M. G. Lawrence

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.

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.

Atmospheric oxidation capacity sustained by a tropical forest

Terrestrial vegetation, especially tropical rain forest, releases vast quantities of volatile organic compounds (VOCs) to the atmosphere, which are removed by oxidation reactions and deposition of reaction products. The oxidation is mainly initiated by hydroxyl radicals (OH), primarily formed through the photodissociation of ozone. Previously it was thought that, in unpolluted air, biogenic VOCs deplete OH and reduce the atmospheric oxidation capacity. Conversely, in polluted air VOC oxidation leads to noxious oxidant build-up by the catalytic action of nitrogen oxides (NOx = NO + NO2). Here we report aircraft measurements of atmospheric trace gases performed over the pristine Amazon forest. Our data reveal unexpectedly high OH concentrations. We propose that natural VOC oxidation, notably of isoprene, recycles OH efficiently in low-NOx air through reactions of organic peroxy radicals. Computations with an atmospheric chemistry model and the results of laboratory experiments suggest that an OH recycling efficiency of 40–80 per cent in isoprene oxidation may be able to explain the high OH levels we observed in the field. Although further laboratory studies are necessary to explore the chemical mechanism responsible for OH recycling in more detail, our results demonstrate that the biosphere maintains a remarkable balance with the atmospheric environment.

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.

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.

Megacity emissions and lifetimes of nitrogen oxides probed from space

Analysis of downwind plume evolution using satellite observations can be used for air pollution estimates. Megacities are immense sources of air pollutants, with large impacts on air quality and climate. However, emission inventories in many of them still are highly uncertain, particularly in developing countries. Satellite observations allow top-down estimates of emissions to be made for nitrogen oxides (NOx = NO + NO2), but require poorly quantified a priori information on the NOx lifetime. We present a method for the simultaneous determination of megacity NOx emissions and lifetimes from satellite measurements by analyzing the downwind patterns of NO2 separately for different wind conditions. Daytime lifetimes are ~4 hours at low and mid-latitudes, but ~8 hours in wintertime for Moscow. The derived NOx emissions are generally in good agreement with existing emission inventories, but are higher by a factor of 3 for the Saudi Arabian capital Riyadh.

Global reactive nitrogen deposition from lightning NOx

We present results of the deposition of nitrogen compounds formed from lightning (LNO x ) using the global chemical transport Model of Atmospheric Transport and Chemistry?Max Planck Institute for Chemistry version. The model indicates an approximately equal deposition of LNO x in both terrestrial and oceanic ecosystems, primarily in the tropics and midlatitudes open ocean, despite much higher intensities of lightning flashes above landmasses. The highest values of deposition are due to wet convective deposition, with highest values concentrated in the tropical continents. Nonconvective wet deposition, associated with large-scale weather patterns, occurs over large areas of the ocean amid lower values per square meter, manifesting the long-range transport of NO y , including long-lived species such as HNO3 at high altitudes and PAN. Dry deposition is concentrated primarily above landmasses, yet oceanic deposition over wide areas is still observed. Combined together, the total LNO x deposition exhibits maximal influx values over land, whereas oceanic deposition over wider areas renders the integrated deposition over both ecosystems almost identical. Peaks of terrestrial deposition values (located in Africa, South America, and Asia) show seasonal variability by meridionally penetrating the northern or southern midlatitude following the corresponding summer hemisphere, in accordance with the migration of LNO x production sites. On land, wet and dry deposition rates are more or less equal with a small bias toward wet deposition, whereas above the ocean, wet deposition is markedly higher because of a small water uptake efficiency and relatively small surface roughness. Further work of modeling additional species and obtaining more information on different compounds is required

Ice nuclei in marine air : bioparticles or dust?

Ice nuclei (IN) concentrations impact clouds, but their sources and distribution in the atmosphere are still not well known. Particularly little attention has been paid to ice nuclei IN sources in marine environments, although anecdotal evidence suggests that IN populations in remote marine regions may be dominated by a biological particles associated with sea spray. It is also known that certain plankton species can act as IN at comparatively high temperatures, while others do not. In this exploratory model study, we aim to bring attention to this long-neglected topic and identify promising target regions for future field campaigns. We assess the possible global distribution of marine biological ice nuclei using a combination of historical observations, satellite data and model output. By comparing simulated marine biological IN distributions and dust IN distributions, we predict strong regional differences in the importance of marine biological IN relative to dust IN. In particular, our analysis suggests that marine biological IN may play a dominant role in determining IN concentrations over the Southern Ocean and possibly over regions of the Arctic, while dust IN likely dominate in continental outflow regions. Marine biological IN may be an important aspect to consider in proposals for marine cloud brightening by artificial sea spray production. Devices for artificial sea-spray production could potentially be designed to include high concentrations of biological particulate matter in the generated spray, which could dramatically increase the concentrations of IN. To our knowledge, this potential has previously not been considered in such proposals. A small number of previous studies indicate that Arctic clouds in particular may be highly sensitive to IN concentrations, with impacts on radiative properties, precipitation and cloud lifetime, but further study will be needed to evaluate the sensitivity of marine clouds to a possible biological IN source. We believe these results will help motivate further study of marine biological IN. Important routes for further study include laboratory measurements of the ice nucleating activity of plankton species and other marine particulate matter, and field studies using modern instrumentation (such as a CFDC combined with mass spectrometry) to more accurately count and analyze IN in the remote marine boundary layer.

The detection of solar proton produced 14 CO

Major solar eruptions (coronal mass ejections) are accompanied by massive ejections of protons. When these charged particles head for the Earth through the interplanetary magnetic field with high flux and energy, a solar proton event (SPE) is recorded. Strong SPEs, in which energetic protons penetrate the atmosphere in large numbers are rare, but do have chemical effects (Crutzen, 1975; Jackman et al., 1990, 2001). They also have nucleonic effects by which they can almost instantaneously increase the atmospheric production of radio-nuclides, including14C (radiocarbon), but this has never been demonstrated. We show, using satellite observations and modeling, that the 2nd most intensive set of SPEs on record, that of August–December 1989, must have caused detectable increases in atmospheric 14CO. This is confirmed by a sequence of peaks in the Baring Head (NZ) time series of14CO observations (Brenninkmeijer, 1993), probably providing a unique indication of production of 14C by solar protons, thus demonstrating the use of SPE 14CO as an atmospheric tracer.

Global Photochemical Model Evaluation using GOME Tropospheric Column Data

The focus of our TROPOSAT-related work has been mainly on comparisons of tropospheric NO2 with output from our global chemistry-transport model MATCH-MPIC. Global monthly NO2 distributions from MATCH-MPIC were compared with GOME NO2 columns in von Kuhlmann (2001). A closer focus on NO2 over Asia has been taken by a joint PhD student with the University of Bremen (T. Kunhikrishnan); the first part of this study has been aimed at improving our understanding of NOX in the troposphere over Asia (especially India), and at developing an improved estimate of the NOX emissions rate from this region, through the use of sensitivity runs with MATCH-MPIC along with GOME tropospheric NO2 columns. We first verified that the retrieval assumptions from GOME (stratospheric zonal symmetry and negligible troposphere NO2 in the Pacific reference sector) hold well for MATCH-MPIC. We then showed that the model simulates the mean NO2 amounts in the region fairly well, and that the NOX lifetime of about 15 hours computed by the model, agrees well with the lifetime derived from the decay of NO2 off the west coast of India. Taken together, these imply that the current total magnitude of NOX emissions in MATCH-MPIC is reasonable. While 60–70 % of the NOX in the lower troposphere over India comes from Indian emissions, in the upper troposphere the NOX comes mainly from outside the region during the winter months and from convective transport and lightning during the summer monsoon.