Organic and inorganic decomposition products from the thermal desorption of atmospheric particles

Abstract

Abstract. Atmospheric aerosol composition is often analyzed using thermal desorption techniques to evaporate samples and deliver organic or inorganic molecules to various designs of detectors for identification and quantification. The organic aerosol (OA) fraction is composed of thousands of individual compounds, some with nitrogen- and sulfur-containing functionality, and often contains oligomeric material, much of which may be susceptible to decomposition upon heating. Here we analyze thermal decomposition products as measured by a thermal desorption aerosol gas chromatograph (TAG) capable of separating thermal decomposition products from thermally stable molecules. The TAG impacts particles onto a collection and thermal desorption (CTD) cell, and upon completion of sample collection, heats and transfers the sample in a helium flow up to 310 °C. Desorbed molecules are refocused at the head of a GC column that is held at 45 °C and any volatile decomposition products pass directly through the column and into an electron impact quadrupole mass spectrometer (MS). Analysis of the sample introduction (thermal decomposition) period reveals contributions of NO+ (m/z 30), NO2+ (m/z 46), SO+ (m/z 48), and SO2+ (m/z 64), derived from either inorganic or organic particle-phase nitrate and sulfate. CO2+ (m/z 44) makes up a major component of the decomposition signal, along with smaller contributions from other organic components that vary with the type of aerosol contributing to the signal (e.g., m/z 53, 82 observed here for isoprene-derived secondary OA). All of these ions are important for ambient aerosol analyzed with the aerosol mass spectrometer (AMS), suggesting similarity of the thermal desorption processes in both instruments. Ambient observations of these decomposition products compared to organic, nitrate, and sulfate mass concentrations measured by an AMS reveal good correlation, with improved correlations for OA when compared to the AMS oxygenated OA (OOA) component. TAG signal found in the traditional compound elution time period reveals higher correlations with AMS hydrocarbon-like OA (HOA) combined with the fraction of OOA that is less oxygenated. Potential to quantify nitrate and sulfate aerosol mass concentrations using the TAG system is explored through analysis of ammonium sulfate and ammonium nitrate standards. While chemical standards display a linear response in the TAG system, re-desorptions of the CTD cell following ambient sample analysis shows some signal carryover on sulfate and organics, and new desorption methods should be developed to improve throughput. Future standards should be composed of complex organic/inorganic mixtures, similar to what is found in the atmosphere, and perhaps will more accurately account for any aerosol mixture effects on compositional quantification.

Analysis of indoor particles and gases and their evolution with natural ventilation

“…The home is located approximately 0.5 km south of US Interstate 44 and 2 km west of US Interstate 55 (Section S1, Figure S1). In addition to nearby highways, potential sources of background outdoor OA include regional power plants and local industry, railroad traffic, and biogenic OA originating from photochemically aged isoprene and monoterpenes emitted largely from forested regions to the southwest of St. Louis …”

“…The TAG's sample collection and analysis methodology have been described in previous work . Briefly, aerosol is collected through an inertial impactor onto the surface of a custom collection and thermal desorption (CTD) cell.…”

“…monoterpenes emitted largely from forested regions to the southwest of St. Louis. 53,54 The study lasted from July 22 to August 4, 2016 and was divided into four sampling periods: The test home was unoccupied during the first three sampling periods. All doors and windows (other than those opened during the 1WO and 2WO sampling periods; Section S1, Figure S2) were closed.…”

“…The TAG's sample collection and analysis methodology have been described in previous work. 45,54 Briefly, aerosol is collected through an inertial impactor onto the surface of a custom col- Table S2. Italicized numbers correspond to higher peak abundances that exceed the y-axis range…”

“…While some material volatilizes during desorption, a fraction of the collected sample is too thermally labile to successfully volatilize and instead decomposes into smaller gas-phase fragments, which elute between minutes 6 and 16 of analysis in the thermal decomposition period of the chromatogram ( Figure 3). 54 In this study, the TAG thermal decomposition period was used to evaluate indoor infiltration of outdoor particles using two outdoor-originating species: sulfates (m/z 64) and isoprene epoxydiol-derived SOA (IEPOX SOA, m/z 82). Since sulfate aerosol has few indoor sources, is minimally reactive, and exhibits low volatility, 59 it has been used in previous indoor air quality studies as a tracer for infiltrating outdoor aerosol.…”