Page 376 - Fundamentals of Air Pollution 3E

IV. Modeling Pollutant Transformations 331

bases from which they were derived. A major shortcoming is the limited
amount of quality smog chamber data available.
The emission inventory and the initial and boundary conditions of pollut-
ant concentrations have a large impact on the ozone concentrations calcu-
lated by photochemical models.
To model a decrease in visibility, the chemical formation of aerosols from
sulfur dioxide and oxidants must be simulated.
In a review of ozone air quality models, Seinfeld (33) indicates that the
most uncertain part of the emission inventories is the hydrocarbons. The
models are especially sensitive to the reactive organic gas levels, speciation,
and the concentrations aloft of the various species. He points out the need
for improvement in the three-dimensional wind fields and the need for
hybrid models that can simulate sub-grid-scale reaction processes to incor-
porate properly effects of concentrated plumes. Schere (34) points out that
we need to improve the way vertical exchange processes are included in
the model. Also, although the current models estimate ozone quite well,
the atmospheric chemistry needs improvement to better estimate the con-
centrations of other photochemical components such as peroxyacyl nitrate
(PAN), the hydroxyl radical (OH), and volatile organic compounds (VOCs).
In addition to the improvement of data bases, including emissions, bound-
ary concentrations, and meteorology, incorporation of the urban ozone
with the levels at larger scales is needed.

C. Regional-Scale Transformations
In order to formulate appropriate control strategies for oxidants in urban
areas, it is necessary to know the amount of oxidant already formed in the
air reaching the upwind side of the urban area under various atmospheric
conditions. Numerous physical and chemical processes are involved in
modeling transformations (35, 36) on the regional scale (several days,
1000 km): (1) horizontal transport; (2) photochemistry, including very slow
reactions; (3) nighttime chemistry of the products and precursors of photo-
chemical reactions; (4) nighttime wind shear, stability stratification, and
turbulence episodes associated with the nocturnal jet; (5) cumulus cloud
effects—venting pollutants from the mixed layer, perturbing photochemical
reaction rates in their shadows, providing sites for liquid-phase reactions,
influencing changes in the mixed-layer depth, perturbing horizontal flow;
(6) mesoscale vertical motion induced by terrain and horizontal divergence
of the large-scale flow; (7) mesoscale eddy effects on urban plume trajector-
ies and growth rates; (8) terrain effects on horizontal flows, removal, and
diffusion; (9) sub-grid-scale chemistry processes resulting from emissions
from sources smaller than the model's grid can resolve; (10) natural sources
of hydrocarbons, NQ X, and stratospheric ozone; and (11) wet and dry
removal processes, washout, and deposition.