GEO-EO-Req ColumnConc for Coverage

Scheffe:

Section 2.1.7 Satellite–Based Air Quality Observing Systems

More descriptive explanation of satellite data use is provided relative to other sections as there has been rapid development of applications over the last decade and still changing

An extensive array of satellite-based systems (Table 6) with the capability of measuring atmospheric column total species has been established by United States and European Satellite programs lead by NASA and NOAA in the United States and the European Space Agency (ESA). A suite of satellites including Aqua, Aura, CALIPSO, OCO, Glory, as well as NOAA- 17, NOAA-18 and NPOESS, have either been launched since about the year 2000 or have other near-term proposed launch dates. Collectively, the remote sensing techniques for measuring columns and/or profiles of aerosols (AOD), O3, CO, CO2, CH4, SO2, nitrogen oxides, CFCs, other pollutants, and atmospheric parameters such as temperature and H2O. Most of these satellites have a near-polar orbit allowing for two passes per day over a given location. When taken together, a group of six satellites (Aqua, Aura, CALIPSO, OCO, as well as CloudSat and PARASOL) coined the A-Train is being configured to fly in a formation that crosses the equator a few minutes apart at around 1:30 local time to give a comprehensive picture of earth weather, climate and atmospheric conditions.

Satellite imagery offers the potential to cover broad spatial areas; however, an understanding of their spatial, temporal and measurement limitations is necessary to determine how these systems complement ground based networks and support air quality management assessments. Temporal characterization. The near polar orbiting tracks of most satellites performing trace gas measurements provides wide spatial coverage of reasonable horizontal (10-50 km) resolution, but delivers only twice daily snapshots of a particular species. Consequently, temporal patterns of pollutants as well as a time-integrated measure of pollutant concentrations cannot be delineated explicitly through satellite measurements alone. The Geostationary satellite platforms such as the GOES systems in NOAA do provide near continuous coverage of physical parameters for weather tracking and forecasting purposes. There are proposed campaigns within NASA and across partnership Federal agencies to deploy geostationary platforms with measurement capabilities for trace gases and aerosols to enhance space based characterization of tropospheric air quality (Fishman et al., 2005).

Spatial Characterization. Polar orbiting satellites typically provide horizontal spatial resolution between 10 and 100km, depending on the angle of a particular swath segment. Spatial resolution less than 10km is possible with geostationary platforms. Characterization of elevated pollutants delivered by satellite systems complements of our ground based in-situ measurement networks – especially considering that a considerable fraction of pollutant mass resides well above Earth’s surface. With few exceptions, Satellite data typically represents a total atmospheric column estimate. For certain important trace gases (e.g., NO2, SO2, HCHO) and aerosols, the majority of mass resides in the boundary layer of the lower troposphere, enabling associations linking column data to surface concentrations or emissions fields. For example, reasonable correlations, especially in the Eastern United States, have been developed between concentrations from ground level PM2.5 stations and aerosol optical depths (AOD) from NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua and Terra satellites (Engel-Cox et al. 2004; Figure 14). The Infusing Satellite Data into Environmental Applications (IDEA, http://idea.ssec.wisc.edu/) site provides daily displays and interpretations of MODIS and surface air quality data. The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite (discussed below) provides some ability to resolve aerosol vertical gradients.

Figure 15. Correlation surfaces between MODIS AOD and hourly PM2.5 surface sites from April - September, 2002 (Engel-Cox, et. al, 2004).

In contrast to aerosols, most ozone resides in the stratosphere. Various techniques have been developed to extract the stratospheric signal to yield a tropospheric ozone residual (TOR), based on known homogeneities in the stratosphere and the use of chemical transport models and multiple measurements. Early approaches (Fishman, 1978) before and during the Total Ozone Mapping Spectrometer (TOMS) studies combined LIMB (angled view to characterize stratosphere) and NADIR (downward view, characterizing total column) techniques to derive tropospheric ozone residuals. The 2004 launch of NASA’s Aura mission with multiple ozone sensors is starting to produce more refined tropospheric ozone maps (e.g., Figure 15). However, delineating boundary layer ozone from free tropospheric reservoirs continues to pose significant interpretation challenges.

Figure 16. Daily averaged tropospheric ozone column levels derived from NASA’s OMI in Dobson Units for June 22, 2005 (courtesy, J. Szykman, EPA and J. Fishman, NASA).

Measurement issues. Most satellite air quality observations are based on spectroscopic techniques typically using reflected solar radiation as a broad source of UV through IR electromagnetic radiation (LIDAR aboard CALIPSO does utilize an active laser as the radiation source). While the science of satellite based measurements of trace gases and aerosols is relatively mature, interferences related to surface reflections, cloud attenuation and overlapping spectra of nearby species require adequate filtering and accounting for in processing remote signals. For example, aerosol events episodes associated with clouds often are screened out in developing in applications involving AOD characterizations through MODIS. Correlations between AOD and surface aerosols generally are better in the Eastern U.S. relative to the West because due to excessive surface light scattering from relatively barren land surfaces. Use of Satellite data in air quality management assessments. Satellite data, particularly fire and smoke plume observations and GOES meteorological data, support various air quality forecasting efforts servicing public health advisories. Forecasting is driven by characterizing the environment in current and immediate (1-3 days) future time frames. Air quality assessments require greater confidence in a systems (e.g., a model) response behavior to longer term, and usually much greater, changes in emissions, land use and meteorology; which requires greater confidence in formulation of numerous physical and chemical processes. Despite these differences, research and application products originally catalyzed by forecasting objectives generally overlap well with retrospective air quality assessment needs, the focus of this discussion.

Satellite products complement existing observational platforms and support the air quality assessment process through:

1. direct observational evidence of regional and long range intercontinental transport,
2. emission inventory improvements through inverse modeling,
3. evaluation of Air Quality Models,
4. tracking emissions trends (accountability), and
5. complementing surface networks through filling of spatial gaps.

As air quality assessments evolve toward embracing more pollutant categories, an attendant need to characterize a variety of spatial (and temporal) scales parallels places demands on developing more compositionally rich characterizations of air pollutants. Satellite technologies combined with partnerships with Federal agencies such as NASA and NOAA are assisting the air quality community by providing data that covers broad spatial regimes in areas lacking ground based monitors and, more importantly, a vertical compliment to our horizontal surface based networks. Although breathing zone monitoring is a rich data source, most pollutant mass resides beyond the representative reach of surface stations. During well mixed conditions with stable pressure systems during the afternoon, pollutant levels aloft often correlate well with surface conditions offering potential for “gap filling” in the surface based networks. Perhaps of greater utility is the use of satellite data to evaluate air quality models used to estimate air quality consequences of future emissions and climate scenarios. Satellite observations can be applied as a constraints on modeled total column mass or emission fields. Satellites support hemispherical and global scale air quality assessments, which are projected to be of increasing importance to North American air quality as both the relative contribution of transported air pollution and air quality-climate interactions increases over the next few decades. The pattern of gradual lowering of air quality standards (Figure 16) also raises the importance of transported air pollution. The 2006 revision of the daily PM2.5 NAAQS from 65 to 35 μg/m3 will increase the relative contribution of trans-oceanic dust transport to violations. Direct observational evidence of long distance transport clearly can be viewed with satellite imagery (Figures 17-18). Satellites often provide the only observation base for evaluating global scale air quality models in regions lacking adequate measurement and emissions inventory resources.