A23D-1565
Aerosol Processing in Mixed-Phase Clouds in ECHAM5-HAM: Comparison of Single-Column Model Simulations to Observations
The global aerosol-climate model ECHAM5-HAM (Stier et al., 2005) has been extended by an explicit treatment of cloud-borne particles. Two additional modes for in-droplet and in-crystal particles are introduced, which are coupled to the number of cloud droplet and ice crystal concentrations simulated by the ECHAM5 double-moment cloud microphysics scheme (Lohmann et al., 2007). Transfer, production and removal of cloud-borne aerosol number and mass by cloud droplet activation, collision scavenging, aqueous-phase sulfate production, freezing, melting, evaporation, sublimation and precipitation formation are taken into account. The model performance is demonstrated and validated with observations of the evolution of total and interstitial aerosol concentrations and size distributions during three different mixed-phase cloud events at the alpine high-altitude research station Jungfraujoch (Switzerland) (Verheggen et al, 2007). Although the single-column simulations can not be compared one-to-one with the observations, the governing processes in the evolution of the cloud and aerosol parameters are captured qualitatively well. High scavenged fractions are found during the presence of liquid water, while the release of particles during the Bergeron-Findeisen process results in low scavenged fractions after cloud glaciation. The observed coexistence of liquid and ice, which might be related to cloud heterogeneity at subgrid scales, can only be simulated in the model when forcing non-equilibrium conditions. References: U. Lohmann et al., Cloud microphysics and aerosol indirect effects in the global climate model ECHAM5-HAM, Atmos. Chem. Phys. 7, 3425-3446 (2007) P. Stier et al., The aerosol-climate model ECHAM5-HAM, Atmos. Chem. Phys. 5, 1125-1156 (2005) B. Verheggen et al., Aerosol partitioning between the interstitial and the condensed phase in mixed-phase clouds, Accepted for publication in J. Geophys. Res. (2007)
A23D-1566
Simulation of Asia Dust and Cloud Interaction Over Pacific Ocean During Pacdex
The effect of dust plume on the Pacific cloud systems and the associated radiative forcing is an outstanding problem for understanding climate change. Many studies showing that dust aerosol might be a good absorber for solar radiation, at the same time dust aerosols could affect the cloud's formation and precipitation by its capability as cloud condensation nuclei (CCN) and ice forming nuclei (IFN). But the role of aerosols in clouds and precipitation is very complex. Simulation of interaction between cloud and dust aerosols requires recognition that the aerosol cloud system comprises coupled components of dynamics, aerosol and cloud microphysics, radiation processes. In this study, we investigated the interaction between dust aerosols and cloud with WRF which coupled with detailed cloud microphysics processes and dust process. The observed data of SACOL (Semi-Arid Climate and Environment Observatory of Lanzhou University) and PACDEX (Pacific Dust Experiment) is used as the initialization which include the vertical distributions and concentration of dust particles. Our results show that dust aerosol not only impacts cloud microphysical processes but also cloud microstructure; Dust aerosols can act as effective ice nuclei and intensify the ice-forming processes. http://climate.lzu.edu.cn/~jhuang/
A23D-1567
Deep Convective Cloud Properties and Aerosol Influences as Observed from A-Train Satellites
Deep convective clouds (DCC) are studied with the Aqua and Terra MODIS cloud products of cloud optical depth (COD), ice particle effective radius (IER), frequency of occurrence, etc. The DCC cloud properties are analyzed together with aerosol retrievals. IER is shown to be affected by cloud top brightness temperature, surface elevation, aerosol concentration and availability of giant cloud condensation nuclei (GCCN). DCCs developed over elevated areas tend to have smaller IER at the cloud top. We attribute this to colder cloud base and thinner cloud depth for mountainous region. Simultaneous observations of aerosols and clouds made by NASA's A-Train provide direct evidence of aerosol-cloud interaction for DCC. Increased level of aerosols reduces IER because of more activated droplets, delayed coalescence processes and possibly homogeneous freezing of numerous small droplets. On the other hand, larger IER is observed for clouds developed near the source of dust. We argue that activation of large dust particles as GCCN and/or as heterogeneous freezing ice nuclei may lead to early glaciation and thus larger IER. We studied the latitudinal variations of IER's dependence on temperature for DCCs and explain it through a simple thermodynamic model. We found a strong connection between DCC frequency pattern and precipitation pattern for several parts of the world. The diurnal variation of DCC top microphysical properties is linked to diurnal precipitation. General properties of DCCs presented in this study and their variations are useful for cloud parameterization in GCM models and our findings of aerosol influence on cloud top properties provides new insight to the aerosol-cloud-precipitation interaction.
A23D-1568
Chemical Characterization of Secondary Organic Aerosol Formed Through Cloud Processing of Methylglyoxal
There is increasing evidence suggesting that secondary organic aerosol (SOA) forms as a result of low volatility product formation in atmospheric aqueous phase reactions. In this work aqueous phase photooxidation experiments between methylglyoxal (an isoprene oxidation product) and hydroxyl radical were conducted to simulate the cloud processing of methylglyoxal. The results verify that, as predicted, oxalic acid forms through cloud processing of methylglyoxal. This work adds to the growing body of literature (Altieri et al., 2006; Carlton et al., 2006; Carlton et al., 2007; Crahan et al., 2004; Warneck, 2003; 2005; Yu et al., 2005) supporting the hypothesis that cloud processing is a substantial source of oxalic acid to the atmosphere. Oxalic acid is the most abundant dicarboxylic acid in the atmosphere and a contributor to SOA. The formation of additional monomer products (e.g., malic acid, succinic acid, glycolic acid) and the development of an oligomer system were also identified through use of a combination of electrospray ionization mass spectrometry (ESI-MS) techniques: a quadrupole ESI-MS, an ion trap ESI-MS-MS, and an ultra-high resolution ESI FT-ICR MS. We propose a mechanism of oligomer formation through esterification of monomers with a hydroxy acid formed from hydroxyl radical initiated reactions. Oligomers were only recently identified as cloud processing products (Altieri et al., 2006), and this work is the first chemical characterization of oligomers formed through cloud processing reactions. The chemical characterization includes the distribution of molecular weights, elemental compositions, structure, and organic mass to organic carbon (OM:OC) ratio. Methylglyoxal is a water- soluble product of both biogenic and anthropogenic hydrocarbon oxidation. The varied and multiple sources of methylglyoxal suggest there is strong potential for these low volatility products (e.g., oxalic acid and oligomers) to significantly contribute to SOA.
A23D-1569
Initial High Spectral Resolution Lidar Results From the Cumulus Humilis Aerosol Processing Study (CHAPS) and Cloud and Land Surface Interaction Campaign (CLASIC)
The Cumulus Humilis Aerosol Processing Study (CHAPS) field campaign, sponsored by the Department of Energy Atmospheric Science Program (ASP), and the Cloud and Land Surface Interaction Campaign (CLASIC), sponsored by the DOE Atmospheric Radiation Measurement (ARM) Program, were performed during June 2007, with the respective goals of studying aerosol and cloud interactions and advancing the understanding of how land surface processes influence cumulus convection in the Oklahoma City region. The campaigns involved several coordinated air- and ground-based remote and in situ sensors, including, among others, the NASA Langley Research Center (LaRC) Airborne High Spectral Resolution Lidar (HSRL) aboard the NASA King Air B-200 aircraft, instruments located at the DOE Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) Climate Research Facility (CRF), and instruments aboard the DOE Gulfstream-1, the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter, and the DOE ARM Cessna 206 aircraft. The aircraft were flown in coordinated patterns over common ground tracks on several occasions, some of which involved passing over the SGP CRF and/or were coordinated with overpasses from satellites in the A-Train constellation. These coordinated measurements enabled coincident observations of aerosol properties near clouds, facilitating analyses of cloud and aerosol interactions and properties across changing atmospheric conditions. Over the course of the CHAPS-CLASIC mission, the NASA LaRC Airborne HSRL instrument flew 22 flights obtaining nearly 70 hours of observations. The Airborne HSRL directly measured aerosol backscatter and depolarization at wavelengths of 532 nm and 1064 nm, and aerosol extinction at 532 nm, allowing for calibrated computation of the aerosol extinction-to-backscatter ratio, wavelength dependence, and depolarization ratio profiles extending from near the surface to about 8 km above ground level. An overview of the NASA LaRC HSRL mission, as well as preliminary results from the HSRL during the CLASIC/CHAPS field campaign will be presented.
A23D-1570
Aerosol Optical and Chemical Properties Within and Without Clouds During an Airborne Field Campaign in Central Oklahoma
The optical properties of aerosol particles are one of the controlling factors in determining direct aerosol radiative forcing. These optical properties depend on the chemical composition and size distribution of the aerosol particles, which can change due to various processes during the particles' lifetime in the atmosphere. Here we present preliminary results showing aerosol optical and chemical properties obtained during the CHAPS field campaign within cloud drops and outside of clouds. The Cumulis Humilis Aerosol Processing Study (CHAPS), sponsored by the DOE Atmospheric Science Program (ASP), took place in the vicinity of Oklahoma City in June, 2007. The intention of the study was to investigate the influence of clouds on aerosols and of aerosol on clouds. Duplicate sets of in-situ aerosol optical instruments were deployed on the ASP G-1 aircraft during the CHAPS campaign. One set of instruments was downstream of an isokinetic inlet designed to sample the ambient aerosol, the other set was downstream of a counterflow virtual impactor (CVI) designed to sample and dry cloud droplets so that the cloud drop nuclei could be studied. Each instrument set comprised a 3-wavelength particle soot absorption photometer (PSAP) and integrating nephelometer to provide spectral aerosol absorption, scattering and back-scattering and a particle counter to obtain aerosol number concentration. In addition, a time-of-flight aerosol mass spectrometer (ToF-AMS) was able to sample on either inlet to provide information about the non-refractory chemical composition of the aerosol and cloud drop residuals. The data presented here will describe both how aerosol optical properties change upstream and downstream of a mid-size conurbation (Oklahoma City) and how ambient aerosol optical properties differ from those of the cloud drop nuclei. These changes in aerosol optical properties will be placed in the context of differences in chemical composition derived from the ToF-AMS. http://asp.labworks.org/
A23D-1571
Errors of Nonlinear Advection Algorithms and Their Implications for Studying the Aerosol- Cloud Interaction
Monotonic numerical schemes are commonly used in modeling tracer transport in the atmosphere to prevent the appearance of non-physical negative values and various flux correction procedures are applied to preserve sharp gradients in the simulated fields. Because these monotonicity and flux correction constrains are nonlinear, the monotonicity for individual species does not guarantee monotonicity for their sum or other linear combination. As a result, artificial (numerical) redistribution of total aerosol number concentration in a 3D model, for example, can mask the effects of real aerosol processing by clouds. The problem is potentially relevant to any model that relies on preserving certain relations between independently advected variables (e.g., multi-moment aerosol/cloud modules, sectional microphysics, multicomponent chemistry). This study quantifies the errors in three widely used advection algorithms by extending traditional single-shape advection tests to a multiple shapes and multiple species with prescribed linear sum. A normalization procedure that preserves positive definiteness of individual variables and monotonicity of their linear combination, such as the total number concentration of aerosol particles, is also discussed.
A23D-1572
Aerosol Chemical Composition and its Effects on Cloud-Aerosol Interactions during the 2007 CHAPS Experiment
Chemical composition of submicron aerosol particles was determined using an Aerodyne Time-of-Flight Aerosol Mass Spectrometer (AMS) outfitted on the DOE G-1 aircraft during the Cumulus Humilis Aerosol Processing Study (CHAPS) conducted in Oklahoma City area in June 2007. The primary objective of CHAPS was to investigate the effects of urban emissions on cloud aerosol interactions as a function of processing of the emissions. Aerosol composition was typically determined at three different altitudes: below, in, and above cloud, in both upwind and downwind regions of the urban area. Aerosols were sampled from an isokinetic inlet with an upper size cut-off of ~1.5 micrometer. During cloud passages, the AMS also sampled particles that were dried from cloud droplets collected using a counter-flow virtual impactor (CVI) sampler. The aerosol mass concentrations were typically below 10 microgram per cubic meter, and were dominated by organics and sulfate. Ammonium was often less than required for complete neutralization of sulfate. Aerosol nitrate levels were very low. We noted that nitrate levels were significantly enhanced in cloud droplets compared to aerosols, most likely resulting from dissolution of gaseous nitric acid. Organic to sulfate ratios appeared to be lower in cloud droplets than in aerosols, suggesting cloud condensation nuclei properties of aerosol particles might be affected by loading and nature of the organic components in aerosols. In-cloud formation of sulfate was considered unimportant because of the very low SO2 concentration in the region. A detailed examination of the sources of the aerosol organic components (based on hydrocarbons determined using a proton transfer reaction mass spectrometer) and their effects on cloud formation as a function of atmospheric processing (based on the degree of oxidation of the organic components) will be presented.
A23D-1573
Measurement of the VOC Environment from an Aircraft Platform during an Aerosol-Cloud Interaction Study Near Oklahoma City
The interaction and relationship between atmospheric aerosols and ambient VOC concentrations is a current area of activity in atmospheric research both in urban and remote environments. Current atmospheric models seriously under predict the amount of secondary organic aerosol (SOA), highlighting the importance of simultaneous particle and VOC measurements and their importance to climate change models. The Cumulus Humilis Aerosol Processing Study (CHAPS) campaign was conducted in June of 2007 near Oklahoma City to investigate the mutual interaction of anthropogenic aerosols and clouds on each other. An important part of this study was characterizing the temporally and spatially resolved VOC environment in which urban aerosols and cumuliform clouds in the Oklahoma City environment exist. We report results from the proton transfer reaction mass spectrometer (PTR-MS) aboard the DOE Gulfstream 1 (G1) research aircraft to perform time-resolved measurements of the ambient VOC species and concentrations in conjunction with the suite of aerosol, trace gas and meteorological instruments aboard the G1. Several classes of VOC's were observed. Benzene, toluene, C2-benzenes and other species indicative of primary urban emissions were found in the Oklahoma City plume, strongly correlated with CO and other inorganic urban trace gases. Oxygenated species such as methanol, acetone and acetaldehyde were observed with no well defined spatial or temporal variation. Finally, species typical of biogenic emissions, isoprene and its oxidation products were seen in broad but definite spatial distributions, not correlated with the urban gas phase products as measured with the PTR-MS and other instrumentation. The concentration levels of these species and possible ground-based sources will be investigated and reported in this presentation. Correlation with aerosol instrumentation such as the Aerodyne aerosol mass spectrometer (AMS), nephelometer and other optical measurements will also be reported, both within and out of cloud environments. These correlations will be examined for possible influences of the local VOC species on the cloud-aerosol systems under study.
A23D-1574
Cloud water and aerosol studies in a background marine environment
The study of aerosol and cloud water chemical composition is essential to understand cloud processing of different compounds, determining which species are more efficiently removed and which ones stay longer in the atmosphere and, therefore, are more important for aerosol climate forcing. As part of the Rain In Cumulus over the Ocean Experiment (RICO), cloud water and aerosol samples were collected in Puerto Rico. We present concentrations of water-soluble ions, total and dissolved organic carbon (TOC and DOC), total nitrogen (TN), and the speciation of nitrogen compounds (amino acids) for water and aerosol samples collected at East Peak and Cape San Juan, Puerto Rico. Mass and elemental/organic carbon (EC, OC) concentrations were also determined for the aerosol samples. The results show average concentrations of TOC and TN in cloud water of about 1.1 mg/L and for DOC about 0.9 mg/L. The DOC/TOC ratio averaged 0.78, indicating that most of the organic compounds present are dissolved in the cloud water. TOC was composed mainly of organic acids (47 percent) and TN of inorganic species (80 percent). With respect to the aerosol samples, the average mass concentration of fine particles (Dp < 1.7 um) was 2.4 ug/m3. EC was found at low-to-non detectable levels (< 0.5 ng/m3). The concentrations of OC, DOC, TOC, and TN ranged from 30 to 100 ng/m3. The size distributions showed that OC and TN were mainly present in the fine particle fractions (Dp < 1 um). The predominant ions for both cloud and aerosol samples were Cl- and Na+, the primary components of sea salt. However, when air masses arrived from Northwest Africa or from islands upwind of Puerto Rico there was a decrease in Na+ and Cl- concentrations and an increase in SO42-, NH3+ and Ca2+ concentrations, likely reflecting anthropogenic and crustal sources of these species. Overall, the average concentrations of all species are similar to those typically found in background (remote) environments; however, these concentrations showed a significant increase when aged pollution from Northwest Africa and/or fresh pollution from neighboring Caribbean islands reached Puerto Rico. Speciation of nitrogen compounds and details about the size distributions results will also be presented at the conference.