Timothy J. Garrett

Increased Arctic cloud longwave emissivity associated with pollution from mid-latitudes

There is consensus among climate models that Arctic climate is particularly sensitive to anthropogenic greenhouse gases and that, over the next century, Arctic surface temperatures are projected to rise at a rate about twice the global mean. The response of Arctic surface temperatures to greenhouse gas thermal emission is modified by Northern Hemisphere synoptic meteorology and local radiative processes. Aerosols may play a contributing factor through changes to cloud radiative properties. Here we evaluate a previously suggested contribution of anthropogenic aerosols to cloud emission and surface temperatures in the Arctic. Using four years of ground-based aerosol and radiation measurements obtained near Barrow, Alaska, we show that, where thin water clouds and pollution are coincident, there is an increase in cloud longwave emissivity resulting from elevated haze levels. This results in an estimated surface warming under cloudy skies of between 3.3 and 5.2 W m-2 or 1 and 1.6 °C. Arctic climate is closely tied to cloud longwave emission, but feedback mechanisms in the system are complex and the actual climate response to the described sensitivity remains to be evaluated.

Emissions from Ships with respect to Their Effects on Clouds

Emissions of particles, gases, heat, and water vapor from ships are discussed with respect to their potential for changing the microstructure of marine stratiform clouds and producing the phenomenon known as ‘‘ship tracks.’’ Airborne measurements are used to derive emission factors of SO 2 and NO from diesel-powered and steam turbine-powered ships, burning low-grade marine fuel oil (MFO); they were ;15‐89 and ;2‐25 g kg21 of fuel burned, respectively. By contrast a steam turbine‐powered ship burning high-grade navy distillate fuel had an SO2 emission factor of ; 6gk g 21. Various types of ships, burning both MFO and navy distillate fuel, emitted from ;4 3 1015 to 2 3 1016 total particles per kilogram of fuel burned (;4 3 1015‐1.5 3 1016 particles per second). However, diesel-powered ships burning MFO emitted particles with a larger mode radius (;0.03‐0.05 mm) and larger maximum sizes than those powered by steam turbines burning navy distillate fuel (mode radius ;0.02 mm). Consequently, if the particles have similar chemical compositions, those emitted by diesel ships burning MFO will serve as cloud condensation nuclei (CCN) at lower supersaturations (and will therefore be more likely to produce ship tracks) than the particles emitted by steam turbine ships burning distillate fuel. Since steam turbine‐powered ships fueled by MFO emit particles with a mode radius similar to that of diesel-powered ships fueled by MFO, it appears that, for given ambient conditions, the type of fuel burned by a ship is more important than the type of ship engine in determining whether or not a ship will produce a ship track. However, more measurements are needed to test this hypothesis. The particles emitted from ships appear to be primarily organics, possibly combined with sulfuric acid produced by gas-to-particle conversion of SO 2. Comparison of model results with measurements in ship tracks suggests that the particles from ships contain only about 10% water-soluble materials. Measurements of the total particles entering marine stratiform clouds from diesel-powered ships fueled by MFO, and increases in droplet concentrations produced by these particles, show that only about 12% of the particles serve as CCN. The fluxes of heat and water vapor from ships are estimated to be ;2‐22 MW and;0.5‐1.5 kg s21, respectively. These emissions rarely produced measurable temperature perturbations, and never produced detectable perturbations in water vapor, in the plumes from ships. Nuclear-powered ships, which emit heat but negligible particles, do not produce ship tracks. Therefore, it is concluded that heat and water vapor emissions do not play a significant role in ship track formation and that particle emissions, particularly from those burning low-grade fuel oil, are responsible for ship track formation. Subsequent papers in this special issue discuss and test these hypotheses.

Drizzle Suppression in Ship Tracks

Abstract Although drizzle was a relatively infrequent occurrence during the Monterey Area Ship Track study, diverse measurements from several sources produced data signals consistent with a reduction in drizzle drops in stratus clouds affected by ship effluents. Concurrent increases in liquid water in the cloud droplet size range, due to redistribution from the drizzle mode, were not always observed, possibly because of the relatively small and often negligible amounts of water in the drizzle mode. Significant changes in cloud droplet size distribution, as well as reductions in drizzle flux and concentrations of drops >50-μm radius, were observed in ship tracks when drizzle was more uniformly present in the ambient cloud. Radiometric measurements showed that increased droplet concentrations in ship tracks, which resulted in reduced droplet sizes, can significantly alter the liquid water path. Radar observations indicated that the reduced reflectivities of ship tracks compared with ambient clouds may be du...

Nephelometer Measurements of the Asymmetry Parameter, Volume Extinction Coefficient, and Backscatter Ratio in Arctic Clouds

A new instrument, a cloud integrating nephelometer (CIN), was used on the University of Washington’s CV580 research aircraft to measure the asymmetry parameter (g), volume scattering coefficient, and backscatter ratio in clouds in situ and in the visible spectrum during the FIRE-ACE/SHEBA field project in the late spring and summer of 1998 in the Arctic. The principle behind the operation of the CIN is described, and error estimates for the measurements are calculated. The average value of g measured with the CIN in all the Arctic clouds was 0.824, which is slightly less than the value expected for the water-containing clouds that dominated the measurements. The average value of g measured in a glaciated cloud, containing mostly large bullet rosette ice crystals, was 0.737. This relatively small value of g is compared to modeled and indirect measurements of g described in the literature, some of which gave similar small values of g.

Evolution of a Florida Cirrus Anvil

Abstract This paper presents a detailed study of a single thunderstorm anvil cirrus cloud measured on 21 July 2002 near southern Florida during the Cirrus Regional Study of Tropical Anvils and Cirrus Layers–Florida Area Cirrus Experiment (CRYSTAL-FACE). NASA WB-57F and University of North Dakota Citation aircraft tracked the microphysical and radiative development of the anvil for 3 h. Measurements showed that the cloud mass that was advected downwind from the thunderstorm was separated vertically into two layers: a cirrus anvil with cloud-top temperatures of −45°C lay below a second, thin tropopause cirrus (TTC) layer with the same horizontal dimensions as the anvil and temperatures near −70°C. In both cloud layers, ice crystals smaller than 50 μm across dominated the size distributions and cloud radiative properties. In the anvil, ice crystals larger than 50 μm aggregated and precipitated while small ice crystals increasingly dominated the size distributions; as a consequence, measured ice water content...

Nitric Acid Uptake on Subtropical Cirrus Cloud Particles

The redistribution of HNO 3 via uptake and sedimentation by cirrus cloud particles is considered an important term in the upper tropospheric budget of reactive nitrogen. Numerous cirrus cloud encounters by the NASA WB-57F high-altitude research aircraft during the Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment (CRYSTAL-FACE) were accompanied by the observation of condensed-phase HNO 3 with the NOAA chemical ionization mass spectrometer. The instrument measures HNO 3 with two independent channels of detection connected to separate forward and downward facing inlets that allow a determination of the amount of HNO 3 condensed on ice particles. Subtropical cirrus clouds, as indicated by the presence of ice particles, were observed coincident with condensed-phase HNO 3 at temperatures of 197-224 K and pressures of 122-224 hPa. Maximum levels of condensed-phase HNO 3 approached the gas-phase equivalent of 0.8 ppbv. Ice particle surface coverages as high as 1.4 x 10 14 molecules cm -2 were observed. A dissociative Langmuir adsorption model, when using an empirically derived HNO 3 adsorption enthalpy of -11.0 kcal mol -1 , electively describes the observed molecular coverages to within a factor of 5. The percentage of total HNO 3 in the condensed phase ranged from near zero to 100% in the observed cirrus clouds. With volume-weighted mean particle diameters up to 700 μm and particle fall velocities up to 10 m s -1 , some observed clouds have significant potential to redistribute HNO 3 in the upper troposphere.

Aerosol Effects on Cloud Emissivity and Surface Longwave Heating in the Arctic

Increases in anthropogenic aerosols in the atmosphere tend to increase the reflectance of solar (shortwave) radiation from water clouds, which can lead to lower surface temperatures. Here an opposing effect whereby aerosols increase the longwave emissivity of thin clouds that adds to the warming of the earth’s surface, is discussed. The latter effect may be particularly important in the Arctic, especially during the winter and early spring when thin stratus clouds are ubiquitous, relatively high levels of anthropogenic pollution are common, and there is little solar radiation.

Assessing the relative contributions of transport efficiency and scavenging to seasonal variability in Arctic aerosol

Regional aerosol concentrations are governed by an evolving balance between aerosol sources and sinks. Here, a simple technique is described for making estimates of the extent to which seasonal aerosol variability is controlled by wet scavenging rather than the efficiency of transport from pollution source regions. Carbon monoxide (CO) is employed as an assumed passive tracer of pollution transport efficiency, to which the magnitude of aerosol light scattering is compared. Because aerosols, unlike CO, are affected by wet scavenging as well as transport efficiency, the ratio of short-term perturbations in these two quantities provides a measure of the relative roles of these two processes. This technique is applied to surface measurements in the Arctic at Barrow, Alaska (71◦N) for the decade between 2000 and 2009. What is found is that a well-known seasonal cycle in ‘Arctic Haze’ is dominated by variability in wet scavenging. Crossing the freezing threshold for warm rain production appears particularly critical for efficiently cleaning the air.

Shortwave, single-scattering properties of arctic ice clouds

The first direct airborne measurements of the asymmetry parameter (g) and extinction coefficient (βext) of clouds are analyzed. The measurements were obtained with a Cloud-Integrating Nephelometer (CIN) in arctic clouds in May and June 1998. The CIN was evaluated by comparing its measurements of the single-scattering properties of water clouds with values from Mie theory using measurements of the droplet size spectra. These results are used to interpret CIN measurements of the single-scattering properties of ice and mixed-phase clouds. For cirrus clouds composed solely of ice crystals the derived value of g was 0.74±0.03 (or 0.76±0.03 if ice crystal faces are assumed to be perfectly smooth and parallel); this value is significantly lower than that calculated assuming the ice crystals to be hexagonal prisms or bullet rosettes. The CIN measurements of βext for cirrus clouds were several times greater than values derived from measurements of the cross-sectional areas and concentrations of the ice crystals. Several possible explanations for the differences between calculated and measured optical properties of the ice clouds are discussed. The measured values of g for mixed-phase clouds depended on the relative concentrations of water and ice. These results are used to show that the albedo of clouds is particularly sensitive to the onset of ice formation. Weak convective clouds and clouds saturated with respect to liquid water generally had higher values of g than those for nonconvective clouds or clouds saturated with respect to ice but subsaturated with respect to liquid water.

Long-Range Transport of Continental Aerosols over the Atlantic Ocean and Their Effects on Cloud Structures

Abstract Airborne measurements of aerosols and cloud microstructures were made in the vicinity of the Azores Islands. Two dichotomous cases are examined: a clean marine air mass and a continentally influenced air mass. The clean marine air mass had relatively low Aitken nucleus (CN) and cloud condensation nucleus (CCN) concentrations, while the continentally influenced air mass had high CN and CCN concentrations. Also, black carbon, sulfate, and SO2 concentrations were significantly higher in the continentally influenced air mass. The continentally influenced air mass had a monomodal aerosol number distribution with a peak at about 0.05-µm radius, whereas the clean marine air mass had a bimodal aerosol number distribution with peaks at about 0.02 and 0.08µm radius.The formation of the aerosol mode at 0.08µm is attributed to aerosols left behind when drizzle drops (which grow efficiently by the collision-coalescence mechanism in clean marine air) evaporate. Stratocumulus clouds in the continentally influen...