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Final Report: Project to Further Research, Develop, and Apply a New Air Pollution Modeling System
EPA Grant Number: R823186Title: Project to Further Research, Develop, and Apply a New Air Pollution Modeling System
Investigators: Jacobson, Mark Z.
Institution: Stanford University
EPA Project Officer: Shapiro, Paul
Project Period: September 1, 1995 through August 31, 1998
Project Amount: $325,701
RFA: Exploratory Research - Chemistry and Physics of Air (1995)
Research Category: Engineering and Environmental Chemistry
Description:
Objective:The primary goals of the project were to (1) improve treatment of chemical and physical processes in a numerical air pollution model and (2) research scientific topics related to air pollution with the improved model. Several new or improved algorithms were developed to accomplish the first goal. These included development of new algorithms to treat condensational and dissolutional growth and improved algorithsms to treat chemical equilibrium and gas chemical ordinary differential equations. Four scientific studies were carried out to accomplish the second goal. These included studies of (1) the effects of aerosols on photolysis coefficients and ozone concentrations in an urban area, (2) the causes of observed downward ultraviolet (UV) reductions in Los Angeles, (3) the effects of soil moisture on temperatures, winds, and pollutant concentrations, and (4) the relative importance of aerosols versus clouds on aqueous oxidation of S(IV). Summary/Accomplishments (Outputs/Outcomes):
Improved treatment of chemical and physical processes
During the project period, several numerical schemes were developed or improved for use in the the air pollution model, GATORM (gas, aerosol, transport, radiation, and meteorological model). First, unconditionally-stable, mass-conserving, and noniterative numerical techniques for simulating condensation and dissolution between the gas phase and multiple size bins of the aerosol phase were developed, and the results were published (Jacobson, 1997c). The dissolution scheme permits the coupling of nonequilibrium gas-aerosol transfer equations with equilibrium calculations within each aerosol size bin.
Second, a chemical equilibrium code was improved and used to show that calcium and magnesium have a large yet different effect on the aerosol size distribution in different regions of Los Angeles. One improvement to the code, EQUISOLV, was the development of a new numerical technique of solving individual equilibrium equations. The technique, the Analytical Equilibrium Iteration (AEI) method, gives the same solutions (to at least 7 decimal places) as the previous technique used, the Mass-Flux Iteration (MFI) method, but consumes 13-48 times less computer time. EQUISOLV was also updated to include treatment of potassium, calcium, magnesium, and carbonate. Previously, it treated only nitrate, ammonium, chloride, sulfate, and sodium. Predictions from the updated code, EQUISOLV II, were compared with data measured by an 8-stage Berner impactor at Long Beach and Riverside during the Southern California Air Quality Study. Base-case predictions of nitrate and ammonium matched size-distributed observations well in nearly all size bins of every case. A sensitivity test indicated that when Ca and Mg were removed from the calculation, coarse-mode nitrate decreased at Long Beach, as expected, to maintain charge balance. At Riverside, removing Ca and Mg had the opposite effect, increasing coarse-mode nitrate, shifting it from the accumulation mode. The reason is explained in terms of mean mixed activity coefficients. The modified code, EQUISOLV II, and the application described above, are described in Jacobson (1998g). The original code, EQUISOLV (Jacobson et al., 1996), was also partly developed as part of this project.
A third advancement was the improvement of SMVGEAR II, a sparse-matrix chemical ordinary differential equation solver. The speed of the solver was improved by 50 - 67% for urban simulations and 14-44% for global simulations on both vector and scalar machines (Jacobson, 1998c). Speedups were due to automatic recalculation of absolute error tolerance as opposed to keeping the absolute tolerance fixed. Changes in errors resulting from the new technique were less than 1%.
A fourth accomplishment was the development of a particle size bin structure for use over a three dimensional grid. The moving-center particle size bin structure, which avoids numerical diffusion during growth but treats nucleation, emissions, and other processes realistically, was developed in Jacobson (1997a), and used in (1997b).
Air pollution studies
The algorithms described above were each tested individually and as a whole as part of the GATORM model. Four mesoscale studies, in which model predictions were compared with observations, are described in Jacobson (1997b, 1998d, e, f). Three of the studies, plus studies by students on this project, are summarized below.
Study of the effects of aerosols on photolysis and ozone
Jacobson (1998d) discusses the effects of size- and composition-resolved
aerosols on photolysis, temperatures, and ozone within and above an urban
airshed. With respect to photolysis, three-dimensional simulations indicated
that (1) in regions of the boundary layer where absorption of ultraviolet (UV)
radiation was strong, aerosols reduced photolysis coefficients of UV-absorbing
gases; (2) in regions of the boundary layer where UV scattering dominated UV
absorption by aerosols, aerosols enhanced photolysis coefficients of UV-
absorbing gases; (3) aerosols increased photolysis coefficients for visible-
absorbing gases since visible scattering always exceeded visible absorption by
aerosols; (4) scattering and weakly absorbing aerosols above the boundary layer
increased photolysis coefficients above the boundary layer for all absorbing
gases; and (5) increases in aerosol absorption extinction within the boundary
layer reduced photolysis coefficients above the boundary layer for all
absorbing gases. Photolysis coefficients changes due to aerosols decreased near-
surface ozone mixing ratios in Los Angeles by 5-8%. With respect to
temperatures, simulations indicated that aerosols increased radiative heating
rates at all altitudes but decreased surface solar irradiances during the day.
Surface irradiance reductions cooled the ground, reducing mechanical and
thermal turbulent heat fluxes back to the boundary layer, cooling near-surface
air, and stabilizing the boundary layer. During the night, aerosols decreased
boundary-layer heating rates but increased downward infrared irradiances to the
ground. Warmer ground temperatures increased mechanical turbulent heat fluxes
to the boundary layer, increasing nighttime near-surface temperatures. Thus,
aerosols affected temperatures primarily through ground-atmosphere turbulent
heat transfer.
Study of the causes of UV irradiance reductions in Los
Angeles
Jacobson (1998f) studied the causes of observed UV irradiance
reductions in Los Angeles. Measurements in 1973 and 1987 showed that downward
ultraviolet (UV) irradiances within the boundary layer in Los Angeles were up
to 50% less than those above the boundary layer. Downward total solar
irradiances were reduced by less than 14% in both cases. It was demonstrated in
the paper that standard particulate absorbers and scatterers could not fully
account for the observed UV reductions. It was hypothesized that absorption by
organic material, most likely nitrated aromatics, benzaldehydes, benzoic acids,
aromatic polycarboxylic acids, phenols, and polycyclic aromatic hydrocarbons,
and absorption by nitrated inorganics, caused much of the additional reductions.
This finding uncovered a previously-ignored source of UV light absorption.
Study of the effects of soil moisture on temperatures, winds, and
pollutants
Jacobson (1998e) studied the effects of soil moisture
initialization in a coupled air quality / meteorological model on temperature
profiles, wind speeds, and pollutant concentrations. Three simulations, each
with different initial soil moisture fields, were run. In one, termed the
baseline simulation, predicted temperatures, wind speeds, and gas / aerosol
pollutant concentrations matched observations most accurately. In the other two
simulations, soil moisture contents were initialized about 4% lower and higher,
respectively, than in the baseline simulation. In the low-moisture case,
predicted temperatures profiles were hotter, near-surface wind speeds were
faster, and near-surface pollutant concentrations were lower than observations
and baseline predictions. In the high soil moisture case, predicted
temperatures were colder, wind speeds were slower, and pollutant concentrations
were higher than observations and baseline predictions. Initial soil moisture
contents affected vertical temperature profiles up to 600 mb altitude after two
days Elevated temperature changes were due in part to changes in sensible heat
fluxes from the surface and in part to changes in elevated heat advection
fluxes. Changes in temperature profiles affected wind speeds and boundary layer
depths, which affected times and magnitudes, respectively, of peak
concentrations. Slower wind speeds, associated with high soil moisture contents,
delayed times of peak concentrations in the eastern Los Angeles basin. Faster
wind speeds, associated with low soil moisture contents, advanced times of peak
concentrations. High soil moisture contents resulted in thinner boundary layer
depths, increasing average near-surface pollutant concentrations, including
that of ozone. Low soil moisture contents resulted in thicker boundary layer
depths, decreasing average concentrations, include that of ozone. At some
locations, changes in the magnitude of peak ozone concentrations depended on
how changes in soil moisture affected ozone precursors and destroyers.
Study of the oxidation pathways of S(VI) as a function of pH and water
content
A postdoctoral student, Jinyou Liang, studied the important
oxidation pathways of S(IV) in aerosols and cloud drops as a function of pH and
liquid water content. He concluded that, for liquid water contents of 2x10-4 to
5 g m-3 and pHs of 0 to 8, H2O2(aq) and O3(aq) provides the major sinks for SO2
in the aqueous phase when the pH ? 6 and > 6, respectively. OH is important
only when pH ? 5 or when H2O2 is depleted. In aerosols, aqueous-phase oxidation
of SO2 is insignificant compared with gas-phase oxidation. However, SO2
oxidation in wet aerosols may be enhanced when the temperature is low (273 K)
and the relative humidity is high. High concentrations of O3 and HCHO in the
gas phase accelerate the oxidation of SO2(g) and enable HOCH2SO3- to build up at
pH > 4. Subsequent oxidation of HOCH2SO3- by OH(aq) is an important source of
S(VI). Jinyou's results have been submitted for publication (Liang and Jacobson,
1998).
Other Studies
A second postdoctoral student, Ping Ding,
worked part-time, for over a year on the project. Dr. Ding implemented a
cumulus parameterization, similar to that of Arakawa and Schubert, but with
multiple cloud-base levels, into a general circulation model at Stanford. This
parameterization has now been implemented into the regional-scale GATORM model;
however, it remains to be tested on a regional scale. Other cumulus
parameterizations will be tested as well in GATORM in the future. Dr. Ding's
work at Stanford is briefly summarized in Ding et al. (1996) and Ding and
Jacobson (1997).
A graduate student, Gerard Ketefian, has been working on the development of a nonhydrostatic version of the meteorological model in GATORM. His work to date is briefly summarized in Ketefian and Jacobson (1995, 1996, 1997).
A graduate student, Frank Freedman, has worked on this project in a study turbulence in the nighttime residual layer over the ocean and land. He wrote a paper for the AMS conference on the subject (Freedman et al., 1997), and is working on a second paper for next year's conference.
A graduate student, Ann Fridlind, has worked tangentially on this project. She has been examining the transfer of gases to the ocean surface and aerosols in the marine boundary layer, which is important for coastal air pollution studies. For this goal, Ann has concentrated on three tasks. The first task was to develop an ocean-atmosphere transport scheme. The scheme combines Monin-Obukhov similarity theory with a standard ocean / atmosphere transfer parameterizations. Her work was presented at the 1996 AGU conference (Fridlind and Jacobson, 1996). The second task was to code up a detailed DMS oxidation mechanism, to test the mechanism against smog chamber data, to update the mechanism with more recent chemical reaction rate data, and to compress the mechanism. She presented results for this task at the 1997 AGU conference (Fridlind and Jacobson, 1997). The third task was to determine whether EQUISOLV II could be used to simulate data from the ACE-1 field campaign. So far, she has compared model predictions with two-stage and seven-stage impactor measurements from NOAA-PMEL. The two- stage results were presented at the 1998 AAAR conference (Fridlind and Jacobson, 1998). She is currently working on the 7-stage data and plans to write a paper on her results.
Conclusions:The most important scientific finding resulting from the project are (1) organics within aerosols are a potentially important source of UV radiation reduction, especially in polluted air, (2) aerosols in Los Angeles are relatively absorbing, tending to reduce photolysis coefficients and ozone concentrations near the ground (however, it should be noted that ozone reductions require high aerosol concentrations; thus, ozone reductions are not benefits of polluted air), (3) soil moisture is one of the most important parameters in determining pollutant concentrations in an urban region.
Journal Articles on this Report: 11 Displayed | Download in RIS Format
| Other project views: | All 27 publications | 13 publications in selected types | All 11 journal articles |
| Type | Citation | ||
|---|---|---|---|
|
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Jacobson MZ. Computation of global photochemistry with SMVGEAR II. Atmospheric Environment, September 1995;29(18):2541-2546. |
R823186 (Final) |
not available |
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Jacobson MZ, Tabazadeh A, Turco RP. Simulating equilibrium within aerosols and nonequilibrium between gases and aerosols. <>Journal of Geophysical Research-Atmospheres 1996;101(D4):9079-9091. |
R823186 (Final) |
not available |
|
|
Jacobson MZ. Development and application of a new air pollution modeling system. 2: Aerosol module structure and design. Atmospheric Environment, January 1997;31(2):131-144. |
R823186 (Final) |
not available |
|
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Jacobson MZ. Development and application of a new air pollution modeling system. 3: Aerosol-phase simulations. Atmospheric Environment, February 1997;31(4):587-608. |
R823186 (Final) |
not available |
|
|
Jacobson MZ. Numerical techniques to solve condensational and dissolutional growth equations when growth is coupled to reversible reactions. Aerosol Science and Technology 1997;27(4):491-498. |
R823186 (Final) |
not available |
|
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Jacobson MZ. Improvement of SMVGEAR II on vector and scalar machines through absolute error tolerance control. Atmospheric Environment, February 1998;32(4):791-796. |
R823186 (Final) |
not available |
|
|
Jacobson MZ. Studying the effects of aerosols on vertical photolysis rate coefficient and temperature profiles over an urban airshed. Journal of Geophysical Research 1998;103(D9):10593-10604. |
R823186 (Final) |
not available |
|
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Jacobson MZ. Studying the effects of calcium and magnesium on size-distributed nitrate and ammonium with EQUISOLV II. Atmospheric Environment, September 1999;33(22):3635-3649. |
R823186 (Final) |
not available |
|
|
Jacobson MZ. Effects of soil moisture on temperatures, winds, and pollutant concentrations in Los Angeles. Journal of Applied Meteorology 1999;38(5):607-616. |
R823186 (Final) |
not available |
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Jacobson MZ. Isolating nitrated and aromatic aerosols and nitrated aromatic gases as sources of ultraviolet light absorption. Journal of Geophysical Research 1999;104(D3):3527-3542. |
R823186 (Final) |
not available |
|
|
Liang J, Jacobson MZ. A study of sulfur dioxide oxidation pathways over a range of liquid water contents, pH values, and temperatures. Journal of Geophysical Research-Atmospheres, June 1999;104(D11):13749-13769. |
R823186 (Final) |
not available |
environmental chemistry, west, tropospheric, VOC , Air, Geographic Area, Scientific Discipline, RFA, Engineering, Chemistry, & Physics, Physics, Chemistry, particulate matter, State, tropospheric ozone, aerosols, California (CA), chemical treatment, ozone, aerosol particles, cloud drop composition, surface solar radiation , environmental monitoring, particulates, chemical equilibrium solver, air modeling, air pollution models, air pollution modeling system, aerosol/ cloud interactions, atmospheric aerosol particles, ambient aerosol
Progress and Final Reports:
Original Abstract