Aging of secondary organic aerosol from small aromatic VOCs: changes in chemical composition, mass yield, volatility and hygroscopicity

Abstract

Abstract. Secondary organic aerosol (SOA) is transformed after its initial formation, but this chemical aging of SOA is poorly understood. Experiments were conducted in the Carnegie Mellon environmental chamber to form and transform SOA from the photo-oxidation of toluene and other small aromatic volatile organic compounds (VOCs) in the presence of NOx. The effects of chemical aging on organic aerosol (OA) composition, mass yield, volatility and hygroscopicity were explored. Higher exposure to the hydroxyl radical resulted in different OA composition, average carbon oxidation state OSC) and mass yield. The OA oxidation state generally increased during photo-oxidation, and the final OA OS C ranged from −0.29 to 0.45 in the performed experiments. The volatility of OA formed in these different experiments varied by as much as a factor of 30, demonstrating that the OA formed under different oxidizing conditions can have significantly different saturation concentration. There was no clear correlation between hygroscopicity and oxidation state for this relatively hygroscopic SOA.

Elemental ratio measurements of organic compounds using aerosol mass spectrometry: characterization, improved calibration, and implications

“…The Improved-Ambient elemental ratios of chamber SOA are higher than previously reported Aiken-Ambient values and the relative change varies with the identity of the precursor. For the α-pinene + O 3 and β-caryophyllene + O 3 SOA , the predicted increase in O : C (H : C) is smaller than that for the isoprene + OH and toluene + OH SOA (Hildebrandt Ruiz et al, 2014). These differences are likely linked to the specific molecular functionalities associated with SOA formed from each precursor.…”

“…Figure 5a shows the effect of calculating Improved-Ambient OS C values with a fragmentation table that includes the H + fragment (see Supplement for details). The Hildebrandt Ruiz et al (2014) correction results in slightly smaller OS C values than obtained with the default AMS fragmentation table due to a small (< 3 %) increase in the Improved-Ambient H : C.…”

“…Figure 5b The robustness of the OS C parameter is largely due to its invariance with respect to dehydration (and hydration) processes (Kroll et al, 2011). The OS C value that is currently calculated from AMS spectra is not strictly invariant with respect to hydration and dehydration because the AMS fragmentation table neglects small amounts of H + formed from fragmentation of H 2 O + ions (Hildebrandt Ruiz et al, 2014). Other fragments, such as OH + , and O + are properly accounted for as discussed above.…”

CCN Data Interpretation Under Dynamic Operation Conditions

“…As an example, we present a data set using SFCA to characterize the CCN activity of secondary organic aerosol (SOA) generated at the Carnegie Mellon University (CMU) SOA chamber (Hildebrandt Ruiz et al 2014). When activated droplet sizes for the chamber aerosol were compared to those for ammonium sulfate calibration aerosol, the chamber aerosol droplets were larger than the ammonium sulfate droplets under identical instrument operation.…”

“…The ability to scan a wide instrument supersaturation range over a short time is a great advantage for measurements, where the aerosol is changing rapidly such as in airborne platforms or environmental chamber facilities (Moore et al 2012a,b;Hildebrandt Ruiz et al 2014;Russell et al 2013). While Moore and Nenes (2009) developed a computational model to theoretically explore the SFCA concept, it is not mature for interpretation of SFCA data on an operational basis.…”