Chemical composition of isoprene SOA under acidic and non-acidic conditions: Effect of relative humidity

Abstract

Abstract. The effect of acidity and relative humidity on bulk isoprene aerosol parameters has been investigated in several studies, however few measurements have been conducted on individual aerosol compounds. While the focus of this study 10 has been the examination of the effect of acidity and RH on secondary organic aerosol (SOA) chemical composition from isoprene photooxidation in the presence of NOx, a detailed characterization of SOA at the molecular level have been also conducted. Experiments were conducted in a 14.5 m 3 smog chamber operated in flow mode. Based on a detailed analysis of mass spectra obtained from GCMS of silylated derivatives in EI and CI modes, and UPLC/ESI/QTOF HRMS, and collision-induced dissociation in the positive and negative ionization modes, we characterized not only 15 typical isoprene products, but also new oxygenated compounds. The analysis showed the presence of methylthreonic acids (mTr) and methyltartaric acids (mTA), proposed recently by our groups as isoprene aging SOA markers. Furthermore, a series of organosulfates (OSs) were tentatively identified including 2mTr-OS, 2mTA-OS and 2mTA nitroxy-OS. Under acidic conditions, the major identified compounds include 2-methyltetrols (2mT), 2-methylglyceric acid (2mGA) and 2mT-OS. Other products identified include epoxydiols, mono-and dicarboxylic acids, OSs, and nitroxy-and 20 nitrosoxy-OSs. The contribution of SOA products from isoprene oxidation to PM2.5 was investigated by analyzing ambient aerosol collected at rural sites in Poland. mTs, mGA and several organosulfates and nitroxy-OS were detected in both the field and laboratory samples. The influence of RH on SOA formation was modest in non-acidic seed experiments. Total SOC decreased with increasing RH. The yields of most compounds decreased, but the concentrations of 2mTA, IEPOX-OS, 2mGA-OS and 2mTr-OS increased with increasing RH. Some components followed this pattern while other were more 25 abundant in non-acidic experiments or behaved in a mixed way, depending on RH.

Enhanced Production of Organosulfur Species during a Severe Winter Haze Episode in the Guanzhong Basin of Northwest China

“…Nevertheless, some studies found that the production of isoprene-derived organosulfates was decreased at high RH levels possibly due to the dilution of particle acidity and sulfate nucleophiles. 55,56 Given that biogenic isoprene emissions are reported to be generally low in Xi'an and the surrounding areas during wintertime, 57−59 the acid-catalyzed uptake of isoprene-derived epoxydiols might be not quite prevalent or at least not a dominant pathway for the OrgS formation in this area.…”