Claire Granier

Increase in tropospheric nitrogen dioxide over China observed from space.

Emissions from fossil fuel combustion and biomass burning reduce local air quality and affect global tropospheric chemistry. Nitrogen oxides are emitted by all combustion processes and play a key part in the photochemically induced catalytic production of ozone, which results in summer smog and has increased levels of tropospheric ozone globally. Release of nitrogen oxide also results in nitric acid deposition, and—at least locally—increases radiative forcing effects due to the absorption of downward propagating visible light. Nitrogen oxide concentrations in many industrialized countries are expected to decrease, but rapid economic development has the potential to increase significantly the emissions of nitrogen oxides in parts of Asia. Here we present the tropospheric column amounts of nitrogen dioxide retrieved from two satellite instruments GOME and SCIAMACHY over the years 1996–2004. We find substantial reductions in nitrogen dioxide concentrations over some areas of Europe and the USA, but a highly significant increase of about 50 per cent—with an accelerating trend in annual growth rate—over the industrial areas of China, more than recent bottom-up inventories suggest.

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.

Atmospheric chemistry of small organic peroxy radicals

Global atmospheric models play a key role in international assessments of the human impact on global climate and air pollution. To increase the accuracy and facilitate comparison of results from such models, it is essential they contain up-to-date chemical mechanisms. To this end, we present an evaluation of the atmospheric chemistry of the four most abundant organic peroxy radicals: CH3O2, C2H5O2, CH3C(O)O2, and CH3C(O)CH2O2. The literature data for the atmospheric reactions of these radicals are evaluated. In addition, the ultraviolet absorption cross sections for the above radicals and for HO2 have been evaluated. The absorption spectra were fitted to an analytical formula, which enabled published spectra to be screened objectively. Published kinetic and product data were reinterpreted, or in some case reanalyzed, using the new cross sections, leading to a self-consistent set of kinetic, mechanistic, and spectroscopic data. Product studies were also evaluated. A set of peroxy radical reaction rate coefficients and products are recommended for use in atmospheric modeling. A three-dimensional global chemical transport model (the Intermediate Model for the Global Evolution of Species, IMAGES) was run using both previously recommended rate coefficients and the current set to highlight the sensitivity of key atmospheric trace species to the peroxy radical chemistry used in the model.

Global Wildland Fire Emission Model (GWEM): Evaluating the use of global area burnt satellite data

[1] The new Global Wildland Fire Emission Model (GWEM) has been developed on the basis of data from the European Space Agency’s monthly Global Burnt Scar satellite product (GLOBSCAR) and results from the Lund-Potsdam-Jena Dynamic Global Vegetation Model (LPJ-DGVM). GWEM computes monthly emissions of more than 40 chemical compounds and aerosols from forest and savanna fires. This study focuses on an evaluation of the GLOBSCAR data set. The GWEM version presented here makes use of the Moderate-Resolution Imaging Spectroradiometer (MODIS) land cover map. Emission totals for the year 2000 are 1741 Tg C, 5716 Tg CO2, 271 Tg CO, 12.52 Tg CH4, 9.09 Tg C (as nonmethane hydrocarbons), 8.08 Tg NOx (as NO), 24.30 Tg PM2.5, 15.80 Tg OC, and 1.84 Tg black carbon. These emissions are lower than other estimates found in literature. An evaluation assesses the uncertainties of the individual input data. The GLOBSCAR product yields reasonable estimates of burnt area for large wildland fires in most parts of the globe but experiences problems in some regions where small fires dominate. The seasonality derived from GLOBSCAR differs from other satellite products detecting active fires owing to the different algorithms applied. Application of the presented GWEM results in global chemistry transport modeling will require additional treatment of small deforestation fires in the tropical rain forest regions and small savanna fires, mainly in subequatorial Africa. Further improvements are expected from a more detailed description of the carbon pools and the inclusion of anthropogenic disturbances in the LPJ model. INDEX TERMS: 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry; 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 1615 Global Change: Biogeochemical processes (4805); KEYWORDS: vegetation fire emissions, global area burnt satellite products, tropospheric chemistry

Assessing future nitrogen deposition and carbon cycle feedback using a multimodel approach: Analysis of nitrogen deposition

n this study, we present the results of nitrogen deposition on land from a set of 29 simulations from six different tropospheric chemistry models pertaining to present-day and 2100 conditions. Nitrogen deposition refers here to the deposition (wet and dry) of all nitrogen-containing gas phase chemical species resulting from NOx (NO + NO2) emissions. We show that under the assumed IPCC SRES A2 scenario the global annual average nitrogen deposition over land is expected to increase by a factor of ∼2.5, mostly because of the increase in nitrogen emissions. This will significantly expand the areas with annual average deposition exceeding 1 gN/m2/year. Using the results from all models, we have documented the strong linear relationship between models on the fraction of the nitrogen emissions that is deposited, regardless of the emissions (present day or 2100). On average, approximately 70% of the emitted nitrogen is deposited over the landmasses. For present-day conditions the results from this study suggest that the deposition over land ranges between 25 and 40 Tg(N)/year. By 2100, under the A2 scenario, the deposition over the continents is expected to range between 60 and 100 Tg(N)/year. Over forests the deposition is expected to increase from 10 Tg(N)/year to 20 Tg(N)/year. In 2100 the nitrogen deposition changes from changes in the climate account for much less than the changes from increased nitrogen emissions.

Mount Pinatubo Aerosols, Chlorofluorocarbons, and Ozone Depletion

The injection into the stratosphere of large quantities of sulfur during the June 1991 eruption of Mount Pinatubo (Philippines) and the subsequent formation of sulfate aerosol particles have generated a number of perturbations in the atmosphere with potential effects on the Earths climate. Changes in the solar and infrared radiation budget caused by the eruption should produce a cooling of the troposphere and a warming of the lower stratosphere. These changes could affect atmospheric circulation. In addition, heterogeneous chemical reactions on the surface of sulfate aerosol particles render the ozone molecules more vulnerable to atmospheric chlorine and hence to man-made chlorofluorocarbons.

The importance of atmospheric chemistry in the calculation of radiative forcing on the climate system

An interactive two-dimensional model of the troposphere, stratosphere, and mesosphere, in which dynamics, radiation, and chemistry are treated interactively, is used to investigate the anthropogenic changes in the steady state chemical composition of the atmosphere since preindustrial times and to assess the associated changes in radiative forcing on climate. The perturbations in the atmospheric oxidation capacity due to anthropogenic emissions of source gases are found to be significant. In the troposphere, an ozone increase of 80–120% at northern midlatitudes and a global decrease of 10–20% in the OH concentration since the preindustrial period are calculated. In the polar lower stratosphere of the southern hemisphere, an ozone depletion since preindustrial times reaching more than 60% during spring is calculated as a result of rapid catalytical destruction of ozone by chlorine radicals in the presence of polar stratospheric clouds. Particular attention is given to the induced changes in radiative forcing. These results stress the potentially important role of chemical feedbacks on climate and indicate that the direct forcing associated with increasing concentrations of greenhouse gases is enhanced by about 30% when these feedbacks are taken into account. On a global average basis, the greenhouse effect of tropospheric ozone represents approximately 17% of the total radiative perturbation. This forcing is characterized by a strong latitudinal dependence, peaking at midlatitudes in the northern hemisphere. The importance of indirect climate forcings by stratospheric ozone (including local cooling of the stratosphere) is confirmed. It is found that the net (solar + infrared) indirect effect of stratospheric ozone changes is to increase the chlorofluorocarbon direct radiative forcing. On the other hand, the change in the longwave forcing associated with water vapor increase in the stratosphere appears to play a minor role.

Impact of heterogeneous chemistry on model predictions of ozone changes

A two-dimensional chemical/transport model of the middle atmosphere is used to assess the importance of chemical heterogeneous processes both in the polar regions (on polar stratospheric clouds (PSCs)) and at other latitudes (on sulfate aerosols). When conversion on type I and type II PSCs of N2O5 into HNO3 and of ClONO2 into reactive forms of chlorine is taken into account, enhanced ClO concentrations lead to the formation of a springtime “ozone hole” over the Antarctic continent. No such major reduction in the ozone column is found in the Arctic region. When conversion of nitrogen and chlorine compounds is assumed to occur on sulfate particles present in the lower stratosphere at all latitudes, significant perturbations in the chemistry are also found. For background aerosol conditions, the concentration of nitric acid is enhanced and agrees with observed values, while that of nitrogen oxides is reduced and agrees less than if heterogeneous processes are ignored in the model calculations. The concentration of the OH radical is significantly increased. Ozone number density appears to become larger between 16 and 30 km but smaller below 16 km, especially at high latitudes. The ozone column is only slightly modified, except at high latitudes where it is substantially reduced if the ClONO2 conversion into reactive chlorine is taken into account. After a large volcanic eruption such as that of Mount Pinatubo in June 1991, these changes are further exacerbated. The ozone budget in the lower stratosphere becomes less affected by nitrogen oxides but is largely controlled by the ClOx and HOx chemistries. A substantial decrease in the ozone column is predicted as a result of the Pinatubo eruption, mostly in winter at midlatitudes and high latitudes. The predicted values depend on the assumption made for the evolution of the aerosol surface area density but is expected to be of the order of 10% at mid-latitudes in February and March 1992. An enhanced mixing ratio of NO2 of the order of 100 parts per trillion by volume and a reduced mixing ratio of NO2 below 25 km should be detected outside the polar vortex, especially in air masses with high levels of volcanic aerosols.

Past and future changes in global tropospheric ozone: Impact on radiative forcing

Calculations by a global three-dimensional chemical transport model of the atmosphere suggest that increased surface emissions of chemical compounds caused by industrial activities at mid-latitudes in the northern hemisphere and by biomass burning in the tropics since the middle of the 19th century have produced an increase in the abundance of tropospheric ozone along with a reduction in the oxidizing capacity of the atmosphere (globally averaged OH concentration reduced by 17% and methane lifetime enhanced by 1.5 years). These perturbations in tropospheric ozone result in a change in annually averaged radiative forcing of 0.37 W m -2 (0.62 W m -2 in the northern hemisphere during the summer months). Future changes (1990-2050) in tropospheric ozone associated with population increase and economic development (primarily in developing countries) are expected to be largest in the tropics, specifically in South and Southeast Asia. Further changes in the oxidizing capacity of the atmosphere could be small if the abundance of tropospheric water vapor increases as a result of anticipated climate change.

Atmospheric impact of NOx emissions by subsonic aircraft: A three-dimensional model study

Three-dimensional model calculations suggest that the worlds fleet of subsonic aircraft has enhanced the abundance of nitrogen oxides in the upper troposphere by up to 20–35% and has produced a significant increase in the ozone concentration in this region of the atmosphere (4% in summer and 1% in winter). In year 2050, on the basis of current scenarios for growth in aviation, the concentration of NOx at 10 km could increase by 30–60% at midlatitudes, and the concentration of ozone could be enhanced by 7% and 2% in summer and winter, respectively (relative to a situation without aircraft effects). The perturbation is not limited to the flight corridors but affects the entire northern hemisphere. The magnitude (and even the sign) of the ozone change depends on the level of background atmospheric NOx and hence on NOx sources (lightning, intrusion from the stratosphere, and convective transport from the polluted boundary layer) and sinks which are poorly quantified in this region of the atmosphere. On the basis of our model estimates, 20% of the NOx found at 10 km (midlatitudes) is produced by aircraft engines, 25% originates from the surface (combustion and soils), and approximately 50% is produced by lightning. For a lightning source enhanced in the model by a factor of 2, the increase in NOx and ozone at 10 km due to aircraft emissions, is reduced by a factor of 2. The magnitude of aircraft perturbations in NOx is considerably smaller than the uncertainties in other NOx sources.