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San Gabriel Wilderness Area |
Site Name
IMPROVE:
SAGA1
Region
Southern California Ranges
Terrain
Terrain is shown in the 2 km terrain
map and the 20 km terrain
map.
The San Gabriel Wilderness Area occupies 34,118 acres on the southern slopes
of the San Gabriel Mountains, one of the Transverse Ranges that lie along an
east-west axis from the Santa Barbara coast to the Mojave Desert creating a
natural barrier between central and southern California. Wilderness
elevations range from about 500 m (1,600 ft) to 2,500 m (8,200 ft). Highest
Wilderness elevations are along the ridge of the San Gabriel Mountains that
forms the northern Wilderness boundary. Lowest Wilderness elevations are
along the West Fork of the San Gabriel River that flows eastward in this
area and forms the southern Wilderness boundary. From the southeast corner
of the Wilderness the San Gabriel River flows southward ~ 10 km (6 mi) into
the Los Angeles Basin between Pasadena and Pomona and in this stretch of San
Gabriel Canyon includes San Gabriel and Morris Reservoirs. The San Gabriel
River valley thus forms the most direct conduit for low elevation urban
pollution transport into the Wilderness.
The San Gabriel IMPROVE site SAGA1 is located just outside the western
Wilderness Area boundary. It is a well-exposed ridge-top site at an
elevation of 1,701 m (5,874 ft), which is in the middle of the range of
Wilderness elevations.
Representativeness
The SAGA1 IMPROVE site should be well representative of aerosol composition
and concentration at San Gabriel Wilderness locations, especially higher
locations. It should also be representative of the impact of Los Angeles
basin emissions within the San Gabriel Mountains generally. There may be
times when lower Wilderness elevations, especially within Devils Canyon in
the western Wilderness and the Bear Creek drainage in the eastern
Wilderness, are contained within the regional marine layer that covers the
Los Angeles basin much of the year, especially from late spring to early
fall. The Los Angeles basin marine layer typically extends vertically to 300
to 600 m (1,000 to 2,000 ft). Elevations in these canyon and valley bottoms
are near 600 m ((1,968 ft), or about 1,000 m (3,000 ft) lower than the SAGA1
IMPROVE site.
Nearby Population/Industrial Centers and Local Sources
The San Gabriel Wilderness is within 10 km of the sprawling and heavily
populated and industrialized South Coast Air Basin and is very subject to
its influence. Nearest Los Angeles area communities are Pasadena, El Monte,
and Pomona.
Nearby Meteorological Data Stations
Nearby meteorological monitoring network stations are shown in the data
network map and at the RAWS
station US Climate Archive site. There are numerous RAWS sites shown in
the data network map although none of them are strictly within San Gabriel
Wilderness Area boundaries. Nearest RAWS sites with hourly data are the
Crystal Lake
(BAER) California RAWS site, elevation 1,661 m (5,448 ft) and the Burro
Canyon (BAER)
California RAWS site, elevation 647 m (2,121 ft). These sites are just
outside the eastern Wilderness boundary and should represent similar
Wilderness elevations, except that wind data may be modified by local
terrain. The Mt.
Wilson No. 2 California COOP site, elevation 1,741 m (5,710 ft) has
long-term data from a well exposed regionally representative site for higher
elevations. There are also numerous meteorological monitoring sites in the
Los Angeles basin that should provide data that is regionally representative
for lower elevations, including the
El Monte SAO
site, and the
Pasadena California COOP site, located 10 to 15 km from the southern
edge of the San Gabriel Wilderness Area.
The San Gabriel Wilderness Area is roughly midway between the San
Diego/Miramar and Vandenburg AFB RAOB sites in southern California both of
which conduct atmospheric soundings twice daily. Data from these sites is
regionally representative for upper air structure.
Wind Patterns
Overall, the prevailing wind direction in the region surrounding San Gabriel
Wilderness is south to southwesterly. This is evident in
Los Angeles wind roses. There are seasonal phenomena
that modify this overall pattern. In the summer a thermal low develops over
the Mohave Desert. During the winter, higher pressure over the Mohave Desert
results in winds blowing from the desert towards the California south coast.
This frequently takes the form of Santa Ana foehn desert winds, which may
carry large amounts of dust (PM10) into the Wilderness Area. Locally and in
the absence of synoptic forcing, wind patterns in the San Gabriel Wilderness
should be characteristic of mountain/valley upslope/downslope circulation,
especially in valley locations in the Devils Canyon and Bear Creek
drainages. Within these drainages, daytime upslope flow will be from the LA
basin towards Wilderness locations, with a potential for transport of urban
aerosols into the area. For nighttime drainage flow the reverse would be
true. Diurnal aerosol concentration patterns resulting from such local
upslope/downslope flow may not be manifested in observations at the SAGA1
site, which is located near a ridge crest west of the Wilderness.
Potential local transport routes into the Wilderness include Los Angeles
Basin emissions transported directly via upslope winds, mixed upwards with
afternoon mixing, or trapped regionally under a persistent subsidence
inversion. There is also a potential for particulate matter associated with
regional and local wildland fires common in the region.
Inversions/Trapping
San Gabriel Wilderness valleys are subject to inversion and trapping of
pollutants during periods of high pressure and stagnation over southern
California. Under some conditions there may be a diurnal pattern to
extinction in the Wilderness, when afternoon vertical upward mixing results
in pollutants being transported into the Wilderness, followed by return
drainage transport at night and decrease in light extinction. A diurnal
pattern could also occur in conjunction with the daily advent of the marine
layer, which typically thickens and advances over the Los Angeles basin
during the night and early morning hours, before retreating to the sea or
"burning off" to hazy sunshine around midday. Such a pattern could persist
until the region is scoured of visibility degrading particulate matter by
synoptic events. Most likely to occur during summer, when strong surface
heating results in afternoon upward mixing to valley elevations. This
phenomenon may not always be seen in SAGA1 IMPROVE site data when the site
is above the inversion height. Note that there is a 1,000 m (3,000 ft)
elevation difference between the SAGA1 site and lowest canyon bottom
Wilderness locations.
Wang and Angell,
1999 describe a region of the U.S. with high frequency of regional
stagnation events centered over the southern California Mojave Desert.
According to long term data the region typically has two or more stagnation
episodes per month from May to October, where an air stagnation episode is
defined as stagnation conditions that persist for 4 days or longer. During
this period, pressure and temperature gradients in the region are weakest,
and wind circulations weakest. Subsidence inversions during these conditions
may trap regional haze with buildup over periods of several days. These
buildup periods should be evident in data from the SAGO1 IMPROVE site.
Climatological Statistics
The Mt. Wilson
No. 2 California COOP site, elevation 1,741 m (5,710 ft) has long-term
data from a well exposed regionally representative site for higher
elevations. Data from the
Pasadena
California COOP site is representative of lower elevation locations.
These and other
Southern California Climate Summaries are available from the
Western Regional Climate Center.
Keywords
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Last updated 13 November 2004
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