No abstract

Absolute secondary organic aerosol (SOA) mass loading (C SOA ) is a key parameter in determining partitioning of semi-and intermediate volatility compounds to the particle phase. Its impact on the phase state of SOA, however, has remained largely unexplored. In this study, systematic laboratory chamber measurements were performed to elucidate the influence of C SOA , ranging from 0.2 to 160 µg m −3 , on the phase state of SOA formed by ozonolysis of various precursors, including α-pinene, limonene, cis-3-hexenyl acetate (CHA) and cis-3-hexen-1-ol (HXL). A previously established method to estimate SOA bounce factor (BF, a surrogate for particle viscosity) was utilized to infer particle viscosity as a function of C SOA . Results show that under nominally identical conditions, the maximum BF decreases by approximately 30% at higher C SOA , suggesting a more liquid phase state. With the exception of HXL-SOA (which acted as the negative control), the phase state for all studied SOA precursors varied as a function of C SOA . Furthermore, the BF was found to be the maximum when SOA particle distributions reached a geometric mean particle diameter of 50-60 nm. Experimental results indicate that C SOA is an important parameter impacting the phase state of SOA, reinforcing recent findings that extrapolation of experiments not conducted at atmospherically relevant SOA levels may not yield results that are relevant to the natural environment.

Secondary organic aerosol (SOA) is ubiquitous in the atmosphere and plays important roles in environmental chemical processes, influencing air quality and the Earth’s radiative budget. In the present work, 1-octen-3-ol (OTL) was identified as one of several prominent green leaf volatiles (GLVs) emitted as a result of sugarcane wounding. GLVs, a subset of volatile organic compounds (VOCs), act as SOA precursors and are a potentially underrepresented source of the overall SOA budget. Here, ozonolysis experiments of OTL standards were carried out in Teflon chambers in conjunction with a scanning mobility particle sizer (SMPS), an electrical low pressure impactor (ELPI+), and a near-infrared laser desorption ionization aerosol mass spectrometer (NIR-LDI-AMS). Under our experimental conditions, the OTL ozonolysis rate constant and aerosol yield were estimated to be 5.00 ± 0.58 × 10–24 cm3 s–1 molecule–1 and 1.03 ± 0.07%, respectively. Bounce factor (BF) calculations based on the ELPI+ data at relative humidity (RH) levels of 5, 30, 60, and 90% suggest that the OTL-derived SOA exhibits largely non-liquid characteristics regardless of RH levels at particle genesis. Furthermore, high RH at particle genesis also appears to decrease the hygroscopicity of the SOA, impacting its ability to activate as cloud droplets. Online chemical analysis of the SOA using a NIR-LDI-AMS supports the production of oxygenated products ranging from 45 to 161 m/z, in addition to prominent oligomers well beyond this m/z range.

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.

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