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GACP Projects

Progress Report for the Period of August 1, 1998 to July 30, 1999:

Form A: ACCOMPLISHMENT REPORT

Name: Mian Chin
Institution: Georgia Tech / NASA GSFC
Title: Global Distribution of Tropospheric Aerosols: A 3-D Model Analysis of Satellite Data

Abstract:
We propose a 3-year research project to investigate the composition and distribution of tropospheric aerosols. A global 3-dimensional model will be used to simulate four major types of tropospheric aerosols: sulfate, dust, sea salt, and carbonaceous aerosols. Our objectives are (1) to interpret satellite aerosol measurements in terms of aerosol sources and concentrations, (2) to define global distributions of the four major aerosol types, and (3) to determine the effects of human activities on tropospheric aerosol loading. The assimilated meteorological fields, generated from the NASA Goddard Earth Observing System Data Assimilation System (GEOS DAS), are used to drive the Goddard Ozone Chemistry Aerosol Radiation and Transport model (GOCART). Because it is a global scale model and uses assimilated meteorological data, GOCART should be one of the best tool to extract aerosol information from the combination of satellite data and field measurements. In general, we will proceed through incorporating individual aerosol sources into the model, calculating concentrations and optical thickness of the four major aerosols, and analyzing satellite and field observations.

Long-term Goals:

  • Assess the radiative forcing by tropospheric aerosols and greenhouse gases
  • Investigate the aerosol-chemistry-climate interactions
  • Understand the processes regulating the concentrations and distributions of aerosols, ozone, and their related species

Objectives in this project:

  1. Interpreting satellite aerosol measurements. Our first objective is to use a global aerosol model to interpret the total column aerosol quantities retrieved from satellite observations (e.g., aerosol optical depth or index). This model should be validated against many field measurements such that the model can be used with reasonable confidence when analyzing the satellite data. Model results can also in turn help reduce uncertainties in satellite retrieval.
  2. Defining global distributions of major aerosol types. Each aerosol type has its own optical property and radiative effects. It is clear that knowledge of only total aerosol optical depth is not sufficient for estimating the aerosol forcing on climate, especially for projecting future climate change due to the change of aerosol sources. Instead, distributions and properties need to be determined for each major aerosol types individually so that the total aerosol optical properties reflect the variations of aerosol composition.
  3. Assessing the contribution of human activities to global aerosol loading. With the exception of sea salt, all the major types of aerosol have very strong anthropogenic sources. Those sources are likely to grow through the next century. However, because anthropogenic sources are concentrated in the continental surface, anthropogenic aerosols could be sufficiently removed within the boundary layer by dry or wet depositions. Hence, the impact of human activities on aerosol loading, in both boundary layer and free troposphere, needs to be investigated and quantified in order to assess the climate change caused by the increase of anthropogenic sources.

Approach:
Our approach includes: (1) incorporating sources of major aerosols into GOCART model; (2) simulating major aerosol types; (3) validation of TOMS retrieval for both absorbing and non-absorbing aerosols; and (4) model evaluation by comparisons with data obtained in several field programs, and model application in interpreting satellite and field observations.

Tasks Completed:

  1. Successfully developed a dust emission module. The original work here is to locate the dust source at the unvegetated surface depression regions. Dust emission depends on surface availability, surface wetness, and 10 meter wind speed. Dust is emitted in four size bins.
  2. Several years of simulations of sulfate and dust aerosols have been completed, which cover periods of several important fields campaign periods, and overlaps with TOMS and AVHRR operational periods.
  3. Model results for sulfate and dust have been evaluated extensively with field observations of concentrations, TOMS retrieval of column distribution patterns, lidar measurements for vertical distributions, and AERONET for optical depth.
  4. Carbonaceous and sea salt aerosols have also been incorporated using the existing algorithms and emission inventories. Preliminary results for black carbon, organic carbon, and sea salt aerosols have been generated.
  5. Conducted simulations of Rn-222 and Pb-210 as tracers for evaluating transport and wet scavenging parameters for aerosol species.

Future Plans (for next two years and beyond):

  1. Improving sulfate and dust simulations
  2. Improving carbonaceous and sea salt emission schemes
  3. Integrating four major aerosols together to calculate total aerosol optical depth
  4. Comparing with TOMS, AVHRR, SeaWiFS, and AERONET
  5. Incorporating aerosol microphysics modules
  6. Addressing direct and indirect radiative forcing by different types of aerosol

Results:

> Sulfate (also sponsored by other projects):
We have conducted tropospheric sulfur simulation for periods of more than 6 years (1990, 1991, 1993, 1994, Nov.1995 - Oct.1997). Model results have been compared extensively with observations from field campaigns (PEM West A, PEM West B, PEM Tropics A, ACE-1, ACE-2, SUCCESS), ground-based monitoring networks (EMEP, EMEFS, AEROCE, SEAREX), and other observations. In general, model captures sulfate and its precursor levels and their spatial and temporal variations, not only at the surface, but also in the vertical (Figure 1 EPS in Form B).

The 1990 global sulfur budget shows (1) about 80% of sulfate sources are from anthropogenic emissions of SO2; (2) wet and dry depositions remove nearly equal amount of sulfur emitted to the atmosphere; (3) chemical loss and dry deposition remove equal amount of SO2 (44% in each processes), but in-cloud oxidation of SO2 is about twice as fast as gas-phase oxidation; (4) contributions of anthropogenic and natural sources to the global sulfate burden are proportional to their source strengths, although there are strong regional differences.

We have also identified several places that the model needs improvement. For example, the oxidation rate of SO2 to sulfate in winter higher latitudes seems too slow; the current parameterization of DMS emission from the ocean seems to overestimate DMS emission at higher latitudes but underestimate it in the tropics; and the wet deposition parameters still need to be tested.

Dust:
Dust simulation shows an excellent match in global distributions and relative abundance with the absorbing aerosol index from the TOMS instrument. A 7-day evolution of a dust plume originated from Sahara in March 1988 were recorded by TOMS aerosol index, and the model reproduces the transport directions, patterns, and development of the dust plume on a daily bases (Figure 2 in Form B). The model also capture the levels and variations at several surface sites which are influenced heavily by the dust transport, for example, Barbados, Bermuda, and Izana (Tenerife). Model calculated optical depths at the sites dominated by dust aerosols agree very well with the ground Sun photometer measurements in AERONET. Comparisons of vertical distribution patterns with LITE backscattering signal are also satisfactory. Results from the dust simulation, i.e., the peak altitude, have been used by the TOMS group to improve their retrieval of aerosol optical depth. One of the problems we recently found is that the model does not remove dust faster enough at the remote regions where the dust levels are expected to be minimum. We are in the process of improving wet scavenging scheme for dust aerosol.

Form B: SIGNIFICANT HIGHLIGHTS

The GOCART model results of sulfate and its precursors are compared in Figure 1 with observations from a EMEP station, a shipcruise measurement during ACE-1, and aircraft measurements during PEM West B. Model captures the daily variations of sulfate for the year 1990 at the EMEP site, and reproduces the short-term spatial and temporal variations of sulfur species measured by ship and aircraft.

The GOCART model results of the evolution of Saharan dust plume in March 1988 are compared with TOMS absorbing aerosol index in Figure 2. The model reproduces the features: (1) the anticyclonic rotation of the plume toward Europe; (2) acrossing Atlantic transport by the middle-level jet.

Form C: PLANS FOR THE SECOND YEAR

Name: Mian Chin
Institution: Georgia Tech / GSFC

In the second year of this project, we will

  1. continuing to improve the sulfate and dust simulations;
  2. improving carbonaceous and sea salt aerosol emission schemes;
  3. simulating carbonaceous and sea salt aerosols;
  4. conducting sensitivity studies;
  5. integrating all major types of aerosol and converting aerosol masses to optical depths; and
  6. working more closely with satellite and AERONET data on aerosol optical properties

Form D: BIBLIOGRAPHY

Name: Mian Chin
Institution: Georgia Tech / GSFC

a. List of publications:

  • Chin, M., R. Rood, S.J. Lin, D. Jacob, J.F. Muller, and A. Thompson, Atmospheric sulfur cycle simulated in the global model GOCART: 1. Model description and global properties, to be submitted to JGR, 1999.
  • Chin, M., et al., Atmospheric sulfur cycle simulated in the global model GOCART: 2. Comparison with observations, to be submitted to JGR, 1999.
  • Chin, M., et al., Atmospheric sulfur cycle simulated in the global model GOCART: 3. Sensitivities to the physical and chemical processes, to be submitted to JGR, 1999.
  • Ginoux, P., M. Chin, I. Tegen, O. Torres, J. Herman, R. Rood, and S.J. Lin, Modeling of mineral dust in the global model GOCART: 1. Sources, transport, and distributions, to be submitted to JGR, 1999.
  • Ginoux, P., M. Chin, D. Savoie, J. Prospero, Modeling of mineral dust in the global model GOCART: 2. Temporal variations and vertical profiles, to be submitted to JGR, 1999.
  • Davis, D., G. Chen, M. Chin, Atmospheric sulfur, in Handbook of Weather, Climate, and Water, McGraw Hill, J. Fishman, ed. for atmospheric chemistry, 1999.
  • Chin, M., D. Considine, et al., Simulation of Rn-222 and Pb-210 in the global model GOCART: Evaluation of Rn-222 emission and Pb-210 wet deposition, to be submitted to JGR, 1999.

c. List of oral presentations:

  • Chin, M., R. Rood, S.J. Lin, D. Jacob, J.F. Muller, and A. Thompson, Simulation of tropospheric sulfate and Pb-210 in the global 3-D model GOCART, Amer. Geophysical Union Spring Meeting 1999, Boston, Mass.
  • Ginoux, P., M. Chin, I. Tegen, J. Herman, O. Torres, D. Winker, B. Holben, and S.J. Lin, Global modeling of mineral dust and its optical thickness, Amer. Geophysical Union Spring Meeting 1999, Boston, Mass.

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