Dominance of solubility over speciation in thermodynamic water uptake model: a sensitivity study for Barrow, Alaska particle hygroscopic growth

The ability of aerosol particles to take up water before supersaturated activation is defined as hygroscopic growth. The particle hygroscopic growth factor (HGF) is required for calculations of aerosol optical properties. To represent HGF in real environments, it is desirable to initialize a thermodynamic water uptake model with ambient aerosol measurements. The assumptions needed to adapt measurements especially those of the organic fraction into the necessary model inputs are tested here. Before HGF could be calculated, measured concentrations of three organic functional groups (alcohol, alkane and acid) were apportioned among five organic compounds (n-nonadecane, adipic acid, oxalic acid, citric acid and levoglucosan) by balancing the moles of each group between compounds. The same method was performed on inorganic ions to obtain electrolyte compositions required by the model. To test the sensitivity of HGF calculations on actual measurements, a subset of submicron particle filter observations from Barrow, Alaska was used to constrain an organic-electrolyte water uptake model. Model results showed larger changes in HGF around the base case (range = 1.34 1.45) when levoglucosan was varied. Smaller magnitude change was observed when citric acid and adipic acid were varied (range = 1.37 1.40). The range in HGF was much smaller when changing individual organic compounds compared to changing the ratio of inorganic to total organic fraction (range = 1.25 1.55). Every sensitivity experiment varied the test fraction in the same increment, allowing for as fair a comparison as possible among experiments. The model appeared to be twice as sensitive to the ratio of organic fraction to inorganic fraction than to changes in the individual organic compounds. Because known solubility is implicitly included in the model, HGF calculations are controlled more by the total soluble fraction than by organic speciation when testing a range centered on average filter concentrations.