Impacts of surface adsorbed catechol on tropospheric aerosol surrogates: heterogeneous ozonolysis and its effects on...

Surface adsorbed organics are ubiquitous components of inorganic tropospheric aerosols and have the potential to alter aerosol chemical and physical properties. To assess the impact of adsorbed organics on water uptake by inorganic substrates, we used diffuse reflectance infrared spectroscopy to compared water adsorption isotherms for uncoated NaCl and α-Al2O3 samples, samples containing a monolayer of adsorbed catechol, and adsorbed catechol samples following ozonolysis. Adsorption of gaseous catechol on to the inorganic substrates produced vibrational features indicating physisorption on NaCl and displacement of surface hydroxyl groups forming binuclear bidentate catecholate on α-Al2O3, with surface concentrations of 2-3 × 10(18) molecules m(-2). Subsequent heterogeneous ozonolysis produced muconic acid at a rate 4-5 times faster on NaCl compared to α-Al2O3, with predicted atmospheric lifetimes of 4.3 and 18 h, respectively, assuming a tropospheric ozone concentration of 40 ppb. Water adsorption isotherms for all NaCl samples were indistinguishable within experimental uncertainty, indicating that these organic monolayers had negligible impact on coadsorbed water surface concentrations for these systems. α-Al2O3-catechol samples exhibited dramatically less water uptake compared to uncoated α-Al2O3, while oxidation of surface adsorbed catechol had no effect on the extent of water uptake. For both substrates, adsorbed organics increased the relative abundance of "ice-like" versus "liquid-like" water, with the effect larger for catechol than oxidized ozonolysis products. These results highlight the importance of aerosol substrate in understanding the heterogeneous ozonolysis of adsorbed polyphenols and suggest such coatings may impair ice nucleation by aluminosilicate mineral aerosol.