10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Chemical Composition and Implications for Viscosity of SOA at Low Temperature
CLAUDIA MOHR, Wei Huang, Cheng Wu, Yvette Gramlich, Harald Saathoff, Aki Pajunoja, Annele Virtanen, Stockholm University
Abstract Number: 818 Working Group: Aerosol Chemistry
Abstract Secondary organic aerosols (SOA) can be transported to or formed in the upper troposphere. Temperature and relative humidity conditions representative of the upper troposphere, however, are rarely simulated in chamber studies, despite their potential importance for the phase state, morphology and chemical composition of SOA particles and thus their cloud formation potential. In the present work, we investigate the chemical composition and viscosity of SOA from α-pinene and toluene formed at conditions corresponding to temperatures of 223 K to 296 K and relative humidities between
We conducted two types of chamber experiments at the AIDA facility at KIT. In most experiments, SOA was directly formed in the dark AIDA chamber kept at different temperatures by reaction of precursors with O3 or OH radicals. For the second type, SOA from α-pinene was first produced in an adjacent, smaller chamber kept at ambient temperature, and then introduced into the AIDA chamber, kept at different temperatures, where for a subset of experiments it was mixed with the SOA produced from toluene. For about half of the experiments, NOx was added to investigate its influence on oxidation products and SOA yield. The chemical composition of SOA particles was analyzed with a chemical ionization mass spectrometer with filter inlet for gases and aerosols (FIGAERO-CIMS), deploying iodide as reagent ion.
Comparisons of particle-phase mass spectra for SOA from the oxidation of mixtures of toluene and α-pinene by OH radicals at three different temperatures indicate that the temperature has a significant impact on the chemical composition for these mixed SOA. Mixing particles from α-pinene and toluene oxidation after separated formation does not seem to drastically change the nature of the organic compounds. This is a somewhat expected result, as the time scales of particle mixing and potential subsequent chemical reaction between particle-phase compounds likely lie beyond the time scales of our few-hour-experiments. However, when gaseous compounds of the two precursors are already mixed during the oxidation process before particle formation takes place, different compounds exhibit high signal. In the presence of NOx, a clear reduction of the SOA yield was observed, especially for compounds with higher masses.
We also present detailed investigations of the shape of signal resulting from the thermal desorption of SOA particles deposited on the Teflon filter in the FIGAERO (thermograms). Single mode thermograms with signal maxima occurring at distinct desorption temperatures (Tmax) are correlated with a compound’s enthalpy of sublimation. Significantly higher Tmax were observed in the sum thermograms for CxHyOz compounds of α-pinene SOA formed at cold and dry conditions compared to humid and/or warm conditions. The results suggest differences in the viscosity of the formed SOA particles depending on experimental conditions, potentially resulting from high molecular weight organics and/or oligomers. Mass spectra comparison for these conditions support this conclusion.
The results provide insights into the influence of changing temperature and humidity conditions on chemical composition and viscosity of anthropogenic and biogenic SOA during its formation and evolution in the atmosphere.