Citation:

Jaoui, M., R. Szmigielski, K. Nestorowicz, A. Kolodziejczyk, K. Sarang, K. Rudzinski, A. Konopka, Tad Kleindienst, E. Bulska, AND M. Lewandowski. Organic Hydroxy Acids as Highly Oxygenated Molecular (HOM) Tracers for Aged Isoprene Aerosol. ENVIRONMENTAL SCIENCE & TECHNOLOGY. American Chemical Society, Washington, DC, 53(24):14516-14527, (2019). https://doi.org/10.1021/acs.est.9b05075

Impact/Purpose:

Isoprene has the highest worldwide emissions of all non-methane hydrocarbons and reacts rapidly in the atmosphere. Due to its aerosol significant forming potential, many experimental investigations have been conducted to examine its contribution to fine particulate matter known to lead to detrimental health effects. Chemical mechanisms have been developed to help explain gas-phase products which form organic aerosol. Later generation oxidation products are of increasing interest since these products may generate a high fraction of the isoprene aerosol mass and new particle growth. Air quality models will continue to be deficient without these pertinent mechanistic details. The compounds reported here provide a rationale to constrain atmospheric models in their predictions of isoprene-derived aerosol and atmospheric oxidant loads.

Description:

Highly oxygenated molecules (HOMs) are a class of compounds associated with secondary organic aerosols exhibiting high oxygen to carbon (O:C) ratios and often originating from the oxidation of biogenic compounds. Here, the photooxidation and ozonolysis of isoprene were examined under a range of conditions to identify HOM tracers for aged isoprene aerosol. The HOM tracers were identified as silylated derivatives by gas chromatography–mass spectrometry and by detecting their parent compounds by liquid chromatography–high resolution mass spectrometry. In addition to the previously observed methyltetrols and 2-methylglyceric acid, seven tracer compounds were identified, including 2-methyltartronic acid (MTtA), 2-methylerythronic acid (2MeTrA), 3-methylerythronic acid (3MeTrA), 2-methylthreonic acid (2MTrA), 3-methylthreonic acid (3MTrA), erythro-methyltartaric acid (e-MTA), and threo-methyltartaric acid (t-MTA). The molecular structures were confirmed with authentic standards synthesized in the laboratory. The presence of some of these HOMs in the gas and particle phases simultaneously provides evidence of their gas/particle partitioning. To determine the contributions of aged isoprene products to ambient aerosols, we analyzed ambient PM2.5 samples collected in the southeastern United States in summer 2003 and at two European monitoring stations located in Zielonka and Godów (Poland). Our findings show that methyltartaric acids (MTA) and 2- and 3-methylthreonic acids (and their stereoisomers) are representative of aged isoprene aerosol because they occur both in the laboratory chamber aerosol obtained and in ambient PM2.5. On the basis of gas chromatography–mass spectrometry (GC-MS) analysis, their concentrations were found to range from 0.04 ng for 3-methylthreonic acid to 6.3 ng m–3 for methyltartaric acid at the southeast site in Duke Forest, NC, USA.

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