Preliminary results from a two-moment aerosol model applied to a three-dimensional model of the global sulfur cycle
Date
1995-12
Authors
Kreidenweis, Sondra M., author
Harrington, Debra Y., author
Walton, John J., author
Penner, Joyce E., author
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Journal ISSN
Volume Title
Abstract
The focus of this work is the development and application of a two-mode, two-moment model of sulfate aerosol dynamics that is coupled to the Lawrence Livermore National Laboratory (LLNL) three dimensional transport, transformation, and deposition model, GRANTOUR. The new aerosol model predicts two moments of the distribution, particle number concentration and particle mass concentration; earlier studies of the global sulfur cycle have predicted only a single moment of the aerosol, the total mass of particulate sulfate. Two modes are used to represent the fine aerosol fraction in this study. The treatment of sulfuric acid vapor is also modified from earlier studies, from a diagnostic variable usually set equal to zero, to a prognostic variable which drives the gas-to-particle conversion in the aerosol model. The parameterized two-moment aerosol model was developed for computational efficiency and the small number of prognostic variables, which reduce storage requirements in the large-scale model. The addition of this simple aerosol scheme nearly tripled the computational time needed to complete a month of simulation, relative to that for a chemistry-only run. The implementation of the aerosol model in GRANTO UR is described, and preliminary steady-state results from perpetual July simulations are presented and discussed. Anthropogenic and natural sources of sulfur gases were used as input, with simple oxidation schemes converting the gases to sulfuric acid. No direct emissions of particulate matter or other sources of aerosol mass were considered. Predictions of aerosol sulfate mass concentrations at the surface are nearly identical to previous results from the single moment aerosol scheme, and are highest in industrialized regions with large anthropogenic emissions. However, the spatial distribution of number concentration is markedly different from that for the particulate mass. In particular, the predicted monthly mean surface number concentrations in continental regions are much lower than observations. This result is interpreted as indicating that direct emissions of particulate matter are the strongest regulator of particle number concentration in such regions, and must be included to improve comparisons with observations. At 355 mb, however, the predicted number concentrations are generally higher than those at the surface, and show a latitudinally banded structure similar to some observations. This result is interpreted as signifying a lessening of the importance of surf ace primary emissions in determining the number concentrations of free tropospheric aerosol. A trajectory analysis is performed for one parcel which was lifted to a 300 mb height after picking up emissions in the marine boundary layer of the western North Atlantic. Several thousand particles cm3 were nucleated in this parcel in the free troposphere. It was later brought to the surface in subsiding air over northern Africa, where it contributed to an unexpectedly high monthly mean number concentration in this region. The analysis, although limited in scope, supports the idea that the free troposphere may serve as a source of aerosol number to the boundary layer.
Description
December 1995.
Rights Access
Subject
Aerosols -- Environmental aspects
Atmosphere