Jean-François Müller

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.

Geographical distribution and seasonal variation of surface emissions and deposition velocities of atmospheric trace gases

The geographical distributions on the global scale of trace gas surface emission and deposition are established on the basis of a variety of technical, geographic, and climatic data. The 5° × 5° resolution maps of the sources and deposition velocities are constructed, which can be used as surface boundary conditions in a three-dimensional chemical/transport model of the troposphere. Special attention is devoted to emissions of CO, NOx, and several nonmethane hydrocarbons (NMHC) and to the fossil fuel emissions of methane. Anthropogenic sources, i.e., the emissions produced or controlled by human activities, represent about 75% or more of the total surface emissions of CO, CH4, SOx and NOx and about two thirds of the total production of atmospheric CO (from surface sources and atmospheric oxidation of hydrocarbons). The possibility arises that methane releases from natural gas exploitation in the USSR are substantially larger than accounted for in previous studies, implying possible important consequences for the methane budget.

Vertical distributions of lightning NOx for use in regional and global chemical transport models

We have constructed profiles of lightning NOχ mass distribution for use in specifying the effective lightning NOχ source in global and regional chemical transport models. The profiles have been estimated for midlatitude continental, tropical continental, and tropical marine regimes based on profiles computed for individual storms in each regime. In order to construct these profiles we have developed a parameterization for lightning occurrence, lightning type, flash placement, and NOχ production in a cloud-scale tracer transport model using variables computed in the two-dimensional Goddard Cumulus Ensemble (GCE) model. Wind fields from the GCE model are used to redistribute the lightning NOχ throughout the duration of the storm. Our method produces reasonable results in terms of computed flash rates and NOχ mixing ratios compared with observations. The profiles for each storm are computed by integrating the lightning NOχ mass across the cloud model domain for each model layer at the end of the storm. The results for all three regimes show a maximum in the mass profile in the upper troposphere, usually within 2–4 km of the tropopause. Downdrafts appear to be the strongest in the simulated midlatitude continental systems, evidenced by substantial lightning NOχ mass (up to 23%) in the lowest kilometer. Tropical systems, particularly those over marine areas, tended to have a greater fraction of intracloud flashes and weaker downdrafts, causing only minor amounts of NOχ to remain in the boundary layer following a storm. Minima appear in the profiles typically in the 2–5 km layer. Even though a substantial portion of the NOχ is produced by cloud-to-ground flashes in the lowest 6 km, at the end of the storm most of the NOχ is in the upper troposphere (55–75% above 8 km) in agreement with observations. With most of the effective lightning NOχ source in the upper troposphere where the NOχ lifetime is several days, substantial photochemical O3 production is expected in this layer downstream of regions of deep convection containing lightning. We demonstrate that the effect on upper tropospheric NOχ and O3 is substantial when the vertical distribution of the lightning NOχ source in a global model is changed from uniform to being specified by our profiles. Uncertainties in a number of aspects of our parameterization are discussed.

IMAGES: A three‐dimensional chemical transport model of the global troposphere

A new three-dimensional chemical transport model of the troposphere is presented. This model, named intermediate model of global evolution of species, has been developed to study the global distributions, budgets, and trends of 41 chemical compounds, including the most important species that determine the oxidation capacity of the atmosphere. The chemical mechanism is made of approximately 125 chemical reactions and 26 photodissociations. The model accounts for surface emissions, chemical transformations, dry and wet deposition, and aerosol reactions of trace constituents. The model is formulated in σ coordinates and includes 25 layers in the vertical. Its horizontal resolution is 5° in longitude and 5° in latitude. To keep the requirements in computer time limited, a simplified representation of the transport is adopted: the advection, solved by a semi-Lagrangian scheme, is driven by monthly mean climatological winds provided by an European Center for Medium-Range Weather Forecasts analysis. The effect of wind variability at timescales smaller than a month is taken into account by an eddy diffusion parameterization. Convection in cumulonimbus clouds is also represented. All input field, such as the distribution of winds, clouds, eddy diffusion coefficients, and the boundary conditions, are monthly means constrained by observational data. The modeled global distributions of species such as methane, carbon monoxide, nitrogen oxides, and ozone are generally in good agreement with observations. The lifetime of methane, which can be regarded as a measure of the oxidizing capacity of the atmosphere, is found to be equal to 11 years, in agreement with recent estimates. The model also shows that the deposition of ozone at the Earths surface (1100 Tg/yr) balances the sum of the net photochemical production (550 Tg/yr) and the flux from the stratosphere (550 Tg/yr). In the case of carbon monoxide, surface emissions (1400 Tg/yr) are approximately 50% larger than in situ production by hydrocarbon oxidation (900 Tg/yr).

Variations in the predicted spatial distribution of atmospheric nitrogen deposition and their impact on carbon uptake by terrestrial ecosystems

Widespread mobilization of nitrogen into the atmosphere from industry, agriculture, and biomass burning and its subsequent deposition have the potential to alleviate nitrogen limitation of productivity in terrestrial ecosystems, and may contribute to enhanced terrestrial carbon uptake. To evaluate the importance of the spatial distribution of nitrogen deposition for carbon uptake and to better quantify its magnitude and uncertainty NO y -N deposition fields from five different three-dimensional chemical models, GCTM, GRANTOUR, IMAGES, MOGUNTIA, and ECHAM were used to drive NDEP, a perturbation model of terrestrial carbon uptake. Differences in atmospheric sources of NO x -N, transport, resolution, and representation of chemistry, contribute to the distinct spatial patterns of nitrogen deposition on the global land surface; these differences lead to distinct patterns of carbon uptake that vary between 0.7 and 1.3 Gt C yr -1 globally. Less than 10% of the nitrogen was deposited on forests which were most able to respond with increased carbon storage because of the wide C:N ratio of wood as well as its long lifetime. Addition of NH x -N to NO y -N deposition, increased global terrestrial carbon storage to between 1.5 and 2.0 Gt C yr -1 , while the missing terrestrial sink is quite similar in magnitude. Thus global air pollution appears to be an important influence on the global carbon cycle. If N fertilization of the terrestrial biosphere accounts for the missing C sink or a substantial portion of it, we would expect significant reductions in its magnitude over the next century as terrestrial ecosystems become N saturated and O 3 pollution expands.

MOZART, a global chemical transport model for ozone and related chemical tracers: 2. Model results and evaluation

In this second of two companion papers, we present results from a new global three-dimensional chemical transport model, called MOZART (model for ozone and related chemical tracers). MOZART is developed in the framework of the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM) and includes a detailed representation of tropospheric chemistry. The model provides the distribution of 56 chemical species at a spatial resolution of 2.8° in both latitude and longitude, with 25 levels in the vertical (from the surface to level of 3 mbar) and a time step of 20 min. The meteorological information is supplied from a 2-year run of the NCAR Community Climate Model. The simulated distributions of ozone (O 3 ) and its precursors are evaluated by comparison with observational data. The distribution of methane, nonmethane hydrocarbons (NMHCs), and CO are generally well simulated by the model. The model evaluation in the tropics stresses the need for a better representation of biomass burning emissions in order to evaluate the budget of carbon monoxide, nitrogen species, and ozone with more accuracy in these regions. MOZART reproduces the NO observations in most parts of the troposphere. Nitric acid, however, is overestimated over the Pacific by up to a factor of 10 and over continental regions by a factor of 2-3. Discrepancies are also found in the simulation of PAN in the upper troposphere and in biomass burning regions. These results highlight shortcomings in our understanding of the nitrogen budget in the troposphere. The seasonal cycle of ozone in the troposphere is generally well reproduced by the model in comparison with ozone soundings. MOZART tends, however, to underestimate O 3 at higher latitudes, and specifically above 300 mbar. The global photochemical production and destruction of ozone in the troposphere are 3018 Tg/yr and 2511 Tg/yr, respectively (net ozone production of 507 Tg/yr). The stratospheric influx of O 3 is estimated to be 391 Tg/yr and the surface dry deposition 898 Tg/yr. The calculated global lifetime of methane is 9.9 years in the annual average.

A three-dimensional study of the tropospheric sulfur cycle

The global tropospheric distributions of seven important sulfur species were simulated with a global three-dimensional chemistry-transport model (IMAGES). Surface emission and deposition velocity maps were established for use as lower boundary conditions in the model. While anthropogenic SO2 emissions are by far the largest sulfur source in the northern midlatitudes, reduced sulfur compounds, notably dimethyl sulfide (DMS) predominate over most remote areas. Simulations were performed for the present-day (∼ 1985) atmosphere. The calculated distributions are compared with available observations. The model results are found to be generally within a factor of (at most) 2–3 of long-term observations. Comparison with campaign measurements is more difficult, mostly due to the strong dependence of sulfur species concentrations on local meteorological conditions. The results, however, indicate the need for future model refinements, especially with respect to biogenic emission estimates and parameterization of cloud processes. A sensitivity study is presented to discuss the uncertainties of the results related to several parameters (the decoupling of wet scavenging and convective transport for soluble species, volcanoes emission and deposition velocities). Results are also discussed in terms of global budgets and related variables and processes. Around 125 Tg S/yr of non-sea-salt (nss) sulfur compounds (DMS, CS2, H2S, COS, and SO2) are injected into the atmosphere. The balance is mainly maintained by nss-sulfates wet and dry deposition, and by SO2 dry deposition (94% of total sulfur deposition). It is found that DMS oxidation represents the main contribution to SO2 chemical production (80% of the chemical sources), and that the major sink of SO2 is provided by in-cloud oxidation (90% of the chemical sinks), under the assumption that all SO2 incorporated into clouds is oxidized. The calculated annual wet deposition of sulfates reaches 3 g S m−2 yr−1 over Europe and North America, while it is usually lower than 0.5 g S m−2 yr−1 in remote parts of the world. Estimations for the global lifetimes are 0.9 day for DMS, 4 days for CS2, 2.2 days for H2S, 0.6 day for SO2, 0.18 day for DMSO, 6.1 days for MSA, and 4.7 days for nss-sulfates.

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.

Data composites of airborne observations of tropospheric ozone and its precursors

Tropospheric data from a number of aircraft campaigns have been gridded onto global maps, forming “data composites” of chemical species important in ozone photochemistry. Although these are not climatologies in the sense of a long temporal average, these data summaries are useful for providing a picture of the global distributions of these species and are a start to creating observations-based climatologies. Using aircraft measurements from a number of campaigns, we have averaged observations of O3, CO, NO, NOx, HNO3, PAN, H2O2, CH3OOH, HCHO, CH3COCH3, C2H6, and C3H8 onto a 5° latitude by 5° longitude horizontal grid with a 1-km vertical resolution. These maps provide information about the distributions at various altitudes, but also clearly show that direct observations of the global troposphere are still very limited. A set of regions with 10°–20° horizontal extent has also been chosen wherein there is sufficient data to study vertical profiles. These profiles are particularly valuable for comparison with model results, especially when a full suite of chemical species can be compared simultaneously. The O3 and NO climatologies generated from measurements obtained during commercial aircraft flights associated writh the MOZAIC and NOXAR programs are incorporated with the data composites at 10–11 km. As an example of the utility of these data composites, observations are compared to results from two global chemical transport models, MOZART and IMAGES, to help identify incorrect emission sources, incorrect strength of convection, and missing chemistry in the models. These comparisons suggest that in MOZART the NOx biomass burning emissions may be too low and convection too weak and that the transport of ozone from the stratosphere in IMAGES is too great. The ozone profiles from the data composites are compared with ozonesonde climatologies and show that in some cases the aircraft data agree with the long-term averages, but in others, such as in the western Pacific during PEM-Tropics-A, agreement is lacking. Finally, the data composites provide temporal and spatial information, which can help identify the locations and seasons where new measurements would be most valuable. All of the data composites presented here are available via the Internet (http://aoss.engin.umich.edu/SASSarchive/).