Pucai Wang

Aerosol optical properties and their radiative effects in northern China

[1] As a fast developing country covering a large territory, China is experiencing rapid environmental changes. High concentrations of aerosols with diverse properties are emitted in the region, providing a unique opportunity for understanding the impact of environmental changes on climate. Until very recently, few observational studies were conducted in the source regions. The East Asian Study of Tropospheric Aerosols: An International Regional Experiment (EAST-AIRE) attempts to characterize the physical, optical and chemical properties of the aerosols and their effects on climate over China. This study presents some preliminary results using continuous high-quality measurements of aerosol, cloud and radiative quantities made at the first EAST-AIRE baseline station at Xianghe, about 70 km east of Beijing over a period of one year (September 2004 to September 2005). It was found that the region is often covered by a thick layer of haze (with a yearly mean aerosol optical depth equal to 0.82 at 500 nm and maximum greater than 4) due primarily to anthropogenic emissions. An abrupt ‘‘cleanup’’ of the haze often took place in a matter of one day or less because of the passage of cold fronts. The mean single scattering albedo is approximately 0.9 but has strong day-to-day variations with maximum monthly averages occurring during the summer. Large aerosol loading and strong absorption lead to a very large aerosol radiative effect at the surface (the annual 24-hour mean values equals 24 W m � 2 ), but a much smaller aerosol radiative effect at the top of the atmosphere (one tenth of the surface value). The boundary atmosphere is thus heated dramatically during the daytime, which may affect atmospheric stability and cloud formation. In comparison, the cloud radiative effect at the surface is only moderately higher (� 41 W m � 2 ) than the aerosol radiative effect at the surface.

Aerosol optical depth (AOD) and Angstrom exponent of aerosols observed by the Chinese Sun Hazemeter Network from August 2004 to September 2005

500, and 650 nm were analyzed for the period of August 2004 to September 2005. The smallest mean AOD (0.15) was found in the Tibetan Plateau where a showed the largest range in value (0.06‐0.9). The remote northeast corner of China was the next cleanest region with AODs ranging from 0.19 to 0.21 and with the largest a (1.16‐1.79), indicating the presence of fine aerosol particles. The forested sites exhibited moderate values of AOD (0.19‐0.51) and a (0.97‐1.47). A surprising finding was that the AOD measured at a few desert sites in northern China were relatively low, ranging from 0.24 to 0.36, and that a ranged from 0.42 to 0.99, presumably because of several dustblowing episodes during the observation period. The AOD observed over agricultural areas ranges from 0.38 to 0.90; a ranges from 0.55 to 1.11. These values do not differ much from those observed at the inland urban and suburban sites where AOD ranges from 0.50 to 0.69 and a ranges from 0.90 to 1.48. Given the geographic heterogeneity and the rapid increase in urbanization in China, much longer and more extensive observations are required.

A simple method to estimate actual evapotranspiration from a combination of net radiation, vegetation index, and temperature

Satellite remote sensing is a promising technique for estimating global or regional evapotranspiration (ET). A simple and accurate method is essential when estimating ET using remote sensing data. Such a method is investigated by taking advantage of satellite measurements and the extensive ground-based measurements available at eight enhanced surface facility sites located throughout the Southern Great Plains (SGP) area of the United States from January 2002 to May 2005. Data analysis shows that correlation coefficients between ET and surface net radiation are the highest, followed by temperatures (air temperature or land surface temperature, Ts), and vegetation indices (enhanced vegetation index (EVI) or normalized difference vegetation index (NDVI)). A simple regression equation is proposed to estimate ET using surface net radiation, air or land surface temperatures and vegetation indices. ET can be estimated using daytime-averaged air temperature and EVI with a root mean square error (RMSE) of ~30 W m?2 and a correlation coefficient of 0.91 across all sites and years. ET can also be estimated with comparable accuracy using NDVI and Ts. More importantly, the daytime-averaged ET can also be estimated using only one measurement per day of temperatures (the daytime maximum air temperature or Ts) with comparable accuracy. A sensitivity analysis shows that the proposed method is only slightly sensitive to errors of temperatures, vegetation indices and net surface radiation. An independent validation was made using the measurements colleted by the eddy covariance method at six AmeriFlux sites throughout the United States from 2001 to 2006. The land cover associated with the AmeriFlux sites varies from grassland, to cropland and forest. The results show that ET can be reasonably predicted with a correlation coefficient that varies from 0.84 to 0.95 and a bias that varies from 3 W m?2 to 15 W m?2 and RMSE varying from ~30 W m?2 to ~40 W m?2. The positive bias partly comes from the energy imbalance problem encountered in the eddy covariance method. The proposed method can predict ET under a wide range of soil moisture contents and land cover types.

Estimation of surface long wave radiation and broadband emissivity using Moderate Resolution Imaging Spectroradiometer (MODIS) land surface temperature//emissivity products

The Moderate Resolution Imaging Spectroradiometer (MODIS) global land surface temperature (LST)/emissivity products supply daily, 8-day, and monthly global temperature and narrowband emissivity data. This article uses these products to calculate the surface long wave radiation of natural objects such as sand, soil, vegetation, etc., based on the Planck function and the Stefan-Boltzmann law. The results show that using the narrowband emissivity of a single band instead of the broadband emissivity results in large errors of up to 100 W m?2 of the calculated long wave radiation. A method to calculate broadband emissivity in the entire TIR spectral region from the narrowband emissivities of the MODIS bands (29, 31, and 32) in the thermal infrared region is proposed. Using the broadband emissivity, the surface long wave radiation could be calculated to an accuracy better than 6 W m?2 in the temperature region of 240–330 K, with a standard deviation of 1.22 W m?2, and a maximum error of 6.05 W m?2 (not considering the uncertainty associated with the MODIS LST/emissivity products themselves). The satellite estimated broadband emissivity was compared with 3-year (January 2001 to December 2003) ground-based measurements of emissivity at Gaize (32.30°N, 84.06°E, 4420 m) on the western Tibetan Plateau. The results show that the broadband emissivity calculated from MODIS narrowband emissivities by this method matches well the ground measurements, with a standard deviation of 0.0085 and a bias of 0.0015.

Influences of urbanization on surface characteristics as derived from the Moderate-Resolution Imaging Spectroradiometer: A case study for the Beijing metropolitan area

Moderate-Resolution Imaging Spectroradiometer (MODIS) global land surface temperature/emissivity (LST), vegetation indices, BRDF/Albedo and land cover products collected for the period of March 2000 to March 2006 are combined with the surface heat fluxes retrieved from MODIS as well as meteorological data to investigate the influence of urbanization associated with the surface characteristics of the city of Beijing. The results show that the use of different rural areas in the urban heat island (UHI) calculation influences the value of UHI and its seasonal variation. Daytime UHI shows a distinct seasonal variation, the maximum during summer being larger than 10°C, while conspicuous negative UHI occurs in winter and spring. Seasonal variation of nighttime UHI is much less. The contrast in thermal inertia between rural and urban areas, anthropogenic heat from the urban area and less latent heat flux over urban areas are the main factors influencing daytime UHI, whereas anthropogenic heat controls the nighttime UHI. Surface broadband emissivity derived from MODIS LST/emissivity for the urban area is nearly equal to the rural areas. Surface albedo over the urban area is 0.03–0.08 less than that of rural areas, but aerosols substantially reduce surface incoming solar radiation over the urban area, which results in the surface absorbed solar radiation being nearly equal for urban and rural areas during autumn. Diurnal variation of UHI demonstrates a distinctively seasonal variation. The accuracy of MODIS LST is investigated and it was found that the influence of satellite view angle on the calculated UHI is small enough to be ignored.

Evaluation and improvement of the MODIS land surface temperature/emissivity products using ground-based measurements at a semi-desert site on the western Tibetan Plateau

Current MODerate‐resolution Imaging Spectroradiometer (MODIS) land surface temperature (LST, surface skin temperature)/emissivity products are evaluated and improvements are investigated. The ground‐based measurements of LST at Gaize (32.30° N, 84.06° E, 4420 m) on the western Tibetan Plateau from January 2001 to December 2002 agree well (mean and standard deviation of differences of 0.27 K and 0.84 K) with the 1‐km Version 004 (V4) Terra MODIS LST product (MOD11A1) generated by the split‐window algorithm. Spectral emissivities measured from surface soil samples collected at and around the Gaize site are in close agreement with the landcover‐based emissivities in bands 31 and 32 used by the split‐window algorithm. The LSTs in the V4 MODIS LST/emissivity products (MYD11B1 for Aqua and MOD11B1 for Terra) from the day/night LST algorithm are higher by 1–1.7 K (standard deviation around 0.6 K) in comparisons to the 5‐km grid aggregated values of the LSTs in the 1‐km products, which is consistent with the results of a comparison of emissivities. On average, the emissivity in MYD11B1 (MOD11B1) is 0.0107 (0.0167) less than the ground‐based measurements, which is equivalent to a 0.64 K (1.25 K) overestimation of LST around the average value of 285 K. Knowledge obtained from the evaluation of MODIS LST/emissivity retrievals provides useful information for the improvement of the MODIS LST day/night algorithm. Improved performance of the refined (V5) day/night algorithm was demonstrated with the Terra MODIS data in May–June 2004.

The Campaign on Atmospheric Aerosol Research Network of China: CARE-China

AbstractBased on a network of field stations belonging to the Chinese Academy of Sciences (CAS), the Campaign on Atmospheric Aerosol Research network of China (CARE-China) was recently established as the country’s first monitoring network for the study of the spatiotemporal distribution of aerosol physical characteristics, chemical components, and optical properties, as well as aerosol gaseous precursors. The network comprises 36 stations in total and adopts a unified approach in terms of the instrumentation, experimental standards, and data specifications. This ongoing project is intended to provide an integrated research platform to monitor online PM2.5 concentrations, nine-size aerosol concentrations and chemical component distributions, nine-size secondary organic aerosol (SOA) component distributions, gaseous precursor concentrations (including SO2, NOx, CO, O3, and VOCs), and aerosol optical properties. The data will be used to identify the sources of regional aerosols, the relative contributions fr...

Identification of sources and formation processes of atmospheric sulfate by sulfur isotope and scanning electron microscope measurements

abundances ( 32 S, 33 S, 34 S, and 36 S) and by detailed X‐ray diffraction and scanning electron microscope (SEM) imaging of aerosol samples acquired at a rural site in northern China from March to August 2005. The comparison of SEM images from coal fly ash and the atmospheric aerosols suggests that direct emission from coal combustion is a substantial source of primary atmospheric sulfate in the form of CaSO4. Airborne gypsum (CaSO4·2H2O) is usually attributed to eolian dust or atmospheric reactions with H2SO4. SEM imaging also reveals mineral particles with soot aggregates adhered to the surface where they could decrease the single scattering albedo of these aerosols. In summer months, heterogeneous oxidation of SO2, derived from coal combustion, appears to be the dominant source of atmospheric sulfate. Our analyses of aerosol sulfate show a seasonal variation in D 33 S( D 33 S describes either a 33 S excess or depletion relative to that predicted from consideration of classical mass‐dependent isotope effects). Similar sulfur isotope variationshavebeenobservedinotheratmosphericsamplesandin(homogenous)gas‐phase reactions. On the basis of atmospheric sounding and satellite data as well as a possible relationship between D 33 S and Convective Available Potential Energy (CAPE) during the sampling period, we attribute the sulfur isotope anomalies (D 33 S and D 36 S) in Xianghe aerosol sulfates to another atmospheric source (upper troposphere or lower stratosphere).

Validation of an UV inversion algorithm using satellite and surface measurements

Ultraviolet radiation in the spectral region between 280 and 315 nm (often referred to as UV-B) is harmful to living organisms. Satellite-based estimation of surface UV-B supplements the sparsely distributed ground-based UV-B monitoring networks. This study is concerned with validation of an inversion algorithm [Li et al., this issue] for retrieving spectrally integrated UV-B (no spectral weighting) and erythemal UV (EUV) (with spectral weighting) fluxes at the surface from satellite. The physical inversion algorithm contains a few analytical expressions and input parameters: the solar zenith angle, ozone amount, albedo at the top of the atmosphere (TOA), and aerosol variables. The algorithm is applied to satellite measurements of total ozone amount and 360 nm reflectance from Meteor 3/TOMS and visible reflectance from NOAA/AVHRR. The retrieved UV-B and EUV fluxes are compared with ground UV observations made at six Canadian UV observation stations with Brewer instruments from 1992 to 1994. Under allsky conditions the comparisons showed very small mean differences and relatively large standard deviations (s.d.): 0.033 W/m 2 (mean) and 0.287 W/m 2 (s.d.) for total UV-B and 3.02 mW/m 2 (mean) and 12.0 mW/m 2 (s.d.) for EUV radiation. The large standard deviations are attributed to the inhomogeneity in sky condition and mobility of cloudy scenes, which renders an inaccurate match between satellite and surface measurements. The comparisons under clear-sky conditions showed very small mean and standard differences. By means of a running average over a period of time, satellite inversion can track the variation of surface-observed UV-B and EUV very well.

Diurnal variability of dust aerosol optical thickness and Angstrom exponent over dust source regions in China

(1) Using 22 months of Sunphotometer Aerosol Optical Thickness (AOT) data collected near the Taklamakan and Gobi dust source regions (Dunhuang, 40.09� N, 94.41� E) in China; we examine the diurnal and seasonal change of dust aerosol properties. Most dust events are during the spring throughearlysummermonthswithaseason-invariantdiurnal change of more than ±10% for AOTand ±30% for Angstrom exponent, with larger AOT and smaller Angstrom exponent values late in the afternoon. These values are much larger when compared to recent studies that have reported a much smaller (±5%) diurnal variability of dust AOT over various AERONET sites where dust is a major contributor to AOT. The differences are largely due to the geographical locations and meteorological conditions and such large diurnal changes of aerosol properties at or near dust source regions may be significant enough for consideration in regionalradiativeforcing,airqualityandnumericalmodeling studies. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 3359 Meteorology and Atmospheric Dynamics: Radiative processes; 3360 Meteorology and Atmospheric Dynamics: Remote sensing. Citation: Wang, J., X. Xia, P. Wang, and S. A. Christopher (2004), Diurnal variability of dust aerosol optical thickness and Angstrom exponent over dust source regions in China, Geophys. Res. Lett., 31, L08107, doi:10.1029/2004GL019580.