Hygroscopicity of secondary organic aerosols formed by oxidation ofcycloalkenes, monoterpenes, sesquiterpenes, and related compounds

[摘要] A series of experiments has been conducted in the Caltech indoor smogchamber facility to investigate the water uptake properties of aerosolformed by oxidation of various organic precursors. Secondary organic aerosol(SOA) from simple and substituted cycloalkenes (C₅-C₈) is producedin dark ozonolysis experiments in a dry chamber (RH~5%). BiogenicSOA from monoterpenes, sesquiterpenes, and oxygenated terpenes is formed byphotooxidation in a humid chamber (~50% RH). Using thehygroscopicity tandem differential mobility analyzer (HTDMA), we measure thediameter-based hygroscopic growth factor (GF) of the SOA as a function of timeand relative humidity. All SOA studied is found to be slightly hygroscopic,with smaller water uptake than that of typical inorganic aerosol substances.The aerosol water uptake increases with time early in the experiments forthe cycloalkene SOA, but decreases with time for the sesquiterpene SOA. Thisbehavior could indicate competing effects between the formation of morehighly oxidized polar compounds (more hygroscopic), and formation oflonger-chained oligomers (less hygroscopic). All SOA also exhibit a smoothwater uptake with RH with no deliquescence or efflorescence. The wateruptake curves are found to be fitted well with an empirical three-parameterfunctional form. The measured pure organic GF values at 85% RH are between1.09–1.16 for SOA from ozonolysis of cycloalkenes, 1.01–1.04 forsesquiterpene photooxidation SOA, and 1.06–1.10 for the monoterpene andoxygenated terpene SOA. The GF of pure SOA (GF_org) in experiments in whichinorganic seed aerosol is used is determined by assuming volume-weightedwater uptake (Zdanovskii-Stokes-Robinson or "ZSR" approach) and using thesize-resolved organic mass fraction measured by the Aerodyne Aerosol MassSpectrometer. Knowing the water content associated with the inorganicfraction yields GF_org values. However, for each precursor, theGF_org values computed from different HTDMA-classified diameters agreewith each other to varying degrees. Comparing growth factors from different precursors, we find thatGF_org is inversely proportional to the precursor molecular weight and SOAyield, which is likely a result of the fact that higher-molecular weightprecursors tend to produce larger and less hygroscopic oxidation products.

[学科分类] 大气科学