Highly Viscous States Affect the Browning of Atmospheric Organic Particulate Matter

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Description

Initially transparent organic particulate matter (PM) can become shades of light-absorbing brown via particle-phase reactions. When semisolid or solid physical states of organic PM prevail, however, the diffusion of reactant molecules can become sufficiently slow to inhibit browning reactions. The production of nitrogen-containing compounds is one important pathway for browning. Herein, organic PM of secondary organic material (SOM) derived from toluene was exposed to ammonia at different values of relative humidity (RH). Experiments show that the production of light-absorbing organonitrogen species from ammonia exposure strongly depended on RH. This RH-dependent behavior can be well captured by a basic model when diffusivities of both small guest molecules and large molecules making up the organic PM were taken into account. Diffusivities of small molecules were directly measured in this study. The approach was to observe rates of water mass uptake or release and to use these rates to calculate diffusivities of the evaporating species. Diffusivities of large molecules were prescribed in the model based on recent measurements for viscosity and evaporation of semi-volatile compounds of SOM. At RH < 20%, although the physical mixing timescale of small molecules can be short in typical-sized aerosol particles, fast browning reactions can still be kinetically limited because of a shallow diffuse-reactive length. Self-diffusivity of large organic molecules can be 4-6 orders of magnitude smaller than the diffusivity of small molecules, which further inhibited the refresh process of reactants near the surface. These results should be accounted for in the modeling of the production of atmospheric brown carbon and its influences of atmospheric energy balance and chemical cycles.