Photosensitised heterogeneous oxidation kinetics of biomass burning aerosol surrogates by ozone using an irradiated rectangular channel flow reactor

by S. M. Forrester, D. A. Knopf
Atmospheric Chemistry and Physics ()
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Abstract

Heterogeneous reaction kinetics involving organic aerosol and atmospheric oxidants such as ozone can be en-hanced under visible or UV irradiation in the presence of a photosensitiser, with subsequent implications for the climate, cloud radiative properties, air quality, and source appoint-ment. In this study we report the steady-state reactive uptake coefficient, γ , of O 3 by levoglucosan and 5-nitroguaiacol act-ing as surrogates for biomass burning aerosol particles, with and without the presence of Pahokee peat acting as a pho-tosensitiser. The reactive uptake has been determined in the dark and as a function of visible and UV-A irradiation and ozone concentration. In addition, γ was determined for 1 : 1, 1 : 10, and 1 : 100 by mass mixtures of Pahokee peat and 5-nitroguaiacol, and for a 10 : 1 : 3 mixture of levoglucosan, Pa-hokee peat, and 5-nitroguaiacol. We developed a novel irra-diated rectangular channel flow reactor (I-RCFR) that was operated under low pressures of about 2–4 hPa, and allowed for uniform irradiation of the organic substrates. The I-RCFR was coupled to a chemical ionisation mass spectrometer and has been successfully validated by measuring the kinetics be-tween various organic species and oxidants. γ of O 3 and lev-oglucosan in the dark and under visible and UV-A irradiation was determined to be in the range of (2–11) × 10 −6 and did not change in the presence of Pahokee peat. The determined γ of O 3 and 5-nitroguaiacol in the dark was 5.7 × 10 −6 and was only enhanced under UV-A irradiation, yielding a value of 3.6 × 10 −5 . γ of the 1 : 1 Pahokee peat/5-nitroguaiacol substrate was enhanced under visible and UV-A irradiation to 2.4 × 10 −5 and 2.8 × 10 −5 , respectively. Decreasing the amount of Pahokee peat in the 5-nitroguaiacol/Pahokee peat substrate resulted in lower values of γ under visible irradi-ation, however, γ was consistent under UV-A irradiation re-gardless of the amount of Pahokee peat. The 10 : 1 : 3 mixture by mass of levoglucosan, Pahokee peat, and 5-nitroguaiacol, under both visible and UV-A irradiation yielded γ values of 2.8 × 10 −5 and 1.4 × 10 −5 , respectively. γ was deter-mined as a function of photon flux for O 3 with the 1 : 1 Pa-hokee peat/5-nitroguaiacol substrate, yielding a linear rela-tionship under both visible and UV-A irradiation. γ of O 3 with the 1 : 1 Pahokee peat/5-nitroguaiacol substrate was de-termined as a function of ozone concentration and exhib-ited an inverse dependence of γ on ozone concentration, commonly interpreted as a Langmuir–Hinshelwood mecha-nism. The reactive uptake data have been represented by a Langmuir-type isotherm. From the O 3 uptake data under vis-ible irradiation, the following fit parameters have been de-rived: k s = (5.5 ± 2.7) × 10 −19 cm 2 s −1 molecule −1 and K O 3 = (2.3 ± 2.0) × 10 −12 cm 3 molecule −1 ; and under UV-A ir-radiation: k s = (8.1 ± 2.0) × 10 −19 cm 2 s −1 molecule −1 and K O 3 = (1.7 ± 0.7) × 10 −12 cm 3 molecule −1 . The oxidative power, or the product of γ and [O 3 ], was determined for O 3 with the 1 : 1 Pahokee peat/5-nitroguaiacol substrate and was in the range of (1.2–26) × 10 6 molecule cm −3 . Atmospheric particle lifetimes were estimated for a 0.4 µm 5-nitroguaiacol particle as a function of visible and UV-A irradiation and ozone concentration. Published by Copernicus Publications on behalf of the European Geosciences Union. 6508 S. M. Forrester and D. A. Knopf: Photosensitised kinetics of biomass burning aerosol

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