Are sesquiterpenes a good source of secondary organic cloud condensation nuclei (CCN)? Revisiting β-caryophyllene CCN

Abstract

Secondary organic aerosol (SOA) was formed in an environmental reaction chamber from the ozonolysis of β-caryophyllene (β-C) at low concentrations (5 ppb or 20 ppb). Experimental parameters were varied to characterize the effects of hydroxyl radicals, light and the presence of lower molecular weight terpene precursor (isoprene) for β-C SOA formation and cloud condensation nuclei (CCN) characteristics. Changes in β-C SOA chemicophysical properties (e.g., density, volatility, oxidation state) were explored with online techniques to improve our predictive understanding of β-C CCN activity. In the absence of OH scavenger, light intensity had negligible impacts on SOA oxidation state and CCN activity. In contrast, when OH reaction was effectively suppressed (> 11 ppm scavenger), SOA showed a much lower CCN activity and slightly less oxygenated state consistent with previously reported values. Though there is significant oxidized material present (O / C > 0.25), no linear correlation existed between the mass ratio ion fragment 44 in the bulk organic mass (<i>f</i>₄₄) and O / C for the β-C-O₃ system. No direct correlations were observed with other aerosol bulk ion fragment fraction (<i>f</i>_<i>x</i>) and κ as well. A mixture of β-C and lower molecular weight terpenes (isoprene) consumed more ozone and formed SOA with distinct characteristics dependent on isoprene amounts. The addition of isoprene also improved the CCN predictive capabilities with bulk aerosol chemical information. The β-C SOA CCN activity reported here is much higher than previous studies (&kappa; < 0.1) that use higher precursor concentration in smaller environmental chambers; similar results were only achieved with significant use of OH scavenger. Results show that aerosol formed from a mixture of low and high molecular weight terpene ozonolysis can be hygroscopic and can contribute to the global biogenic SOA CCN budget.