Environmental Engineering Reference
In-Depth Information
and predicted ozone and ozone precursor concentrations for balloon soundings and
aircraft spirals; time series comparisons of predicted and observed concentrations
from transverse aircraft flights and tall towers; and spatial plots of predicted
concentrations at selected levels aloft with observations over plotted.
The results of the data analyses and model performance evaluations were used
to test 11 widely held hypotheses that have been proposed over the past decade.
Some hypotheses were rejected based on the analyses while other hypotheses
required additional diagnostic and sensitivity simulations with CAMx to test.
3. Findings and Recommendations
Based on the analyses performed, we reached the following key findings.
•
Ozone concentrations aloft were observed to be well above background
during the two episodes with concentrations exceeding 100 ppb in the upper-
mixed layer (below 2 km). During the July-August episode, ozone concen-
trations between 70 and 80 ppb were observed to persist in deep layers above
3 km for multiple days.
•
Aloft ozone were generally underpredicted by CAMx when observed
concentrations were greater than 70 ppb and overpredicted when observed
concentrations were less than 50 ppb. While CAMx explained more than 75%
of the variance in observations in the first 100 m above the surface. it explained
less than 50% of the variance in observations at altitudes above 1 km agl.
•
Mass flux analysis of the CAMx results based on hybrid meteorology (MM5-
CALMET) showed that domain-wide ozone mass was dominated by contri-
butions from the model's top boundary. Further analysis indicated that the
hybrid meteorology introduced artificial areas of convergence resulting in
localized loss of mass through the top boundary. Analysis of mass fluxes with
MM5 meteorology indicated that circulations near the southern model boundary
may have resulted in the loss of ozone aloft through that boundary and thus,
the size of the modeling domain may be too small.
•
Maximum afternoon temperatures predicted for the September episode
appeared to be biased low, which could impact peak ozone production.
•
Improvements in MM5 meteorology, vertical resolution in the model, photo-
lysis rates at high elevations, nighttime mixing processes, and plume rise for
wildfires did not significantly improve aloft ozone model performance.
•
The current state of knowledge about the contributors to wildfire emissions
(e.g., location, size, fuels, consumption, time rate-of-change, speciation, and
plume rise) is uncertain and it may be several years before the fire science
community will be able to provide better estimates of fire emissions for the
air quality modeling community.
