Hygroscopicity of polycatechol and polyguaiacol secondary organic aerosol in sub- and supersaturated water vapor environments

Polycatechol and polyguaiacol are light-absorbing and water-insoluble particles that efficiently form from iron-catalyzed reactions with aromatic compounds from biomass burning emissions. Little quantitative information is known about their water uptake and cloud or haze droplet formation ability. In this study, polycatechol and polyguaiacol particles were synthesized in the laboratory, and their cloud condensation nucleation efficiencies were investigated under sub- and supersaturated relative humidity (RH) conditions using a hygroscopicity tandem differential mobility analyzer (H-TDMA) and a cloud condensation nuclei counter (CCNC), respectively. Experimental results show that both polymeric materials are slightly hygroscopic and that their single hygroscopicity parameter (κ) ranges from 0.03 to 0.25, which is within the κ range for secondary organic aerosols (SOA). Polycatechol is more hygroscopic than polyguaiacol, which is explained by differences in their structure. Polyguaiacol has similar water uptake as other insoluble organic compounds, and droplet formation is modelled well with Brunauer-Emmett-Teller (BET) or Frankel Hill Hershey-Adsorption Isotherm theory (FHH-AT). Both polymeric materials are not strongly surface active in range of 0.5 to 30 g L-1, and thus differences in subsaturated and supersaturated hygroscopicity measurement are not attributed to the presence of surface-active materials. Instead, it is due to the solubility limits of both chemicals and H-TDMA being driven by water adsorption. The implications of these results are discussed in the context of aerosol-cloud interactions from the hygroscopicity of aerosols from primary and secondary sources.