Thermal decomposition in thermal desorption instruments: importance of thermogram measurements for analysis of secondary organic aerosol

Understanding the chemical composition of secondary organic aerosol (SOA) is crucial for explaining sources and fate of this important aerosol class in tropospheric chemistry. Further, determining SOA volatility is key in predicting its atmospheric lifetime and fate, due to partitioning from and to the gas phase. We present three analysis approaches to determine SOA volatility distributions from two field campaigns in areas with strong biogenic emissions, a Ponderosa pine forest in Colorado, USA, from the BEACHON-RoMBAS campaign, and a mixed forest in Alabama, USA, from the SOAS campaign. We used a high-resolution-time-of-flight chemical ionization mass spectrometer (CIMS) for both campaigns, equipped with a micro-orifice volatilization impactor (MOVI) inlet for BEACHON and a filter inlet for gases and aerosols (FIGAERO) for SOAS. These inlets allow near simultaneous analysis of particle and gas-phase species by the CIMS. While gas-phase species are directly measured without heating, particles undergo thermal desorption prior to analysis. Volatility distributions can be estimated in three ways: (1) analysis of the thermograms (signal vs. temperature); (2) via partitioning theory using the gas- and particle-phase measurements; (3) from measured chemical formulas via a group contribution model. Comparison of the SOA volatility distributions from the three methods shows large discrepancies for both campaigns. Results from the thermogram method are the most consistent of the methods when compared with independent AMS-thermal denuder measurements. The volatility distributions estimated from partitioning measurements are very narrow, likely due to signal-to-noise limits in the measurements. The discrepancy between the formula and the thermogram methods indicates large-scale thermal decomposition of the SOA species. We will also show results of citric acid thermal decomposition, where, in addition to the mass spectra, measurements of CO, CO2 and H2O were made, showing thermal decomposition of up to 65% of the citric acid molecules.