Efficient Isoprene Secondary Organic Aerosol Formation from a Non-IEPOX Pathway

Abstract

With a large global emission rate and high reactivity, isoprene has a profound effect upon atmospheric chemistry and composition. The atmospheric pathways by which isoprene converts to secondary organic aerosol (SOA) and how anthropogenic pollutants such as nitrogen oxides and sulfur affect this process are subjects of intense research because particles affect Earth's climate and local air quality. In the absence of both nitrogen oxides and reactive aqueous seed particles, we measure SOA mass yields from isoprene photochemical oxidation of up to 15%, which are factors of 2 or more higher than those typically used in coupled chemistry climate models. SOA yield is initially constant with the addition of increasing amounts of nitric oxide (NO) but then sharply decreases for input concentrations above 50 ppbv. Online measurements of aerosol molecular composition show that the fate of second-generation RO2 radicals is key to understanding the efficient SOA formation and the NOx-dependent yields described here and in the literature. These insights allow for improved quantitative estimates of SOA formation in the preindustrial atmosphere and in biogenic-rich regions with limited anthropogenic impacts and suggest that a more-complex representation of NOx-dependent SOA yields may be important in models.

Characterizing source fingerprints and ageing processes in laboratory-generated secondary organic aerosols using proton-nuclear magnetic resonance (<sup>1</sup>H-NMR) analysis and HPLC HULIS determination

“…However, the range of chemical shifts of the background bands characterize molecular species with more electronegative groups (leading to more deshielded H atoms) than methyltetrols: the band of methylic protons extends to 1.7 ppm (with respect to 1.12-1.13 of methyltetrols) and the band of alkoxy groups (HC-O) extends to 4.3 ppm. The results of Liu et al (2016), indicating that non-IEPOX isoprene SOA includes peroxide equivalents of methyltetrols, are in agreement with these findings. However, Riva et al (2016) reported only perox- ides for non-IEPOX SOA in unseeded experiments and no methyltetrols.…”

“…The actual stability of isoprene hydroperoxides in the aerosol itself is largely uncertain; therefore the discrepancy between the findings presented in this study and the results of Riva et al (2016) cannot be clarified at this stage. In addition, neither Liu et al (2016) nor Riva et al (2016) reported the presence of carboxylic or keto groups, while our data clearly indicate that these (and/or other acyl groups) are found in the unresolved mixtures of non-IEPOX isoprene SOA and are responsible for the signal band between 2.0 and 2.6 ppm. Still, this band is much less intense than that of alkoxyls, which is opposite to what observed for α-pinene SOA, where acyls are by far the main oxygenated aliphatic functional group.…”

Modelling organic aerosol concentrations and properties during ChArMEx summer campaigns of 2012 and 2013 in the western Mediterranean region

“…However, using AMS measurements, particlephase IEPOX was not observed during the ChArMEx campaign over an isoprene-emitting forest in the south of France, suggesting it might have formed organosulfates (Freney et al, 2017). Several studies also showed the importance of non-IEPOX pathway for isoprene oxidation in low-NO x environment Liu et al, 2016). Although ELVOCs may also form from isoprene oxidation, the yields may be low .…”

Isoprene derived secondary organic aerosol in a global aerosol chemistry climate model

“…This compound was identified under low NO x , meaning HO 2 dominated conditions (Berndt et al, 2016) and neutral aerosol pH (Liu et al, 2016;DAmbro et al, 2017a).…”

“…After exploring the IEPOX iSOA formation pathway, experimental studies could also identify non-IEPOX iSOA formation pathways via the highly oxidized, very low volatile isoprene product dihydoxy dihyodroperoxide (C 5 H 12 O 6 , ISOP(OOH)2) (Riva et al, 2016;Liu et al, 2016;Berndt et al, 2016;DAmbro et al, 2017a). This compound was identified under low NO x , meaning HO 2 dominated conditions (Berndt et al, 2016) and neutral aerosol pH (Liu et al, 2016;DAmbro et al, 2017a).…”