Calculating CCN Activity of Ambient ATOFMS Measurements During a Period of Changing Meteorological Conditions Kaitlyn Suski et al.1 Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, CA, USA. Aerosols have both direct and indirect effects on climate. The direct effect is characterized by a particle’s ability to scatter and absorb radiation. These particles can also nucleate cloud droplets. Once activated, these cloud condensation nuclei (CCN) can go on to form clouds, which also scatter and absorb radiation (the indirect aerosol effect). Changing aerosol size and chemistry will affect CCN activity and cloud microphysical properties. Here we show that CCN activity can change dramatically in a short amount of time due to shifting winds and changing meteorological conditions. During a study in Riverside, CA in September of 2007, single particle dual polarity Aerosol Time-of-Flight Mass Spectrometry (ATOFMS) measurements were made that showed three time periods where the aerosol chemistry was dominated by one particle type. A large increase in salt concentration was observed when the winds were coming from the west. Then, a sudden shift to winds from the east resulted in a large concentration of elemental carbon mixed with organic carbon (ECOC), followed closely by a rise in biomass burning particles. A Kohler theory model was used to estimate the number of particles likely to activate and a ratio of activated to total particles was calculated for particles assuming the dominant particle type made up all of the particles (CCNpure) and including all of the particle types (CCNmixed). These calculations suggest that the salt and ECOC periods' CCN activity agreed fairly well with CCNmixed/CCNpure of 1.00 and 0.998, respectively. The biomass period however had less agreement with CCNmixed/CCNpure of 0.988. Our results show that local CCN activity is highly variable on a short time scale and that the dominant particle type can be used to predict CCN activity if it is 50 percent or more of the total particles. Introduction Clouds play an important role in the radiative balance of the Earth. Depending on cloud droplet properties, clouds can scatter or absorb solar, IR and microwave radiation resulting in a warming or cooling of the Earth's surface. Cloud droplets form on aerosols, which when activated are known as cloud condensation nuclei (CCN). The ability of a particle to serve as a CCN is based on its water solubility, chemical composition, and size, as described by the Kohler theory [Kohler, 1936]. Kohler theory models give a good approximation of the critical supersaturation necessary for a particle to activate or take up water. Aerosol chemistry and concentration vary greatly spatially and temporally, which complicate predictions of CCN activity. CCN predictions can be simplified if we can make assumptions about CCN using only the dominant particle type. The dominant particle type in a given location can change dramatically in a short amount of time due to changing meteorological conditions, thus changing the number of CCN active particles. ATOFMS [Gard et al., 1997] has been used to measure aerosol distributions in real time. Two scattering laser beams are used to estimate particle size utilizing the Time-of-Flight technique [Gard et al., 1997] and a Nd-YAG laser is used to ionize the particle to measure the positive and negative ions present. It has been shown [Song et al., 1999, Fergenson et al., 2001] that positive ions can be used to determine the source of the aerosol, while the negative ions can be used to estimate the amount and type of atmospheric processing or aging that has occurred. Methods ATOFMS was used to measure ambient air during a September 2007 field study in Riverside, CA. Changes in wind direction caused anomalous increases in particle concentrations, with the dominant particles types changing rapidly. On September 4, 2007 Santa Ana winds brought salts, likely dry lake bed salt, in from the east. Then, on September 6, 2007 the winds came from the west and a large increase in ECOC concentration was observed, followed by an increase in biomass burning particles.Figure 1. Relative Fraction of particle types for each time period: Salt, ECOC, and Biomass. In this three day period, the dominant particle types changed rapidly, which according to Kohler Theory, [Kohler, 1936] would have significantly modified local CCN activity. The ATOFMS data was separated into three time periods based on the three distinct dominant particle types. Dividing the data in this way allowed an evaluation of the effects of chemical composition on CCN activity. In this study, a Kohler Theory model was used to predict an activated fraction of particles using the dominant fraction of particles as the only CCN source, which was termed CCNpure, and using all of the particle types as CCN sources, termed CCNmixed. The two predicted activated fractions from CCNpure and CCNmixed were compared to see if CCN activity can be predicted accurately using only the dominant particle type as the CCN source. This is an important question to answer because there are a lot of uncertainties in estimating aerosol indirect effects and model calculations can benefit from any simplifications that can be made. The indirect effect is characterized by aerosol influence on cloud formation and the resulting changes in radiative properties. In the submicron size range, CCN activity is influenced mostly by particle chemistry [Kohler, 1936]. Therefore, changing the chemistry of a particle will change the CCN activity and thus the type of cloud droplet that could form. The CCN activity of the three main particle types in the study was calculated using a Kohler Theory Model developed by Gregory Roberts [Roberts et al., 2002]. The model predicts the number of particles that could activate at a given supersaturation based on chemical composition and size using equation 1 [Seinfeld and Pandis, 2006]: ln(pw(Dp)/p0) = (4Mw σw/RTρwDp) – (6nsMw/πρwD3p) (1) Where Mw is the molecular weight of water, ρw is the density of water, Dp is the particle diameter, p0 is the vapor pressure over a flat surface at a given temperature, ns is moles of solute and σw is the surface tension of water. Salt ECOC BiomassDensity (g/cm^3) 2.165 1.9 1.618 Solubility (g/mL H2O) 0.36 0.0052 infinite van Hoff Factor 2 1 1 Molecular Weight (g/mol) 58.44 252 162.14
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