A11A-0023
Diurnal temperature range over Europe between 1950 and 2005
It has been widely accepted that diurnal temperature range (DTR) decreased on a global scale for the second half of the twentieth century. In contrast, we show that the long-term trend has reversed from decrease to increase during the 1970s in Western Europe and during the 1980s in Eastern Europe. The analysis is based on the high- quality dataset of the European Climate Assessment and Dataset Project, from which we selected about 200 stations, covering the area from Iceland to Algeria and from Turkey to the European part of Russia for 1950 to 2005. We investigate national and regional means as well as the pan-European mean with respect to trends and reversal periods. 17 of the 24 investigated regions including the pan-European mean show a significant increase since 1990 at the latest. Of the remaining 7 regions, 2 show a non-significant increase, 3 a significant decrease and the remaining 2 no significant trend. The long-term change in DTR is considered to depend on both, incoming shortwave radiation and outgoing long-wave radiation, the former of which has undergone a change from dimming to brightening. Consequently we discuss the connections between DTR, shortwave radiation and sulfur emissions which are thought to be amongst the most important factors influencing the incoming solar radiation through the primary and secondary aerosol effect. We find reasonable agreement between trends in SO2 emissions, radiation and DTR in areas affected by high pollution. Consequently we conclude that the long- term trends in DTR are mostly determined by emissions and the incoming solar radiation.
A11A-0024
Photochemical Cycling of Humic-Like Substances in Atmospheric Aerosols
Colored, humic-like substances (HULIS) arising from the biodegradation of organic detritus are widespread in natural surface waters, where they ultimately undergo solar photolysis into small alpha-dicarbonylic species, such as glyoxal, glyoxylic and pyruvic acids. Diversely generated and chemically dissimilar HULIS are also found in the atmospheric aerosol. How are significant levels of colored HULIS produced and sustained in the concentrated aerosol phase under intense solar irradiation? Here, this issue is tackled by investigating the solar photolysis of aqueous pyruvic acid (PA) solutions at concentrations representative of the atmospheric aerosol using UV-absorption, high resolution electrospray mass, and nuclear magnetic resonance spectrometries. Under such conditions, PA is not photodegraded but yields polyfunctional polymers, whose mass and UV-absorption spectra remain unaffected after 3, 8 and 22 h photolysis. Unless diluted, these polymers undergo condensation/polymerization in the post-photolysis period into mass < 700 Da species that absorb in the visible, and are bleached upon resuming irradiation. The re- photolyzed solutions recover the mass and UV-absorption spectra of first photolyzed solutions. Whereas initial pH has no effect on the mechanism of reaction, ammonium bisulfate, a major component of the aerosol, markedly influences these processes. These findings suggest that the chemical identity and concentration levels of complex organic substances in the aerosol are the result of dynamic photochemical processing in the condensed phase.
A11A-0025
Chemical Characteristics of Water-Soluble Organic Carbon in the Asian Outflow
Semi-continuous measurements of water-soluble organic carbon (WSOC) and organic carbon (OC) in PM2.5 were made at Gosan, Korea, in March-April 2005. On average, the WSOC/OC mass ratio for all air masses observed at Gosan was 0.30+/-0.12. WSOC correlated well with CO (r2=0.54) in Chinese outflow, suggesting that a major part of the observed WSOC and/or their precursors was of combustion origin. The relationship between the increase of WSOC and O3 suggests that the observed WSOC was mostly secondary product. To interpret the measured organic compounds, thermal analyses of organic standards were made in the laboratory. Thermograms of a single standard of water-soluble organic species showed that carbon that evolved at high temperatures (600-870C) was generally associated with water-soluble compounds having high molecular weights (MWs) on the order of hundreds, while carbon that evolved at low temperatures (<300C) generally had MWs of less than ~180 g mol-1. Positive matrix factorization (PMF) analysis revealed three organic compound groups (low, medium, and highly refractory compounds) based on the OC thermograms. On average, highly and low refractory compound groups accounted for 79% and 21% of the WSOC mass, respectively, at Gosan. Highly-refractory compound groups significantly contributed to WSOC regardless of air-mass origin. The results of the laboratory experiments imply that a large fraction of these highly-refractory compound groups was likely associated with high MW compounds. For water-insoluble organic carbon (=OC-WSOC), medium and low refractory compound groups accounted for 60% and 40%, respectively, consistent with the results of the laboratory experiments.
A11A-0026
Climate Impact of Carbonaceous and Sulfate Aerosols in an Interactive Size-Resolving Aerosol-Climate Model
A multi-mode, two-moment aerosol model has been incorporated in the NCAR CAM3.0 to study the impact of anthropogenic aerosols on the global climate system. Seven aerosol modes, namely three sulfate, one external black carbon, one primary organic carbon, one sulfate/black carbon mixed mode and one sulfate/organic carbon mixed mode, are included in the model. Each aerosol mode predicts both number and mass concentration and thus aerosol size distribution which is critical to aerosol microphysics and optical property. When aerosols are assumed to be internally mixed, their chemical and optical properties are different than when they are externally mixed. Model results suggest that the major portions of black carbon and sulfate masses exist as mixed aerosols while two thirds of organic carbon mass is also in mixed form. In this study, multiple 60-year long simulations are carried out to examine the impact of the carbonaceous and sulfate aerosols in the climate system. Strong positive atmospheric forcing and negative surface forcing by the mixed aerosols affect the atmospheric thermal structure and consequently alter values of important parameters including cloud cover, height of planetary boundary layer, surface heat fluxes, and precipitation. The detailed results will be presented and discussed.
A11A-0027
A modeling study of direct radiative forcing due to carbonaceous aerosol and its effects on the climate of Eastern Asia
The global direct radiative forcing due to carbonaceous aerosol and its effects on the climate of Eastern Asian have been investigated by using CAM3 (Community Atmosphere Model Version 3) developed by the National Center for Atmospheric Research (NCAR). The simulation results in this paper show that carbonaceous aerosol brings about a negative forcing at both of the top of atmosphere (TOA) and the surface in clear sky, and the global means of the forcing are -0.24W/m2 and -1.31W/m2, respectively. However, it produces a positive radiative forcing at TOA and a weaker negative forcing at the surface in the cloudy sky, and the global means of the forcing are +0.08W/m2 and -0.96W/m2, respectively. It indicates that cloud can change the sign of the forcing at TOA, and make the negative surface forcing to be weakened. The carbonaceous aerosol has distinct effects on the climate of Eastern Asia in summer, like the total cloud cover, surface temperature and total precipitation. The atmospheric general circulation of the Northern Hemisphere can also be influenced by it. It is found in this paper that the Hadly cell is weakened and polar cell is strengthened in summer by the existing of carbonaceous aerosol. The observational fact that the precipitation is enhanced in the North part of China and is decreased in the South part of China in summer in recent years is explained by the changes of wind field and atmospheric general circulation due to carbonaceous aerosol in this work. Key wards: Carbonaceous Aerosol; Radiative Forcing; CAM3; Climate of Eastern Asia
A11A-0028
High Spectral Resolution Aerosol Properties: New Information for Calculating Aerosol Radiative Forcing
Aerosol radiative forcing estimates have consistently carried a high level of uncertainty. This may be in part due to incomplete knowledge of the spectral dependence of aerosol properties throughout the solar spectrum. Measurements are typically made at a few wavelengths and extrapolated across the solar spectrum using assumed relationships. We explore the utility of high spectral resolution radiometer data for retrieving aerosol properties at a higher level of accuracy than properties obtained with commonly deployed instruments at lower spectral resolution. The sensitivity of the wavelength dependence of aerosol properties for different aerosol types is examined using a radiative transfer model. Correlations with commonly observed wavelengths provide the information contained in portions of the spectrum that are not typically available from routine measurements. We also examine data obtained from a spectro-radiometer that contains features that are not evident in routine measurements at limited wavelengths, suggesting additional information that may me extracted from high spectral resolution data. Comparisons with ground-based and airborne in situ observations of aerosol properties are used in verification. This analysis shows the utility of spectral measurements in the field on a routine basis and provides guidance for the deployment and processing of data from spectral instruments in the future.
A11A-0029 [WITHDRAWN]
Quantification of the Direct Aerosol Effect Using Ground Based Radiative Flux Measurements at Cabauw (The Netherlands)
Aerosols affect the Earth's climate profoundly by changing the amount of sunlight at the surface due to both, the direct and indirect aerosol effects. Mishchenko et al. (2007) showed that the aerosol optical thickness (AOT) decreased on a global scale since the beginning of the 1990s, even though regional aerosol column densities vary considerably due to both, natural events and especially anthropogenic activities. The decreasing global AOT will likely increase the surface solar radiative heat flux which eventually reduces the counterbalance to greenhouse gas warming and potentially contribute to the warming trend of the last decade. Using surface solar radiative flux measurements and AOTs derived from sun-photometer measurements at Cabauw we show the magnitude of the aerosol effect for cloud free conditions. This so-called direct aerosol effect is quantified by subtracting modelled radiative fluxes for cloud-free and aerosol-free conditions from radiative fluxes as measured at the BSRN station Cabauw (The Netherlands). To exclude measurements where clouds are visible above the horizon we applied several (longwave and shortwave) cloud detection algorithms and examined sky images taken by an 2-π hemispheric camera. The mean direct aerosol effect was in 2006 about -30 W/m2. During the same period the direct aerosol effect accounted for a reduction of the surface solar radiative flux on average by about 120 W/m2 per unit AOT. The poster presentation will contain an uncertainty analysis of the direct aerosol effect and an airmass trajectory analysis to determine the aerosol origin and some information about its composition. From these results we will derive an aerosol climatology for The Netherlands, which is useful for validation purposes of the regional climate model RACMO.
A11A-0030
MODIS Aerosol Optical Depth retrieval over South America: sensitivity on modeled aerosol from improved AERONET regional climatology
Atmospheric aerosols affect climate directly, through absorption and scattering of solar radiation, and indirectly, altering cloud formation mechanisms and properties. The radiation balance is a critical component of climate system and an increase in aerosol concentration causes a net change in radiation budget. Estimating aerosol direct radiative forcing combines, as input to radiative transfer codes, aerosol optical and microphysical models obtained through surface remote sensing and aerosol optical depth (AOD) from satellite data. Besides information on aerosol, atmospheric and surface conditions play also significant role along the estimation process. The purpose of this work is to study the sensitivity of AOD MODIS retrievals over South America to regional aerosol properties from AERONET ground-based data, and to typical atmospheric and surface conditions observed from satellite. Since May 2007, INPE counts on a regional system of aerosol retrieval from MODIS, based on NASA/GSFC algorithms. A new version of this product is under development, with changes on aerosol properties assumed inside the retrieval. Long-term measurements (1999-2006) from 9 AERONET sites over South America were used by LFA (Laboratory of Atmospheric Physics, USP) to produce a new set of regional aerosol models, allowing better characterization of aerosol types over this region. These models were combined with AOD from MODIS data as input to SBDART. The obtained results show that sensitivity of retrieved AOD on modeled microphysical and optical properties is much larger than that on atmospheric vertical profile. For instance, differences between the typical single scattering albedo observed at different AERONET sites create large discrepancies on regional scale MODIS retrievals. The results allow us to conclude that improvements on aerosol optical and microphysical modeling inside MODIS aerosol retrievals are critical in order to obtain a much more reliable tool on estimating aerosol radiative forcing. The use of AERONET climatology to produce regional aerosol models appears as a key method to this goal.
A11A-0031
The potential impacts of pollution on a non-drizzling stratus deck: Does aerosol number matter more than type?
In this presentation results from a cloud-resolving model that can efficiently examine the impact of aerosols on non-drizzling stratus clouds will be shown. Because the model tracks aerosols and cloud droplets in a Lagrangian framework, it does not suffer from numerical errors associated with advection and unlike most Eulerian approaches, the method can accurately track cloud boundaries as they move across a grid cell. The model is able to reproduce cloud water mixing ratio and vertical velocity statistics from the DYCOMS-II field program. The ability of the model to assess the impact of changes in aerosol number and composition on a stratus deck will be presented. Specifically, by using activation curves appropriate for soluble, insoluble, or a mixture of both types of aerosols and for certain extreme aerosol regimes, i.e., a majority of the aerosols are hydrophobic, limiting situations will be examined in this numerical framework that may, in fact, occasionally occur in the atmosphere, e.g., an oceanic stratus field downwind of a large urban area. Expectedly, results from these simulations support previous ship track observations that for increasing aerosol numbers cloud number concentrations increase, whereas cloud droplet radii decrease near cloud top. But, these simulations also suggest that the correlation between cloud number concentration and aerosol number concentration may be a function of aerosol type. http://aerosols.lanl.gov/
A11A-0032
Numerical Simulations of Aerosol Effects on Precipitation Pathways in Deep Convective Clouds
The Regional Atmospheric Modeling System (RAMS; version 4.3) is used to simulate two-dimensional idealized deep convection using a two-moment bulk microphysics scheme for two cases: the "clean" case is initialized with a low concentration (100/cc) of aerosol serving as cloud condensation nuclei (CCN), while the "polluted" case has a higher concentration of CCN (1000/cc). In this study, the microphysical sources and sinks for each precipitation type are studied to reveal the variations in precipitation pathways between the clean and polluted clouds. We will also repeat these simulations for varying thermodynamic profiles. Preliminary results from these initial simulations show that the more polluted cloud produces less precipitation, at least during its initial phase prior to the onset of secondary convection, which is consistent with other numerical studies of the effects of aerosol on the quantity of precipitation output. Furthermore, warm rain processes are the dominant source for rain in the clean case throughout the simulation, whereas rain sources in the polluted case quickly become dominated by ice processes. This presentation will focus on the dominant pathways for rain and how they vary between the clean and polluted cases, and with changing thermodynamic conditions.
A11A-0033
The Role of Adiabaticity in the Aerosol First Indirect Effect
Aerosol indirect effects are the most uncertain of the climate forcing mechanisms that have operated through the industrial period. Several studies have demonstrated modifications of cloud properties due to aerosols and corresponding changes in shortwave and longwave radiative fluxes under specific cloud conditions, but some recent studies have indicated that cloud dynamical processes such as entrainment-mixing may be the primary modulator of cloud optical properties in certain situations. For example, day-to-day variations of the cloud drop effective radius determined from the ground-based remote sensing at the Southern Great Plains were found to be weakly associated with the variations in aerosol loading as characterized by its light-scattering coefficient at the surface, implying that other processes were impacting the cloud radiative properties. To study these other impacts, we extend a previous study to investigate the role of changes in liquid water path (LWP) and the effective radius in single layer stratiform clouds that are induced by entrainment-mixing processes and their effects on cloud radiative properties. We quantify the degree of entrainment-mixing in terms of the adiabaticity defined as the ratio of the observed cloud liquid water path to the corresponding adiabatic value. The cloud optical depth is, as expected, governed primarily by LWP, but that adiabaticity is the next most influential factor. In contrast, the effective radius is found to be equally sensitive to adiabaticity and LWP. In adiabatic clouds the aerosol first indirect effect is clearly observed and related to independent measures of aerosol loading. In sub-adiabatic clouds the aerosol first indirect effect is not readily observed; this may in some circumstances be due to interference from heterogeneous mixing processes that change the droplet number density in a manner that attenuates the effect.
A11A-0034
Modeling Aerosol Effects on Shallow Cumuli and Turbulent Activities Under Various Meteorological Conditions
To determine conditions over the Indian Ocean for which cloud fields are most susceptible to modification from aerosols and to study how turbulent activities and shallow cumuli vary for different meteorological scenarios, the National Center for Atmospheric Research Eulerian-semi-Lagrangian (EULAG) three-dimensional large-eddy simulation model was initialized using data collected during the Indian Ocean Experiment (INDOEX). Radiosonde data were used to construct 6 soundings encompassing the range of temperature and humidity observed in the trade-wind boundary layer. By then adding the characteristics (height, depth and strength) of either a typical transition layer (TL), a strong inversion layer (IL) or no stable layer a total of 18 meteorological scenarios were produced. Separate simulations were conducted using EULAG assuming pristine and polluted conditions (i.e., cloud droplet number concentrations, aerosol extinction profiles and single-scattering albedos) using INDOEX observations. For the range of meteorological conditions observed during INDOEX, sensitivity studies showed that the semi- direct effect always dominated indirect effects, producing a positive daytime mean net indirect forcing varying between 0.2 and 4.5 W m-2. The simulations showed that changes in the environmental relative humidity (RH) and the presence of the TL had critical impacts on the cloud properties, turbulence and lateral detrainment rates, and on how aerosols affect these quantities. The net indirect forcing was larger when the RH was higher and in the absence of any dry and stable layers. It was reduced to less than 1.2 W m-2 when the TL was present. The impact of the IL was dependent on convective strength which increases with increasing RH. In fact, changes in meteorological factors had larger impacts on the simulated cloud properties than did the presence of anthropogenic aerosols, indicating large uncertainties can be introduced when solely using observations of aerosols and clouds made under different meteorological conditions to quantify aerosol effects.
A11A-0035
The Influence of Mixing Processes on the Climate Forcing of Marine Stratocumulus
Recent studies have shown that mixing processes may influence the estimation of the indirect effect through two processes: 1)Increases in cloud condensation nuclei (CCN) concentration can enhance mixing between cloudy and clear air, which modifies the cloud liquid water content (LWC), thereby impacting the change in cloud optical depth and hence the indirect effect. 2)Mixing influences the cloud drop distribution, with the mixing scenario, i.e. rate of mixing relative to the rate of evaporation, governing the change in the distribution. The influence of the mixing scenario on the indirect effect is very uncertain, while the influence of both the mixing scenario and the change in LWC due to changes in CCN concentration has not been addressed. In this work, we investigate the role of both of these processes in the estimation of the indirect effect. This investigation is undertaken with a 3-D large eddy simulation model with fully integrated size bin resolved cloud microphysics (BR-LEM). We present results from BR-LEM simulations of non-precipitating marine Sc in which the CCN concentration is varied between 100 and 1000 cm-3. We simulate the homogeneous and inhomogeneous mixing scenarios for each CCN concentration using a newly developed bin microphysical mixing scheme. We demonstrate that mixing processes do influence the estimation of the indirect effect. We discuss the importance of this influence relative to the impact of increasing CCN concentration alone
A11A-0036
Pollution Effects on Condensation Evaporation and Cloud Coverage of Warm Cumulus Cloud
Aerosols can affect cloud lifetime and dimensions in opposing ways. It was shown that the cloud's lifetime can increase due to rain suppression or decrease due to enhanced evaporation. A numerical cloud model is used to study the pollution effects on moderate-sized, coastal, convective clouds. The results show that polluted convective clouds produce larger rain drops, due to a more efficient collection process. The evaporation process is more significant at the margins of the polluted clouds (compared to the clean cloud) and it is mostly from the small droplets mode in the size distribution. We measure the overall effect on the integrated cloud coverage.
A11A-0037
Indirect effects of mineral dust on warm clouds
Cloud evolution is affected profoundly by aerosols and associated microphysical processes and by cloud dynamics and thermodynamics. To isolate dynamic influences from microphysical impacts, we stratify warm cloud measurements of MODIS, AMSR, and CERES in four key ways: cloud precipitation regime, cloud top temperature (or height), cloud liquid water path, and aerosol number concentration. We found that correlations, which appear to demonstrate Twomey effect, strongly depend on the cloud precipitation regime and cloud top height. Our estimated aerosol indirect effect (AIE) for precipitating clouds with cloud top height above the freezing level is -0.14 (r=0.75, p<0.03) while the AIE for shallow precipitating clouds is not statistically significant. In contrast, for non-precipitating clouds, clouds that directly interact with the dust layer show strong Twomey effects with AIE of -0.15 (r=0.89, p<0.01), while clouds with cloud top above the freezing level show insignificant Twomey effects. We also estimate radiation forcing of dust aerosols directly from satellite measurements. The direct shortwave (SW) radiation effect of Saharan dusts at solar zenith angle of 21.6° is 47.96+6.21 wm-2 per unit AOD with correlation coefficient of 0.88. The indirect SW forcing of Saharan dust is 34.44+4.91 wm-2 per unit AOD for clouds with LWP of 100 gm-2. Our study points out the importance of cloud dynamics and aerosol vertical transport on the aerosol indirect effect. Without carefully segregating cloud precipitation regime and cloud top height, the estimated AIE can vary substantially and have much smaller correlation coefficients.
A11A-0038
An LES model study of the indirect aerosol effects of marine stratocumulus clouds
Stratocumulus clouds that cover one-fourth of the entire global ocean exert significant impact on the global radiation balance and the earth's climate. Their high albedo (30-40%) compared with the ocean background (10%) gives rise to large deficits in the absorbed solar radiative flux at the top of the atmosphere, while their low altitude prevents significant compensation in thermal emission. Importantly, the radiative properties of clouds are crucially affected by the concentrations of the atmospheric particles onto which the cloud droplets form, i.e., cloud condensation nuclei (CCN). This study investigates the CCN impacts on stratocumulus cloud development using the Cooperative Institute for Mesoscale Meteorological Studies (CIMMS) Large Eddy Simulation (LES) model with size-resolving microphysics and attempts to understand how turbulence interacts with cloud microphysics. The CCN spectra are from observations over typical maritime, continental and extreme continental air masses; CCN concentrations at 1% supersaturation are 163, 1023 and 5292 cm-3, respectively. The cloud droplet number concentration, cloud optical depth, and albedo increase but the cloud droplet effective radius decreases with the increase of CCN concentration. High CCN concentration enhances cloud reflectivity (albedo) by increasing the cloud droplet number concentration, leading to a cooling effect. There was no precipitation for the continental and extreme continental clouds while the maritime cloud precipitated after 130 minute integration. Although precipitation may depend on various environmental factors, stratocumulus clouds seem to be mainly affected by cloud microphysics, i.e., cloud droplet number concentration. The domain average shortwave cloud radiative forcing for maritime, continental, and extreme continental clouds are -204.13, -283.27, and -307.99 W m-2, respectively. More details will be discussed at the conference.
A11A-0039
Aerosol / Cloud Interactions Using The Nasa Global Modeling Initiative
The aerosol indirect effect (AIE) is one of the largest sources of uncertainty in assessments of anthropogenic climate change. The objective of this study is to assess the uncertainties in indirect forcing and autoconversion of cloud water to rain from differences in meteorological fields, emission scenarios, parameterizations of cloud droplet formation, and aerosol microphysics. The uncertainty in AIE and autoconversion is assessed with the NASA Global Modeling Initiative (GMI). GMI is ideal for this study as different model components (such as meteorological fields and chemical mechanisms) can easily be interchanged under the same model framework to capture the first AIE, and its sensitivity to parameterizations, meteorological fields, emission scenario and aerosol microphysics. "Present day" and "preindustrial" simulations were carried out using the University of Michigan and AEROCOM emission inventories. Meteorological fields are provided by two global climate models (the NASA GEOS4 finite volume and the Goddard Institute for Space Studies version II') and the NASA Data Assimilation Office. Cloud droplet number concentration (CDNC) was calculated by implementing both diagnostic and physically based droplet parameterizations. Computed CDNC is used to calculate the cloud optical depth, the autoconversion rate and the mean net whole-sky shortwave incoming flux at the surface using a modified version of the FAST-J algorithm. Derived cloud properties, such as cloud optical thickness and effective radius are compared with satellite products from MODIS platform. Our results suggest that differences in meteorological fields, cloud droplet activation parameterizations, emission scenarios and aerosol microphysics could account for more than 30% variability in forcing estimates for the first indirect effect and up to 50% in autoconversion rates. AIE is mostly sensitive to CDNC parameterization; meteorology is of lesser importance.
A11A-0040
How well does aerosol index represent the number of cloud condensation nuclei in global models?
Aerosol index (the product of Angstrom exponent and aerosol optical depth) has been used as a proxy for cloud condensation nuclei (CCN) to study cloud/aerosol interactions in global models and in satellite data. However, aerosol size, composition, and meteorological conditions can have different impacts on aerosol index and CCN. In this study, we investigate the correlation between aerosol index and number of CCN in a global climate- aerosol model. Several different configurations of aerosol model components are used: one with predicted aerosol mass only; one with both predicted aerosol mass and sulfate aerosol number; one with predicted aerosol mass and nitrate in aerosol. The effect of predicted sulfate aerosol size and nitrate aerosols on correlations between aerosol index and the number of CCN is examined.
A11A-0041
Towards the Evaluation of Cloud-Aerosol Interactions using Super-Droplet Method
We have developed a novel, probabilistic, particle based cloud physics model, named Super-Droplet Method (SDM). SDM enables accurate calculation of cloud physics with less demanding cost in computation, and readily incorporate various processes of cloud physics, such as, several sorts of soluble/insoluble CCNs, and their chemical reactions. Though several validations and further development is still necessary, we expect that SDM provides a new approach to the quantitative evaluation of the indirect aerosol forcing on the climate change.
A11A-0042
Condensed Phase Organic Photochemistry in Atmospheric Aerosols
Many organic compounds that have been either observed or proposed to exist in atmospheric aerosols have significant UV-vis absorption cross-sections in the atmospheric window (< 290nm). Thus, photo-initiated reactions of these compounds may be important sinks for the particle-phase reactants as well as sources for new organic products in both the particle and gas phases. Currently, relatively little is known about the chemical fate of most organic compounds that absorb UV-visible radiation in the condensed phase, especially under the highly acidic conditions typical of sulfate aerosols in the upper troposphere and lower stratosphere. Here we present Attenuated Total Reflectance FTIR and UV-vis spectra that demonstrate chemical changes upon UV-vis irradiation of various organic compounds in sulfuric acid/water solutions under a range of acidities relevant for atmospheric aerosol particles and cloud droplets. For example, in aqueous methylglyoxal solution, the FTIR spectra show that UV-vis radiation alters the speciation of methylglyoxal among its various monomer (hydrated and dehydrated) and polymer forms. Concomitant changes in the preliminary UV-vis spectra confirm a changing distribution of carbonyl moieties and show that irradiation causes a decrease in absorption at atmospherically available wavelengths below 300 nm. These results are used to address the effects that particle-phase organic photochemistry may have on the optical properties and/or cloud processing of atmospheric aerosols.
A11A-0043
Analysis of aerosol properties from the AERONET over East Asia for a remote sensing
Most satellite-based algorithms utilize Look-Up Table (LUT) approach, precalculated with radiative transfer models which are based on many assumptions. These assumptions can sometimes lead to significant error in satellite-based algorithms in aerosol products. All available daily level 2.0 AERONET data over East Asia are compiled to investigate monthly and seasonal variations of aerosol properties (aerosol optical depth, aerosol size distribution of the particle volume, and complex refractive index). Correlation between the aerosol properties and precipitable water from the AERONET are also analyzed over the East Asia. For heavily-loaded case of aerosols, in particular, monthly means of aerosol optical depth show different pattern from that of normal cases. Most aerosol properties show seasonal pattern which strongly appears under heavy aerosol-loaded condition and is affected by the relative humidity. The precipitable water affects complex refractive index and aerosol size distribution. From this analysis, the extinction coefficients are calculated using the MIE code. Finally, it is shown that the total precipitable water affects the values of extinction coefficient. Results in this study can be applied directly to improve the retrieval accuracy of the aerosol-products for the satellite-based algorithm by providing realistic aerosol model in LUT calculation.
A11A-0044
Effects of Aerosol Optical Depth on diffuse UV and visible radiation
Ultraviolet radiation (UV, 300-367nm) was measured with a UV-multifilter rotating shadowband radiometer (UV- MFRSR) at Yonsei University, Seoul (37.57°N, 126.97°) for 7 months from January to July 2006 and visible irradiance (400-700 nm) also measured with a MFRSR for 12 months of 2006 at the same station. Spectral UV_AOD and vis_AOD were retrieved using the Langley method and Beer-Bouguer-Lambert's law, and compared with AOD obtained from Skyradiometer to validate their values. The diffuse and direct irradiance were analyzed to investigate the dependence on total optical depth (TOD) and aerosol optical depth (AOD). The direct-horizontal solar irradiance decreases exponentially as the optical depth increases according to the Beer- Bouguer-Lambert's Law. As the TOD and AOD increase, the diffuse-horizontal UV radiation gradually increases and shows a maximum value at some critical optical depth for a given SZA. Similar analysis was performed on the relation between the diffuse irradiance and AOD. RAF(radiation amplification factor) was used to correct the ozone effects on UV. These results provide empirical equations for the amount of diffuse irradiance in UV and visible wavelengths.
A11A-0045
An examination of oxidant amounts on secondary organic aerosol formation and aging
The effect of HOx radicals (OH and HO2) and ozone (O3) on secondary organic aerosol (SOA) formation and aging has been studied. Experiments were performed in the presence and in the absence of oxygen in a small chamber (~18 liters) for several organic precursor gases, including m-xylene, alpha-pinene and ethylbenzene. The HOx source was the UV photolysis of humidified air or nitrogen and was measured with the Penn State GTHOS (Ground-based Tropospheric Hydrogen Oxides Sensor). The precursor gases concentration was monitored with an online GC-FID. The aerosol mass was then quantified by a Tapered Element Oscillating Microbalance (TEOM). Typical oxidant mixing ratios were (0.??? -10) ppm for O3, (10-400) pptv for OH and (0.1-4) ppb for HO2. The SOA yields for different oxidant conditions will be discussed.
A11A-0046
New Aerosol and Cloud Products from the NASA Micro Pulse Lidar Network (MPLNET)
The NASA Micro Pulse Lidar Network (MPLNET) is a federated network of Micro Pulse Lidar (MPL) systems designed to measure aerosol and cloud vertical structure continuously, day and night, over long time periods required to contribute to climate change studies and provide ground validation for models and satellite sensors in the NASA Earth Observing System (EOS). At present, there are thirteen permanent sites worldwide, and five more to be completed soon. Numerous temporary sites have been deployed in support of various field campaigns since the start of MPLNET in 2000. Most MPLNET sites are co-located with sites in the NASA Aerosol Robotic Network (AERONET) to provide both column and vertically resolved aerosol and cloud data. Here we present a collection of new MPLNET aerosol and cloud products that are available. Scene classification is now provided continuously, including identification of multiple cloud layer heights (base and top), planetary boundary layer height, and the height of the highest aerosol layer. Our existing aerosol products have been enhanced to include continuous aerosol extinction profiles throughout the day (previously only available at AERONET observation times). PBL heights are generated using a wavelet technique. Cloud layer heights and subsequent optical depths are now provided to the limit of detection capability. New thick cloud optical depths in excess of 10 are also provided using a novel technique based on the lidar background signal. We will present an overview of the new products and methodology as well as data examples from several diverse sites in our network. http://mplnet.gsfc.nasa.gov
A11A-0047
A non-linear model for aerosol-cloud interactions
The interactions of aerosol and cloud, and their effects on radiation are complex; many aspects are poorely understood. The complexity is due to the aerosol-cloud interaction mechanism which takes place at smaller spatial scales. Existing parameterizations of this effect are derived using idealized cloud dynamics, aerosol composition and size distribution. However, satelite retrievals of cloud microphysical properties from the Moderate Resolution Imaging Spectroradiometer (MODIS) provide some insight into this mechanism. In our study we will use the information from satelite retrievals and construct a multilayer cloud-aerosol interaction model to study the effect on surface radiation. The results will be compared with satelite measurements and available cloud-aerosol interaction models.
A11A-0048
Aerosol radiative properties observed over M'Bour in Senegal during dry season – AMMA 2006 experiment
The presented study was devoted to characterization of airborne aerosol optical properties and radiative effect on climate during the AMMA intensive experiment in M'Bour, Senegal, 2006. The outbreaks of biomass burning aerosols during the dry season occur on a background of frequent dust storms in the region. We analyzed several aerosol events that took place from January to March 2006, which allowed characterization of airborne mineral dust and mixture of biomass burning aerosols with dust. Climatology of key aerosol parameters for the analyzed dry season of the AMMA campaign showed high variability of aerosol sizes and spectral absorption, although aerosol loading was below the average. The LIDAR profiles imply two layers structure for aerosol mixing events and one layer for a dust event. We used atmospheric radiative transfer forward model employed in AERONET retrieval code for assessing aerosol radiative effect and efficiency of different aerosol types observed over the site. For example, the mixture of dust with small strongly absorbing particles yield aerosol radiative efficiency at the surface up to twice higher than of dust alone. In addition, we evaluated the importance of accounting for desert dust particle non-sphericity in simulation of aerosol radiative effects. Indeed, the non-spherical particle model employed in AERONET retrieval results in robust improvements in reproducing atmospheric radiances measured during desert dust events. In radiative forcing simulations the effects of particle non-sphericity are usually neglected. Our test showed that accounting for particle non-sphericity can be significant in the cases of the pronounced desert dust events where diffused radiation dominates in total downward flux. The measurements showed that for some dust episodes the diffused radiation contribution is at least 65 % of the total downward radiation during a day. For such situations the neglect of the aerosol particles non-sphericity can results in overestimations of instantaneous aerosol radiative effect at the surface by the value of 8 to 21 Wm-2 for AOD at 440 nm ranging from 0.5 to 2.0.