Satellite/ground remote sensing of aerosols for monitoring air quality and quantifying aerosol climate forcing and sources. See the following research projects for details.
As part of this project it was defined new top-down approach that spatially constrains the amount of aerosol emissions using satellite (Moderate Resolution Imaging Spectroradiometer (MODIS)) observed radiances with the adjoint of a chemistry transport model (GEOS-Chem). Read related paper
As part of this project a new algorithm, using the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite reflectance and aerosol single scattering properties simulated from a chemistry transport model (GEOS-Chem), was developed to retrieve aerosol optical thickness (AOT) over land in China during the spring dust season. Read paper
As part of this project A vector radiative transfer model was used in conjunction with the line-by-line radiative transfer model and the database of high-resolution transmission (HITRAN) molecular absorption to simulate the degree of linear polarization of skylight in cloud-free conditions... Read paper
Diurnal Radiative Forcing of Dust Aerosols over the Atlantic Ocean
Wang, J., S.A. Christopher, J.S. Reid, H. Maring, D. Savoie, B.H. Holben, J.M. Livingston, P.B. Russell, and S.K. Yang, GOES-8 retrieval of dust aerosol optical thickness over the Atlantic Ocean during PRIDE, J. Geophys. Res., 108 (D19), doi:10.1029/2002JD002494, 2003. (pdf file)
Christopher, S.A., J. Wang, Q. Ji, and S.-C. Tsay, Estimation of Shortwave Dust Aerosol Radiative forcing during PRIDE, J. Geophys. Res., 108 (D19), 8596, doi:10.1029/2002JD002787, 2003. (pdf file)
Some of the goals of this project include:
With the high temporal resolution of GOES8, the diurnal change of dust aerosol forcing can be derived, and more accurate estimation of aerosol forcing on regional climate can be achieved.
Using all available in situ measurements to characterize aerosol optical and radiative proprieties. The data used in this study includes aerosol size distribution measurements, light scattering/absorbing coefficients (from Nephelometer), and vertical aerosol profile (aerosol concentration and optical thickness from air-borne instruments).
Create look-up table in which the reflectance at the top of atmosphere is calculated as a function of different surface reflectance, aerosol optical thickness, and different satellite-earth geometries.
A multi-channel threshold method is developed to remove the cloud, and detect dust pixels.
For a cloud-free pixel, the AOT is retrieved by finding the best bit between GOES8 reflectance and the pre-calculated reflectance in the look-up table. The validation is performed by comparing GOES8 with ground-based and air-borne Sunphotometer measured AOTs. A good agreement was found.
To calculate the dust radiative forcing, the broad-band dust optical properties are further derived from the mutli-spectral aerosol optical thickness inferred from AERONET Sunphtometer.
Using derived dust optical properties and a house-modified delta four stream radiative transfer model, we found an excellent agreement between calculated and measured downward shortwave fluxes (including global, direct, and diffuse components), which ensure the reliability of further using GOES8 AOT to calculate dust forgings.
Diurnal Change of Aerosols Optical Thickness in East Asia Retrieved from Geostationary Meteorology Satellite (GMS)
Wang, J., S.A. Christopher, F. Brechtel, J. Kim, B. Schmid, J. Redemann, P.B. Russell, P. Quinn, and B.N. Holben, Geostationary Satellite Retrievals of Aerosol Optical Thickness during ACE-Asia,J. Geophys. Res., 108 (23), doi:10.1029/2003JD003580, 2003. (pdf file).
Observations show that dust aerosols from the Takla Makan and Gobi deserts in northwest China can be transported to Korea, and even across the Pacific Ocean to the United States [Husar et al., 2001, Herman et al., 1997] and Canada [McKendry et al., 2001]. Due to rapid economic growth, the emission of industrial pollutants has increased in the East Asian regions [Bergin et al., 2001].
The aerosols in this region include sulfate, dust, soot and sea salt, in a highly mixed condition, producing a complex aerosol loading in the troposphere. Some studies have shown that aerosols might be an important factor for the regional cooling in the Sichuan basin in Southern China [Luo et al., 2001; Li et al., 1995] and drought in northern China [Menon et al., 2002]. Since there are few observations available over the oceans, satellite measurements, due to their large spatial coverage (e.g, polar-orbit satellite) and high temporal resolution (e.g., geostationary satellite), provide a unique tool for quantifying aerosol properties and spatial distributions. However, to reliably retrieve aerosol properties from satellite measurements, ground and aircraft measurements are needed to constrain the satellite retrieval processes and validate the satellite results. This study demonstrates such a strategy, with emphasis on estimating the day-time diurnal change of aerosol radiative forcing, by using geostationary satellite data and other measurements during the ACE-Asia Intensive Observation Period (IOP), April 01-April 30, 2001. ACE-Asia was conducted off the coast of East China, Korea, and Japan from late March to early May 2001. A detailed description of this campaign is given by Huebert et al . In contrast with the time-invariant aerosol models used in previous satellite retrieval studies [e.g.,Wang et al., 2003; Rao et al., 1989], aerosol properties in this study are calculated as a function of space and time (called the dynamic aerosol model) by incorporating an aerosol climatology for East Asia as well as observed aerosol properties from ground and in-situ measurements during ACE-Asia. The detailed description of the retrieval method and validations of the retrieval algorithms can be found in the reference listed above. The following is the case demo of our AOT product.
Exploring the Potential of Satellite Data for Air Quality Applications
Wang, J., and S.A. Christopher, Intercomparison between satellite-derived aerosol optical thickness and PM2.5 mass: Implication for air quality studies, Geophys. Res. Lett., 108 (21), doi:10.1029/2003GL018174, 2003.(pdf file)
Christopher, S. A. and J. Wang, 2003: Exploring the Potential of Satellite Data for Air Quality Applications, Eos Trans. AGU, 84(46), Fall Meet. Suppl., Abstract A11E-0033, 2003. (pdf file, download first and then open it).
Particular matters, or aerosols, reduce visibility, affect human health, and also cause several ecological effects. As defined by Environment Protection Agency (EPA), the dry mass content of particular matter with aerodynamic diameter less than 2.5 µm (PM2.5) in the atmosphere is an important parameter for the evaluation of air quality. However, the large spatiotemporal variations of particular matter make it a challenge to judge the air quality and issue prompt health alert from the current ground-based measurement network, especially when the aerosol events come from sources outside the U.S. The launch of EOS TERRA and AQUA satellite provides an unprecedented opportunity to monitor the air pollution over the globe. The intent of this study is to explore the potential of satellite aerosol datasets for air quality applications.
2. Hypothesis and Methodology
Aerosols with diameters around 1 ~2µm are efficient in scattering the visible light. During MODIS passing time (locally, 10:30AM for TERRA and 1:30 for AQUA) in clear sky conditions, the atmospheric boundary layer is well mixed. Hence, the MODIS visible reflectance and its column aerosol optical thickness (AOT) retrievals can be used as indicators of the PM2.5 mass at the surface. In this study, we compared MODIS AOT with the ground-based PM2.5 hourly measurements. For each comparison, MODIS AOT time is centered around the PM2.5 observation time period. The final goal of this comparison is to evaluate the quality of MODIS AOTs in the context of air quality applications before they are assimilated into the air quality models. This is important because evaluation of data quality is a critical step in the data assimilation processes.
3. Data and Study Area
4. A Case Demo. (more cases and analysis can be found in our paper and poster)
The following figures show a heavy haze event identified by the spatial distribution of MODIS AOT. Also shown is the linearly derived Air Quality Index (AQI) and the 700mb geopotential heights. Grey regions are areas where MODIS AOT is not available due to possible sun glint or cloud contamination.
Diurnal Variability of Dust Aerosol Optical Thickness and Angstrom Exponent over Dust Source Regions in China
Collaborated with Drs. X. Xia and P. Wang in the Institute of Atmospheric Physics, Chinese Academy of Sciences, We recently wrote a paper on the dirunal variability of dust optical properties over the dust source regions in China. The paper has been accepted for publication in Geophys. Res. Lett. Here is the reference of the paper: Reference:
Wang, J., X. Xia, P.Wang, and S. A. Christopher, Diurnal Variability of Dust Aerosol Optical Thickness and Angstrom Exponent over Dust Source Regions in China, Geophys. Res. Lett., 31, 10.1029/2003GL019580, 2004. (pdf file).
The following are some brief descriptions and highlights.
1. Major finding
Using 22 months of Aerosol Optical Thickness (AOT) data collected from a Sunphotometer located near the Takalamakan and Gobi dust source regions (Dunhung, 40.09°N, 94.41°E) in 1999 and 2000, we examine the diurnal and seasonal change of dust AOT and Angstrom exponent. Most dust events are during the Spring through early Summer months and our analysis shows a season-invariant diurnal change of more than ±10% for AOT and ±30% for Angstrom exponent, with larger AOT and smaller Angstrom exponent values late in the afternoon. Our results are different and complimentary to recent studies [e.g., Smirnov et al., 2002; Kaufman et al., 2000] that have reported a much smaller (±5%) diurnal variability of dust AOT over various AERONET sites where dust is a major contributor to AOT, but lacked the long-term observation data in the Chinese dust source regions. Large diurnal changes of AOT and Angstrom exponent may be significant enough to be carefully considered in radiative forcing, air quality and numerical modeling studies.
2. Why do we concern about the dust optical properties,
especially in East Asian dust source region?
The effect of aerosols on climate is one of the largest uncertainties in current global climate models [Hansen et al., 1997]. Current understanding of the radiative forcing of dust aerosols is limited, especially over dust source regions where ground observations are sparse and polar orbiting, multi-spectral satellite retrievals at visible to near-infrared wavelengths are often difficult due to the high surface albedo [Kaufman et al., 2002]. While several studies and field experiments have been conducted to study Saharan dust aerosols [Tanré et al., 2003], the widely prevalent dust events ("yellow sand") from the Taklamakan and Gobi deserts in Northwest China, have only gained attention recently [e.g. Husar et al., 2001]. In addition, previous experiments such as ACE-Asia [Hubert et al., 2003] examined aerosol properties thousands of kilometers downwind from the Taklamakan and Gobi deserts. To our knowledge, no long-term or mid-term systematic observations of dust radiative properties in the dust source region over China have been presented.
Using AOT data from the Aerosol Robotic Network (AERONET), recent studies have indicated that the diurnal variability of dust AOT is small (<5%) over various observation sites such as Cape Verde [Kaufman et al., 2000; Smirnov et al., 2002] that is about 500km downwind from the Saharan dust source region. On the other hand, several cases studies from both ground observations [e.g., Levin et al., 1980] and geostationary satellite retrievals [e.g., Wang et al., 2003] indicate that the diurnal variation of dust aerosols is relatively larger (>15%) and are important for dust radiative forcing calculations [Christopher et al., 2003]. Our current study is different from previous research since we use Sunphotometer inferred aerosol optical thickness (SP AOT) data collected near the Chinese dust source regions over a longer time period to analyze the diurnal and seasonal variations of dust AOT and Angstrom exponent.
3. What makes our observation site and this study unique?
4. Please find the analysis results in our paper.
Reference: Wang, J., and S. T. Martin, Satellite characterization of urban aerosols: Importance of including hygroscopicity and mixing state in the retrieval algorithms, J. Geophys. Res.,doi:10.1029/2006JD007705, accepted, 2007. (pdf file)
Satellite retrieval of aerosols is an inversion problem. Assumption of surface reflectance and aerosol optical properties (such as single scattering albedo and phase function) have to be pre-described during the computing of look table that links the satellite-measured reflectance with those retrieval parameters such as aerosol optical thickness and effective radius. In this paper, we investigate how the retrieval accuracy can be affected by those assumptions, in particular, the assumption on urban aerosol optical properties.
Recent field experiments in India and Mexico city have shown that the urban aerosols are most likely composed of internally mixed black carbon and sulfate-nitrate aerosols, and their single scattering albedo can be as low as 0.80 or smaller and is sensitive to the ambient relative humidity. In here, we investigate how the aerosol mixing state and ambient relative humidity, which is neglected in the majority of aerosol retrieval algorithms, will affect the retrieval accuracy of aerosol optical thickness and size.
2. Results (see details in the manuscript)
The following figure shows the isopleths of the ratio of the retrieved aerosol optical thickness assuming external mixing with optical properties at 70% RH to that assuming internal mixing with optical properties at either 70% RH or variable with RH.
The Effect of Non-Sphericity on Geostationary Satellite Retrievals of Dust Aerosols
Collaborated with Drs. Xiong Liu (Harvard University), Jeff Reid (NRL),
and Hal Maring (Miami Univ.),
, and I recently published an article in GRL on
the dust non-sphericity effect in the satellite aerosol retrievals.
One advantage in using geostationary satellite data is its high temporal resolutions. It revisits the same area every 30 minutes, and the change of solar zenith angle for a certain location provides an opportunity to examine the satellite retrievals over a wide range of scattering angles, even for the same dust layer on the same day. (see conceptual figure on the left).
Details can be found in our paper and recent poster:
Wang, J., X. Liu, S.A. Christopher, J.S. Reid, E.A. Reid, and H. Maring, The effects of non-sphericity on geostationary satellite retrievals of dust aerosols, Geophys. Res. Lett., 30, doi:10.1029/2003GL018697, 2003. (pdf file) Wang, J., S A. Christopher, Xiong Liu, Jeffrey S. Reid,Elizabeth Reid, Hal Maring. The effect of non-sphericity on GOES-8 dust aerosol retrievals during PRIDE, Eos Trans. AGU, 84(46), Fall Meet. Suppl., Abstract A11E-0011, 2003. (pdf file, download first and then open it)
Will consideration of non-spherical effects improve the satellite retrievals, if all the required data to characterize aerosol optical properties are given in the same temporal-spatial domain?
4. Results and Conclusion
The retrieval results using different phase functions are shown in the following figure. Here listed some conclusions.