN. There was a good correlation between NCCN (at 0.49% supersaturation) and aerosol optical depth (AOD) (500 nm), that is especially strong for fine‐mode aerosols (Angstrom exponent (a) > 0.8). This relationship can be fitted with a power law function. The changes of NCCN with various factors are explained. A dust event was identified showing a significant increase in NCN and a dramatic decrease in the NCCN/NCN ratio, implying that dust particles do not increase NCCN much, despite mixing with other anthropogenic aerosols.

Seasonal statistical characteristics of aerosol optical properties at a site near a dust region in China

[1]xa0A ground-based sky radiometer was used to measure direct and diffuse solar irradiances at Dunhuang, China from January 1999 to March 2001. The aerosol optical thickness (AOT), Angstrom exponent (α), volume size distributions, single scattering albedo and refractive index of aerosols were simultaneously retrieved using the “SKYRAD” inversion code and their seasonal variations and statistical characteristics were studied. The results reveal that during the study period, the AOT at Dunhuang varied seasonally, with the maximum AOT occurring in the spring and the minimum AOT occurring in the fall. The variation in α showed an opposite pattern, with a minimum in the spring and a maximum in the fall. A simple exponential function can express the relationship between AOT and α. The frequency distributions of AOT and α approximately follow a lognormal probability distribution and a normal probability distribution, respectively. The aerosol volume size distributions can be characterized by the sum of two lognormals distributions, and represent an accumulation mode with a radius of about 0.25 um, and a coarse mode with a radius of about 7.7 um. A pseudomode with a radius of about 1.69 um located between the accumulation mode and coarse mode is present in the springtime. The single scattering albedo showed a slight increasing trend with wavelength in spring, summer and autumn and a decreasing trend with wavelength in winter. The maximum value of the real part of the refractive index occurs at 400 nm, and the minimum value occurs at 875 nm for all seasons. Two Gaussian models were developed to describe the frequency distribution of the real part of the aerosol refractive index; the results indicate that the differences between the fits are greatest in the spring. The statistical characteristics of the frequency distributions of aerosol properties might provide a way to identify and estimate aerosol optical properties in areas located near dust regions.

A dynamic simulation study of the impacts of enhanced UV-B radiation on winter wheat photosynthetic production and dry matter accumulation

By using the system-dynamic software package STELLA, a simulative study was conducted on the photosynthetic productivity and change of the accumulated dry matter of wheat population, related to enhanced UV-B radiation that influences photosynthesis. The effects of daily temperature and crop physiological age on the photosynthesis rate were considered comprehensively. Statistical analysis was undertaken to assess the fitness between simulations and observations of accumulated dry matter by means of root-mean-square error and relative error. Results show that there is no significant difference between simulations and observations, indicating that the STELLA software is effective in imitating the growth. This software provides a new approach in establishing a simulative model for crop growth under the conditions of various climate and environment.

Cloud-Base Distribution and Cirrus Properties Based on Micropulse Lidar Measurements at a Site in Southeastern China

The cloud fraction (CF) and cloud-base heights (CBHs), and cirrus properties, over a site in southeastern China from June 2008 to May 2009, are examined by a ground-based lidar. Results show that clouds occupied the sky 41% of the time. Significant seasonal variations in CF were found with a maximum/minimum during winter/summer and similar magnitudes of CF in spring and autumn. A distinct diurnal cycle in the overall mean CF was seen. Total, daytime, and nighttime annual mean CBHs were 3.05±2.73 km, 2.46±2.08 km, and 3.51±3.07 km, respectively. The lowest/highest CBH occurred around noon/midnight. Cirrus clouds were present ∼36.2% of the time at night with the percentage increased in summer and decreased in spring. Annual mean values for cirrus geometrical properties were 8.89±1.65 km, 9.80±1.70 km, 10.73±1.86 km and 1.83±0.91 km for the base, mid-cloud, top height, and the thickness, respectively. Seasonal variations in cirrus geometrical properties show a maximum/minimum in summer/winter for all cirrus geometrical parameters. The mean cirrus lidar ratio for all cirrus cases in our study was ∼ 25±17 sr, with a smooth seasonal trend. The cirrus optical depth ranged from 0.001 to 2.475, with a mean of 0.34±0.33. Sub-visual, thin, and dense cirrus were observed in ∼12%, 43%, and 45% of the cases, respectively. More frequent, thicker cirrus clouds occurred in summer than in any other season. The properties of cirrus cloud over the site are compared with other lidar-based retrievals of midlatitude cirrus cloud properties.