Jose M. Rodriguez

Nitrogen and sulfur deposition on regional and global scales: A multimodel evaluation

We use 23 atmospheric chemistry transport models to calculate current and future (2030) deposition of reactive nitrogen (NOy, NHx) and sulfate (SOx) to land and ocean surfaces. The models are driven by three emission scenarios: (1) current air quality legislation (CLE); (2) an optimistic case of the maximum emissions reductions currently technologically feasible (MFR); and (3) the contrasting pessimistic IPCC SRES A2 scenario. An extensive evaluation of the present-day deposition using nearly all information on wet deposition available worldwide shows a good agreement with observations in Europe and North America, where 60–70% of the model-calculated wet deposition rates agree to within ±50% with quality-controlled measurements. Models systematically overestimate NHx deposition in South Asia, and underestimate NOy deposition in East Asia. We show that there are substantial differences among models for the removal mechanisms of NOy, NHx, and SOx, leading to ±1 σ variance in total deposition fluxes of about 30% in the anthropogenic emissions regions, and up to a factor of 2 outside. In all cases the mean model constructed from the ensemble calculations is among the best when comparing to measurements. Currently, 36–51% of all NOy, NHx, and SOx is deposited over the ocean, and 50–80% of the fraction of deposition on land falls on natural (nonagricultural) vegetation. Currently, 11% of the worlds natural vegetation receives nitrogen deposition in excess of the “critical load” threshold of 1000 mg(N) m−2 yr−1. The regions most affected are the United States (20% of vegetation), western Europe (30%), eastern Europe (80%), South Asia (60%), East Asia (40%), southeast Asia (30%), and Japan (50%). Future deposition fluxes are mainly driven by changes in emissions, and less importantly by changes in atmospheric chemistry and climate. The global fraction of vegetation exposed to nitrogen loads in excess of 1000 mg(N) m−2 yr−1 increases globally to 17% for CLE and 25% for A2. In MFR, the reductions in NOy are offset by further increases for NHx deposition. The regions most affected by exceedingly high nitrogen loads for CLE and A2 are Europe and Asia, but also parts of Africa.

Multimodel simulations of carbon monoxide: Comparison with observations and projected near-future changes

We analyze present-day and future carbon monoxide (CO) simulations in 26 state-of-the-art atmospheric chemistry models run to study future air quality and climate change. In comparison with near-global satellite observations from the MOPITT instrument and local surface measurements, the models show large underestimates of Northern Hemisphere (NH) extratropical CO, while typically performing reasonably well elsewhere. The results suggest that year-round emissions, probably from fossil fuel burning in east Asia and seasonal biomass burning emissions in south-central Africa, are greatly underestimated in current inventories such as IIASA and EDGAR3.2. Variability among models is large, likely resulting primarily from intermodel differences in representations and emissions of nonmethane volatile organic compounds (NMVOCs) and in hydrologic cycles, which affect OH and soluble hydrocarbon intermediates. Global mean projections of the 2030 CO response to emissions changes are quite robust. Global mean midtropospheric (500 hPa) CO increases by 12.6 ± 3.5 ppbv (16%) for the high-emissions (A2) scenario, by 1.7 ± 1.8 ppbv (2%) for the midrange (CLE) scenario, and decreases by 8.1 ± 2.3 ppbv (11%) for the low-emissions (MFR) scenario. Projected 2030 climate changes decrease global 500 hPa CO by 1.4 ± 1.4 ppbv. Local changes can be much larger. In response to climate change, substantial effects are seen in the tropics, but intermodel variability is quite large. The regional CO responses to emissions changes are robust across models, however. These range from decreases of 10–20 ppbv over much of the industrialized NH for the CLE scenario to CO increases worldwide and year-round under A2, with the largest changes over central Africa (20–30 ppbv), southern Brazil (20–35 ppbv) and south and east Asia (30–70 ppbv). The trajectory of future emissions thus has the potential to profoundly affect air quality over most of the worlds populated areas.

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.

Ozone response to enhanced heterogeneous processing after the eruption of Mt. Pinatubo

Increases in aerosol loading after the Pinatubo eruption are expected to cause additional ozone depletion. Even though aerosol loadings were highest in the winter of 1991–1992, recent analyses of satellite and ground-based ozone measurements indicate that ozone levels in the winter of 1992–1993 are the lowest recorded in recent years, raising the question of the mechanisms responsible for such behavior. We have incorporated aerosol surface areas derived from the Stratospheric Aerosol and Gas Experiment II (SAGE-II) measurements into our two-dimensional model. Inclusion of heterogeneous chemistry on these enhanced aerosol surfaces yields maximum ozone reductions during the winter of 1992–1993 in the Northern Hemisphere, consistent with those derived from observations. This delayed behavior is due to the combination of the non-linear nature of the impact of heterogeneous reactions as a function of aerosol surface area, and the long time constants for ozone in the lower stratosphere. If heterogeneous mechanisms are primarily responsible for the low 1992–1993 ozone levels, we expect ozone concentrations to start recovering in 1994.

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

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.

Trifluoroacetic acid from degradation of HCFCs and HFCs: A three‐dimensional modeling study

Trifluoroacetic acid (TFA; CF3 COOH) is produced by the degradation of the halocarbon replacements HFC-134a, HCFC-124, and HCFC-123. The formation of TFA occurs by HFC/HCFC reacting with OH to yield CF3COX (X = F or Cl), followed by in-cloud hydrolysis of CF3COX to form TFA. The TFA formed in the clouds may be reevaporated but is finally deposited onto the surface by washout or dry deposition. Concern has been expressed about the possible long-term accumulation of TFA in certain aquatic environments, pointing to the need to obtain information on the concentrations of TFA in rainwater over scales ranging from local to continental. Based on projected concentrations for HFC-134a, HCFC-124, and HCFC-123 of 80, 10, and 1 pptv in the year 2010, mass conservation arguments imply an annually averaged global concentration of 0.16 μg/L if washout were the only removal mechanism for TFA. We present 3-D simulations of the HFC/HCFC precursors of TFA that include the rates of formation and deposition of TFA based on assumed future emissions. An established (GISS/Harvard/ UCI) but coarse-resolution (8° latitude by 10° longitude) chemical transport model was used. The annually averaged rainwater concentration of 0.12 μg/L (global) was calculated for the year 2010, when both washout and dry deposition are included as the loss mechanism for TFA from the atmosphere. For some large regions in midnorthern latitudes, values are larger, 0.15–0.20 μg/L. The highest monthly averaged rainwater concentrations of TFA for northern midlatitudes were calculated for the month of July, corresponding to 0.3–0.45 μg/L in parts of North America and Europe. Recent laboratory experiments have suggested that a substantial amount of vibrationally excited CF3CHFO is produced in the degradation of HFC-134a, decreasing the yield of TFA from this compound by 60%. This decrease would reduce the calculated amounts of TFA in rainwater in the year 2010 by 26%, for the same projected concentrations of precursors.

Aerosol particle evolution in an aircraft wake: Implications for the high‐speed civil transport fleet impact on ozone

Previous calculations of the ozone impact from a fleet of high-speed civil transports (HSCTs) have been carried out by global two-dimensional (2-D) models [Bekki and PyIe, 1993; Pitari et al., 1993] which have not included explicit wake processing of sulfur species. This processing could be important for the global sulfate aerosol and ozone perturbations [Weisenstein et al., 1996]. For an HSCT scenario with emission indices of NOx and sulfur equal to 5 and 0.4, respectively, and a cruise speed of Mach 2.4 [Stolarski and Wesoky, 1993b], the Atmospheric and Environmental Research (AER) 2-D model gives 0.50–1.1% as the range of the annually averaged O3 column depletion at 40°–50°N. This range is determined by the extreme assumption that emitted SO2 is diluted into the global model grid box either as gas or as 10 nm sulfate particles. A hierarchy of models is used here to investigate the impact of processes in the wake on the calculated global ozone response to sulfur emissions by a proposed HSCT fleet. We follow the evolution of aircraft emissions from the nozzle plane using three numerical models: the Standard Plume Flowfield-II/Plume Nucleation and Condensation model (SPF-II/PNC), an AER far wake model incorporating microphysics of aerosol particles, and the AER global 2-D chemistry-transport model. Particle measurements in the wake of the Concorde [Fahey et al., 1995a] are used to place constraints on sulfur oxidation processes in the engine and the near field. To explain the Concorde measurements, we consider cases with different fractions of SO3 (2%, 20%, and 40%) in the sulfur emissions at the nozzle plane and also the possibility of other unknown heterogeneous or homogeneous oxidation processes for SO2 in the wake. Assuming similar characteristics for the proposed HSCT fleet, the global ozone response is then calculated by the 2-D model. Using the model-calculated wake processing of sulfur emissions under the above assumptions and constrained by the Concorde particle measurements, the range of annually averaged O3 column depletion at 40°–50°N is reduced from 0.5–1.1% to 0.75–1.0%. Our analysis shows that the global ozone response is more sensitive to the assumed partitioning of sulfur emissions between SO2 and SO3 at the nozzle plane than to the wake dilution rate. Outstanding uncertainties and recommendations for further wake-sampling experiments are also discussed.

The Global Modeling Initiative assessment model: Application to high-speed civil transport perturbation

(3-D) chemical transport model (CTM) was applied to assess the impact of a fleet of high-speed civil transports (HSCTs) on abundances of stratospheric ozone, total inorganic nitrogen (NOv), and H20. This model is specifically designed to incorporate a diversity of approaches to chemical and physical processes related to the stratosphere in a single computing framework, facilitating the analysis of model component differences, modeling intercomparison and comparison with data. A proposed HSCT fleet scenario was adopted, in which the aircraft cruise in the lower stratosphere, emitting nitrogen oxides (NOx) and water (H20). The model calculated an HSCT-induced change in Northern and Southern Hemisphere total column ozone of +0.2% and +0.05%, respectively. This change is the result of a balance between an increase in local ozone below approximately 25 km and a decrease above this altitude. When compared to available NOy observations, we find that the model consistently underestimates lower stratospheric NO v. This discrepancy is consistent with the model bias toward less negative ozone impact, when cohapared to results from other models. Additional analysis also indicates that for an HSCT assessment it is equally important for a model to accurately represent the lower stratospheric concentrations of ozone and H20. The GMI model yields good agreement in comparisons to ozone data for present-day conditions, while H20 is constrained by climatology as much as possible; thus no further biases would be expected from these comparisons. Uncertainties due to discrepancies in the calculated age of air compared to that derived from measurements, and of the impact of emissions on heterogeneous and polar chemistry, are difficult to evaluate at this point.

Effects on stratospheric ozone from high‐speed civil transport: Sensitivity to stratospheric aerosol loading

The potential impact of high-speed civil transport (HSCT) aircraft emissions on stratospheric ozone and the sensitivity of these results to changes in aerosol loading are examined with a two-dimensional model. With aerosols fixed at background levels, calculated ozone changes due to HSCT aircraft emissions range from negligible up to 4-6% depletions in column zone at northern high latitudes. The magnitude of the ozone change depends mainly on the NO(x) increase due to aircraft emissions, which depends on fleet size, cruise altitude, and engine design. The partitioning of the odd nitrogen species in the lower stratosphere among NO, NO2, N2O5, is strongly dependent on the concentration of sulfuric acid aerosol particles, and thus the sensitivity of O3 to NO(x) emissions changes when the stratospheric aerosol loading changes. Aerosol concentrations 4 times greater than background levels have not been unusual in the last 2 decades. Our model results show that a factor of 4 increase in aerosol loading would significantly reduce the calculated ozone depletion due to HSCT emissions. Because of the neutral variabiltiy of stratospheric aerosols, the possible impact of HSCT emissions on ozone must be viewed as a range of possible results.