Alternative Approaches for PM2.5 Mapping: Visibility as a Surrogate Stefan Falke AAAS Science and Engineering Fellow U.S. EPA - Office of Environmental Information Rudolf Husar Washington University in St. Louis Center for Air Pollution Impact and Trend Analysis (CAPITA)

Visibility – Extinction Coefficient Relationship Visual range observations provide a good indicator or air quality but have the disadvantage of being inversely related to aerosol concentrations. A more suitable measure is haziness or extinction coefficient, b ext. where K is the Koschmieder constant. b ext = b abs + b scat The extinction coefficient is in units of km -1 and is proportional to the concentration of light scattering and absorbing aerosols and gases. The raw visibility data needs to be filtered to eliminate and correct for - weather influences (fog, precipitation) - high humidity - visual range threshold Fine particles are efficient at scattering light and are strongly correlated with extinction coefficients, after correcting for meteorological conditions

Visibility as a PM2.5 Surrogate PM 2.5 = 7.6  g/m 3 PM 2.5 = 21.7  g/m 3 PM 2.5 = 65.3  g/m 3 Adapted from Malm, “An Introduction to Visibility” Correlation Falke.. PMCorrelation Falke.. PM = 200 FBext

Method for Aiding Mapping when PM2.5 monitors are spatially sparse Fine Mass Concentrations 1/r2 Interpolation Extinction Coefficient 1/r2 Interpolation Bext Fine Mass 1/r2 Interpolation Bext Aided FM = Fine Mass Bext x Bext

ASOS Visibility Network There are over 900 ASOS sensors including over 70 duplicate monitors (black dots). By design the ASOS network is uniformly distributed while the PM25 FRM network is more concentrated in urban areas.

ASOS Visibility Measurements Prior to 1994, visual range was recorded hourly by human observations Human observations were replaced with automated light scattering instruments for of the Automated Surface Observing System (ASOS) The ASOS sensor measures the extinction coefficient as one-minute averages Forward scatter ASOS visibility sensor photocell detector projector Lens-to-lens 3.5 feet

Application of ASOS for PM2.5 Mapping Currently, available only at a quantized resolution of 18 binned ranges with a visual range upper bound of 10 miles, even though the instrument can provide meaningful data up to miles. However, in the near future, it is anticipated that ASOS data will be available at their full resolution on the web in “real-time.” Even at full resolution, they are of limited use in the western U.S. The application to “real-time” mapping (hourly or less) needs to be studied Application to small spatial scales (urban areas) also needs more evaluation ASOS data have great potential in aiding PM2.5 mapping

Difficulties in Mapping PM2.5 Inadequate spatial coverage Inadequate temporal coverage – standard is once every 3 days “Realistic” interpolation methods Limitations Visibility observations have a very good uniform coverage across country Visibility observations made hourly or every 10 minutes Visibility as surrogate adds physically based extrapolation around measured “anchor” points. Benefits of Visibility Surrogate