A41D-0732
Albedo Measurements and Optical Sizing for Single Aerosol Particles
Atmospheric particles affect the radiative balance of the Earth by their interactions with solar and terrestrial radiation. A useful metric for this effect is the single-scattering albedo, which is the ratio of scattering to extinction. Black carbon is a climatically important class of atmospheric particles due to its appreciable absorption of radiation. The mixing states of the black carbon particles affect the amount of absorption and thus the albedo values, however this information is absent in bulk measurements. To address this issue an instrument has been developed to carry out scattering and extinction measurements simultaneously on single atmospheric particles. Particles are introduced into a frequency stabilized, diode pumped ring laser cavity. The light scattered by the particles is collected by a spherical/ellipsoidal scattering cell. Also, light scattered in the forward and backward directions is collected in independent measurement channels. The ratio of the forward scattering to total scattering is used for optical sizing of the particles. The extinction due to the particles is obtained by measuring the depletion in the cavity transmission signal. Both the scattering and extinction measurements are triggered by individual particles and are combined for a measure of the albedo for each particle. Characterization of the instrument has been carried out with laboratory-generated particles of known sizes and optical properties with modifications being made for eventual field deployment.
A41D-0733
Optimizing Thermal-Optical Analysis for Atmospheric Black Carbon (BC): Determining the Beer-Lambert Mass Without a Fixed Mass Absorption Coefficient for BC
In thermal-optical transmission analysis (TOT), laser light passing through a particle-laden filter is monitored while carbonaceous material is removed in several heating steps and measured by flame ionization detection. In a helium atmosphere, the laser signal is attenuated by the pyrolysis of organic carbon (OC). Later, while carbon is removed in an oxidizing atmosphere, the laser signal returns to its value prior to pyrolysis (split point), whereupon the amount of carbon equivalent to the native BC is measured. Since pyrolyzed OC may actually evolve beyond the split point, the specific absorption cross sections of pyrolyzed OC and native BC must be equivalent. Moreover, OC pyrolysis must be sufficient so that unpyrolyzed OC is not measured as BC beyond the split point. Using response surfaces models of the apparent specific absorption cross sections for pyrolyzed OC and what the instrument measures as native BC, we determined the thermal conditions for establishing the equivalence of the apparent cross sections while insuring sufficient pyrolysis of OC. In this way, we have optimized TOT for BC mass based on the Beer-Lambert Law but without the need for an absolute mass absorption coefficient (or an absolute attenuation coefficient) for BC. Optimal thermal conditions for the equivalence of the cross sections were indicated by the intersection of the response surfaces. Concurrently, optimal conditions for sufficient pyrolysis of OC were indicated by a plateau in the response surface for the BC cross section. Modeling was based on extensive analyses of PM2.5 samples collected from Atlanta, Los Angeles, and Seattle. Although this work was reviewed by EPA and approved for publication, it may not necessarily reflect official Agency policy.
A41D-0734
Measurements of Mass Concentrations of Black Carbon by Using Four Instruments at the Summit of Mount Tai, in the Center of the North China Plain in June 2006
Although the emission rate of black carbon (BC) aerosols from the North China Plain (NCP) has been estimated to be high and it would impact climate severely, the BC mass concentrations at a regionally representative location in the plain have been scarcely observed. During a comprehensive field campaign performed at the summit of Mount Tai (36.26 N, 117.11 E, 1534 m asl) in June 2006, BC (or elemental carbon, EC) concentrations were measured by four instruments: an Aethalometer, a Multi-Angle Absorption Photometry (MAAP) instrument, a Particle Soot Absorption Photometer, and a semi-continuous ECOC analyzer with two thermal protocols (IMPROVE and NIOSH). The hourly BC concentration (PM2.5) as measured by the MAAP instrument ranged from -0.1 to 40.8 μgC m-3, with an average of 3.7 μgC m-3. For both of the first 20-day and the latter 10-day periods during the campaign, for which all the instruments provided PM1 and PM2.5 measurements, respectively, we found strong correlations (R2 >0.88) for all the pairs with which >30 hours of coincident observations were made. The slopes and the intercept values of regression lines ranged from 1.00 to 1.42 and from -0.2 to +0.8 μgC m-3, respectively. This general agreement suggested that we were able to determine the BC concentrations regionally representative over the NCP with an uncertainty of ±50% and thereby reduce the uncertainty factor of >4 associating with the BC emission rate from China. The temporal variation of BC concentrations in June 2006 was dominated by the influence from crop (winter wheat) residue burning after the harvest. The relative importance of BC in comparison to CO2 in terms of heating of the atmosphere over this region is also discussed.
A41D-0735
Size-Segregated Aerosols in the Brazilian Amazon: Chemical Composition using ICP-OES, IC, LTM and Thermal-Optical Analyses
Aerosols from biomass burning increase the number of cloud condensation nuclei, reducing cloud droplet size and, therefore, modifying rainfall location and intensity. The size-resolved chemical composition of these aerosols is important to understand these processes. As part of the (Large Scale Biosphere Atmosphere Experiment in Amazonia - Smoke Aerosols, Clouds, Rainfall and Climate: Aerosols from Biomass Burning Perturb Global and Regional Climate) (LBA-SMOCC) we collected aerosol samples at a pasture site in the Amazon Basin. The sampling period (September to November 2002) included the end of the dry season, the transition period, and the beginning of the wet season. A 13-stage Dekati low-pressure impactor was used to collect particles with Dp between 0.03 μm and 10 μm. Gravimetric analyses were performed to determine the total aerosol mass concentrations. Inductively coupled plasma with optical emission spectroscopy (ICP-OES) was used for the determination of the mass concentrations of Al, B, Ca, Fe, K, Mg, Mn, Na, Si, and Zn. Ion chromatography (IC) was used for the following water-soluble ions Na+, NH4+, K+, Mg2+, Ca2+, Cl-, NO3-, and SO42-. For the analysis of the carbonaceous fraction we used a light transmission method together with thermal-optical analyses. Preliminary results showed that the mass concentrations of the fine fraction (Dp <1 μm) during the whole period ranged from 5.1 to 144.2 μg m-3. The inorganic fraction concentrations determined by the ICP-OES showed values between 0.1 and 11.8 μg m-3. The highest concentrations for the total mass and the inorganic fraction were observed during the dry season due to proximity of the fires to the sampling location. The most abundant elements measured by the ICP-OES were Si, K, and Al during the dry season and Si during the beginning of the wet period. The inorganic fraction, as determined from the ICP-OES, represents 5%, 8%, and 18% of the total mass for the dry, transition, and wet periods, respectively, showing that the inorganic fraction is more significant for the wet season. Further, we present a complete set of the ICP-OES results for the three periods together with the IC results and we discuss the contribution of the carbonaceous fraction.
A41D-0736
Optical Analysis of Coated and Uncoated Soot: Data for Global Climate Change Models
Atmospheric aerosols play a fundamental role in Earth's atmospheric chemistry and climate. Soot is an absorbing aerosol, though the magnitude of that absorption has largely been determined by measuring the optical properties of uncoated soot. It has been proposed that coated soot might absorb radiation more efficiently than uncoated soot, thus warming the climate more than previously suspected. For this study, soot is generated in a well-controlled Santoro-Style diffusion flame burner with ethylene as the fuel, and has been successfully coated with dibutyl phthalate (DBP). DBP has a refractive index of 1.490 (real part), which is similar to the refractive index of sulfuric acid (n=1.426) at 589 nm. DBP is substituted for the commonly found sulfate coated particles for several reasons including safety and instrument integrity. By changing the temperature of the DBP, the vapor pressure of the DBP is changed and consequently, the coating thickness can be changed. The aerosols are measured with a differential mobility analyzer (DMA). From the DMA the aerosols are sent to a condensation particle counter for size distributions or are analyzed. Optical analysis performed with a multi-pass extinction and scattering cell (MPESC) where the total scattering and extinction cross-sections are measured. The MPEC uses a 632nm laser and a 100 m path length. The absorbance cross-section is calculated from those measurements. This study improves upon previous optical studies of soot by using soot particle sizes that are representative of primary soot particles in the atmosphere providing a more solid and viable result.
A41D-0737
correction to numerica advection of moments of the particle size distribution in eulerian model
Quadrature method of moments(QMOM) offers a alternative more efficient than sectional methods and more accurate than modal methods. If QMOM is incorporated into eulerian model, invalid moments set are produced by nonlinear transport methods when valid moments are transported as seperate tracers. A non-negative least squares (NNLS) soultion eliminates the problem without requiring modification of the transport algorithm. The evaluation of NNLS for two representative advection schemes in one dimension was done for 10E4 test cases.
A41D-0738
Parameterization of the effect of sub-grid scale aerosol dynamics on aerosol number emission rates
One of the major challenges in simulating the aerosol number concentration and number size distribution in the atmosphere is the description of aerosol dynamics near sources of primary particles. These emission "hot spots" may be metropolitan areas in global models or large point sources in global and regional models. Most models currently simulate the average particle number concentration in the grid cell, spreading the effect of the hot spot unrealistically across the cell. However, coagulation is a nonlinear process and numerical "dilution" of the emitted particles in the full grid cell introduces potentially significant bias in the model results. Unfortunately, simulation of the rapid dilution of particles as they disperse away from their source together with their coagulation, removal, and growth or evaporation is prohibitively expensive for regional and global chemical transport models. In this study, we develop a method for the parameterization of the sub-grid scale aerosol dynamics. This method calculates the probability that a given particle emitted inside the grid cell will survive and be available for transfer outside the cell. This survival probability is calculated theoretically as a function of the emitted particle size, the pre-existing aerosol size distribution in the grid cell, the meteorological conditions, and the size of the grid cell. The net number of particles effectively emitted to the grid cell can then be calculated by multiplying the size dependent emission rate in the inventory with this survival probability. The method simultaneously conserves mass by adding the mass of particles lost by coagulation to the larger particle sizes. The approach is grid-size independent and can be used in models of all scales. Its results compare favorably with the predictions of a detailed one-dimensional aerosol dynamics and chemistry model under a variety of atmospheric conditions.
A41D-0739
Hygroscopic, Morphological, and Chemical Properties of Agricultural Aerosols
Agricultural fugitive dust is a significant source of localized air pollution in the semi-arid southern Great Plains. In the Texas Panhandle, daily episodes of ground-level fugitive dust emissions from the cattle feedlots are routinely observed in conjunction with increased cattle activity in the late afternoons and early evenings. We conducted a field study to characterize size-selected agricultural aerosols with respect to hygroscopic, morphological, and chemical properties and to attempt to identify any correlations between these properties. To explore the hygroscopic nature of agricultural particles, we have collected size-resolved aerosol samples using a cascade impactor system at a cattle feedlot in the Texas Panhandle and have used the Environmental Scanning Electron Microscope (ESEM) to determine the water uptake by individual particles in those samples as a function of relative humidity. To characterize the size distribution of agricultural aerosols as a function of time, A GRIMM aerosol spectrometer and Sequential Mobility Particle Sizer and Counter (SMPS) measurements were simultaneously performed in an overall size range of 11 nm to 20 µm diameters at a cattle feedlot. Complementary determination of the elemental composition of individual particles was performed using Energy Dispersive X-ray Spectroscopy (EDS). In addition to the EDS analysis, an ammonia scrubber was used to collect ammonia and ammonium in the gas and particulate phases, respectively. The concentration of these species was quantified offline via UV spectrophotometry at 640 nanometers. The results of this study will provide important particulate emission data from a feedyard, needed to improve our understanding of the role of agricultural particulates in local and regional air quality.
A41D-0740
Cloud Condensation Nuclei and aerosol properties at two forest sites in South America
Cloud condensation nuclei and aerosol properties were measured at two forest sites in South America. The first site is the LBA tower North of Manaus in Amazonia and the second site is at the Atlantic Forest in Southeast Brazil. Both sites are located in primary forest. Aerosol concentration is about 12 ug/m³ for PM10 and 2-4 ug/m³ for PM2.5. Most of the aerosol mass is at the coarse mode, loaded with natural biogenic aerosol. Aerosol particle number is at 300-700 particles per cc. It is remarkable the similar values for light scattering and absorption at both sites. The CCN concentrations at various supersaturations are similar for both forest site, as well as the ratio between CCN and total aerosol particle number measured using a TSI CPC. Aerosol size distributions measured using an TSI SMPS shows that no new particle formation events were observed in both sites. Aerosol composition is dominated by organic components at both sites, with potassium indicating that biogenic aerosol dominates the sources. Very low soil dust was observed in both sites. Also sulfate and nitrate concentrations are low. We will discuss the diurnal variability of measured properties and a detailed comparison of tropical and subtropical aerosol properties.
A41D-0741
Hygroscopic Growth Potential and Cloud Condensation Nuclei Activation of Mixed Aerosols
Organic matter comprises a significant fraction of submicron aerosol mass in the atmosphere and it is often well mixed with inorganic components in solution. The water-soluble organic fraction in aerosol particles, known to be similar in structure to fulvic acid (FA), may affect the hygroscopic growth of inorganic aerosol particles. To determine the effect of organic matter on water uptake by sea salt and sulfate aerosols, the water vapor pressures of aqueous FA and mixed FA \ NaCl and FA \ (NH4)2SO4 bulk solutions were measured using a vapor pressure apparatus. Water vapor pressures measured over mixed solutions were within the uncertainty limits of the values for pure inorganic solutions, suggesting that organic matter present in low concentrations cannot alter the hygroscopic growth of inorganic aerosols. We use a modified Köhler equation and FA surface tension measurements to further show that, contrary to previous studies, water-soluble organic matter may not lower the supersaturation required for inorganic particle activation into cloud droplets.
A41D-0742
Water Activity Limits the Hygroscopic Growth Factor of Organic Aerosols
In this work we study the hygroscopic behavior of organic aerosols, which has important implications for Earth's climate. The hygroscopic growth factor (HGF) is defined as the ratio of the diameter of a spherical particle when it is exposed to dry conditions to that at humid conditions. We present a new formulation to express the HGF of an aerosol particle as a function of water activity (aw) in the aqueous phase. This new formulation matches reported HGFs for common inorganic salts and water-miscible organic particles that are known to deliquesce into aqueous drops at high relative humidities (RH). Many studies use tandem differential mobility analyzers (TDMA) to determine the HGF of organic aerosols. For example, Brooks et al. used a TDMA to measure a HGF of 1.2 for 2 μm phthalic acid (PA) particles at 90% RH (aw= 0.9). However, water activity limits the growth of a particle that can be attributed to water uptake. We have assembled a vapor pressure apparatus to measure aw of aqueous solutions at room temperature. Measured water activities for PA, used in our growth formulation, yield a HGF of ~ 1.0005 for 2 μm PA particles at 90% RH. Comparing our results against Brooks et al. suggests that TDMA experiments may grossly overestimate the HGF of PA particles since water activity limits this growth to below 1.0005. Alternatively, we suggest that the adsorption of a negligible mass of water by a highly porous PA particle can lead to an apparent growth in particle size by changing its morphology. Other studies also use TDMAs to measure HGFs of secondary organic aerosols (SOAs). HGFs reported for SOAs are very similar to PA, suggesting that the observed growth may be due to morphological changes in particle size rather than water uptake as commonly assumed. We built a smog chamber where an organic precursor, such as d-limonene, reacts with nitrogen oxides under UV radiation to produce SOAs. We compare the HGFs for SOAs obtained with our method to those obtained with TDMA experiments. Our results suggest that TDMAs may provide erroneous HGFs for non-spherical, porous organic particles.
A41D-0743
Changes in Aerosol and Aerosol Direct Radiative Effects Near Clouds
Aerosols and their radiative effects change in the vicinity of clouds. The high relative humidity (RH) of the environment near low-level clouds causes hygroscopic aerosols to swell, thereby changing their optical properties. Aircraft observations of relative humidity and particle concentrations taken during INDOEX are used to document the increases in RH and the changes in particle concentrations in the vicinity of clouds. These changes along with the chemical composition of the aerosol are used to estimate the changes in optical properties and the effect of these changes on the aerosol direct radiative effects as a function of distance from low-level clouds. Observations from the multichannel cloud radiometer (MCR) during INDOEX are used to compare the calculated changes with those observed. Part of the changes in the observed radiances are due to changes in particle concentrations and to particle growth, but part is also due to the increased illumination of the cloud-free column as a result of radiation reflected by the sides and tops of nearby clouds. Visible and near infrared radiances from the MCR are used to estimate the relative magnitudes of the different contributions to the changes. As such effects decrease with distance from cloud, daytime CALIPSO lidar observations are used to determine the sizes of cloud-free ocean regions in which low-level clouds reside. This distribution provides the probability of distances to cloud for aerosols in the cloud-free regions. The observed changes and the distribution of sizes for cloud-free ocean regions are used to estimate the contribution of the changes in the direct radiative effects of aerosol in the vicinity of low-level clouds to the total direct radiative effects of aerosols for oceans. These changes are estimated to be comparable to the ~1 Wm-2 uncertainty in the ~4.6 Wm-2 direct aerosol effect derived from CERES observations.
A41D-0744
Investigation of Aerosol-Cloud Interactions Using a Chemical Transport Model Constrained by Satellite Observations
This study investigates aerosol-cloud interactions in marine clouds, using a combined approach of global chemical transport modeling for cloud droplet concentrations and satellite remote sensing for cloud liquid water path. The simulated aerosol and cloud droplet concentrations are validated with long-term surface and in situ aircraft measurements for a variety of polluted and relatively clean conditions. The column-integrated cloud liquid water path is constrained by the SSM/I satellite microwave measurements. As a result, the calculated cloud optical thickness (COT) for marine warm clouds displays similar geographical patterns and seasonal variations to two independent retrievals by MODIS and CERES satellite experiments. The fundamental difficulty in validating the simulated COT values is that the two satellite data differ significantly (by 42% to 110%) from each other even for zonal means, and the satellite derived cloud effective radius (Re) is systematically larger (by a factor of 1.5 to 2) than the in-situ aircraft data. The simulated Re is in closer agreement with aircraft data, thus the calculated zonal mean COTs are larger than CERES and MODIS values by factors of 1.5 to 3. The modeled hemispherical mean cloud properties show large systematic differences, i.e., the cloud drop number density is a factor of 2 larger; the liquid water path (LWP) is larger by 12%; and COT is about 30% larger in the northern hemisphere (NH). Most of these asymmetries are driven by anthropogenic aerosols; however, they are not indicated by the satellite observations. Assuming that uncertainties in satellite retrievals are not masking the inter-hemispheric asymmetries, we speculate that this is due to the biogenic source of marine organic aerosols over the southern hemisphere (SH). At constant LWP, a same perturbation in aerosol concentration leads to the largest enhancement in COT over the remote SH oceans. Therefore, the neglected marine biogenic organic aerosols may play an important role in the estimated SH cloud radiative forcing. We also showed that the aerosol dispersion effect on cloud droplet size spectrum reduces the Twomey effect substantially; and the smaller cloud susceptibility obtained at larger cloud LWP implies a possible negative correlation between cloud geometric thickness and droplet concentration.
A41D-0745
On the Importance of Nitrate as IAE-Agent in West-Europe
We measured the activation of nitrate-aerosol in our large flow-through cloud chamber. The chamber produces a number of cloud droplets that is representative for that in stratocumulus in our region. The experiments proceed as follows. Aerosol size-spectra are determined both in the air entering the cloud chamber and at the exit of the chamber. From the difference in the spectra, in combination with the number of cloud droplets formed, we deduce which particles served as CCN. On-line chemical analysis shows that in polluted air the amount of nitrate in the CCN is similar to the concentration of sulphate. The chamber is strategically located at the coast of the North-Sea, where arctic air from the North contains a low concentration of CCN; in polluted air coming from other directions the CCN loadings are higher. The difference in the number of cloud droplets that are formed in the two types of air is an empirical measure for the regional IAE.
A41D-0746
A global climatology of clean and polluted clouds
Global cloud observations from CloudSat and Aura MLS are studied in combination with the new aerosol observations from CALIPSO and carbon monoxide (CO) measurements from Aura MLS to investigate global distribution and seasonal variation of clean and pollution contaminated clouds. Our study will focus on the cirrus cloud in the upper troposphere where MLS ice water content and CO measurements are available. Our approach is to use MLS CO to classify cirrus clouds as "clean" or "polluted", and to use CALIPSO aerosol to identify convective systems contaminated by aerosol particles. We define a "polluted" cloud by two criteria: one uses the coincident MLS CO measurements greater than a certain background value and the other uses the aerosol observations from CALIPSO. Surface emission source of CO and aerosol will be examined in parallel. This study compiles the climatologies of two sets of "polluted" clouds. These climatologies will be compared with that of the clean clouds to delineate how surface pollution alters the properties of upper tropospheric clouds. The results will be compared with model simulations as well. Preliminary analyses find high CO concentration co-exists with deep convective cores and cirrus anvils. Aerosols, however, are found collocated mostly with cirrus away from convective cores, where precipitation is not strongest. The spatial distribution and seasonal variation of "polluted" clouds will be presented at the meeting.
A41D-0747
Effects of Aerosols on Clouds and Precipitation in the UCLA GCM
The UCLA atmospheric general circulation model (AGCM) has been used to investigate the effect of aerosols on the simulated cloud fields and precipitation. This model includes an efficient and physically based radiation parameterization scheme specifically developed for application to clouds and aerosols. Including a background aerosol optical depth of 0.2 produces a decrease in precipitation in the tropics as a result of decreased temperature contrast between this area and the mid- to high latitudes, which suppresses tropical convection. This decrease has corrected an overestimate in precipitation of about 0.34-0.4 mm day-1 in the UCLA AGCM simulations. The total cloudiness is reduced by about 2.0% for both January and July simulations. Reduction in the total cloudiness is consistent with the positive solar forcing at the top of the atmosphere such that the induced cloud feedback dominates aerosol direct effect. Since aerosols stem from local sources, we further investigated the effect of an extremely polluted area such as that occurring in China on climate simulation. The experiment with increased aerosol optical depths in China shows a noticeable enhancement in the July precipitation in the southern part of China and Indian areas due to cooling in the midlatitudes that leads to the strengthening of the Hadley circulation. A series of climate experiments incorporating various aerosol types have also been performed. Large dust particles and black carbon in China would heat the air column in the mid- to high latitudes that tends to shift the simulated precipitation inland, i.e., toward the Himalayas. The climatic effects of aerosols on the radiative budget, temperature, and precipitation fields are not only produced through their direct radiative forcings, but also affected by the subsequent modulation of cloud fields formed in the model. This ¡°indirect" effect through clouds generated in the UCLA AGCM is comparable to direct aerosol radiative forcing. Finally, we are testing the parameterization of ice clouds and aerosols and their radiative forcings in the most recent Weather Research Forecasting (WRF) model that has been used as the foundation for the construction of a regional climate model encompassing land, coastal ocean, and the atmosphere, specifically designed for application to California.
A41D-0748
Parametric Sensitivity Analysis in a New Cloud Resolving Aerosol Model
Mathematical models of stratus clouds have become more complex due to an increasing demand for realistic quantitative simulations. Most models employ parameterizations to approximate micro-physical processes that take place within a cloud. These parameterizations contain a number of parameters whose values and impact to the model solution is unknown. This presents a problem for the predictive quality of the numerical simulation of the model. Sensitivity analysis helps identify the most relevant parameters providing valuable insight into which of these should be accurately determined. We present the implementation of parametric sensitivity analysis to a newly developed high-resolution cloud resolving aerosol model for stratus clouds. Identifying the relevant parameters significantly contributes towards the improvement of the current cloud aerosol model. The sensitivity of the model with respect to its various parameters, such as activation time scales and fall speed of water droplets, is computed. Additionally, we examine the sensitivity with respect to the artificial diffusion coefficients that are present in the discretization. The model solution as well as the sensitivities are computed using the SUite of Nonlinear and DIfferential/ALgebraic equation Solvers (SUNDIALS) package.
A41D-0749
Assessment of the Dehydration-Greenhouse Feedback Over the Arctic During Winter
The effect of pollution-derived sulphuric acid aerosols on the aerosol-cloud-radiation interactions is investigated over the Arctic for February 1990. Observations suggest that acidic aerosols can decrease the heterogeneous nucleation rate of ice crystals and lower the homogeneous freezing temperature of haze droplets. Based on these observations, we hypothesize that the cloud thermodynamic phase is modified in polluted air mass (Arctic haze). Cloud ice number concentration is reduced, thus promoting further ice crystal growth by the Bergeron-Findeisen process. Hence, ice crystals reach larger sizes and low-level ice crystal precipitation from mixed-phase clouds increases. Enhanced dehydration of the lower troposphere contributes to decrease the water vapour greenhouse effect and cool the surface. A positive feedback is created between surface cooling and air dehydration, accelerating the cold air production. This process is referred to as the dehydration-greenhouse feedback (DGF). Simulations performed using an arctic regional climate model for February 1990, February and March 1985 and 1995 are used to assess the potential effect of the DGF on the Arctic climate. Results show that the DGF has an important effect over the Central and Eurasian Arctic, which is the coldest part of the Arctic with a surface cooling ranging between 0 and -3K. Moreover, the lower tropospheric cooling over the Eurasian and Central Arctic strengthens the atmospheric circulation at upper level, thus increasing the aerosol transport from the mid-latitudes and enhancing the DGF. Over warmer areas, the increased aerosol concentration (caused by the DGF) leads to longer cloud lifetime, which contributes to warm these areas. It is also shown that the maximum ice nuclei reduction must be of the order of 100 to get a significant effect.
A41D-0750
The Role of Arctic Cold Lows in Generating Thin Ice Clouds: A Comparison between CloudSat-CALIPSO and NARCM Simulations
CloudSat and CALIPSO observations reveal two types of very extensive thin ice clouds in Polar regions: thin precipitating ice cloud layers (TIC-2) and lofted thin ice cloud layers of smaller crystals (TIC-1). The vertical distribution of the TIC-1 and TIC-2 clouds is the result of a combination of transport and microphysical processes experienced by the air mass along its trajectory. An important dynamical source of the vertical transport is the presence of dominant quasi-stationary cyclones known as cold lows over the Arctic Ocean. They originate from mid latitudes active storms that transported large amount of heat, moisture and aerosols into the high Arctic. Decaying cold lows produce slow adiabatic lifting of moisture and aerosol together, mixing aerosol 5 to 8 km deep into the arctic troposphere. The resulting stratified aerosol layers from these systems have been observed between 3 and 6 km forming with the cold air mass an aerosol dome across the Arctic basin. Their frequency and concentration often exceed that of most polluted region of the world at the same altitude. NARCM model simulations provide evidences that aerosols and water vapor are lifted together by cold low systems, from low level convergence in the Arctic atmosphere. This slow aerosol-cloud interaction over several days effectively allows for a dehydration-greenhouse feedback (DGF) process with implications on the Arctic climate. In this study, the role of cold lows and the significance of the Arctic thin ice clouds in relation to aerosols will be discused.