Determining Aerosol Volatility Parameters Using a “Dual Thermodenuder” System: Application to Laboratory-Generated Organic Aerosols

Abstract

Thermodenuders (TD) are a tool widely used for measuring aerosol volatility in the laboratory and field. Extracting the parameters that dictate organic aerosol volatility from TD data is challenging because gas-particle partitioning rarely reaches equilibrium inside a TD operating under atmospheric conditions, thus a wide variety of parameter sets can explain observed evaporation. Component volatilities (as represented by saturation vapor pressure, C sat ), cannot be directly extracted due to uncertainties in potential limitations to mass transfer (represented by mass accommodation coefficient, a) and components' enthalpies of evaporation (DH vap ). To address these limitations, we have developed a "dual TD" experimental approach in which one line uses a temperature-stepping TD (TS-TD) with a relatively long residence time (RT) and the other operates isothermally at variable residence time (VRT-TD). Data from this approach are used in tandem with an optimizing evaporation kinetics model to extract the values of parameters dictating volatility (C sat , and associated values of DH vap and a). The system was evaluated using laboratory generated dicarboxylic acid aerosols (adipic acid and succinic acid). Excellent agreement with previously published evaporation data collected with other TD systems was observed. Parameter values reported in the literature for the tested acids vary widely, but our results are generally consistent with those from studies that allow for nonunity values of a. For example, our results suggest that a for these aerosols are of order 0.1, in agreement with results determined by Saleh et al. (2009Saleh et al. ( , 2012. Modeling results suggest that the addition of VRT-TD data provides tighter constraint on feasible DH vap and a values. The dual TD approach presented here does not rely on equilibration in the TD and thus can be directly applied to extract volatility parameters for more complex laboratory and ambient organic aerosol systems.

Downwind evolution of the volatility and mixing state of near-road aerosols near a US interstate highway

“…Similarly, in literature, different ranges of ΔHvap values are reported for different aerosol systems (Epstein et al, 2010;May et al, 2013c;Ranjan et al, 2012). TD data collected at varying (T, Rt) provides additional constraints on feasible γe and ΔHvap values (Saha et al, 2015(Saha et al, , 2017bSaha and Grieshop, 2016). TD data over a wide range in (T, Rt) space were collected during the I-40 summer campaign at the near-road trailer, and are shown in Figure 30 S2 (a-d).…”

“…The volatility distribution extraction framework used here is similar to that described in Saha et al (2015). The resulting fit empirically describes the particle volatility distribution using a volatility basis set (VBS) framework (Donahue et al, 2006(Donahue et al, , 2012, where the material is lumped over a logarithmically spaced set of C* (effective saturation concentration) bins 15 at a reference temperature of 25°C.…”

“…Because TD data at different residence 25 times provides additional constraints on the volatility parameter extraction process (Saha et al, 2015(Saha et al, , 2017b, TD/SMPS data (10-400 nm) using the MT-TD setup were collected over a wide range of temperature and Rt conditions (T= 60,90,120 °C;Rt = 9,13,19,30 s) during some of the summer campaign at the roadside trailer. An extra flow controller was used to vary Rt (Saha et al, 2015). In all measurements, a silica gel diffusion dryer was placed upstream of TD inlets and aerosol instruments to maintain relative humidity (RH) < 30-40%.…”

“…excludes the non-volatile population. Empirical particle loss correction factors (Saha et al, 2015), estimated as a function of TD operating conditions (T, Rt), were applied to the VFR from integrated SMPS volume and MFR from ACSM data. Because VFR for size-selected particles was calculated from the change in mode diameter, particle loss correction factors are not required in this calculation.…”

“…Before the TD inlet, an initial gas-particle equilibrium at ambient temperature (summer 30°C, winter 5 °C) and campaign-average aerosol mass loading (COA ~5µg m -3 ) were assumed. The Clausius-Clapyeron equation was applied to calculate temperature 20 dependent C* (Saha et al 2015).…”