Satellite data have been available to complement ground-based measurements for certain pollutants since the mid-1990s, but their use has been rare in the air quality management community. A number of barriers limit the adoption of satellite data outside of a fairly narrow group of atmospheric researchers, including: the fact that that satellites measure the “column” of air above the surface, rather than ground-level concentrations; that data from existing satellites are not available on an hourly basis akin to monitors; that satellite data are not provided at the resolution or gridding appropriate for utilization by regional air quality modelers; and that satellite data do not fit into the methods for compliance with the Clean Air Act set forth by the U.S. Environmental Protection Agency.
Still, ground-based measurements have limitations. With the exception of major cities, monitors are sparsely located across the United States. While most urban areas have at least one monitor for ozone (O3) and fine particulate matter (PM2.5), rural areas typically have no monitors, and there are no monitors over water bodies. Outside of the U.S., even large cities may have few or no air pollution monitors. For pollutants that regularly fall below the NAAQS, like carbon monoxide (CO), nitrogen dioxide (NO2), and sulfur dioxide (SO2), monitor placement is especially limited, even though these pollutants react in the atmosphere to create O3 and/or PM2.5, and serve as key markers for regulated emissions.
Satellite data are available from two types of platforms: polar-orbiting and geostationary. Polar-orbiting satellites cover the whole earth, with coverage typically once a day or less. Geostationary satellites rotate with the earth, and “see” the region with which they rotate. This allows geostationary satellites to observe more frequently over a smaller area (e.g., North America). From the perspective of a local air manager, the frequency of coverage can be as high as sub-hourly for geostationary instruments as compared to typical once-per-day or less for polar-orbiting satellites.
There are three main ways to use satellite data for policy applications: for qualitative applications, for quantitative applications, and for more advanced analysis.
- For qualitative understanding, satellite images allow air quality managers to see and communicate spatial patterns, atmospheric transport, and trends in air pollution. Visualizations from satellite data depict transport of wildfire smoke and dust plumes across regions, continents, and even oceans. Maps of NO2 across the U.S. show clear patterns of cities and suburbs, even major interstate highways and railroads. Both NASA Worldview and Giovanni are well suited to these types of qualitative application.
- Satellite data can also be used to quantify change and relative abundance. The measurement units from satellite instruments typically reflect column densities (e.g., molecules/cm2) or optical properties (e.g., the unitless AOD metric, which varies from 0 to 1). These units do not compare directly with atmospheric composition unit (e.g., molecules/cm3, ug/m3, or mixing ratio). However, the percentage change or the ratio of two related species provides a metric that may be directly compared, as shown for NO2 in the 20 largest U.S. cities (right-hand panel of the NO2 plots shown for each city).
- Beyond qualitative and simple quantitative calculations, satellite data support a wide range of advanced analysis, especially combining with complementary data sources. Satellite data are well suited to evaluate photochemical grid models, and to support the derivation of ground-level pollution estimates in unmonitored areas. Particulate matter estimates derived from AOD products retrieved from multiple satellite instruments are used in public health studies. These types of advanced analysis measures usually require that users download satellite data and customize analysis to suit applications.
To facilitate the wider use of satellite data, a number of platforms have emerged to support data access. We have found that NASA WorldView and NASA Giovanni provide satellite data in a manner that is particularly relevant to air quality managers. NASA WorldView provides near-real-time data on a daily basis, well suited to event analysis, in a clean, intuitive interface. NASA Giovanni is a more flexible platform that allows averaging over time, the creation of a range of plot types and formats, and comparing among variables. More advanced users typically benefit from an investment in capacity-building to use datasets of interest in a flexible data-analysis software such as Matlab, GIS, or the free NCAR Command Language (NCL). By downloading satellite data and creating maps and plots with an analysis software, the user can directly compare satellite retrievals with other datasets (monitor values, model simulations, etc.). This approach also allows the user to create custom visualizations, and to access wider range of satellite data products.
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–Adapted from Tracey Holloway and Arlene Fiore Opportunities and challenges in using satellite data for U.S. air quality management.