Air Stagnation Advisories:
A Research Joint Venture Review
Aaron M. Pollyea, Joseph J. Charney, Warren E. Heilman, and Jeffrey A. Andresen
Introduction
Some National Weather Service (NWS) offices, U.S. Forest Service Smoke Management
Programs, and state air pollution regulatory agencies issue Air Stagnation Advisories (ASA) when
meteorological conditions conducive to a buildup of pollutants in the atmospheric boundary layer are
expected to persist for more than 24 hours. The criteria for issuing an ASA vary according to local
meteorological and physiographic conditions, air quality sensitivities, and administrative requirements
and directives of the issuing agencies. Generally speaking, existing ASA criteria do not reference a
standardized scientific definition of air stagnation, primarily because the atmospheric science
community has not established a rigorous definition for the phenomenon and the meteorological
conditions that contribute to its occurrence. This review was an initial attempt to document the criteria
for issuing an ASA and to compare those criteria against our understanding of the meteorological
conditions that promote air stagnation. Furthermore, this study wished to assess if existing forecasting
techniques and tools could effectively diagnose the potential for air stagnation events, to recommend
new tools or techniques for diagnosing air stagnation events, and finally, to define a scientific standard
against which the existing ASA criteria could be compared.
In order to collect information on how and when ASAs are issued operationally, a list of
organizations that have in the past issued air stagnation advisories or have an interest in the subject of air
stagnation events was compiled. More than 170 inquiries were made either directly (by phone or email)
or indirectly (email through NWS regional headquarters) during the fall of 2009 regarding air stagnation
advisory guidelines and protocols. Responses were ultimately obtained from 20 organizations, generally
from the western seaboard of the country. The responses varied greatly in content, ranging from specific
criteria in an individual peer-reviewed paper to uncertainty about responsibility or details to information
on other organizations that may or may not issue the advisories.
Current Advisory Issuance
The information that was collected indicated a wide range of conditions under which ASAs are
issued. Significant differences were found even within the same agencies. For example, the three
National Weather Service Weather Forecast Offices (WFOs) in Oregon that responded to our request all
required 72 hours of their specific conditions to occur before an advisory would be issued, but those
requirements were quite different at each WFO. Specific mixing height levels were a requirement at
each of the three offices, but each office had different specific height thresholds (in this case, 1000,
1500, and 2000 feet). Two of the offices had upper wind threshold speeds (both at 5 knots), and two
considered the presences of surface inversions. One office allowed for light precipitation in the form of
drizzle, freezing drizzle or flurries to occur. In neighboring Washington State, the NWS criteria are
completely different. The Seattle WFO calls for an advisory when conditions persist as little as 36 hours,
winds less than 10 knots, mixing layer at or below 1500 feet, and the temperature must be at or above
87°F for at least two consecutive days.
A further query was made to both National Weather Service Offices and state level Departments
of Environmental Quality/Protection. Of the 38 responses that were returned, 3 groups would issue these
with a combination of a forecast tool as well as measurements of either particulate matter or ground
level ozone. 28 responded that they issued Air Quality Alerts only at the request of another agency or
from data originating from another agency. 5 responded that they do not issue Air Quality or Air
Stagnation alerts at all.
The issuing of Air Quality Alerts was a common response to the question posed. These alerts
were entirely based upon the Environmental Protection Agency’s Air Quality Index (AQI). The AQI is
generated from real time monitoring of pollutants and information from the National Weather Service on
whether conditions in the area will be conductive to the increase of pollutant concentrations.
Current State of the definition of Air Stagnation Events.
In general, air stagnation events are associated meteorologically with the presence of low-level
inversions or relatively high atmospheric stability, low wind speeds, and the proximity/influence of
synoptic-scale anticyclones. Many of the office or agency definitions of air stagnation events stem from
the work of Korshover (1976) and Angell (1982). The same ideas and definitions were then brought
forward into the study by Wang and Angell (1999), which is arguably the most comprehensive spatial
study of air stagnation events in the scientific literature.
The definitions used in these studies were modified over time from an initial mix of objective
and subjective measurements (Korshover, 1976) to a fully objective system (Wang and Angell, 1999).
Wang and Angell developed the following specific criteria:
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Derived surface level geostrophic wind speed is less that 8 m/s for at least 4 days. In
general, this would correspond to observed surface wind speeds (at anemometer level)
less than about 4 m/s. This criterion was based on the results of studies of the Donora,
Pennsylvania smog disaster of 1948 (Fletcher, 1949; Willett, 1949; Hewson, 1951).
The maximum surface wind speed threshold is increased to 4.4 m/s during the 4-day
period if an inversion exists in the boundary layer (defined as a positive change in
temperature with height below 850 mb).
With the above criteria, the event is thrown out if there was any precipitation (including
trace amounts) during the period.
The event was also thrown out if the wind speed at the 500 mb level exceeded 13 m/s.
Overall, Wang and Angell found that this combination of conditions tended to limit the events
meteorologically to 500 mb level high pressure ridges and warm-core high pressure systems since cold
core high pressure systems tend not to persist long enough.
Continuation of Air Stagnation Event Study.
We have identified the following six major air stagnation events for future study:
December 10th-11th 2005 in the Pacific Northwest
November 28th-29th 2006 in New York, New York
July 10th 2008 in central Nevada
January 18th-20th 2009 in the Pacific Northwest
January 1st 2010 in the pan handle region of Texas and Oklahoma.
August 22nd and 23rd 2010 in the pan handle region of Texas and Oklahoma.
The events, which occur in different regions, seasons, and meteorological environments, can be
used to determine whether a single set of criteria (e.g. Wang and Angell, 1999) are capable of identifying
air stagnation conditions for different regions of the United States. The North American Regional
Reanalysis (NARR) can provide the meteorological conditions that prevailed during these events, which
can then be compared against air stagnation criteria.
Upon completion of the analyses of the individual events, the NARR dataset can also be used to
document the historical frequency and distribution of meteorological conditions consistent with air
stagnation.
References
Fletcher, R.D. 1949: The Donora smog disaster. A problem in atmospheric pollution. Weatherwise.
June, 1949.
Hewson, E.W., 1951: Atmospheric Pollution. Compendium of Meteorology, T.F. Malone, Ed., American
Meteorological Society, 1139-1157.
Korshover, J.,1976: Climatology of stagnating anticyclones east of the Rocky Mountains, 1936-1975.
NOAA Tech. Memo. ERL ARL-55
Wang, J.X.L., and Angell, J. 1999: Air Stagnation climatology for the United States (1948-1998).
NOAA/Air Resources Laboratory ATLAS No. 1
Willett, H.D. 1949: Some meteorological aspects of air pollution. Trans. Conf. on Industrial Wastes,
Ind. Hyg. Found., Amer., Trans. Bull. No. 13, Mellon Institute, Pittsburg, PA.