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Abstract
Thermal desorption measurements, including thermal desorption mass spectrometry, are often used to determine the volatility and chemical composition of secondary organic aerosol (SOA). Accurately interpreting such measurements requires understanding the response of SOA to heat. Using optical microscopy, we monitored catechol + O₃ SOA during heating at mild temperatures (36-52 °C). Catechol + O₃ SOA is a type of SOA formed in wildfire plumes. Surprisingly, the SOA particles appeared to boil when heated to these temperatures. We identified acetone and CO₂ as dominant species emitted from the SOA during heating, implying decomposition of the SOA components. Using mass spectrometry techniques, we observed catechol dimers to be the major product in unheated SOA and observed the degradation of these dimers after heating. Viscosity calculations suggested the mixing time of acetone and CO₂ within the particles was 11 h and 1 h at temperatures of 36 and 52 °C, respectively. The observed boiling can be explained by the production and slow mixing of acetone and CO₂ within the SOA particles when subjected to mild temperatures. Our results underscore the importance of considering decomposition, high viscosities, and slow mixing times when interpreting thermal desorption measurements of SOA, even when heating to mild temperatures.
Supplementary materials
Title
Supporting Information
Description
The Supporting Information contains relevant chemical structures and mechanisms, schematics of the imaging experiments, additional mass spectrometry data, a discussion of possible contamination from other SOA experiments, and videos captured of catechol SOA boiling.
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Title
Video S1
Description
Imaging of catechol SOA during a temperature ramp experiment (corresponding to Fig. 1 in the main text). The particle was attached to a needle to capture side-view images. The initial area-equivalent diameter of the particle (at t = 0) was 132 µm, which grew to a maximum of 259 µm at 1725 s. Note that the temperature values in the video haven’t been corrected for the offset of the temperature-controlled cell, so the actual temperatures are ~0-5 °C higher. The correct temperatures are shown in Fig. 1.
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Video S2
Description
Imaging of catechol SOA during heating to 36 °C (corresponding to Fig. 2 in the main text). The particle was attached to a needle to capture side-view images. The initial area-equivalent diameter of the particle (at t = 0) was ~150 µm. Note that the temperature values in the video haven’t been corrected for the offset of the temperature-controlled cell, so the actual temperatures are ~1 °C higher than shown here. The correct temperatures are shown in Fig. 2.
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Title
Video S3
Description
Imaging of catechol SOA during heating to 52 °C (corresponding to Fig. 3 in the main text). The particle was placed on a hydrophobic glass slide to capture top-view images. The initial area-equivalent diameter of the particle (at t = 0) was ~150 µm. Note that the temperatures in the video haven’t been corrected for the offset of the temperature-controlled cell, so the actual temperatures are ~2 °C higher than shown here. The correct temperatures are shown in Fig. 3.
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