HR: 0800h
AN: A41D-0742    [Abstracts]
TI: Water Activity Limits the Hygroscopic Growth Factor of Organic Aerosols
AU: * Rodriguez, L I
EM: luziro@stanford.edu
AF: Stanford University - Geophysics Department, Mitchell Building, Room 360 397 Panama Mall, Stanford, CA 94305, United States
AU: Cabrera, J A
EM: cabreraj@stanford.edu
AF: Stanford University - Geophysics Department, Mitchell Building, Room 360 397 Panama Mall, Stanford, CA 94305, United States
AU: Golden, D
EM: david.golden@stanford.edu
AF: Stanford University - Mechanical Engineering Department, Building 520, Stanford, CA 94305, United States
AU: Tabazadeh, A
EM: azadeht@stanford.edu
AF: Stanford University - Geophysics Department, Mitchell Building, Room 360 397 Panama Mall, Stanford, CA 94305, United States
AB: In this work we study the hygroscopic behavior of organic aerosols, which has important implications for Earth's climate. The hygroscopic growth factor (HGF) is defined as the ratio of the diameter of a spherical particle when it is exposed to dry conditions to that at humid conditions. We present a new formulation to express the HGF of an aerosol particle as a function of water activity (a_w) in the aqueous phase. This new formulation matches reported HGFs for common inorganic salts and water-miscible organic particles that are known to deliquesce into aqueous drops at high relative humidities (RH). Many studies use tandem differential mobility analyzers (TDMA) to determine the HGF of organic aerosols. For example, Brooks et al. used a TDMA to measure a HGF of 1.2 for 2 μm phthalic acid (PA) particles at 90% RH (a_w= 0.9). However, water activity limits the growth of a particle that can be attributed to water uptake. We have assembled a vapor pressure apparatus to measure a_w of aqueous solutions at room temperature. Measured water activities for PA, used in our growth formulation, yield a HGF of ~ 1.0005 for 2 μm PA particles at 90% RH. Comparing our results against Brooks et al. suggests that TDMA experiments may grossly overestimate the HGF of PA particles since water activity limits this growth to below 1.0005. Alternatively, we suggest that the adsorption of a negligible mass of water by a highly porous PA particle can lead to an apparent growth in particle size by changing its morphology. Other studies also use TDMAs to measure HGFs of secondary organic aerosols (SOAs). HGFs reported for SOAs are very similar to PA, suggesting that the observed growth may be due to morphological changes in particle size rather than water uptake as commonly assumed. We built a smog chamber where an organic precursor, such as d-limonene, reacts with nitrogen oxides under UV radiation to produce SOAs. We compare the HGFs for SOAs obtained with our method to those obtained with TDMA experiments. Our results suggest that TDMAs may provide erroneous HGFs for non-spherical, porous organic particles.
DE: 0305 Aerosols and particles (0345, 4801, 4906)
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting