Model studies over the Baltimore-Washington
area for the July 1995 high ozone episode were conducted to investigate
the impact of the presence of urban aerosols on photochemical ozone production.
A Single Column Chemical Transport Model (SCCTM) was designed using SMVGEAR
II, a sparse-matrix, vectorized Gear-type first order ordinary differential
equation solver [Jacobson, 1995] and convective and turbulent mixing
routines driven by the assimilated data from the Goddard Earth Observing
System data assimilation system (GEOS-1 DAS). It was shown by Dickerson
et al. [1997] that scattering aerosols can accelerate ozone production
in the PBL due to enhancement of the NO2 photolysis rate. We
found that this effect is highly sensitive to the aerosol composition and
vertical distribution, which both vary in time. We conducted calculations
with diurnal variation of aerosol distribution and interactively calculated
photolysis rates. The calculated ozone mixing ratio is in good agreement
with observed ozone at the Greenbelt Maryland site. Absorbing aerosols
in the PBL destabilize the atmosphere by radiative heating, thereby enhancing
vertical mixing and affecting chemical processes in the PBL significantly.
A Single Column Climate Model (SCCM) was coupled with the SCCTM to include
this important feedback. Sensitivity studies using the SCCTM have shown
a strong sensitivity of ozone concentration to changes in the turbulent
mixing and the convective ventilation of the PBL caused by aerosol. We
have also developed a 3-D stretched-grid transport and chemistry model
based on an extension of the Goddard Chemical Transport Model (GCTM) horizontal
transport algorithm [Lin and Rood, 1996] and coupled it with the
SMVGEAR II chemistry model. With this model we have conducted a simulation
of the July 1995 high ozone episode. We focus the resolution on the northeastern
part of the US. The model produces a highly resolved description of surface emission,
chemical processes, and transport in this highly populated region. Our studies show the important role of turbulent mixing and convective ventilation of the PBL in photochemical ozone production and the sensitivity of photochemistry to the altitude and optical properties of the aerosol particles.
Jacobson M. Z., Computation of global photochemistry
with SMVGEAR II, Atmospheric Environment 29, 2541-2546, 1995.
Lin S.-J., and R. B. Rood, Fast multidimensional
flux form semi-Lagrangian transport schemes on the sphere, Mon. Weather
Rev., 124, 2046-2070, 1996.
Symposium on Interdisciplinary Issues in Atmospheric Chemistry
Dickerson R. R., S. Kondragunta, G. Stenchikov,
K. L. Civerolo, B. G. Doddridge, and B. N. Holben, The impact of aerosols
on solar ultraviolet radiation and photochemical smog, Science,
278, 827-830, 1997.