Under the Clean Air Act, the Environmental Protection Agency (EPA) is required to establish a nationally uniform air quality index for the reporting of air quality.
In 1976, the EPA established this index, then called the Pollutant Standards Index, for use by state and local communities across the country.
The Index provides information on pollutant concentrations for ground-level ozone, particulate matter, carbon monoxide, sulfur dioxide, and nitrogen dioxide.
On July 18, 1997, the EPA revised the ozone and particulate matter standards, in light of a comprehensive review of new scientific evidence including refined fine particulate matter standards.* Any program which is designed to improve air quality must devise tools in which emissions, meteorology, air chemistry and transport are understood.
Clearly, the complexity of this task requires measurements at both regional and mesoscale ranges, as well as on a continental scale to investigate long range transport.
Unfortunately, determination of fine particulate matter (PM) concentrations is particularly difficult since an accurate measurement of PM2.5 relies on costly equipment which cannot provide the complete transport story and the mixing and dispersion of particulate matter is much more complex than that for trace gases.
Besides the need for accurate measurements as a way of documenting air quality standards, the EPA is required in the near future to implement a 24 hour Air Quality Forecast.
Current forecast tools are usually based on emission inventories and meteorological forecasts, but significant work is being done in trying to assimilate both ground measurements as well as satellite measurements into these schemes.
Clearly, the 'Holy Grail' would be the capability of assimilating full 3D (+ time) measurements.
However, since satellite measurements are primarily passive, only total air column properties such as aerosol optical depth can be retrieved.
In particular, it is not possible to determine the vertical layering of aerosols in the troposphere from passive remote sensing measurements.
Therefore, the connection with air pollution is very poor.
Furthermore, the vertical structure of the aerosol is very important in assessing transport events and how they mix with the Planetary Boundary Layer (PBL).
The need to fill this data gap and supply vertical information on plume detection has led to the launch of the Cloud Aerosol Lidar and Infrared Pathfinder Satellite (CALIPSO) space borne lidar system, which can in principle provide vertical profiles of aerosol backscatter that can be used in the assimilation schemes.
One particular problem which needs to be addressed, is the fact that the relationship between the optical scattering coefficients (or AOD) and the PM2.5 mass is not simple.
Finally, regarding non-attainment of National Ambient Air Quality Standards (NAAQS), it has also been shown that a significant portion of the PM2.5 aerosol mass can be due to non-local sources.
This fact is critical in assessing the appropriate strategy in emission controls, as part of the state implementation plan (SIP) to come into compliance.
However, these studies are usually based on statistical analysis tools such as Positive Factor Analysis (PFA), and are not applicable to any single measurement.
In addition, little is known about the impact of episodic long range transport as a possible mechanism for affecting local pollution.
Such a mechanism cannot be investigated by statistical means or by any existing air transport models which do not consider high altitude plumes (aerosol layers), and must be studied solely with an appropriate suite of measurements including the simultaneous use...
