All of the questions below are linked to descriptive answers in this chapter of the Giovanni Online Users Manual regarding the Giovanni Aerosol Optical Thickness Comparison instances (Daily and Monthly). Click the question of interest.
Aerosol optical thickness, AOT, (also called aerosol optical depth, AOD) is a measure of radiation extinction due to the interaction of radiation with aerosol particles in the atmosphere, primarily due to the processes of scattering and absorption. Aerosol optical thickness is dimensionless. This parameter indicates the attenuation a beam of radiation as it passes through a layer of the atmosphere which contains aerosols.
A more technical definition: "Aerosol Optical Thickness" is the degree to which aerosols prevent the transmission of light.
The aerosol optical depth or optical thickness (tau, τ) is defined as the integrated extinction coefficient over a vertical column of unit cross section. Extinction coefficient is the fractional depletion of radiance per unit path length (also called attenuation especially in reference to radar frequencies). The optical thickness along the vertical direction is also called normal optical thickness (compared to optical thickness along slant path length).
No. Ground-based instruments can be used to measure AOT in the atmosphere above the instrument. The advantage of satellite sensors is that they provide a view of AOT over a large area, rathen than measurements of AOT taken from the ground, which indicate AOT above that location only.
Atmospheric aerosols are caused by several different processes, and can be produced naturally or by human activities. Some types of aerosols are soot and ash caused by fires (which can be either natural or human-caused); desert and soil dust; ash and chemical species (notably sulfur dioxide, SO2) from volcanoes or fossil fuel burning for energy production; marine aerosols, such as suspended sea salt due to wave action, or dimethyl sulphide from some types of phytoplankton; and "smogs", a pollution haze augmented by reactions of chemical species in the atmosphere, which can include photochemical reactions induced by sunlight. ("Vog" is a similar natural phenomenon produced by volcanic emissions.)
Satellite sensor data are available in both the Daily and Monthly instances from the Moderate Resolution Imaging Spectroradiometer (MODIS) on the Earth Observing System (EOS) Terra and Aqua satellites and the Multiangle Imaging SpectroRadiometer (MISR) on the Terra satellite. The Daily instance also includes data from the Ozone Measuring Instrument on the EOS Aqua satellite. The Monthly instance also includes data from the the Medium Resolution Imaging Spectrometer (MERIS) on Envisat.
The Daily instance features PM2.5 data from the Environmental Protection Agency, which is acquired by ground-based monitoring instruments.
The model data is from the NASA Goddard Chemistry Aerosol Radiation and Transport (GOCART) model, featuring Black Carbon and Dust aerosol data.
The spatial area of interest can be selected by either of the two methods described below
Java applet map: The Java applet map can be utilized to "click and drag" a selection box to designate the spatial area of interest.
Geographic coordinates: The geographic coordinates of the corners of the spatial area box can be input individually. This method can be used if Java is not enabled. Note that decimal and not degree (seconds and minutes) coordinates must be entered.
To select data parameters for analysis, click the box next to the parameter of interest. Every parameter selected will generate an individual visualization. In the case of time-series data, the data can be converted to a single multiple time-series plot by clicking the "Yes" radio button for the Overlay Flag.
The starting date (year/month/day), and the ending date (year/month/day) for the analysis are selected using the calendrical drop-down menus on the Giovanni interface page, which are located below the Parameter Selection interface. Error messages will be generated if the beginning month or the end month are out of the data set time range, or if the end month is before the beginning month. The beginning dates and end dates for the data in the AOT Comparison instances are shown next to each data product.
The plotting preference options are described in the Plot Preferences section of the Online Users Manual. Clicking the "Edit Preferences" button allows plotting preference options to be entered prior to the generation of the initial visualization.
After the initial output has been generated, the temporal and spatial ranges (constraints) and plotting preferences can be changed, and new output generated, using the "Submit Refinements" button.
To download the results of an analysis, click on the "Download Data" link above the visualizations on the Results page. "Initial Data Retrieval" results are the data files acquired to perform the analysis. The results are listed under the name of the operation which generated the visualization.
To download an individual results data file, click on the icon for the desired data format (see below). To download more than one data file in Batch mode, click the desired data files and then click "Download in Batch". To acquire the actual visualization image, click the link to the GIF image.
Three numerical and two image data output formats are available. Numerical data is available in HDF, netCDF, and ASCII. Image data output formats are GIF and KMZ. If Google Earth is installed on a user's home computer, clicking the KMZ icon will allow immediate load and display of the visualization in Google Earth.
All of the data from the satellite sensors is available globally, and any size area up to the full globe may be selected. Note that daily data usually will exhibit gaps of missing data between the daily scanning coverage of a particular sensor. Monthly data is averaged over the entire month and will usually show much less missing data.
The PM2.5 data product is available only over the United States.
The time range that can be selected is limited by the period of data availability; the periods of data availability for each data product are listed in the data product table in the Giovanni instance. Selection of a time range for which data is not available may result in an error message. For time series plots, periods of missing data should be displayed as gaps in the time series. Selection of longer time ranges may take considerably more time for initial data acquisition from the archive (fetching) and may require a longer period of elapsed time for processing.