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Measuring and modeling the hygroscopic growth of two humic substances in mixed aerosol particles of atmospheric relevance

by I. R. Zamora, M. Z. Jacobson
Atmospheric Chemistry and Physics ()
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The hygroscopic growth of atm. particles affects atm. chem. and Earth's climate. Water-sol. org. carbon (WSOC) constitutes a significant fraction of the dry submicron mass of atm. aerosols, thus affecting their water uptake properties. Although the WSOC fraction is comprised of many compds., a set of model substances can be used to describe its behavior. For this study, mixts. of Nordic aquatic fulvic acid ref. (NAFA) and Fluka humic acid (HA), with various combinations of inorg. salts (sodium chloride and ammonium sulfate) and other representative org. compds. (levoglucosan and succinic acid), were studied. We measured the equil. water vapor pressure over bulk solns. of these mixts. as a function of temp. and solute concn. New water activity (aw) parameterizations and hygroscopic growth curves at 25°C were calcd. from these data for particles of equiv. compn. We examd. the effect of temp. on the water activity and found a max. variation of 9% in the 0-30 °C range, and 2% in the 20-30 °C range. Five two-component mixts. were studied to understand the effect of adding a humic substance (HS), such as NAFA and HA, to an inorg. salt or a saccharide. The deliquescence point at 25 °C for HS-inorg. mixts. did not change significantly from that of the pure inorg. species. However, the hygroscopic growth of HA/inorg. mixts. was lower than that exhibited by the pure salt, in proportion to the added mass of HA. The addn. of NAFA to a highly sol. solute (ammonium sulfate, sodium chloride or levoglucosan) in water had the same effect as the addn. of HA to the inorg. species for most of the water activity range studied. Yet, the water uptake of these NAFA mixts. transitioned to match the growth of the pure salt or saccharide at high aw values. The remaining four mixts. were based on chem. compn. data for different aerosol types. As expected, the two solns. representing org. aerosols (40% HS/40% succinic acid/20% levoglucosan) showed lower water uptake than the two solns. representing biomass burning aerosols (25% HS/27% succinic acid/18% levoglucosan/30% ammonium sulfate). However, interactions in multicomponent solns. may be responsible for the large variation of the relative water uptake of identical mixts. contg. different HSs above a water activity of 0.95. The ZSR (Zdanovskii, Stokes, and Robinson) model was able to predict reasonably well the hygroscopic growth of all the mixts. below aw = 0.95, but produced large deviations for some multicomponent mixts. at higher values. [on SciFinder(R)]

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