A34C-01 INVITED
The Importance of Water Uptake by Aerosols in the Climate Change Problem
It is well understood that aerosol species have and are continuing to play a central role in the radiative forcing of the climate system. While the role of single-scattering properties of aerosols on climate is generally well- recognized, a key factor that governs the aerosol optical property viz., the hygroscopic growth has received insufficient attention particularly in terms of its role in the climatic impacts due to aerosols. A sensitivity investigation is performed that quantitatively highlights the consequence of the growth of sea-salt-organic carbon mixtures for radiative forcing. Next, we employ the GFDL coupled atmosphere-ocean model to study specifically the aerosol radiative forcing and climate response arising due to the hygroscopic features of sulfate aerosols as they have increased from preindustrial to present-day. We make use of observations of optical depth and surface concentrations to evaluate the reliability of the simulated hygroscopic growth. Regional climate responses in Europe, Asia and Africa are examined, with a focus on temperature, hydrological cycle and surface energy budgets. The importance of hygroscopicity in the climate change problem is put in perspective by comparing the climatic effects with those due to aerosol absorption as well as with those caused by the infrared-absorbing long- lived greenhouse gases. Further, we explore the climate consequence arising from the scenarios of the future emissions of aerosols and the associated hygroscopicity effects.
A34C-02
The importance of aerosol water for air pollution effects on weather and climate
We apply a new concept to study air pollution effects on weather and climate, which is based on thermodynamic principles that explain hydration and osmosis - including the required transformation of laboratory based concepts to atmospheric conditions. Under ambient conditions the equilibrium relative humidity (ERH) determines the saturation molality, solute and solvent activities (and activity coefficients), and the aerosol associated water mass, sine the water content is fixed by ERH for a given aerosol concentration and type. As a consequence, aerosol water drives the gas/liquid/solid aerosol partitioning, ambient aerosol size-distributions and directly links aerosol hygroscopic growth into fog, haze and clouds. Various modeling results indicate that a) our new concept is not limited to dilute binary solutions, b) sensitive aerosol properties such as the pH of binary and mixed inorganic/organic salt solutions up to saturation can be computed accurately, and c) that anthropogenic emissions can be directly linked to visibility reduction, cloud formation and climate forcing, if we explicitly account for the aerosol water mass. Our new concept is more explicit than the traditional CCN concept as it abandons the use of ambiguous terms such as "marine" and "continental" aerosols, and refines lumped categories such as mineral dust, biomass burning, sea salt, organic or sulfate aerosols currently used in atmospheric modeling. Despite, our concept is computationally very efficient as it allows solving the whole gas/liquid/solid aerosol partitioning analytically without numerical iterations. It is therefore especially suited for regional high resolution, or global climate applications. http://www.atmos-chem-phys.net/7/3163/2007/acp-7-3163-2007.html
A34C-03 INVITED
Effects of Organic Particle Types and Mixtures on Aerosol Water
Some organic compounds are hydrophobic, limiting their water uptake in aerosol particles. If such compounds represent a significant fraction of atmospheric particle numbers, then the expected direct aerosol radiative forcing is smaller than if all particles grow with the efficiency of sulfate in humid conditions. Internal mixtures of organic compounds with sulfate can take up water efficiently. The distribution of organic compounds among particles can have an important effect on aerosol water. Recent measurements by near-edge x-ray absorption fine spectrum (NEXAFS) scanning transmission microscopy (STXM) show that organic particles include many different compositions, including fourteen distinct organic compositions. Yet ambient measurements often reflect internally mixed particles with similar hygroscopic properties. This apparent contradiction increases the uncertainty with which future changes in aerosol composition impact climate.
A34C-04
Retrieval of Aerosol Liquid Water Path and Hygroscopic Growth from the AERONET database
Aerosol water uptake in the atmosphere alters aerosol size distributions, changes aerosol optical properties, and consequently impacts the aerosol radiative forcing. The parameterization of water uptake in aerosol transport models is empirical and unchecked, largely because of the lack of available measurements. We have developed a retrieval of the aerosol liquid water path (LWP) and hygroscopic growth using the AERONET database, and apply our results at over 200 AERONET locations. The real refractive index of prevalent atmospheric aerosols at the 500 nm wavelength ranges from 1.50 for sea salt to 1.56 for dust. Liquid water, on the other hand, has a real refractive index of 1.33 at 500 nm. We use this disparity in refractive indices to determine the equivalent water fractions for aerosol mixtures associated with the AERONET refractive index retrievals. Once the aerosol water fraction is known, we can also determine the hygroscopic growth factor, aerosol LWP, and the dry aerosol loading in the atmospheric column. Our aerosol LWP results are highly correlated with the following parameters: fine mode volume concentration, aerosol optical thickness (440 nm), aerosol water fraction, and coarse mode volume concentration (average correlation coefficients for all 200 sites ranges from 0.47 for the coarse mode concentration to 0.78 for the fine mode concentration). The regressions of these parameters have different slopes at each of the AERONET sites, which suggests a LWP sensitivity to aerosol composition. Aerosol liquid water path is not highly correlated with the mode radii or the fine mode volume fraction (average correlation coefficients for all sites range from 0.11 for the coarse mode median radius to 0.27 for the fine mode volume fraction). We compiled monthy-averaged statistics at over 200 AERONET locations based upon the daily-average AERONET retrievals; our results are consistent with regional and seasonal expectations. Aerosol water fractions range from a low value of 0.14 at the Etosha Pan site (Namibia) to a high value of 0.73 at the Anmyon site (South Korea) in August, which is consistent with the arid and humid climates of those two sites. Likewise, the hygroscopic growth factor ranges from 1.05 at Etosha Pan to 1.69 at Anmyon for that month. The aerosol water fraction does not necessarily dominate the aerosol LWP (the effective radius and aerosol loading are also important), so these two sites do not represent the extreme aerosol LWPs. Rather, Beijing had the highest average LWP for August, with a value of 120 mg m-2, and Tinga Tingana in Australia had the lowest August average with a value of 5 mg m-2. We will present the regional and seasonal variability of these retrieved parameters at several representative AERONET sites. We will also discuss the sensitivity of this retrieval to a variety of aerosol mixtures.
A34C-05
Measurements of Aerosol Hygroscopic Growth From Eight Different Regions and Aerosol Types
The change in the aerosol scattering coefficient with relative humidity has been measured at eight regions across the Earth over a time period of a few months to eight years. The measurement sites include the Arctic, Northeastern North America, Central US, Northwestern US, the Korean Peninsula, Indian Ocean, African Sahel and Central Europe. The aerosol types in these studies include mixtures of dust, smoke, pollution and marine aerosol. The covariance in the aerosol hygroscopic growth factor with other aerosol properties and source regions will be discussed as well as implications for direct and indirect radiative forcing.
A34C-06
The Influence of Sodium Methanesulfonate on Hygroscopic and Reactive Properties of NaCl Particles
The most important precursor for sulfate aerosol over the oceans is gaseous dimethyl sulfide (DMS), which is produced by phytoplankton and subsequently emitted to the atmosphere where it is oxidized yielding variety of products. In the past, much attention has been given to the physico-chemical properties of the mixed sea salt/sulfate particles that can be formed as a result of DMS-to-SO2-to-H2SO4 reaction sequence. For many years, effective conversion of DMS to particulate sulfate was assumed to be the dominant reaction pathway for DMS in the marine boundary layer. However, recent modeling studies and field observations indicate that under certain conditions DMS does not predominantly convert to sulfate, but rather ends up in sea salt particles as methanesulfonic sodium salt (CH3SO3Na), which previously have been considered only of minor importance. In contrast to the NaCl and Na2SO4 salts, CH3SO3Na is a surfactant and presence of this specie in sea salt particles can lower the surface tension, impact molecular processes at the air/water interface, thus affecting heterogeneous reactivity of sea salt aerosol as well as CCN activity. The presented work is focused on combined experimental and modeling studies aimed at characterizing the hygroscopic properties and chemical reactivity of mixed NaCl/CH3SO3Na particles. Heterogeneous reaction kinetics of gaseous nitric acid with particles were investigated with a novel Particle-on-Substrate Stagnation Flow Reactor approach under conditions, including particle size, relative humidity and reaction time, directly relevant to the atmospheric chemistry of sea salt particles. The reaction kinetics was followed by observing chloride depletion in the particles by computer-controlled scanning electron microscopy with energy- dispersive X-ray analysis (CCSEM/EDX). We also investigated the influence of CH3SO3Na on hygroscopic property of NaCl particles by Micro-FTIR. In addition, scanning electron microscopy (SEM) mapping, time-of-flight secondary ion mass spectrometry (TOF-SIMS) imaging and depth profiling, and Molecular Dynamic (MD) simulation were performed to provide a better understanding of layered structures of deliquesced and dry NaCl/CH3SO3Na particles and assess their effects on chemical reactivity and hygroscopicity. Experimental results show that presence of CH3SO3Na has little influence on hygroscopic properties of NaCl particles. However, it could alter reactive uptake of HNO3 onto NaCl to varying degrees, depending on the RH and amount of CH3SO3Na present in particles.
A34C-07 INVITED
The Importance of Aerosol Water
Aerosol plays a central role in atmospheric processes, and understanding its interplay with atmospheric water is crucial for quantifying its indirect and direct climatic effects. This talk will give an overview of the impact of aerosol water (either liquid or adsorbed) on the ability of aerosol to nucleate cloud droplets and ice. We will also discuss the importance of aerosol phase and relative humidity history in controlling the mass partitioning of semivolatile compounds. Results taken from in-situ and laboratory experiments will illustrate the importance of knowing aerosol-water interactions and particle phase state.