ASOS Visibility Data Evaluation and Analysis
Background
Over the past decades visibility data have been used to document the spatial and temporal
pattern of atmospheric haziness over the US and also as a surrogate for the pattern and
trends of fine particle mass. Starting in 1994-6, NOAA National Weather Service (NWS)
in cooperation with FAA has been replacing human observers of visual range with
automated light scattering instruments that can automatically monitor the haziness with
high dynamic range, accuracy, and precision. The instrumental visibility data are part of
the Automated Surface Observing System, ASOS. In 2001, the ASOS system consists of
over 860 sites distributed rather uniformly over the country as seen in the station location
map (Fig. 1). The network is operated jointly by FAA (535 sites), NWS (315) with
participation from DOD.
Fig 1. Location of ASOS monitoring stations over the US. In 2000, over 800 stations
were distributed uniformly over the US.
The ASOS sensor measures the extinction coefficient as one-minute averages. The
visibility sensors in the new Automated Surface Observing Systems (ASOS),
(http://www.nws.noaa.gov/asos/vsby.htm) is a Belfort Model 6220 forward scatter
instrument that illuminates the ambient air by a Xenon flash lamp and detects the forward
scattering from dust, smoke, haze, as well as from rain, fog, and snow particles. The
instrument is an accurate and reliable way of monitoring light scattering due to the
variety of particles. The forward scattering signal can be meaningfully converted into
visibility, i.e. the distance to which objects are discernable. The ASOS package also
monitors, temperature, dew point, precipitation, and etc., which allows the separation of
fog, rain, and snow from dust, smoke, and haze. Hence, it is also possible to relate the
Sensor Equivalent Visibility (SEV) reported by ASOS to the in-situ concentrations of
fine particle dust, smoke, and haze.
Figure 2. Automated Surface Observing Systems (ASOS), and the Belfort Model 6220
forward scatter instrument
According to the manufacturer, Belfort Instruments (http://www.belfort-inst.com) the
specifications of the instrument include a visibility range from 17 ft to 30 miles, with and
accuracy of +/- 10% of the light scattering signal. This range and sensitivity makes the
instrument a suitable augmentation of the filter-based 24-hr average fine particle
monitoring system now being implemented by EPA in response to the new PM2.5
standard. A unique contribution of the ASOS system is that it monitors the ambient
aerosol concentration in-situ, i.e. in the state as particles are inhaled rather than on filters
following equilibration to standard laboratory conditions. The other unique feature is the
fast, one-minute time response.
At ASOS-equipped airports pilots can examine the visibility pattern using the full
resolution of the Belfort scattering instrument (up to 30 miles visual range in 1 to 10minute resolution. The one-minute resolution, ASOS forward light scattering data are
stored on the on-site computer for 12 hours.
Unfortunately, the ASOS visibility data are not archived at this high resolution. Rather,
the visibility values are quantized. The 18 distinct visual range bins are: <1/4, 1/4, 1/2,
3/4, 1 1/4, 1 1/2, 1 3/4, 2, 2 1/2, 2, 4, 5, 6, 7, 8, 9,10+. The number of visibility bins is
marginally sufficient for most air quality applications. However, the main limitation of
the archived data arises from the fact that the ASOS visibility values are truncated at 10
miles by declaring that the last bin contains all the occurrences above the 10 mile visual
range. The instrument can provide meaningful data to 20-30 miles visual range. The
average daytime visibility over the Eastern US is well above >10 miles. Hence, the
archived ASOS data can only reveal the spatial and temporal pattern during the hazy
episodes. This limits the detailed pattern and trend analysis of regional haze.
ASOS Data Usages and Data Evaluation.
The primary purpose of the ASOS data is to provide the necessary visual range
information aviation safety. For that purpose, the truncated visibility data are evidently
appropriate. However, the ASOS light scattering data many other benefits:
 It can document hazardous road conditions during smoke, dust and intense haze
 It can be used to evaluate aerosol forecast models, including data assimilation
 ASOS data can support research on PM sources and transport
 The ASOS system is already in place.

The performance and the characteristics of the ASOS sensors were tested by the
operational Agencies (FAA, NWS) but the results of such test are not available to us.
Richards et al., (1996) have demonstrated that the full resolution ASOS visibility sensor
along with the other monitored meteorological parameters can be used to derive fine
particle mass concentrations during non-precipitating periods and relative humidity less
than 80%.
The data of Richards et al., 1996 indicate that the ASOS sensor can provide meaningful
data to visibility of 20-30 miles. If the full resolution data were available the ASOS data
could be used to document the fine particle pattern during both clean and hazy periods
over the eastern US. The ASOS sensor does not seem suitable for visibility/fine particle
monitoring over the pristine Southwest with average visibility well in excess of 30 miles.
Bradley and Lewis (1998) and Ramsey (2000) have performed a limited comparisons of
the ASOS sensor to the previously used human observer approach. The data indicate
general consistency of the two methods but also show significant deviations particularly
at very low visibilities. This suggests combining the old human observer data with that
ASOS data for long-term trend analysis requires a detailed calibration study.
Proposed Approach
The proposed work consists of two major tasks:
1. Evaluation of ASOS Visibility Data as a PM Surrogate
2. ASOS Data Analysis over the Southeastern US: Summer 2000 Huston Study
Task 1: Evaluation of ASOS Visibility Data as a PM Surrogate
Weather Filters and Humidity Correction
The ASOS data undergo extensive automated quality control by the Weather Service.
However, many interferences, largely due to weather makes the raw ASOS data
unsuitable as a surrogate for PM2.5. For this reason, additional filters need to be applied
to the ASOS data.
The weather filter eliminated visibility records when the obstruction to vision could be
attributed to weather, i.e. hydrometeors associated meteorological phenomena. Records
that contain flags for rain, fog, or precipitation need to be eliminated. Furthermore, the
daily record when the difference between temperature and dew point was <2.2 C were
also eliminated. This temperature spread corresponds to about 90% relative humidity.
Finally, an “ice fog” and “blowing snow” filter need to be applied that eliminates extreme
cold and windy conditions (temperature <-29 C and wind speed >16 km/hr).
Our past experience is with the weather-filters developed for the human-gathered weather
data. As part of this work, the weather filters will be re-examined and adopted for the
automated ASOS data.
Aggregation into Hourly and Daily Averages
The ASOS monitoring data are transmitted at one-minute resolution. For some
applications, including fast changing weather the one-minute resolution is highly
desirable for the interpretation of the phenomenon involved. Example applications
include the identification of dust fronts; smoke plumes near major fires or pollution
plumes near urban-industrial centers. The fine-scale temporal resolution of ASOS should
allow separation of aerosol plumes from the background aerosol based on temporal
structure of the 1-minute data. Since the spatial and temporal pattern of pollutant
concentrations are related through the wind speed, the ASOS data should allow
estimating the size of aerosol puffs passing over the sensor.
As part of the proposed analysis the fine time resolution of the ASOS data will be
evaluated with the goal of characterizing the aerosol fine structure.
For many applications the “noisy” one-minute resolution ASOS data are not appropriate.
This is the case for regional scale analysis as well as for seasonal and long-term pattern
analysis. The aggregation of the ASOS light scattering data into hourly and daily
aggregates is made difficult by the strong non-linearity associated with the humidity
effects. As part of the proposed work alternative aggregation schemes will be evaluated
along with recommendations for routine processing.
ASOS Scattering- PM2.5 Relationship
One of the key purposes of this work is to evaluate the suitability of ASOS light
scattering data as a surrogate for PM2.5 concentrations. Research over the 30 years has
demonstrated that light scattering measurements using integrating nephelometers
correlate well with fine particle mass, except under humid and dusty conditions. It is
expected that the forward scattering ASOS sensor will also correlate well with fine mass.
In fact, in a pilot study conducted by Richards et al., 1996, it was found that hourly
TEOM PM 2.5 data at south Boston, MA correlated well with ASOS visibility sensor
light scattering coefficient from Logan Airport. The 24 hr average of TEOM PM2.5 and
ASOS surrogate PM2.5 had a correlation R2 = 0.9. (Fig)
Fig. 3A. Comparison of the hourly ASOS scattering data with TEMO PM2.5
measurement in Boston. B). Comparison of daily average scattering and PM2.5.
(Richards et. al, 1996)
As part of this work, the ASOS scattering data will be compared to the existing PM2.5
data from FRM and hourly TEOM/Beta gauge sensors. Currently there are about 600
PM2.5 FRM filter samplers that report data every 3 days. (Fig. 4). Unfortunately, there
are only a limited number (<50) of hourly PM2.5 samplers through out the country.
Fig 4. Monitors reporting in the AIRS database at least 6 months of PM2.5 data sampled
every 3rd day from 1999 – 2000 (FRM).
The visibility and PM samplers are generally not collocated and a detailed data
comparison is not possible. However, the available data will allow the rough evaluation
of the PM2.5 – ASOS relationship over different regions, seasons, weather and aerosol
conditions. Such analysis will be conducted for the locations that are most suitable for
intercomparison.
Systematic and Random Measurement Errors
The ASOS data will be evaluated for systematic and random errors. Stations that
consistently deviate from the surrounding stating values will be examined for possible
systematic errors. Also, the temporal pattern at the available sites will be examined for
possibly spurious behavior. The systematic and random measurement error analysis will
be focused on the sites with duplicate sensors.
Task 2. Analysis of the ASOS Data over the Southeastern US:
Summer 2000 Huston Study
Spatial Pattern of Hourly ASOS Data during the Houston Aerosol
Study
The ASOS data will be analyzed in detail to support 2000 Texas Air Quality Study
(http://www.utexas.edu/research/ceer/texaqs/visitors/study.html). The study involved
measurements at approximately 20 sites in the Houston area. In Figure Fig 5 squares
represent the main ground chemistry sites and the base of aircraft operations. Circles are
particulate matter sites. Triangles are additional meteorological and chemical sites.
Fig. 5. PM Sampler locations during the 2000 Houston Aerosol Study.
The ASOS data will be mapped for the entire duration of the Texas study, AugustOctober, 2000. The ASOS scattering data will be presented as
 Raw light scattering coefficient, Bscat
 Scattering coefficient with data during rain, fog and (90%+) humidity
removed, FBext
 Relative humidity-corrected scattering coefficient, RHBExt.
The ASOS data values, along with the available regional-scale PM2.5 values will provide
a regional context for the intensively monitored Houston data.
Comparison of ASOS Pattern with Routine Aerosol Data
The spatial and temporal pattern of the ASOS data in the Houston region will be
compared to the monitoring data during the Houston Aerosol Study. Given the intensive
aerosol sampling network, it is hoped that data from several monitoring sites will be
suitable for comparison with the ASOS data.
Comparison with Special measurements During the Huston Aerosol
Study
The Houston Aerosol Study included aircraft and other special sampling. The aircraft
data provide the vertical aerosol structure, while the ASOS and other surface data cover
the spatial and temporal pattern at the surface.
As part of this work, we will collaborate with the participants of the Houston study in
jointly analyzing the special aerosol measurement and the ASOS data. We will also
contribute to the analysis of the high-resolution satellite data from the 8 wavelength
SeaWiFS sensor. The daily color satellite data will provide estimates of the vertical
aerosol optical depth over the region.
Deliverables
Report: Evaluation of ASOS Visibility Data as a PM Surrogate
Report: Analysis of the ASOS Data over the Southeastern US during the 2000 Huston
Study
Data: ASOS data filtered for weather, humidity and spurious data.
Project Personnel, Period and Budget
The project will be conducted by Rudolf B. Husar, Professor of Mechanical Engineering
and Director of the Center for Air Pollution Impact and Trend Analysis (CAPITA),
Washington, University, St. Louis. He will be assisted by Graduate Research Assistants,
Sean Raffuse and Jeremy Colson.
The project will be conducted between September 2001- May 2002. Throughout the
project, there will close interaction with the Project Officer, James F. Meagher.
The Budget of the 9 month long project is $50,000. Tasks 1 and 2 are budgeted equally at
$25,000 each.
References
ASOS: http://www.faa.gov/asos/asos.htm
Richards L.W., Dye T.S., Arthur M., Byars M.S. (1996) Analysis of ASOS Data for
Visibility Purposes, Final Report STI-996231-1610-FR, EPA Work Assignment 2-4.
Contract Number 63D30064, prepared for Systems Applications International, Inc., San
Rafael, CA.
Bradley, J.B., and R. Lewis, 1998: Comparability of ASOS and Human Observations,
14th International Conference on Interactive Information Processing Systems (IIPS),
American Meteorological Society, Phoenix, AZ, Paper 10.1
Ramsay, A. C., 2000: Comparison of ASOS and Observer Ceiling Height and Visibility
Values. Report to then National Weather Service ASOS Program Office under Contract
Number 50-DGNW-6-90001.