Relating hygroscopicity and composition of organic aerosol particulate matter

Abstract

A hygroscopicity tandem differential mobility analyzer (HTDMA) was used to measure the water uptake (hygroscopicity) of secondary organic aerosol (SOA) formed during the chemical and photochemical oxidation of several organic precursors in a smog chamber. Electron ionization mass spectra of the non-refractory submicron aerosol were simultaneously determined with an aerosol mass spectrometer (AMS), and correlations between the two different signals were investigated. SOA hygroscopicity was found to strongly correlate with the relative abundance of the ion signal <i>m/z</i> 44 expressed as a fraction of total organic signal (<i>f</i>₄₄). <i>m/z</i> 44 is due mostly to the ion fragment CO₂⁺ for all types of SOA systems studied, and has been previously shown to strongly correlate with organic O/C for ambient and chamber OA. The analysis was also performed on ambient OA from two field experiments at the remote site Jungfraujoch, and the megacity Mexico City, where similar results were found. A simple empirical linear relation between the hygroscopicity of OA at subsaturated RH, as given by the hygroscopic growth factor (GF) or "ϰ_org" parameter, and <i>f</i>₄₄ was determined and is given by ϰ_org = 2.2 &times; <i>f</i>₄₄ &minus; 0.13. This approximation can be further verified and refined as the database for AMS and HTDMA measurements is constantly being expanded around the world. The use of this approximation could introduce an important simplification in the parameterization of hygroscopicity of OA in atmospheric models, since <i>f</i>₄₄ is correlated with the photochemical age of an air mass.