Debra K. Weisenstein

Sensitivity of ozone to bromine in the lower stratosphere

Measurements of BrO suggest that inorganic bromine (Br_y) at and above the tropopause is 4 to 8 ppt greater than assumed in models used in past ozone trend assessment studies. This additional bromine is likely carried to the stratosphere by short-lived biogenic compounds and their decomposition products, including tropospheric BrO. Including this additional bromine in an ozone trend simulation increases the computed ozone depletion over the past ∼25 years, leading to better agreement between measured and modeled ozone trends. This additional Br_y (assumed constant over time) causes more ozone depletion because associated BrO provides a reaction partner for ClO, which increases due to anthropogenic sources. Enhanced Br_y causes photochemical loss of ozone below ∼14 km to change from being controlled by HO_x catalytic cycles (primarily HO_2+O_3) to a situation where loss by the BrO+HO_2 cycle is also important.

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.

A comparison of observations and model simulations of NOx/NOy in the lower stratosphere

Extensive airborne measurements of the reactive nitrogen reservoir (NO_(y)) and its component nitric oxide (NO) have been made in the lower stratosphere. Box model simulations that are constrained by observations of radical and long-lived species and which include heterogeneous chemistry systematically underpredict the NO_x (= NO + NO_2) to NO_y ratio. The model agreement is substantially improved if newly measured rate coefficients for the OH + NO_2 and OH + HNO_3 reactions are used. When included in 2-D models, the new rate coefficients significantly increase the calculated ozone loss due to NO_x and modestly change the calculated ozone abundances in the lower stratosphere. Ozone changes associated with the emissions of a fleet of supersonic aircraft are also altered.

Impact of heterogeneous chemistry on model‐calculated ozone change due to high speed civil transport aircraft

Heterogeneous chemistry could have a very significant effect on the predicted impact of engine exhaust from high speed civil transport (HSCT) aircraft on atmospheric ozone. Two-dimensional models including only gas phase chemistry indicate that deposition of nitrogen oxides from aircraft exhaust in the lower stratosphere would significantly perturb the natural nitrogen budget, most likely resulting in ozone depletion. The model calculates that an injection of 1 megaton of NO 2 per year at 17-20 km would decrease the column ozone by 3-6% at northern mid latitudes using gas phase chemistry only

Toward a better quantitative understanding of polar stratospheric ozone loss

Previous studies have shown that observed large O3 loss rates in cold Arctic Januaries cannot be explained with current understanding of the loss processes, recommended reaction kinetics, and standard assumptions about total stratospheric chlorine and bromine. Studies based on data collected during recent field campaigns suggest faster rates of photolysis and thermal decomposition of ClOOCl and higher stratospheric bromine concentrations than previously assumed. We show that a model accounting for these kinetic changes and higher levels of BrO can largely resolve the January Arctic O3 loss problem and closely reproduces observed Arctic O3 loss while being consistent with observed levels of ClO and ClOOCl. The model also suggests that bromine catalysed O3 loss is more important relative to chlorine catalysed loss than previously thought.

Potential impact of SO2 emissions from stratospheric aircraft on ozone

Renewed interest in the potential impact of stratospheric aircraft on atmospheric ozone has focused on emissions of nitrogen oxides (NO x ). This work shows that enhancement of the sulfate aerosol layer by aircraft emissions of sulfur could be more significant to the ozone impact than emission of NO x , especially when emissions of NO x in future engines are reduced by a factor of three from present engine designs. Our calculations show that increases in the aerosol surface area of the stratosphere by factors of two to three are expected if significant amounts of aircraft-emitted sulfur are converted to sulfuric acid and undergo homogeneous nucleation in the aircraft plume. This possibility is supported by both in situ stratospheric observations and plume/wake modeling.

Aviation Fuel Tracer Simulation: Model Intercomparison and Implications

An upper limit for aircraft-produced perturbations to aerosols and gaseous exhaust products in the upper troposphere and lower stratosphere (UT/LS) is derived using the 1992 aviation fuel tracer simulation performed by eleven global atmospheric models. Key findings are that subsonic aircraft emissions: 1) have not be responsible for the observed water vapor trends at 40°N; 2) could be a significant source of soot mass near 12 km, but not at 20 km, 3) might cause a noticeable increase in the background sulfate aerosol surface area and number densities (but not mass density) near the northern mid-latitude tropopause, and 4) could provide a global, annual mean top of the atmosphere radiative forcing up to +0.006 W/m² and −0.013 W/m² due to emitted soot and sulfur, respectively.

Measurements of the NO y ‐N2O correlation in the lower stratosphere: Latitudinal and seasonal changes and model comparisons

The tracer species nitrous oxide, N2O, and the reactive nitrogen reservoir, NOy, were measured in situ using instrumentation carried aboard the NASA ER-2 high altitude aircraft as part of the NASA Airborne Southern Hemisphere Ozone Expedition/Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAESA) and Stratospheric Tracers of Atmospheric Transport (STRAT) missions. Measurements were made throughout the latitude range of 70°S to 60°N over the time period of March to October 1994 and October 1995 to January 1996, which includes the period when the Antarctic polar vortex is most intense. The correlation plots of NOy with N2O reveal compact, near-linear curves throughout data obtained in the lower stratosphere (50 mbar to 200 mbar). The average slope of the correlation, ΔNOy/ΔN2O, in the southern hemisphere (SH) exhibited a much larger seasonal variation during this time period than was observed in the northern hemisphere (NH). Between March and October in the potential temperature range of 400 K to 525 K, the correlation slope in the SH midlatitudes increased by 28%. A smaller but still positive increase in the correlation slope was observed for higher-latitude data obtained within or near the edge of the SH polar vortex. At NH midlatitudes the correlation slope did not significantly change between March and October, while between October and January the slope increased by +7%. The larger SH midlatitude increase is consistent with ongoing descent throughout the winter and spring and also suggests that denitrification, the irreversible loss of HNO3 through sedimentation of cloud particles, is not a significant term (<10–15%) in the budget of NOy at SH midlatitudes during the wintertime. A secular increase in the correlation slope is ruled out by comparison with SH data obtained during the 1987 Airborne Antarctic Ozone Expedition (AAOE) aircraft campaign. These results suggest that a seasonal cycle exists in the correlation slope for both hemispheres, with the SH correlation slope returning to the April value during the SH spring and summer. Changes in stratospheric circulation also probably play a role in both the SH and the NH correlation slope seasonal cycles. Comparisons with two-dimensional model results suggest that the slope decreases when the denitrified Antarctic vortex is diluted into midlatitudes upon vortex breakup in the spring and that through the descent of stratospheric air, the slope recovers during the following fall/winter period.

Use of satellite data to constrain the model‐calculated atmospheric lifetime for N2O: Implications for other trace gases

The source gases, such as nitrous oxide (N2O) and chlorofluorocarbons (CFCs), are released into the atmosphere at the Earths surface and are removed mainly by photolysis in the stratosphere. The atmospheric abundance of a source gas is proportional to its emission rate and atmospheric lifetime. The lifetime is, in turn, determined by the local photochemical removal rate of the gas and the efficiency of transport that carries the gas from where it is emitted to the dominant photochemical removal region. The latter is particularly important for source gases for which the removal is restricted to the tropical stratosphere, a region where both the photolysis rates and concentrations are highest. We calculate that approximately 80% Of N2O is removed in the stratosphere between 30°N and 30°S. Using the data for N2O obtained from the stratospheric and mesospheric sounder (SAMS) instrument on the Nimbus 7 satellite to constrain the model-calculated distributions, we concluded that previous models may have underestimated the magnitude of vertical transport over the tropics and that the calculated lifetimes for N2O and CFC source gases could be 30% shorter than previously reported values. The calculated lifetime for N2O of 110 years would imply a source strength of 13×106 tons (N) yr−1, compared to a source strength of 9.2×106 tons (N) per year for a lifetime of 160 years. A shorter lifetime for the CFCs (47 years for CFC-11 and 95 years for CFC-12) would imply a more rapid decrease in the atmospheric chlorine content once the CFC emissions are stopped, making it possible to reach the pre-ozone hole value of 2 ppbv as early as 2045. Accurate determination of the lifetime of CFC-11 is particularly important, since the lifetime is used in the definitions of the ozone depletion potentials (ODP), chlorine loading potentials (CLP), and greenhouse warming potentials (GWP) of the replacement chemicals for the CFCs. A shorter lifetime for CFC-11 would elevate the magnitudes of ODP, CLP, and GWP for these chemicals.

Kinetics of reactions of ground state nitrogen atoms (4S3/2) with NO and NO2

The discharge flow technique has been used with resonance fluorescence detection of N atoms to study the fast radical-radical reaction of ground state nitrogen atoms (^4S_(3/2)) with NO and NO_2. The rate constants obtained are (in units of cm^3 molecule^(−1) s^(−1)) k_1 = (2.2±0.2) × 10^(−11) exp[(160±50)/T] in the temperature range 213 K ≤ T ≤ 369 K for N + NO → N_2 + O and k_2 = (5.8±0.5) × 10^(−12) exp [(220±50)/T] in the temperature range 223 K ≤ T ≤ 366 K for N + NO_2 → N_2O + O. The reported error limits are at the 95% confidence level. The reaction kinetics are consistent with other radical-radical reactions, essentially no enthalpic barrier is observed. Substitution of the measured rate of R_1 for the value recommended hi the latest Jet Propulsion Laboratory compendium [DeMore et al., 1992] results in a small change in the concentration of ozone predicted in a two-dimensional photochemical model. Modeled ozone concentrations are higher (approximately 1%) in the high-latitude upper stratosphere as a result of a 3–10% reduction in the calculated concentrations of NO_y.