this five-year period, the highest annual dust concentration was
78 mg/m3 in 2003 and the lowest annual concentration was 41 mg/
m3 measured in 2007. This range of values measured with the
TEOM agrees well with the range of annual values obtained in the
same region using more conventional gravimetric sampling
techniques.
A visual inspection of the daily time series suggests that ambient
dust concentration on the Llano Estacado is normally low but occasionally
punctuated with extreme values associated with dust
storms. This conclusion is further supported by an analysis of
measured hourly dust concentration values. The overall distribution
of hourly observations is positively skewed with a mode of
18 mg/m3
, a median of 28 mg/m3
, and a mean concentration of 55 mg/
m3
.
Diurnal patterns of dust concentration were computed by
separately averaging hourly values for a given “time of day” for all
days within the 5-yr sampling period. The results of this analysis
reveal that the overall diurnal distribution follows a cyclical pattern
with relatively high levels of dust concentration during daylight
hours and relatively low values at night and in the early morning.
The lowest values of hourly dust concentration tend to be associated
with the early morning period between 04:00 and 06:00.
Following the early morning low point, there is a general rise in
dust concentration as the sun rises, reaching a peak in the afternoon
around 14:00. Relatively high dust levels are maintained from
around 11:00 to around 16:00 followed by a decline of dust concentration
in the evening, eventually reaching a low point in the
early morning at 05:00.
Diurnal patterns of blowing dust are influenced by diurnal
variations of key climatic factors. Generally, we find relatively
strong winds during the day and lighter winds at night and in the
early morning. The morning increase in wind speed is associated
with the rising sun and the resulting rise in incoming solar
radiation. Solar radiation produces thermal instability and enhances
the mixing of high momentum winds from the upper levels
of the atmosphere to the surface layer. Solar radiation can also
reduce the critical threshold by drying the surface and reducing
relative humidity.
Diurnal patterns of blowing dust were also calculated for each of
the four seasons. These seasonal distributions were found to follow
similar patterns with larger dust levels during the day and relatively
low dust levels at night and early morning. However, a close
inspection of the seasonal distributions revealed important differences.
The most conspicuous change of the diurnal pattern
occurred in the summer where the difference between peak afternoon
values and early morning values was considerably less
than that of other seasons. The fact that there is a less significant
diurnal variation of dust concentration in the summer suggests that
this season lacks major dust storms so that the ambient dust concentration
tends to hover near the mean concentration during the
summer months.