Abstract
Abstract. Secondary organic aerosol (SOA) can reside in the atmosphere for a week or more. While its initial formation from the gas-phase oxidation of volatile organic compounds tends to take place in the first few hours after emission, SOA can continue to evolve chemically over its atmospheric lifetime. Simulating this chemical aging over an extended time in the laboratory has proven to be challenging. We present here a procedure for studying SOA aging in laboratory chambers that is applied to achieve 36 h of oxidation. The formation and evolution of SOA from the photooxidation of m-xylene under low-NOx conditions and in the presence of either neutral or acidic seed particles is studied. In SOA aging, increasing molecular functionalization leads to less volatile products and an increase in SOA mass, whereas gas- or particle-phase fragmentation chemistry results in more volatile products and a loss of SOA. The challenge is to discern from measured chamber variables the extent to which these processes are important for a given SOA system. In the experiments conducted, m-xylene SOA mass, calculated under the assumption of size-invariant particle composition, increased over the initial 12–13 h of photooxidation and decreased beyond that time, suggesting the existence of fragmentation chemistry. The oxidation of the SOA, as manifested in the O:C elemental ratio and fraction of organic ion detected at m/z 44 measured by the Aerodyne aerosol mass spectrometer, increased continuously starting after 5 h of irradiation until the 36 h termination. This behavior is consistent with an initial period in which, as the mass of SOA increases, products of higher volatility partition to the aerosol phase, followed by an aging period in which gas- and particle-phase reaction products become increasingly more oxidized. When irradiation is stopped 12.4 h into one experiment, and OH generation ceases, minimal loss of SOA is observed, indicating that the loss of SOA is either light- or OH-induced. Chemical ionization mass spectrometry measurements of low-volatility m-xylene oxidation products exhibit behavior indicative of continuous photooxidation chemistry. A condensed chemical mechanism of m-xylene oxidation under low-NOx conditions is capable of reproducing the general behavior of gas-phase evolution observed here. Moreover, order of magnitude analysis of the mechanism suggests that gas-phase OH reaction of low volatility SOA precursors is the dominant pathway of aging in the m-xylene system although OH reaction with particle surfaces cannot be ruled out. Finally, the effect of size-dependent particle composition and size-dependent particle wall loss rates on different particle wall loss correction methods is discussed.
Highlights
Organic aerosol (OA) constitutes 20–90 % of all submicron particles in the atmosphere, and up to 80 % of this is classified as secondary organic aerosol (SOA) (Zhang et al, 2007; Murphy et al, 2006)
Ng et al (2011) determined a correlation between f43 and H:C and, combined with a correlation between f44 and O:C (Aiken et al, 2008), mapped the triangular region in f44–f43 space onto the Van Krevelen diagram. They found that for ambient OA classified as oxygenated OA (OOA) and laboratory chamber-generated SOA the H:C and O:C evolution toward the apex of the triangle tends to fall along a line with a slope of −0.5 on a Van Krevelen diagram
Two experiments with the same initial conditions and 18 h of irradiation were performed to assess the reproducibility of initial conditions and SOA production (Table 2)
Summary
Organic aerosol (OA) constitutes 20–90 % of all submicron particles in the atmosphere, and up to 80 % of this is classified as secondary organic aerosol (SOA) (Zhang et al, 2007; Murphy et al, 2006). Ng et al (2011) determined a correlation between f43 and H:C and, combined with a correlation between f44 and O:C (Aiken et al, 2008), mapped the triangular region in f44–f43 space onto the Van Krevelen diagram They found that for ambient OA classified as oxygenated OA (OOA) and laboratory chamber-generated SOA the H:C and O:C evolution toward the apex of the triangle tends to fall along a line with a slope of −0.5 on a Van Krevelen diagram. Kroll et al (2009) found that for oxidation of squalane (C30H36) particles, functionalization reactions (addition of polar functional groups) dominated at low OH exposure, and fragmentation reactions (scission of C-C bonds in the carbon skeleton) dominated as OH exposure increased They observed an O:C ratio of 0.45 after 35.8 squalene OH oxidation lifetimes. The effects of size-dependent particle composition on particle wall loss correction methods are discussed
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