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.
Highlights
Reaction kinetics data provide key parameters for both air quality and climate models
The tracer and positive matrix factor (PMF) approaches can at times agree (Kroll, 2014) and the tracer and molecular-ion approaches can sometimes agree as well (Smith et al, 2009), our results suggest that the heterogeneous kinetics of organic aerosol (OA) is underestimated when a non-molecular-ion peak is used as the tracer to measure the particle-phase concentration of OA based on unit-mass resolution (UMR)-AMS
The current results suggest that the lifetime of OA estimated in models due to heterogeneous oxidation might be overestimated for a reaction system where the products are highly similar to the reactant and the kinetic data derived by individual non-molecular m/z tracers of OA
Summary
Reaction kinetics data provide key parameters for both air quality and climate models. They are required to compute the trace gas and particulate matter (PM) content of the atmosphere (Kolb et al, 2010) and to evaluate the atmospheric lifetime and fate for individual species. Previous studies have found that heterogeneous reactions with OH, in particular, can lead to an increase in density, cloud condensation nuclei (CCN) activation (George and Abbatt, 2010) and optical extinction (Cappa et al, 2011) of organic particulate matter. There is a growing interest in understanding the mechanism of PM transformation through heterogeneous reactions including oxidation and determining the rates at which organic aerosols are chemically transformed in the atmosphere
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