Highly Viscous States Affect the Browning of Atmospheric Organic Particulate Matter

Abstract

Initially transparent organic particulate matter (PM) can become shades of light-absorbing brown via atmospheric particle-phase chemical reactions. The production of nitrogen-containing compounds is one important pathway for browning. Semisolid or solid physical states of organic PM might, however, have sufficiently slow diffusion of reactant molecules to inhibit browning reactions. Herein, organic PM of secondary organic material (SOM) derived from toluene, a common SOM precursor in anthropogenically affected environments, was exposed to ammonia at different values of relative humidity (RH). The production of light-absorbing organonitrogen imines from ammonia exposure, detected by mass spectrometry and ultraviolet–visible spectrophotometry, was kinetically inhibited for RH < 20% for exposure times of 6 min to 24 h. By comparison, from 20% to 60% RH organonitrogen production took place, implying ammonia uptake and reaction. Correspondingly, the absorption index k across 280 to 320 nm increased from 0.012 to 0.02, indicative of PM browning. The k value across 380 to 420 nm increased from 0.001 to 0.004. The observed RH-dependent behavior of ammonia uptake and browning was well captured by a model that considered the diffusivities of both the large organic molecules that made up the PM and the small reactant molecules taken up from the gas phase into the PM. Within the model, large-molecule diffusivity was calculated based on observed SOM viscosity and evaporation. Small-molecule diffusivity was represented by the water diffusivity measured by a quartz-crystal microbalance. The model showed that the browning reaction rates at RH < 60% could be controlled by the low diffusivity of the large organic molecules from the interior region of the particle to the reactive surface region. The results of this study have implications for accurate modeling of atmospheric brown carbon production and associated influences on energy balance.

Predicting the glass transition temperature and viscosity of secondary organic material using molecular composition

“…Note that this relation is not accurate for predicting the bulk diffusivity of water and small molecules and it may also underestimate the diffusivity of organic molecules in a highly viscous matrix by a few orders of magnitudes (Champion et al, 2000;Shiraiwa et al, 2011;Power et al, 2013;Marshall et al, 2016;Bastelberger et al, 2017;Reid et al, 2018). The particle phase state, viscosity, and bulk diffusivity have been shown to affect the gas uptake and chemical transformation of organic compounds due to the kinetic limitations of bulk diffusion Abbatt et al, 2012;Zhou et al, 2013;Slade and Knopf, 2014;Arangio et al, 2015;Davies and Wilson, 2015;Wang et al, 2015;Berkemeier et al, 2016;Marshall et al, 2016;Liu et al, 2018;Pratap et al, 2018;Zhang et al, 2018), which may facilitate the long-range transport of organic compounds embedded in viscous or glassy particles (Shrivastava et al, 2017b;Mu et al, 2018). Molecular motion can be hindered in a highly viscous matrix, slowing down photochemical reactions in particles (Lignell et al, 2014;Hinks et al, 2016).…”

Effect of Bulk Composition on the Heterogeneous Oxidation of Semi-Solid Atmospheric Aerosols

“…Their reactions with atmospheric oxidizers, such as the OH radical, lead to chemical aging and potentially to the release of new volatile components to the atmosphere. In addition, the optical properties of these aerosols will greatly depend on the chemical composition and physical structure of the particle surface [11,14].…”

“…Although there are large uncertainties concerning their composition and phase state, they are believed to be mainly biogenic secondary particles formed from forest fires, biomass burning, and biofuel combustion [3][4][5][6][7][8]. Chemical and physical changes due to the multiphase processes between the atmosphere and the particles have impacts on climate, air quality, visibility, and human health [2,[9][10][11][12][13][14]. Due to the particle high viscosity (ranging from 10 2 to 10 12 Pa s), such changes are limited by slow bulk diffusion over a long time-scale [15,16].…”

Production and Measurement of Organic Particulate Matter in the Harvard Environmental Chamber

“…One challenge for laboratory studies, which are necessarily of limited scope and time, is to simulate the atmospheric evolution of gas and particle phase species. Residence times must be long enough that compounds in both gas and particles phases can undergo oxidation and multiphase reaction as they would in ambient environments 4,5,6,7,8 . Another challenge is to work in the laboratory at concentrations sufficiently low that represent the ambient environment 9,10,11 .…”