Impact of Thermal Decomposition on Thermal Desorption Instruments: Advantage of Thermogram Analysis for Quantifying Volatility Distributions of Organic Species

Abstract

We present results from a high-resolution chemical ionization time-of-flight mass spectrometer (HRToF-CIMS), operated with two different thermal desorption inlets, designed to characterize the gas and aerosol composition. Data from two field campaigns at forested sites are shown. Particle volatility distributions are estimated using three different methods: thermograms, elemental formulas, and measured partitioning. Thermogram-based results are consistent with those from an aerosol mass spectrometer (AMS) with a thermal denuder, implying that thermal desorption is reproducible across very different experimental setups. Estimated volatilities from the detected elemental formulas are much higher than from thermograms since many of the detected species are thermal decomposition products rather than actual SOA molecules. We show that up to 65% of citric acid decomposes substantially in the FIGAERO-CIMS, with ∼20% of its mass detected as gas-phase CO, CO, and HO. Once thermal decomposition effects on the detected formulas are taken into account, formula-derived volatilities can be reconciled with the thermogram method. The volatility distribution estimated from partitioning measurements is very narrow, likely due to signal-to-noise limits in the measurements. Our findings indicate that many commonly used thermal desorption methods might lead to inaccurate results when estimating volatilities from observed ion formulas found in SOA. The volatility distributions from the thermogram method are likely the closest to the real distributions.

Effects of gas–wall partitioning in Teflon tubing and instrumentation on time-resolved measurements of gas-phase organic compounds

“…The separation in time can then be exploited via manual analyses or factor analysis techniques (e.g., Ulbrich et al, 2009). This is equivalent to a "poor-person's chromatography," but using ambient temperature and Teflon surfaces that avoid thermal decomposition of multifunctional species that can occur in gas chromatography due to use of high temperatures (Stark et al, 2017).…”

Constraining nucleation, condensation, and chemistry in oxidation flow reactors using size-distribution measurements and aerosol microphysical modelling

“…Although The TD-EIMS reported ambient concentrations decadally between C*=10 1 to C*=10 7 g m -3 , differences in mass concentrations per 15 bin were small Hunter et al, 2017) and thus our assumed division should be within experimental uncertainty. The initial total aerosol mass (as measured by the AMS) is evenly divided between the C*=10 -4 to C*=10 -2 g m -3 bins, consistent with the overall low-volatility of the ambient OA (Stark et al, 2017); the C*=10 0 g m -3 bin is assumed to have an initial concentration of 0 g m -3 ; Figure 2a shows an example of the initial ambient partitioning between the volatility bins for a case from the BEACHON campaign. Monoterpene (MT) and isoprene concentrations are simulated 20 explicitly outside of the VBS (though their reaction products enter the VBS as discussed earlier).…”

Chlorine-initiated oxidation of n-alkanes under high NOx conditions: Insights into secondary organic aerosol composition and volatility using a FIGAERO-CIMS

“…The Tmax shift phenomena have been observed in several FIGAERO-CIMS studies (D'Ambro et al, 2017;Gaston et al, 2016;Huang et al, 2018a;Lopez-Hilfiker et al, 2015;Thompson et al, 2017). Explanations vary for the observed Tmax shifts.…”

“…Theory and operation of the CIMS are described in detail elsewhere (Aljawhary et al, 2013;Bertram et al, 2011;Lee et al, 2014;Wang and Hildebrandt Ruiz, 2017). The CIMS inlet is coupled to a filter inlet for gases and aerosols (FIGAERO), which has been 10 used in a number of studies to investigate the chemical composition and volatility of particle-phase compounds (D'Ambro et al, 2017;Gaston et al, 2016;Huang et al, 2018a;Lee et al, 2016;Lopez-Hilfiker et al, 2014Stark et al, 2017;Thompson et al, 2017). The FIGAERO system alternated between two operational modes.…”

“…The temperature at the peak desorption signal, Tmax correlates with the enthalpy of sublimation (dHsub) and the saturation vapor pressure (C * ) of 20 the compound (Lopez-Hilfiker et al, 2014. However, some compounds may exhibit bimodal or multimodal behavior, with a second desorption mode occurring at much higher temperatures than expected, which is often interpreted as the result of thermal fragmentation or decomposition of larger oligomeric compounds (Huang et al, 2018a;Lopez-Hilfiker et al, 2014;Stark et al, 2017;Wang et al, 2016).…”