Chemical aging of <i>m</i>-xylene secondary organic aerosol: laboratory chamber study
Secondary organic aerosol (SOA) can reside in the atmosphere for a week or more. While its initial forma- tion from the gas-phase oxidation of volatile organic com- pounds tends to take place in the first few hours after emis- sion, SOA can continue to evolve chemically over its at- mospheric lifetime. Simulating this chemical aging over an extended time in the laboratory has proven to be challeng- ing. 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 functionaliza- tion leads to less volatile products and an increase in SOA mass, whereas gas- or particle-phase fragmentation chem- istry 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 par- ticle composition, increased over the initial 12–13 h of pho- tooxidation 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 behav- ior 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 experi- ment, and OH generation ceases, minimal loss of SOA is ob- served, indicating that the loss of SOA is either light- or OH- induced. Chemical ionization mass spectrometry measure- ments of low-volatility m-xylene oxidation products exhibit behavior indicative of continuous photooxidation chemistry. Acondensed chemical mechanism ofm-xylene oxidation un- der low-NOx conditions is capable of reproducing the gen- eral behavior of gas-phase evolution observed here. More- over, 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 al- though OH reaction with particle surfaces cannot be ruled out. Finally, the effect of size-dependent particle composi- tion and size-dependent particle wall loss rates on different particle wall loss correction methods is discussed.