Technical Note: Application of positive matrix factor analysis in heterogeneous kinetics studies utilizing the mixed-phase relative rates technique

Abstract

Abstract. The mixed-phase relative rates approach for determining aerosol particle organic heterogeneous reaction kinetics is often performed utilizing mass spectral tracers as a proxy for particle-phase reactant concentration. However, this approach may be influenced by signal contamination from oxidation products during the experiment. In the current study, the mixed-phase relative rates technique has been improved by combining a positive matrix factor (PMF) analysis with electron ionization aerosol mass spectrometry (unit-mass resolution), thereby removing the influence of m / z fragments from reaction products on the reactant signals. To demonstrate the advantages of this approach, the heterogeneous reaction between OH radicals and citric acid (CA) was investigated using a photochemical flow tube coupled to a compact time-of-flight aerosol mass spectrometer (C-ToF-AMS). The measured heterogeneous rate constant (k2) of citric acid toward OH was (3.31 ± 0.29) × 10−12 cm3 molecule−1 s−1 at 298 K and (30 ± 3)% relative humidity (RH) and was several times greater than the results utilizing individual m / z fragments. This phenomenon was further evaluated for particulate-phase organophosphates (triphenyl phosphate (TPhP), tris-1,3-dichloro-2-propyl phosphate (TDCPP) and tris-2-ethylhexyl phosphate (TEHP)), leading to k2 values significantly larger than previously reported. The results suggest that heterogeneous kinetics can be significantly underestimated when the structure of the products is highly similar to the reactant and when a non-molecular tracer is measured with a unit-mass resolution aerosol mass spectrometer. The results also suggest that the heterogeneous lifetime of organic aerosol in models can be overestimated due to underestimated OH uptake coefficients. Finally, a comparison of reported rate constants implies that the heterogeneous oxidation of aerosols will be dependent upon a number of factors related to the reaction system, and that a single rate constant for one system cannot be universally applied under all conditions.

OH-initiated heterogeneous oxidation of tris-2-butoxyethyl phosphate: implications for its fate in the atmosphere

“…S5). This is consistent with our previous conclusion that the evaporation of a different OPE, triphenyl phosphate (TPhP), is negligible in the reactor even though it has a higher vapor pressure than TBEP (Liu et al, 2014b). This suggests that evaporation of TBEP in the reactor has little influence on kinetic calculations.…”

“…In our previous work, we measured the heterogeneous rate constants (k 2,OH ) for several OPEs, (TPhP, TEHP and TD-CPP,) with the mixed-phase relative rates technique, which utilized methanol as a reference compound for the OH concentration determination (Liu et al, 2014b). Unfortunately, the oxidized products of TBEP significantly contribute to the methanol signal when measured with a protontransfer mass spectrometer (PTR-MS).…”

“…Furthermore, recent studies have found that SOA (secondary organic aerosol) is a semisolid phase with high viscosity (Abramson et al, 2013;Vaden et al, 2011) and that aged SOA demonstrated a slower evaporation rate than fresh or uncoated SOA (Vaden et al, 2011). If OPEs are internally mixed with or coated by SOA (resulting in a core-shell morphology) during transport, the evaporation rate of OPEs may be further reduced, and/or the reactivity of OPEs towards OH may be slowed (as observed in reactions of benzo[a]pyrene and O 3 coated with SOA (Zhou et al, 2013) and TPhP coated with oxalic acid (Liu et al, 2014b)). In particular, the measurements of TPhP in PM in remote regions (Möller et al, 2012), despite its dominant gas-phase loss contribution (based upon our model results), highlight the effect of multicomponent particle mixtures on the kinetics of particle degradation.…”

“…In most of these studies, the concentration of the particle-phase compound of interest was measured with an aerosol mass spectrometer (AMS), utilizing specific mass spectral fragments as a tracer for the particulate compound, while the gaseous reference compound was usually monitored with other instrumentation. In our previous work, we demonstrated that the larger the tracer fragment chosen, the larger the k 2 value is derived if the products are highly similar to the reactant and a unit mass resolution (UMR) AMS is utilized (Liu et al, 2014b). It was also demonstrated that an approach using positive matrix factorization (PMF) analysis improves the accuracy of the rate constant determination for selected organophosphate flame retardants found in particles (Liu et al, 2014a).…”

Heterogeneous chemistry and reaction dynamics of the atmospheric oxidants, O3, NO3, and OH, on organic surfaces

“…The authors hypothesized the formation of radicals at the particle surface that are able to migrate through the SOA matrix to the squalane support where further reactions occur. Flow-tube studies of heterogeneous reactions of hydroxyl radicals with organic aerosols ranging from those present in soot 50,117,[123][124][125] and biomass-burning aerosols, [126][127][128] to those from herbicides, 129 pesticides, 130,131 and flame retardants, 132,133 have been performed to obtain information such as product structure, 117,123,125,130,134 yield and volatilization, 123,126 changes in hygroscopic properties 124 and particle size, 124,131 as well as kinetic information such as reactive and degradative rate constants 50,126,127,[129][130][131][132][133][134] and reactive uptake coefficients. 50,117,125,126,132,135 These reactive uptake coefficients are generally large, but vary significantly between 0.19 and 2.0, 50,117,132,135 although even higher values have been reported.…”