Progress Report February, 2002

Evaluation of the Light Scattering Data from the ASOS Network

Submitted by

Rudolf B. HusarCenter for Air Pollution Impact and Trend Analysis

February 18, 2002

Submitted to

James F. MeagherNOAA Aeronomy Laboratory R/AL

Boulder Colorado

ASOS STATUS REPORT

• ASOS All 569 FAA-sponsored and 313 NWS Automated Surface Observing System sites are commissioned. For a state-by-state listing of commissioned sites, see "Map of Automated Weather Observing Sites and Current Weather." For your convenience, we have included the ground-to-air frequencies and telephone numbers of these systems. The ASOS systems at each site operate to comply with the "Weather Observation Service Standards." A Next Generation Runway Visual Range (NGRVR) interface to ASOS was developed and has completed the Operational Test & Evaluation testing. There are currently 120 RVR long line sites planned with 72 sites operational.

• ASOS "TEST" MESSAGE The following information is being promulgated to clear up some possible misunderstanding regarding the ASOS "Test" message: After an ASOS system is installed at a site, it undergoes a testing and evaluation period to ensure that the system is operating properly before it is commissioned as an operational system. During this pre-commissioning period, the ASOS generated weather message includes the word "TEST". Although the weather data in the test message may be accurate, it does not undergo the same level of quality assurance checks as a commissioned site does, and therefore should not be used operationally. Once the site is commissioned, the test message is removed and the ASOS becomes fully operational.

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)

Data Quality Assessment

• Co-located ASOS Sensors – absolute calibration

• Lower detection limit (0.05 km-1, 50 km visual range)

• Random sensor malfunctions

• File format problems

Comparison of Sites with Duplicate ASOS Sensors

At the St. Louis Lambert airport, the Bext from the two ASOS sensors track well (right, top)

However, the absolute magnitude of the Bext values differ by a factor

When the ASOS sensor values are multiplied by a 1.66, the two signals are virtually identical (right, bottom)

This indicates that at this site, the absolute calibration of the sensors differs by a factor of 1.66

Bext Comparison at other Duplicate Sites

The signal pattern of adjacent sensors (at different runways) is consistent. However, the calibration differences between sensors may be up to a factor two.More rigorous calibration could reduce the calibration error.

ASOS Bext Threshold: 0.05 km(-1)

• The Bext values below 0.05 km-1 are reported as 0.05. • Based on Koschmieder, the ASOS lower detection limit is ~ 50 mile visual range• In the pristine SW US, the ASOS threshold distorts the data• However over the East and Western US, the Bext signal is well over the detection limit

Evidence of ASOS Data Problems

The ASOS data for Temperature and Dewpoint appear to be erratic for some stations

The problems include constant values, spikes and rapid step changes.

Data Problems: Bad data records

• The main data reading problems are due to bad records

• Some records for some stations are not fixed length

• Cause of the bad data records need to be identified

ASOS Data Pattern Analysis

• Diurnal Cycle

• RH Dependence of Bext

• ASOS Bext – PM2.5 Relationship

Typical Diurnal Pattern of Bext, Temperature and Dewpoint

• Typically, Bext shows a strong nighttime peak due to high relative humidity.

• Most of the increase is due to water absorption by hygroscopic aerosols. At RH >90% , the aerosol is mostly water

• At RH < 90%, the Bext is mostly influenced by the dry aerosol content; the RH effect can be corrected.

Macon, GA, Jul 24, 2000

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

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

Seasonal Average Diurnal Bext Pattern

• For each minute of the day, the data were averaged over June, July and August, 2000

• Average Bext was calculated for – Raw, as reported

– For data with RH < 90%

– RH < 90% and RH Corrected

• Based on the three values, the role of water can be estimated for each location

Location of ASOS and Nearby Hourly PM2.5 Sites

• There are no co-located ASOS and PM2.5 sites

• The stations are not co-located but in the same city

• Hourly PM2.5 data are compared to the filtered and RH-corrected one minute Bext

ASOS-Hourly PM2.5 Allentown, PA

ASOS-Hourly PM2.5 Des Moines, IO

ASOS-Hourly PM2.5 Grand Rapids, MI

• In Grand Rapids, MI, July, the relationship is good.

• Occasional spikes of Bext are probably weather events not adequately filtered

ASOS-Hourly PM2.5 Islip Long Island, NY

ASOS-Hourly PM2.5 Toledo, OH

ASOS-Hourly PM2.5 San Diego, CA

ASOS-Hourly PM2.5 Islip Long Island, NY

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

???

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?

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

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

Summary – Tentative Conclusions

• There are data for at least 220 Weather Service ASOS stations

• Format problems with the data files forced us to discard 30-40 % of readings

• To use the ASOS data as PM2.5 surrogate, RH filtering and correction can be applied. These procedures need more calibration.

• Comparison of RH-filtered and corrected Bext with hourly PM2.5 at several cities is most encouraging.

• ASOS will be a meaningful PM surrogate of PM2.5 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.