Abstract
Abstract. Amorphous (semi-)solid organic aerosol particles have the potential to serve as surfaces for heterogeneous ice nucleation in cirrus clouds. Raman spectroscopy and optical microscopy have been used in conjunction with a cold stage to examine water uptake and ice nucleation on individual amorphous (semi-)solid particles at atmospherically relevant temperatures (200–273 K). Three organic compounds considered proxies for atmospheric secondary organic aerosol (SOA) were used in this investigation: sucrose, citric acid and glucose. Internally mixed particles consisting of each organic and ammonium sulfate were also investigated. Results from water uptake experiments followed the shape of a humidity-induced glass transition (Tg(RH)) curve and were used to construct state diagrams for each organic and corresponding mixture. Experimentally derived Tg(RH) curves are in good agreement with theoretical predictions of Tg(RH) following the approach of Koop et al. (2011). A unique humidity-induced glass transition point on each state diagram, Tg'(RH), was used to quantify and compare results from this study to previous works. Values of Tg'(RH) determined for sucrose, glucose and citric acid glasses were 236, 230 and 220 K, respectively. Values of Tg'(RH) for internally mixed organic/sulfate particles were always significantly lower; 210, 207 and 215 K for sucrose/sulfate, glucose/sulfate and citric acid/sulfate, respectively. All investigated SOA proxies were observed to act as heterogeneous ice nuclei at tropospheric temperatures. Heterogeneous ice nucleation on pure organic particles occurred at Sice = 1.1–1.4 for temperatures below 235 K. Particles consisting of 1:1 organic-sulfate mixtures took up water over a greater range of conditions but were in some cases also observed to heterogeneously nucleate ice at temperatures below 202 K (Sice= 1.25–1.38). Polynomial curves were fitted to experimental water uptake data and then incorporated into the Community Aerosol Radiation Model for Atmospheres (CARMA) along with the predicted range of humidity-induced glass transition temperatures for atmospheric SOA from Koop et al. (2011). Model results suggest that organic and organic/sulfate aerosol could be glassy more than 60% of the time in the midlatitude upper troposphere and more than 40% of the time in the tropical tropopause region (TTL). At conditions favorable for ice formation (Sice > 1), particles in the TTL are expected to be glassy more than 50% of the time for temperatures below 200 K. Results from this study suggests that amorphous (semi-)solid organic particles are often present in the upper troposphere and that heterogeneous ice formation on this type of particle may play an important role in cirrus cloud formation.
Highlights
Organic compounds are ubiquitous in tropospheric aerosol and often account for a large portion of aerosol mass (e.g., Froyd et al, 2010; and Zhang et al, 2007)
Water uptake is marked by the appearance of a broad O-H peak in the range of Raman frequencies associated with liquid water (∼ 3100 and ∼ 3600 cm−1)
Deliquescence of the citric acid crystal proceeded over a small relative humidities (RH) interval (76.2– 79.5 %) as water was quickly absorbed into the bulk crystal
Summary
Organic compounds are ubiquitous in tropospheric aerosol and often account for a large portion of aerosol mass (e.g., Froyd et al, 2010; and Zhang et al, 2007). A growing body of literature has shown that certain types of organic aerosol may exist as amorphous glasses or semi-solids at atmospherically relevant temperatures and relative humidities (RH) (Bones et al, 2012; Murray, 2008; Mikhailov et al, 2009; Saukko, 2012; Shiraiwa et al, 2011, 2012; Tong et al, 2011; Virtanen et al, 2010; Zobrist et al, 2008). The viscosity of a liquid is on the order of 10−3 Pa s and between 102 and 1012 Pa s for semi-solid substances (Shiraiwa et al, 2011). The transition between a semi-solid and glass is often defined according to a characteristic glass transition temperature (Tg), below which the viscosity of the substance is > 1012 Pa s
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