Chemical aging of <i>m</i>-xylene secondary organic aerosol: laboratory chamber study

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-NO x 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 OHinduced. 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-NO x 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.

Application of the Statistical Oxidation Model (SOM) to Secondary Organic Aerosol formation from photooxidation of C<sub>12</sub> alkanes

“…H 2 O 2 photolysis is comparably much slower, which facilitates operation at a nearly constant [OH] throughout the low-NO x experiments. To access long oxidation times (up to 32 h) in the low-NO x experiments, the methodology set out by Loza et al (2012) was followed. As a result, the time series of OA mass concentrations (C OA ) under low-NO x conditions have a "gap" of ∼6 h (typically from hours 18-24).…”

Analysis of secondary organic aerosol formation and aging using positive matrix factorization of high-resolution aerosol mass spectra: application to the dodecane low-NO<sub>x</sub> system

“…Since the wall loss rate is a function of diameter (Fig. 13, Loza et al, 2012), the size of the aerosol and composition at that size are important for factor dependent wall-loss corrections. Panel b of Fig.…”

“…Since factor 1 is the first to condense onto the aerosol, some of its mass is lost more rapidly than factor 2, which condenses later onto larger particles (which are lost by deposition at a slower rate). The mass fractions of factors 1 and 2 were calculated for each size bin for 12 mass distributions; this information was used to adapt the wall deposition calculations carried out by Loza et al (2012) for individual factor wall loss corrections.…”

Kinetic multi-layer model of gas-particle interactions in aerosols and clouds (KM-GAP): linking condensation, evaporation and chemical reactions of organics, oxidants and water

“…Chemical reactions can proceed in both gas and condensed phase but it is often difficult to discriminate gas, surface and bulk reactions. Moreover, the relative importance of them for secondary organic aerosol formation and aging is poorly understood (Moise and Rudich, 2000;Kalberer et al, 2004;McNeill et al, 2008;Hallquist et al, 2009;George and Abbatt, 2010a;Fry et al, 2011;Salo et al, 2011;Loza et al, 2012). So far resistor model formulations are widely used to describe and investigate heterogeneous reactions and multiphase processes in laboratory, field and model studies of atmospheric chemistry (Hanson, 1997;Finlayson-Pitts and Pitts, 2000;Worsnop et al, 2002;Anttila et al, 2006;King et al, 2009;Xiao and Bertram, 2011).…”