Ivar S. A. Isaksen

Multimodel ensemble simulations of present-day and near-future tropospheric ozone

Global tropospheric ozone distributions, budgets, and radiative forcings from an ensemble of 26 state-of-the-art atmospheric chemistry models have been intercompared and synthesized as part of a wider study into both the air quality and climate roles of ozone. Results from three 2030 emissions scenarios, broadly representing optimistic, likely, and pessimistic options, are compared to a base year 2000 simulation. This base case realistically represents the current global distribution of tropospheric ozone. A further set of simulations considers the influence of climate change over the same time period by forcing the central emissions scenario with a surface warming of around 0.7K. The use of a large multimodel ensemble allows us to identify key areas of uncertainty and improves the robustness of the results. Ensemble mean changes in tropospheric ozone burden between 2000 and 2030 for the 3 scenarios range from a 5% decrease, through a 6% increase, to a 15% increase. The intermodel uncertainty (±1 standard deviation) associated with these values is about ±25%. Model outliers have no significant influence on the ensemble mean results. Combining ozone and methane changes, the three scenarios produce radiative forcings of -50, 180, and 300 mW m-2, compared to a CO 2 forcing over the same time period of 800-1100 mW m-2. These values indicate the importance of air pollution emissions in short- to medium-term climate forcing and the potential for stringent/lax control measures to improve/worsen future climate forcing. The model sensitivity of ozone to imposed climate change varies between models but modulates zonal mean mixing ratios by ±5 ppbv via a variety of feedback mechanisms, in particular those involving water vapor and stratosphere-troposphere exchange. This level of climate change also reduces the methane lifetime by around 4%. The ensemble mean year 2000 tropospheric ozone budget indicates chemical production, chemical destruction, dry deposition and stratospheric input fluxes of 5100, 4650, 1000 and 550 Tg(O 3 ) yr-1, respectively. These values are significantly different to the mean budget documented by the Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report (TAR). The mean ozone burden (340 Tg(O 3 )) is 10% larger than the IPCC TAR estimate, while the mean ozone lifetime (22 days) is 10% shorter. Results from individual models show a correlation between ozone burden and lifetime, and each models ozone burden and lifetime respond in similar ways across the emissions scenarios. The response to climate change is much less consistent. Models show more variability in the tropics compared to midlatitudes. Some of the most uncertain areas of the models include treatments of deep tropical convection, including lightning NO x production; isoprene emissions from vegetation and isoprenes degradation chemistry; stratosphere-troposphere exchange; biomass burning; and water vapor concentrations. Copyright 2006 by the American Geophysical Union.

Radiative forcing in the 21st century due to ozone changes in the troposphere and the lower stratosphere

ranging from 0.40 to 0.78 W m 2 on a global and annual average. The lower stratosphere contributes an additional 7.5–9.3 DU to the calculated increase in the ozone column, increasing radiative forcing by 0.15–0.17 W m 2 . The modeled radiative forcing depends on the height distribution and geographical pattern of predicted ozone changes and shows a distinct seasonal variation. Despite the large variations between the 11 participating models, the calculated range for normalized radiative forcing is within 25%, indicating the ability to scale radiative forcing to global-mean ozone column change. INDEX TERMS: 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 0341 Atmospheric Composition and Structure: Middle atmosphere—constituent transport and chemistry (3334) Citation: Gauss, M., et al., Radiative forcing in the 21st century due to ozone changes in the troposphere and the lower stratosphere, J. Geophys. Res., 108(D9), 4292, doi:10.1029/2002JD002624, 2003.

Solar Proton Events: Stratospheric Sources of Nitric Oxide

The production of nitric oxide (NO) in the stratosphere during each of the solar proton events of November 1960, September 1966, and August 1972 is calculated to have been comparable to or larger than the total average annual production of NO by the action of galactic cosmic rays. It is therefore very important to consider the effect of solar proton events on the temporal and spatial distribution of ozone in the stratosphere. A study of ozone distribution after such events may be particularly important for validating photochemical-diffusion models.

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.

Fresh air in the 21st century

Ozone is an air quality problem today for much of the worlds population. Regions can exceed the ozone air quality standards (AQS) through a combination of local emissions, meteorology favoring pollution episodes, and the clean-air baseline levels of ozone upon which pollution builds. The IPCC 2001 assessment studied a range of global emission scenarios and found that all but one projects increases in global tropospheric ozone during the 21st century. By 2030, near-surface increases over much of the northern hemisphere are estimated to be about 5 ppb (+2 to +7 ppb over the range of scenarios). By 2100 the two more extreme scenarios project baseline ozone increases of >20 ppb, while the other four scenarios give changes of -4 to +10 ppb. Even modest increases in the background abundance of tropospheric ozone might defeat current AQS strategies. The larger increases, however, would gravely threaten both urban and rural air quality over most of the northern hemisphere.

European scientific assessment of the atmospheric effects of aircraft emissions

The purpose of this report prepared on behalf of the European Commission is to review the current understanding of chemical and dynamical processes in the upper troposphere and lower stratosphere, and to assess how these processes could be perturbed as a result of current and future aircraft emissions. Specifically, perturbations in the atmospheric abundance of ozone and in climate forcing, as predicted by atmospheric models, will be presented. The goal is to compile and evaluate scientific information related to the atmospheric impact of subsonic and supersonic aircraft emissions and to review the state of knowledge concerning the various aspects of this problem. In Section 2, the issues and scientific questions relevant to the problem of aircraft perturbations will be presented. The key physical and chemical processes occurring in the troposphere and stratosphere will be discussed in Section 3. Estimates of air traffic and aircraft emissions will be given in Section 4. Section 5 and 6 will review the understanding of the atmospheric impact of aircraft emissions at small and large scale, respectively. The effect of aircraft emissions on climate forcing will be discussed in Section 7. Finally, conclusions will be provided in Section 8.

Aviation radiative forcing in 2000: An update on IPCC (1999)

New estimates of the various contributions to the radiative forcing (RF) from aviation are presented, mainly based on results from the TRADEOFF project that update those of the Intergovernmental Panel on Climate Change (IPCC, 1999). The new estimate of the total RF from aviation for 2000 is approximately the same as that of the IPCC’s estimate for 1992. This is mainly a consequence of the strongly reduced RF from contrails, which compensates the increase due to increased traffic from 1992 to 2000. The RF from other aviationinduced cirrus clouds might be as large as the present estimate of the total RF (without cirrus). However, our present knowledge on these aircraft-induced cirrus clouds is too poor to provide a reliable estimate of the associated RF. Zusammenfassung Neue Abschatzungen der einzelnen Beitrage zum Strahlungsantrieb des Luftverkehrs werden vorgestellt, die im Wesentlichen auf Ergebnissen des TRADEOFF-Projektes beruhen und die die IPCC-Abschatzungen (1999) aktualisieren. Der neue Wert fur den gesamten Strahlungsantrieb des Luftverkehrs im Jahr 2000 ist in etwa gleich gros wie die IPCC-Abschatzung fur das Jahr 1992. Das ist im Wesentlichen eine Folge des stark reduzierten Strahlungsantriebes durch Kondensstreifen, wodurch der Anstieg aufgrund der Zunahme des Verkehrs von 1992 bis 2000 kompensiert wird. Der Antrieb durch andere luftverkehrsinduzierte Wolken konnte ebenso gros sein wie die neue Abschatzung fur den gesamten Strahlungsantrieb (ohne Zirren). Jedoch ist unser heutiges Wissen uber diese luftverkehrsinduzierten Wolken nicht gut genug, um belastbare Aussagen uber den damit verbundenen Strahlungsantrieb zu machen.

Estimation of the direct radiative forcing due to sulfate and soot aerosols

The direct radiative forcings due to tropospheric sulfate and fossil fuel soot aerosols are calculated. The change in the atmospheric sulfate since preindustrial time is taken from a recent three-dimensional chemistry transport model calculation. A multistream radiative transfer code and observed atmospheric input data is used. The direct radiative forcing due to sulfate is calculated to −0.32 W/m2. Our results for global and annual mean radiative forcing have been compared with results from other model studies. We have assumed a linear relationship between the concentration of fossil fuel soot and sulfate aerosols. The resulting radiative forcing due to soot particles is 0.16 W/m2. Two types of mixtures of sulfate and soot are further assumed. The calculated single scattering albedo is compared to observations.

A global three‐dimensional chemical transport model for the troposphere: 1. Model description and CO and ozone results

A global three-dimensional photochemical tracer/transport model (CTM) of the troposphere has been developed. The model is based on the NASA/Goddard Institute for Space Studies (GISS) CTM with the incorporation of an extensive photochemical scheme. The model resolution is 8° latitude and 10° longitude with nine vertical layers below 10 hPa. One year of meteorological data from a free running GCM (NASA/GISS), including advective winds and convection frequency with 8-hour time resolution, is used as input. Transport of species by advection, convection, and diffusion is included in the model. The chemical scheme consists of 49 components, 85 thermal reactions, and 16 photolytic reactions. The chemical scheme is solved by the quasi steady state approximation (QSSA) method with iterations and chemical families, with a time step of 30 min. The model simulates well the lower tropospheric distribution of key species like carbon monoxide, nonmethane hydrocarbons (NMHCs), and ozone. The model is also able to simulate the important pattern of background NOx distribution [see Jaffe et al. this issue]. In the upper troposphere, coarse model resolution gives some discrepancies between modeled ozone concentrations and observations, especially at high latitudes. A global tropospheric ozone budget is presented. Net ozone production is found in the boundary layer and in the upper troposphere. In the middle free troposphere there is a close balance between chemical loss and production of ozone, giving a small net ozone loss. Hydroxyl (OH) concentrations are found to be sensitive to parameterization of cloud effects on photolysis rates. A global mean tropospheric OH concentration of 1.1×106 molecules/cm3 is calculated, which is about 15% higher than recent estimates from analysis of CH3CCI3 observations indicate.

Climatic forcing of nitrogen oxides through changes in tropospheric ozone and methane; global 3D model studies

Abstract A three-dimensional global chemical tracer model and a radiation transfer model have been used to study the role of NOx emissions for radiative forcing of climate. Through production of tropospheric O3, NOx emissions lead to positive radiative forcing and warming. But by affecting the concentration of OH radicals, NOx also reduces the levels of CH4, thereby giving negative forcing and cooling. The lifetime of NOx varies from hours to days, giving large spatial variations in the levels of NOx. We have selected geographical regions representing different chemical and physical conditions, and chemical and radiative effects of reducing NOx emissions by 20% in each region are studied. Due to nonlinearities in the O3 chemistry as well as differences in convective activity, there are large geographical differences in the effect of NOx on O3 as well as variations in the annual profile of the changes. The effect of NOx emissions on methane is also found to depend on the localisation of the emissions. The calculated ozone and methane forcing are of similar magnitude but of opposite sign. The methane effect acts on a global scale with a delay of approximately a decade, while the ozone effect is of regional character and occurs during weeks.