Humidity drives spontaneous OH oxidation of organic particles

Abstract

We report evidence that organic aerosols containing carboxylic acids can be spontaneously oxidized in the dark under normal atmospheric conditions due to interfacial hydroxyl radical production. Product formation is negligible under dry conditions and increases with increasing relative humidity. In a dioxygen-free environment, the oxidation efficiency is substantially decreased. Size-resolved measurements show an increase in the reactivity and product formation yields for smaller particles, correlated with their surface-to-volume ratio. Our findings suggest that spontaneous hydroxyl radical production at the air-water interface of organic nanodroplets may be an important pathway in their oxidation, especially during nighttime.

Fig. 1.. Chemical structures of citric acid (CA), maleic acid (MA), and trans-Aconitic acid (trans-AA).

Fig. 2.. Products formation at various humidity levels.

(Top) Product concentrations as a function of RH, when CA particles were exposed to increasing levels of humidity. The RH increases as the color scale fades. (Bottom) Product concentration normalized by the measured total particle mass, also as a function of RH.

Fig. 3.. Product yields at various particle sizes.

Malonic acid (red diamonds) and malic acid (blue circles) formation yields, quantified by liquid chromatography–mass spectrometry, as a function of the particle surface-to-volume ratio [3/radius (r)] in CA. Solid and open symbols represent measurements performed using AAC and DMA, respectively. The radius is calculated on the basis of the electrical Dm. Products at 3/r higher than 0.08 were not detectable using DMA due to the low total mass.

Fig. 4.. Evaluation of the role of O₂.

Comparison plot of the product concentrations in the presence (purple) and absence (green) of O₂, produced in CA particles.

Fig. 5.. Atmospheric impact.

OA decay rates due to (i) the spontaneous OH oxidation for the range of product yield of 4 × 10⁻³ to 16 × 10⁻³ hour⁻¹ (orange region), (ii) the heterogeneous oxidation by OH uptake at [OH]_g = 3 × 10⁵ to 3 × 10⁶ molecules cm⁻³, at altitudes of 0 to 2 km (green and purple), and (iii) the heterogeneous oxidation by OH uptake at [OH]_g = 3 × 10⁶ molecules cm⁻³, at altitudes of 3 to 5 km and k_uptake of 2 × 10⁻¹³ to 6 × 10⁻¹³ cm³ molecules⁻¹ s⁻¹ (gray region; gray solid line is the mean value). Data for OH reactive uptake were reproduced from the study of Rasool et al. (47).