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

Abstract. Heterogeneous reaction kinetics involving organic aerosol and atmospheric oxidants such as ozone can be enhanced under visible or UV irradiation in the presence of a photosensitiser, with subsequent implications for the climate, cloud radiative properties, air quality, and source appointment. In this study we report the steady-state reactive uptake coefficient, γ, of O₃ by levoglucosan and 5-nitroguaiacol acting as surrogates for biomass burning aerosol particles, with and without the presence of Pahokee peat acting as a photosensitiser. 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, Pahokee peat, and 5-nitroguaiacol. We developed a novel irradiated 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 between various organic species and oxidants. γ of O₃ and levoglucosan 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 O₃ 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 irradiation, however, γ was consistent under UV-A irradiation regardless 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 determined as a function of photon flux for O₃ with the 1 : 1 Pahokee peat/5-nitroguaiacol substrate, yielding a linear relationship under both visible and UV-A irradiation. γ of O₃ with the 1 : 1 Pahokee peat/5-nitroguaiacol substrate was determined as a function of ozone concentration and exhibited an inverse dependence of γ on ozone concentration, commonly interpreted as a Langmuir–Hinshelwood mechanism. The reactive uptake data have been represented by a Langmuir-type isotherm. From the O₃ uptake data under visible irradiation, the following fit parameters have been derived: k_s = (5.5 ± 2.7) × 10⁻¹⁹ cm² s⁻¹ molecule⁻¹ and K_O3 = (2.3 ± 2.0) × 10⁻¹² cm³ molecule⁻¹; and under UV-A irradiation: k_s = (8.1 ± 2.0) × 10⁻¹⁹ cm² s⁻¹ molecule⁻¹ and K_O3 = (1.7 ± 0.7) × 10⁻¹² cm³ molecule⁻¹. The oxidative power, or the product of γ and [O₃], was determined for O₃ with the 1 : 1 Pahokee peat/5-nitroguaiacol substrate and was in the range of (1.2–26) × 10⁶ 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.