1. Evaluation of the Light Scattering Data from the ASOS Network Rudolf B. Husar Center for Air Pollution Impact and Trend Analysis January 2, 2002

2. The ASOS Package and the Forward Scattering Visibility Sensor The ASOS Instrument Package Belford Model 220 forward scattering sensor

3. Data Problems The main data reading problems were due to bad records Some records for some stations are not fixed length We will need to identify the cause of the bad records

4. ASOS Stations from FAA, NWS and Archived at NCDC For this analysis data for 220 stations were available from the NCDC archive These ASOS sites are mostly NWS sites, uniformly distributed over the country (Imagine if we could get the entire set, including the DOD sites, not listed)

5. Diurnal Cycle of Relative Humidity and Bext The diurnal RH cycle causes the high Bext values in the misty morning hours The shape of the RH-dependence is site (aerosol) dependent – needs work Relative Humidity Bext

6. Adopted RH Correction Curve (To be validated for different locations/seasons) -The ASOS Bext value are filtered for high humidity -Values at RH >= 80% is not used -Later we will try to push the RH correction to 90%) -The Bext is also corrected for RH: RHCorrBext = Bext/RHFactor RH is calculated from T – Temperature, deg C and D – Dewpoint, deg C RH = 100*((112-(0.1*T)+D)/(112+(0.9*T))) 8

7. ASOS Bext – EPA PM2.5 Mass Comparison: Grand Rapids, MI Hourly PM2.5 data are compared to the filtered and RH-corrected one minute Bext The stations are not co-located but in the same city In Grand Rapids, MI, Jun, Sep, Oct, the relationship is good. Occasional spikes of Bext are probably weather events not adequately filtered

8. ASOS Bext – EPA PM2.5 Mass Comparison: Greensborough, NC The PM2.5 station is in Winston-Salem while the ASOS Bext is at Greensborough. The ASOS-PM2.5 relationship for Jun, Sep and Oct is good. Occasional spikes of Bext are not yet explained ???

24. ASOS Bext – EPA PM2.5 Mass Comparison: Pensacola, FL At Pensacola, FL, Aug, Sep, Oct, the relationship is more noisy. There are more Bext spikes (to be filtered? Will see) The Bext/PM2.5 is higher. ASOS calibration problem?

25. ASOS Bext – EPA PM2.5 Mass Comparison: Odessa, TX At the arid site Odessa, TX, the relationship is poor: –more Bext ‘noise’ –systematically higher Bext/Pm2.5 The higher Bext will be interesting to explore. –Is it coarse particle dust? –Will se

26. ASOS Bext – EPA PM2.5 Mass Comparison: Albuquerque, NM At the pristine Albuquerque, NM site, the PM2.5 values much lower than in the East The lower detection limit of the ASOS Bext is about 0.05 km -1. Clearly, over the pristine West, the ASOS will hit the lower detection limit Lower detection limit: 0.05

27. Summary – Tentative Conclusions There are data for at least 220 ASOS stations Format problems with the data files forced us to discard 10-20 % of readings To use the ASOS data as PM2.5 surrogate, filtering for high RH and RH correction is needed. These procedures need more calibration. RH-filtered and corrected Bext were compared to hourly PM2.5 for several cities. The results so far are most encouraging – ASOS will be a meaningful PM surrogate. ASOS can provide PM concentration estimates with high time and spatial resolution over the Eastern US. For the pristine Southwest, the utility of ASOS is questionable. The next steps will focus on further comparisons and calibrations.