Aerosol Optical Depth
Aerosols are liquid or solid particles suspended in the atmosphere. They are a key component of smog. Such smog reduces air quality. In addition to urban/industrial sources of aerosols, they are also produced by volcanic ash eruptions, dust "storms," and forest fires and other burning to clear agricultural land.
Aerosols affect human health and the environment. High concentrations of aerosols, when inhaled, lead to upper respiratory diseases including asthma. They decrease visibility and lead to unsafe conditions for transportation. The Environmental Protection Agency (EPA) estimates that more than 106 million people in the United States live in areas of poor air quality, costing about $143 billion dollars per year in hospital expenditures.
Aerosols modify the energy budget of the earth-atmosphere system in several ways. They directly scatter and absorb solar and thermal infrared radiation modify cloud amount, life time, and other properties and therefore indirectly change the Earth-leaving radiation. Absorption of radiant energy by aerosols leads to heating of the troposphere and cooling of the surface, which can change the relative humidity and atmospheric stability thereby influencing cloud formation and precipitation. Consequently, aerosols can influence land surface process, the global surface temperature, climate and the hydrological cycle, and ecosystems.
The product is a measure of the solid and/or liquid particles suspended in the air including dust, sand, volcanic ash, smoke, and urban/industrial aerosols. The aerosol optical depth (AOD) measures the amount of light lost due to the presence of aerosols on a vertical path through the atmosphere. More formally the AOD is the extinction (scattering + absorption) vertical optical thickness of aerosols. Aerosol Particle Size (APS) is the measurement of the bimodal size distribution of the aerosol population in terms of the effective radius and effective variance of each mode.
There is a mathematical relationship between the aerosol optical thickness and the wavelength of the light. This equation contains an exponential variable, called the Ångström exponent (AE). The Ångström exponent can be computed from the optical thickness measured at two different wavelenghts. The AE is inversely related to the average size of the particles (large particles = low AE, small particles = high AE). As a result, for this GOES-R data product, the AE is reported as a substitute for the average particle size.
Improvements and Benefits
GOES-R aerosol products will be more accurate than current GOES products (GOES-R ABI accuracy is ~10% compared to current GOES AOD at ~20%). Identification of the aerosol size parameter is not possible with the GOES sensors currently in operation. In contrast to the one channel used by the current GOES, GOES-R ABI provides five channels between 0.47 and 2.25 ?m suitable for retrieving aerosol properties over land and ocean. Additionally, the availability of these products at a 5-minute interval will be beneficial to the user as the products can be tailored to 15-minute or 30-minute composites to fill the data gaps associated with clouds. The use of the near real-time fire and smoke aerosol emissions in operational numerical air quality prediction models will greatly enhance the accuracy of forecast guidance. The combination of numerical forecast guidance and near real-time satellite aerosol imagery will benefit field forecasters in their air quality warnings and alerts. Accumulation of the satellite data over a long time period and extending the current GOES record is also useful for air quality assessment work done by the EPA .
GOES-R AOD using proxy data
How does it work? - Algorithm
Since the aerosols scatter and absorb light, when the concentration is high, they are easily visible in the satellite imagery. The challenge is to distinguish aerosols from clouds and from bright surfaces in the background. This is done by comparing values from multiple wavelengths in the visible light and thermal infrared portion of the spectrum. The 2.1 μm channel is transparent to most aerosols, so this channel is used to obtain the contribution of the surface to the signal received at the satellite. Several infrared channels are used to detect clouds.
The aerosol optical depth can then be determined by comparing visual wavelength radiances to values pre-computed for the ABI instrument for the potential range of AOD values. This matching of actual observations to values previously calculated for the range of observable values is often referred to as using a "look up table." This strategy is efficient in performing the complex calculations for the full range of observable conditions once and saving those results for relatively easy matching to subsequent actual observations.
See the GOES-R ATBD page for all ATBDs.
How are the results compared to existing data? - Calibration and Validation
The primary means of validation is the comparison with measurements made from the ground, specifically from the Aerosol Robotic Network (AERONET). The limited number of ground locations where high quality ground observations of aerosol properties are made restricts the geographical extent of this type of validation. Comparison with (independent) satellite-based aerosol products tests the consistency of ABI retrievals under real conditions. Spatial and temporal matching of the products involved in such a comparison, however, is not trivial.
A more technical validation presentation, (PDF, 18.06 MB) is also available.