Courtney J. Scott

A two-dimensional model of sulfur species and aerosols

A two-dimensional model of sulfate aerosols has been developed. The model includes the sulfate precursor species H2S, CS2, DMS, OCS, and SO2. Microphysical processes simulated are homogeneous nucleation, condensation and evaporation, coagulation, and sedimentation. Tropospheric aerosols are removed by washout processes and by surface deposition. We assume that all aerosols are strictly binary water-sulfuric acid solutions without solid cores. The main source of condensation nuclei for the stratosphere is new particle formation by homogeneous nucleation in the upper tropical troposphere. A signficant finding is that the stratospheric aerosol mass may be strongly influenced by deep convection in the troposphere. This process, which could transport gas-phase sulfate precursors into the upper troposphere and lead to elevated levels of SO2 there, could potentially double the stratospheric aerosol mass relative to that due to OCS photooxidation alone. Our model is successful at reproducing the magnitude of stratospheric aerosol loading following the Mount Pinatubo eruption, but the calculated rate of decay of aerosols from the stratosphere is faster than that derived from observations.

In situ observations of NOy, O3, and the NOy/O3 ratio in the lower stratosphere

Extensive in situ measurements of reactive nitrogen (NO y ) and ozone (O 3 ) were made in the lower stratosphere over a broad latitude range (60°N-70°S) during two different seasons (March and October) in 1994. Both NO y and O 3 mixing ratios show a strong latitude dependence, with values increasing toward the poles. The NO y /O 3 ratio reveals a high-gradient region near the tropics that is not well-represented in standard 2-D photochemical transport models. Improving the representation by changing the horizontal eddy-diffusion coefficients near the tropics has important implications for the predicted impacts of aircraft emissions on stratospheric O 3 .

On the relation between stratospheric chlorine/bromine loading and short‐lived tropospheric source gases

Current methods for estimating the concentrations of inorganic chlorine/bromine species (Cly/Bry) in the stratosphere due to decomposition of tropospheric source gases assume that the Cly/Bry concentration in the stratosphere is determined mainly by the balance between production from in situ oxidation of the source gases in the stratosphere and removal by transport of Cly/Bry out of the stratosphere. The rationale being that for source gases whose lifetimes are of the order of several months or longer the concentration of Cly/Bry in the troposphere is small because they are produced at a relatively slow rate and also removed efficiently by washout processes. As a result of the small concentration, the rate at which Cly/Bry is transported to the stratosphere is expected to be small compared to the in situ stratospheric production. Thus the transport of Cly/Bry from the troposphere contributes little to the stratospheric concentration. In contrast, the origin of stratospheric Cly/Bry from reactive source gases with tropospheric lifetimes comparable to the washout lifetime of Cly/Bry (of the order of 10–30 days) in the troposphere is distinctly different. The in situ source in the stratosphere is expected to be significantly smaller because only a small portion of the source gas is expected to survive the troposphere to be transported into this region. At the same time these short-lived source gases produce appreciable amounts of Cly/Bry in the troposphere such that transport to the stratosphere offers a larger source for stratospheric Cly/Bry than in situ production. Thus, for reactive source species, simple methods of estimating the concentration of stratospheric Cly/Bry that ignore the tropospheric contribution will seriously underestimate the loading. Therefore estimation of the stratospheric Cly/Bry loading requires not only measurements of tropospheric source gases but also measurements of Cly/Bry at the tropopause. This paper illustrates the mechanism by using results from a two-dimensional chemistry-transport model. However, in view of the importance of tropospheric transport on stratospheric loading the detailed values should be further evaluated using a three-dimensional model with appropriate treatment of convective transport.

Ozone depletion potential of CH3Br

The ozone depletion potential (ODP) of methyl bromide (CH3Br) can be determined by combining the model-calculated bromine efficiency factor (BEF) for CH3Br and its atmospheric lifetime. This paper examines how changes in several key kinetic data affect BEF. The key reactions highlighted in this study include the reaction of BrO + HO2, the absorption cross section of HOBr, the absorption cross section and the photolysis products of BrONO2, and the heterogeneous conversion of BrONO2 to HOBr and HNO3 on aerosol particles. By combining the calculated BEF with the latest estimate of 0.7 year for the atmospheric lifetime of CH3Br, the likely value of ODP for CH3Br is 0.39. The model-calculated concentration of HBr (∼0.3 pptv) in the lower stratosphere is substantially smaller than the reported measured value of about 1 pptv. Recent publications suggested models can reproduce the measured value if one assumes a yield for HBr from the reaction of BrO + OH or from the reaction of BrO + HO2. Although the DeMore et al. [1997] evaluation concluded any substantial yield of HBr from BrO + HO2 is unlikely, for completeness, we calculate the effects of these assumed yields on BEF for CH3Br. Our calculations show that the effects are minimal: practically no impact for an assumed 1.3% yield of HBr from BrO + OH and 10% smaller for an assumed 0.6% yield from BrO + HO2.

Transport between the tropical and midlatitude lower stratosphere : Implications for ozone response to high-speed civil transport emissions

Several recent studies have quantified the air exchange rate between the tropics and midlatitudes in the lower stratosphere using airborne and satellite measurements of chemical species. It is found that the midlatitude air is mixed into the tropical lower stratosphere with a replacement timescale of 13.5 months (with 20% uncertainty) for the region from the tropopause to 21 km [Volk et al., 1996] and at least 18 months for the region of 20–28 km [Schoeberl et al., 1997]. These estimates are used to adjust the horizontal eddy diffusion coefficients, Kyy, in a two-dimensional chemistry transport model. The value of Kyy previously used to simulate the subtropical barrier, 0.03 × 106 m2/s, generates an exchange time of about 4 years, and the model without subtropical barrier (Kyy = 0.3 × 106 m2/s) has an exchange time of 5 months. Adjusting the Kyy to 0.13 × 106 m2/s from the tropopause to 21 km and 0.07 × 106 m2/s above 21 km produces the exchange timescales which match the estimates deduced from the measurements. The subtropical barrier prevents the engine emissions of the high-speed civil transport (HSCT) aircraft from being transported into the tropics and subsequently lifted into the upper atmosphere or mixed into the southern hemisphere. The model results show that the calculated ozone response to HSCT aircraft emissions using the Kyy adjusted to observed mixing rates is substantially smaller than that simulated without the subtropical barrier.