Heterogeneous OH oxidation kinetics of amorphous organic aerosol surrogates at typical tropospheric temperatures

Abstract:

The chemical aging of organic aerosol (OA) can be significantly affected by its phase state that is modulated by OA composition, relative humidity, and temperature. To specifically quantify the temperature effect on the OH heterogeneous oxidation kinetics, we determined the reactive uptake coefficients (γ) of OH by organic aerosol surrogates (i.e., glucose, glucose/hexanetriol mixture, canola oil, triacontane, and squalane) under dry conditions for temperatures from 213 to 313 K. A temperature-controlled flow reactor coupled to a chemical ionization mass spectrometer is applied to derive γ. The glass transition temperature (T_g) of the substrates was estimated using the poke-flow technique, and the corresponding viscosities are expressed with the modified Vogel-Fulcher-Tammann equation. Best estimates for the organic species’ self-molecular diffusion and OH diffusion in the condensed-phase are derived via a fractional Stokes-Einstein equation. OH uptake experiments demonstrated that the major change in reactivity occurs at temperature above T_g. To resolve the competitive contributions of surface and bulk reactions to the total reactivity, uptake kinetics were determined for triacontane that remains solid and for squalane that transitions from semi-solid to liquid over the examined temperature range. Application of the classical resistor model allowed us to further discriminate the dominant parameters (e.g., surface desorption energy, reacto-diffusive length, OH diffusion coefficient, and Henry’s law constant) that yield observed temperature dependency of γ. Results of 1 to 10 hour OH exposure experiments will be reported, further illustrating how uptake kinetics evolve with time at temperatures of 233 K and 293 K. These findings are crucial to better understand the chemical aging process of OA at temperatures relevant for the troposphere, and thus to better evaluate its impact on source apportionment, air quality and climate.