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Present and future impact of aircraft, road traffic and shipping emissions on global tropospheric ozone

In this study, the LMDz-INCA climate-chemistry model and up-to-date global emission inventories are used to investigate the "present" (2000) and future (2050) impacts of transport emissions (road traffic, shipping and aircraft) on global tropospheric ozone. For the first time, both impacts of emissions and climate changes on transport-induced ozone are investigated. The 2000 transport emissions are shown to mainly affect ozone in the Northern Hemisphere, with a maximum increase of the tropospheric column of up to 5 DU, from the South-eastern US to Central Europe. The impact is dominated by road traffic in the middle and upper troposphere, North of 40° S, and by shipping in the northern lower troposphere, over oceanic regions. A strong reduction of road emissions and a moderate (B1 scenario) to high (A1B scenario) increase of the ship and aircraft emissions are projected by the year 2050. As a consequence, LMDz-INCA simulations predict a drastic decrease in the impact of road emissions, whereas aviation would become the major transport perturbation on tropospheric ozone, even in the case of a very optimistic aircraft mitigation scenario. The A1B emission scenario leads to an increase of the impact of transport on zonal mean ozone concentrations in 2050 by up to +30% and +50%, in the Northern and Southern Hemispheres, respectively. Despite a similar total amount of global NOx emissions by the various transport sectors compared to 2000, the overall impact on the tropospheric ozone column is increased everywhere in 2050, due to a sectoral shift in the emissions of the respective transport modes. On the opposite, the B1 mitigation scenario leads to a significant reduction (by roughly 50%) of the ozone perturbation throughout the troposphere compared to 2000. Considering climate change, and according to scenario A1B, a decrease of the O3 tropospheric burden is simulated by 2050 due to climate change (−1.2%), whereas an increase of ozone of up to 2% is calculated in the upper troposphere in the inter-tropical zone, due to enhanced lightning activity. A global impact of similar magnitude is simulated for the transport-induced ozone burden perturbation (−1.6%). As a result, the future increase in global ozone due to changes in anthropogenic emissions is lowered by 12% and by 4%, for the background and the transport-induced ozone, respectively. However, positive and negative climate effects are obtained on ozone, depending on the season, region and altitude, with an increase of the transport-induced ozone perturbation (+0.4 DU) in the already most affected area of Northern Hemisphere.
Enhancements of gravity wave amplitudes at midlatitudes during sudden stratospheric warmings in 2008

Two minor and one major stratospheric warming happened in January and February 2008 when the polar vortex was shifted toward midlatitudes. The analysis of temperature profiles from radiosondes in Payerne (Switzerland) during this period reveals an enhancement of gravity wave amplitudes between 25 and 30 km altitude especially during the two minor warmings around 20 January and 1 February. Increases of gravity wave amplitudes in the mid-stratosphere are associated with a strong tropopause jet and the presence of the polar vortex edge over Switzerland.
Light-absorbing impurities in Arctic snow

Absorption of radiation by ice is extremely weak at visible and near-ultraviolet wavelengths, so small amounts of light-absorbing impurities in snow can dominate the absorption of solar radiation at these wavelengths, reducing the albedo relative to that of pure snow, contributing to the surface energy budget and leading to earlier snowmelt. In this study Arctic snow is surveyed for its content of light-absorbing impurities, expanding and updating the 1983–1984 survey of Clarke and Noone. Samples were collected in Alaska, Canada, Greenland, Svalbard, Norway, Russia, and the Arctic Ocean during 1998 and 2005–2009, on tundra, glaciers, ice caps, sea ice, frozen lakes, and in boreal forests. Snow was collected mostly in spring, when the entire winter snowpack is accessible for sampling. Sampling was carried out in summer on the Greenland Ice Sheet and on the Arctic Ocean, of melting glacier snow and sea ice as well as cold snow. About 1200 snow samples have been analyzed for this study. The snow is melted and filtered; the filters are analyzed in a specially designed spectrophotometer system to infer the concentration of black carbon (BC), the fraction of absorption due to non-BC light-absorbing constituents and the absorption Ångstrom exponent of all particles. This is done using BC calibration standards having a mass absorption efficiency of 6.0 m2 g−1 at 550 nm and by making an assumption that the absorption Angstrom exponent for BC is 1.0 and for non-BC light-absorbing aerosol is 5.0. The reduction of snow albedo is primarily due to BC, but other impurities, principally brown (organic) carbon, are typically responsible for ~40% of the visible and ultraviolet absorption. The meltwater from selected snow samples was saved for chemical analysis to identify sources of the impurities. Median BC amounts in surface snow are as follows (nanograms of carbon per gram of snow): Greenland 3, Arctic Ocean snow 7, melting sea ice 8, Arctic Canada 8, subarctic Canada 14, Svalbard 13, Northern Norway 21, western Arctic Russia 27, northeastern Siberia 34. Concentrations are more variable in the European Arctic than in Arctic Canada or the Arctic Ocean, probably because of the proximity to BC sources. Individual samples of falling snow were collected on Svalbard, documenting the springtime decline of BC from March through May. Absorption Ångstrom exponents are 1.5–1.7 in Norway, Svalbard, and western Russia, 2.1–2.3 elsewhere in the Arctic, and 2.5 in Greenland. Correspondingly, the estimated contribution to absorption by non-BC constituents in these regions is ~25%, 40%, and 50% respectively. It has been hypothesized that when the snow surface layer melts some of the BC is left at the top of the snowpack rather than being carried away in meltwater. This process was observed in a few locations and would cause a positive feedback on snowmelt. The BC content of the Arctic atmosphere has declined markedly since 1989, according to the continuous measurements of near-surface air at Alert (Canada), Barrow (Alaska), and Ny-Ålesund (Svalbard). Correspondingly, the new BC concentrations for Arctic snow are somewhat lower than those reported by Clarke and Noone for 1983–1984, but because of methodological differences it is not clear that the differences are significant. Nevertheless, the BC content of Arctic snow appears to be no higher now than in 1984, so it is doubtful that BC in Arctic snow has contributed to the rapid decline of Arctic sea ice in recent years.
Reconciliation of measurements of hygroscopic growth and critical supersaturation of aerosol particles in central Germany

Aerosol physical, chemical and hygroscopic properties were measured in a range of airmasses during COPS (Convective and Orographically-induced Precipitation Study) ground-based in June and July of 2007 at the Hornisgrinde mountain site in the Black Forest, Southwest Germany. Non-refractory aerosol composition was measured with an Aerosol Mass Spectrometer, simultaneous to hygroscopic growth factors at 86% relative humidity and CCN activity measurements for particles of dry (<20%) diameters 27 to 217 nm, with particle water uptake exhibiting substantial variability with time and with particle size. Variability in the measurements of hygroscopic growth factor and critical supersaturation for particles of similar sizes indicates significant compositional impact on particle water affinity. Critical supersaturation prediction using a single parameter hygroscopicity approximation derived from measured HTDMA mean growth factors deviate, beyond measurement uncertainties, from critical supersaturations derived from CCN measurements. These led to differences averaging around 35% in the number of CCN (NCCN) for the most reliable measurements depending on averaging methodology, often very much larger for individual time periods. This indicates aspects of water uptake behaviour unresolved in this experiment by the single parameter representation which, depending on its origin, may have important consequences on its generalised use.
Global fire emissions and the contribution of deforestation, savanna, forest, agricultural, and peat fires (1997–2009)

New burned area datasets and top-down constraints from atmospheric concentration measurements of pyrogenic gases have decreased the large uncertainty in fire emissions estimates. However, significant gaps remain in our understanding of the contribution of deforestation, savanna, forest, agricultural waste, and peat fires to total global fire emissions. Here we used a revised version of the Carnegie-Ames-Stanford-Approach (CASA) biogeochemical model and improved satellite-derived estimates of area burned, fire activity, and plant productivity to calculate fire emissions for the 1997–2009 period on a 0.5° spatial resolution with a monthly time step. For November 2000 onwards, estimates were based on burned area, active fire detections, and plant productivity from the MODerate resolution Imaging Spectroradiometer (MODIS) sensor. For the partitioning we focused on the MODIS era. We used maps of burned area derived from the Tropical Rainfall Measuring Mission (TRMM) Visible and Infrared Scanner (VIRS) and Along-Track Scanning Radiometer (ATSR) active fire data prior to MODIS (1997–2000) and estimates of plant productivity derived from Advanced Very High Resolution Radiometer (AVHRR) observations during the same period. Average global fire carbon emissions according to this version 3 of the Global Fire Emissions Database (GFED3) were 2.0 Pg C year−1 with significant interannual variability during 1997–2001 (2.8 Pg C year−1 in 1998 and 1.6 Pg C year−1 in 2001). Globally, emissions during 2002–2007 were relatively constant (around 2.1 Pg C year−1) before declining in 2008 (1.7 Pg C year−1) and 2009 (1.5 Pg C year−1) partly due to lower deforestation fire emissions in South America and tropical Asia. On a regional basis, emissions were highly variable during 2002–2007 (e.g., boreal Asia, South America, and Indonesia), but these regional differences canceled out at a global level. During the MODIS era (2001–2009), most carbon emissions were from fires in grasslands and savannas (44%) with smaller contributions from tropical deforestation and degradation fires (20%), woodland fires (mostly confined to the tropics, 16%), forest fires (mostly in the extratropics, 15%), agricultural waste burning (3%), and tropical peat fires (3%). The contribution from agricultural waste fires was likely a lower bound because our approach for measuring burned area could not detect all of these relatively small fires. Total carbon emissions were on average 13% lower than in our previous (GFED2) work. For reduced trace gases such as CO and CH4, deforestation, degradation, and peat fires were more important contributors because of higher emissions of reduced trace gases per unit carbon combusted compared to savanna fires. Carbon emissions from tropical deforestation, degradation, and peatland fires were on average 0.5 Pg C year−1. The carbon emissions from these fires may not be balanced by regrowth following fire. Our results provide the first global assessment of the contribution of different sources to total global fire emissions for the past decade, and supply the community with an improved 13-year fire emissions time series.
Stratocumulus cloud thickening beneath layers of absorbing smoke aerosol

Marine stratocumulus cloud properties, and the free-tropospheric environment above them, are examined in NASA A-Train satellite data for cases where smoke from seasonal burning of the West African savannah overlay the persistent southeast Atlantic stratocumulus cloud deck. CALIPSO space-borne lidar observations show that features identified as layers of aerosol occur predominantly between 2 km and 4 km. Layers identified as cloud features occur predominantly below 1.5 km altitude and beneath the layer of elevated smoke aerosol. The diurnal mean shortwave heating rates attributable to the absorption of solar energy in the aerosol layer is nearly 1.5 K d−1 for an aerosol optical thickness value of 1, and increases to 1.8 K d−1 when the smoke resides above clouds owing to the additional component of upward solar radiation reflected by the cloud. As a consequence of this heating, the 700 hPa air temperature above the cloud deck is warmer by approximately 1 K on average for cases where smoke is present above the cloud compared to cases without smoke above cloud. The warmer conditions in the free-troposphere above the cloud during smoke events coincide with cloud liquid water path values that are greater by 20 g m−2 and cloud tops that are lower for overcast conditions compared to periods with low amounts of smoke. The observed thickening and subsidence of the cloud layer are consistent with published results of large-eddy simulations showing that solar absorption by smoke above stratocumulus clouds increases the buoyancy of free-tropospheric air above the temperature inversion capping the boundary layer. Increased buoyancy inhibits the entrainment of dry air through the cloud-top, thereby helping to preserve humidity and cloud cover in the boundary layer. The direct radiative effect of absorbing aerosols residing over a bright cloud deck is a positive radiative forcing (warming) at the top of the atmosphere. However, the greater liquid water path for cases of smoke overlaying cloud contributes an additional negative semi-direct radiative forcing (cooling) of climate in locations such as the southeast Atlantic Ocean owing to the enhanced albedo of the thicker cloud.
The vapor pressures and activities of dicarboxylic acids reconsidered: the impact of the physical state of the aerosol

We present vapor pressure data of the C2 to C5 dicarboxylic acids deduced from measured evaporation rates of single levitated particles as both, aqueous droplets and solid crystals. The data of aqueous solution particles over a wide concentration range allow us to directly calculate activities of the dicarboxylic acids and comparison of these activities with parameterizations reported in the literature. The data of the pure liquid state acids, i.e. the dicarboxylic acids in their supercooled melt state, exhibit no even-odd alternation in vapor pressure, while the acids in the solid form do. This observation is consistent with the known solubilities of the acids and our measured vapor pressures of the supercooled melt. Thus, the gas/particle partitioning of the different dicarboxylic acids in the atmosphere depends strongly on the physical state of the aerosol phase, the difference being largest for the even acids. Our results show also that, in general, measurements of vapor pressures of solid dicarboxylic acids may be compromised by the presence of polymorphic forms, crystalline structures with a high defect number, and/or solvent inclusions in the solid material, yielding a higher vapor pressure than the one of the thermodynamically stable crystalline form at the same temperature.
Analysis of SAGE II ozone of the middle and upper stratosphere for its response to a decadal-scale forcing

Stratospheric Aerosol and Gas Experiment (SAGE II) Version 6.2 ozone profiles are analyzed for their decadal-scale responses in the middle and upper stratosphere from September 1991 to August 2005. The profile data are averaged within twelve, 20°-wide latitude bins from 55° S to 55° N and at twelve altitudes from 27.5 to 55.0 km. The separate, 14-yr data time series are analyzed using multiple linear regression (MLR) models that include seasonal, 28 and 21-month, 11-yr sinusoid, and linear trend terms. Proxies are not used for the 28-mo (QBO-like), 11-yr solar uv-flux, or reactive chlorine terms. Instead, the present analysis focuses on the periodic 11-yr terms to see whether they are in-phase with that of a direct, uv-flux forcing or are dominated by some other decadal-scale influence. It is shown that they are in-phase over most of the latitude/altitude domain and that they have max minus min variations between 25° S and 25° N that peak near 4% between 30 and 40 km. Model simulations of the direct effects of uv-flux forcings agree with this finding. The shape of the 11-yr ozone response profile from SAGE II also agrees with that diagnosed for the stratosphere over the same time period from the HALOE data. Ozone in the middle stratosphere of the northern subtropics is perturbed during 1991−1992 following the eruption of Pinatubo, and there are pronounced decadal-scale variations in the ozone of the upper stratosphere for the northern middle latitudes presumably due to dynamical forcings. The 11-yr ozone responses of the southern hemisphere appear to be free of those extra influences. The associated linear trend terms from the SAGE II analyses are slightly negative (−2 to −4%/decade) between 35 and 45 km and nearly constant across latitude. This finding is consistent with the fact that ozone is estimated to have decreased by no more than 1.5% due to the increasing chlorine from mid-1992 to about 2000 but with little change thereafter. It is concluded that a satellite, solar occultation measurement provides both the signal sensitivity and the vertical resolution to record the stratospheric ozone response to the forcing from the solar uv-flux, as well as those due to any other long-term changes.
A minimum bulk microphysics

Cloud microphysics present extreme complexities, and even under bulk approaches, the formulation tends to be involved. A minimum microphysics is proposed, aimed at applications for geophysical fluid dynamics, by a maximum simplification of a standard bulk formulation. The proposed formulation is also independently derived by a simple phenomenological argument. The formulational structure of the bulk microphysics is also discussed. The autoconversion process formulation is discussed separately in a phenomenological manner, because a formal application of the bulk approach becomes involved. Four major possible formulations for autoconversion are identified. The proposed formulation is tested with a nonhydrostatic anelastic model under segmentally-constant approximation (NAM-SCA).
Cloud thermodynamic phase inferred from merged POLDER and MODIS data

The global spatial and diurnal distribution of cloud properties is a key issue for understanding the hydrological cycle, and critical for advancing efforts to improve numerical weather models and general circulation models. Satellite data provides the best way of gaining insight into global cloud properties. In particular, the determination of cloud thermodynamic phase is a critical first step in the process of inferring cloud optical and microphysical properties from satellite measurements. It is important that cloud phase be derived together with an estimate of the confidence of this determination, so that this information can be included with subsequent retrievals (optical thickness, effective particle radius, and ice/liquid water content). In this study, we combine three different and well documented approaches for inferring cloud phase into a single algorithm. The algorithm is applied to data obtained by the MODIS (MODerate resolution Imaging Spectroradiometer) and POLDER3 (Polarization and Directionality of the Earth Reflectance) instruments. It is shown that this synergistic algorithm can be used routinely to derive cloud phase along with an index that helps to discriminate ambiguous phase from confident phase cases. The resulting product provides a semi-continuous index ranging from confident liquid to confident ice instead of the usual discrete classification of liquid phase, ice phase, mixed phase (potential combination of ice and liquid particles), or simply unknown phase clouds. The index value provides simultaneously information on the phase and the associated confidence. This approach is expected to be useful for cloud assimilation and modeling efforts while providing more insight into the global cloud properties derived from satellite data.