Abstract
Abstract. The 2-methyltetrols have been widely chosen as chemical tracers for isoprene-derived secondary organic aerosols. While they are often assumed to be relatively unreactive, a laboratory study reported that pure erythritol particles (an analog of 2-methyltetrols) can be heterogeneously oxidized by gas-phase OH radicals at a significant rate. This might question the efficacy of these compounds as tracers in aerosol source-apportionment studies. Additional uncertainty could arise as organic compounds and inorganic salts often coexist in atmospheric particles. To gain more insights into the chemical stability of 2-methyltetrols in atmospheric particles, this study investigates the heterogeneous OH oxidation of pure erythritol particles and particles containing erythritol and ammonium sulfate (AS) at different dry inorganic-to-organic mass ratios (IOR) in an aerosol flow tube reactor at a high relative humidity of 85 %. The same reaction products are formed upon heterogenous OH oxidation of erythritol and erythritol–AS particles, suggesting that the reaction pathways are not strongly affected by the presence and amount of AS. On the other hand, the effective OH uptake coefficient, γeff, is found to decrease by about a factor of ∼20 from 0.45±0.025 to 0.02±0.001 when the relative abundance of AS increases and the IOR increases from 0.0 to 5.0. One likely explanation is the presence of dissolved ions slows down the reaction rates by decreasing the surface concentration of erythritol and reducing the frequency of collision between erythritol and gas-phase OH radicals at the particle surface. Hence, the heterogeneous OH reactivity of erythritol and likely 2-methyltetrols in atmospheric particles would be slower than previously thought when the salts are present. Given 2-methyltetrols often coexist with a significant amount of AS in many environments, where ambient IOR can vary from ∼1.89 to ∼250, our kinetic data would suggest that 2-methyltetrols in atmospheric particles are likely chemically stable against heterogeneous OH oxidation under humid conditions.
Highlights
The photochemical oxidation of isoprene is one of major sources of atmospheric secondary organic aerosols (SOA), which can potentially affect the regional and global air quality (Claeys, 2004; Carlton et al, 2009; Wennberg et al, 2018)
There remains considerable uncertainty in how inorganic salts alter the heterogeneous reactivity of organic compounds, which governs the chemical lifetime of organic compounds in the atmosphere
The heterogeneous reactivity of erythritol toward gas-phase oxidants such as hydroxyl (OH) radicals could be slower in erythritol–ammonium sulfate (AS) particles compared to pure erythritol particles, depending on the concentration of erythritol and AS
Summary
The photochemical oxidation of isoprene is one of major sources of atmospheric secondary organic aerosols (SOA), which can potentially affect the regional and global air quality (Claeys, 2004; Carlton et al, 2009; Wennberg et al, 2018). Hu et al (2016) have investigated the heterogenous reactivity of ambient IEPOX-SOA (consisting of 2-methyltetrols, C5-alkene triols, organosulfate, etc.) towards gas-phase OH radicals in the southeastern US and the Amazon and reported the reaction kinetics for IEPOX-SOA based on the decay of C5H6O+ ion (a tracer ion for IEPOX-SOA in ambient particles) in their AMS measurements They calculated on average a more than 2-week (19 ± 9 d) atmospheric lifetime of IEPOX-SOA against heterogeneous OH oxidation based on rate constant of 4.0 ± 2.0 × 10−13 cm molecule−1 s−1 and averaged ambient OH concentration of 1.5×106 molecule cm−3. The results of this work will provide further insights into the chemical stability of 2-methyltetrols in atmospheric particles against heterogeneous OH oxidation
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