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Heterogeneous reactions between nitrate radicals + organic coatings and nitrate radicals + soot Mak, Jackson

Abstract

The reactive uptake of nitrate radicals (NO3) by self-assembled monolayers (SAM) and methane soot was investigated using a flow tube reactor coupled to a chemical ionization mass spectrometer (CIMS) as the detector. The detection of NO3 was accomplished by using I⁻ as the reagent ion, and the rate constant for the reaction of N03 + I⁻ was determined to be (6.5 ± 5.0) x 10⁻¹⁰ cm³ • molecule⁻¹ • s⁻¹. The performance of the experimental approach was evaluated by measuring the rate constant for the bimolecular reaction between NO3 and NO. The rate constant for this reaction was determined to be (2.7 + 0.2) x 10⁻¹¹ cm³ - molecule⁻¹ -s⁻¹, which is in excellent agreement with the literature data. Self-assembled monolayers were used as proxies for organic aerosols. The reactive uptake coefficients were determined to be (8 ± 3) x 10⁻⁴ for octadecanethiol (ODT) on gold and (1.3 + 0.2) x 10⁻³ for octadecyltrichlorosilane (OTS) on glass. Although calculations show the loss of NO3 to organic coatings is of minor importance as a sink of NO3 , they do, however, suggest NO3 is as efficient as OH in processing surfaces of saturated hydrocarbons. In addition, the surface of an OTS monolayer was probed with Fourier transform infrared (FTIR) spectroscopy prior to and following exposure to NO3. The spectra indicate there was no fragmentation of the monolayer. Heterogeneous reactions between NO3 and methane soot were investigated. From the experimental results, a lower limit of 0.03 was determined for the reactive uptake coefficient for this reaction. Calculations show the depletion of NO3 by soot may be an important NO3 sink in the atmosphere in urban areas. Furthermore, infrared (IR) absorption spectra show significant changes to the soot surface after exposure to NO3. Functional groups such as epoxides, carbonyls, organic nitrites and nitrates were observed after treatment.

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