Utilizing an Electrical Low Pressure Impactor to Indirectly Probe Water Uptake via Particle Bounce Measurements 

Abstract

Abstract. Secondary organic aerosol (SOA), formed through oxidation of volatile organic compounds (VOCs), display complex viscosity and phase behaviors influenced by temperature, relative humidity (RH), and chemical composition. Here, the efficacy of a multi-stage electrical low pressure impactor (ELPI) for indirect water uptake measurements was studied for ammonium sulfate (AS) aerosol, sucrose aerosol, and α-pinene derived SOA. All three aerosol systems were subjected to greater than 90 % chamber relative humidity, with subsequent analysis indicating persistence of particle bounce for sucrose 10 aerosol of 70 nm (initial dry diameter) and α-pinene derived SOA of number geometric mean diameters between 39 nm and 136 nm (initial dry diameter). On the other hand, sucrose aerosol of 190 nm (initial dry diameter) and AS aerosol down to 70 nm (initial dry diameter) exhibited no particle bounce at elevated RH. Partial drying of aerosol within the lower diameter ELPI impaction stages, where inherent and significant RH reductions occur, is proposed as one explanation for particle bounce persistence.

Reply on RC1

“…Our ERH and DRH values are slightly higher compared to published literature values likely due to the relatively higher rate of RH increase (during deliquescence) and decrease (during efflorescence), which has been shown to shift measured glass transition relative humidity values for glassy solids (Mikhailov et al, 2009). Nevertheless, full and clear deliquescence and efflorescence of the AS aerosol was observed utilizing this modified method.…”

“…While the particle bounce of AS aerosol behaved as expected as a function of RH, the same was not observed for sucrose aerosol. For sucrose, a DRH of approximately 85% is well established and measurable water uptake starting at 30 -40 % RH has been observed (Robinson et al, 2014), but the ERH has proven difficult to determine (Zobrist et al, 2011;Samain et al, 2017). Unlike AS, Δi fractional, t for polydisperse sucrose particles (initial dry number GMD = 65 nm) did not approach zero past the DRH (Fig.…”

“…Uptake of enough water can solubilize a particle if RH becomes high enough, facilitating its response to changes in gas-phase composition. Thus, RH, along with chemical composition and temperature, has been deemed a controlling factor of aerosol phase state (Koop et al, 2011;Krieger et al, 2012;Martin, 2000;Reid et al, 2018).…”