Secondary Organic Aerosol Formation from Acyclic, Monocyclic, and Polycyclic Alkanes

Abstract

A large number of organic species emitted into the atmosphere contain cycloalkyl groups. While cyclic species are believed to be important secondary organic aerosol (SOA) precursors, the specific role of cyclic moieties (particularly for species with multiple or fused rings) remains uncertain. Here we examine the yields and composition of SOA formed from the reaction of OH with a series of C10 (cyclo)alkanes, with 0-3 rings, in order to better understand the role of multiple cyclic moieties on aerosol formation pathways. A chamber oxidation technique using high, sustained OH radical concentrations was used to simulate long reaction times in the atmosphere. This aging technique leads to higher yields than in previously reported chamber experiments. Yields were highest for cyclic and polycyclic precursors, though yield exhibited little dependence on number of rings. However, the oxygen-to-carbon ratio of the SOA was highest for the polycyclic precursors. These trends are consistent with aerosol formation requiring two generations of oxidation and 3-4 oxygen-containing functional groups in order to condense. Cyclic, unbranched structures are protected from fragmentation during the first oxidation step, with C-C bond scission instead leading to ring opening, efficient functionalization, and high SOA yields. Fragmentation may occur during subsequent oxidation steps, limiting yields by forming volatile products. Polycyclic structures can undergo multiple ring opening reactions, but do not have markedly higher yields, likely due to enhanced fragmentation in the second oxidation step. By contrast, C-C bond scission for the linear and branched structures leads to fragmentation prior to condensation, resulting in low SOA yields. The results highlight the key roles of multigenerational chemistry and susceptibility to fragmentation in the formation and evolution of SOA.

Effect of oxidant concentration, exposure time, and seed particles on secondary organic aerosol chemical composition and yield

“…2 (e.g., H/C, O/C) were independent of the NO x levels used in the environmental chambers in the region studied, as has been observed in previous studies (Chhabra et al, 2011). The nitrogen-to-carbon (N/C) ratio ranged from 0.031 to 0.054 for SOA produced in the MIT chamber with added NO x (Hunter et al, 2014) but was not characterized for other measurements shown in Fig. 2.…”

“…whereas α-pinene forms SOA with moderate yields (Eddingsaas et al, 2012) and JP-10 forms SOA with mass yields greater than unity (Hunter et al, 2014;Lambe et al, 2012). Yields of alkane SOA do not display a systematic NO x dependence (Loza et al, 2014); thus, to first order we assume different NO x levels between the MIT chamber and PAM reactor do not influence our comparison of measured JP-10 SOA yields.…”

“…The decrease in SOA yield subsequent to increase as a function of OH exposure is possibly due to gas-phase species carbon-carbon bond breaking from continued oxidation or heterogeneous OH oxidation reactions at high OH exposure (Hunter et al, 2014;Lambe et al, 2012;Loza et al, 2012); this trend is most clearly evident in the flow reactor studies. These observations suggest that the first step in SOA formation is oxidation of gas-phase species leading to subsequent condensation.…”

“…al., 2015). Black markers indicate data from (Kroll et al, 2006;Chhabra et al, 2010;Eddingsaas et al, 2012), and Ng et al (2007) obtained in the Caltech chamber and data from Hunter et al (2014) obtained in the MIT chamber.…”

Dimensionality-reduction techniques for complex mass spectrometric datasets: application to laboratory atmospheric organic oxidation experiments

“…Air quality and climate change are major threats to the quality of millions of human lives across the globe (IPCC, 2014;Landrigan et al, 2018). An important scientific component of both topics is the photooxidation chemistry of organic compounds in the atmosphere, which can lead to the formation of ozone and fine particulate matter, both of which can affect the radiative budget of the atmosphere and can harm human health.…”