Analytical Chemistry Seminar: Weiwei Hu and Zhe Peng
Jointly sponsored by the Department of Chemistry and Biochemistry, CIRES, and the Environmental Program
Characterization of a Real-Time Tracer for Isoprene Epoxydiols-Derived Secondary Organic Aerosol (IEPOX-SOA) from Aerosol Mass Spectrometer Measurements
Weiwei Hu et al. - Cooperative Institute for Research in Environmental Sciences, and Department of Chemistry and Biochemistry, University of Colorado, Boulder
Secondary organic aerosol (SOA) can be formed from isoprene epoxydiols (IEPOX), compounds that are produced from isoprene oxidation under low-NO conditions. IEPOX-SOA can account for a substantial fraction of organic aerosol (OA) in biogenic-influenced areas. In this study, IEPOX-SOA was identified from measurements at the forested Southeast U.S. supersite (Centreville, AL) during the Southern Oxidant and Aerosol Study (SOAS) using Positive Matrix Factorization (PMF) of aerosol mass spectrometer (AMS) measurements. The SOAS results clearly show that the fraction of OA measured at C5H6O+ (fC5H6O) in AMS spectra is a good tracer for IEPOX-SOA and correlates with the well-known methyltetrol tracers of this chemistry. Other field and chamber studies are included in the analysis to investigate the robustness of fC5H6O as an IEPOX-SOA tracer in AMS data. We observed clearly higher fC5H6O (3x10-3 - 25x10-3) in OA from regions with strong isoprene emissions vs those from urban and biomass-burning plumes (0 - 3.5x10-3 with an average of 1.75x10-3). In isoprene-influenced areas, fC5H6O in ambient OA positively correlates with the relative contribution of IEPOX-SOA to OA and decreases with OA aging. The kOH for destruction of IEPOX-SOA via heterogeneous oxidation in aerosol phase is approximately 5.3x10-13 cm3 molec－1 s－1, with life time is around 14.5 days (assuming average OH concentration=1.5x106 molecular cm-3). Volatility analysis using a thermal denuder shows that IEPOX-SOA is not more volatile than bulk SOA, contrasting with the high volatility of the tracers identified. Finally, we develop a simplified method to estimate ambient IEPOX-SOA mass concentrations as a function of fC5H6O, which is shown to perform well compared to the full PMF method. When only unit mass resolution data is available as in ACSM data, the method performs less well because of increased interferences from other ions at m/z 82. Estimated IEPOX-SOA concentrations in the southeastern U.S. from an aircraft campaign (SEAC4RS)correlate well with the IEPOX-related species detected by other techniques over a wide range of conditions, which confirms the usefulness of our method. IEPOX-SOA accounts for 14 - 17% of the OA in the SE U.S. during the summer according to both the SEAC4RS and SOAS results.
Oxidation flow reactors for the study of atmospheric chemistry systematically examined by modeling
Zhe Peng - Cooperative Institute for Research in Environmental Sciences, and Department of Chemistry and Biochemistry, University of Colorado, Boulder
Oxidation flow reactors (OFRs) using OH produced from low-pressure Hg lamps at 254 nm(OFR254) or both 185 and 254 nm (OFR185) are commonly used in atmospheric chemistry and other fields. OFR254 requires the addition of externally formed O3 since OH is formed from O3 photolysis, while OFR185 does not since O3 is formed in the reactor and OH can also be formed from H2O photolysis. In this study, we use a plug-flow kinetic model to investigate OFR properties under a very wide range of conditions applicable to both field and laboratory studies. We show that the radical chemistry in OFRs can be characterized as a function of UV light intensity, H2O concentration, and total external OH reactivity (e.g., from VOCs, NOx, and SO2). In OFR185, OH exposure is more sensitive to external OH reactivity than in OFR254, because injected O3 in OFR254 promotes the recycling of HO2 to OH, making external perturbations to theradical chemistry less significant. There has been some speculation in the literature about whether 'non-tropospheric chemistry' (photolysis at 185 or 254 nm, and/or reactions with O(1D) and O(3P)) may play an important role in these OFRs. For field studies in forested regions or the urban area of Los Angeles, reactants of atmospheric interest are predominantly consumed by OH. Nontropospheric oxidants contribute to the degradation of some species under conditions of low H2O concentration and/or very high external OH reactivity. This appears to have been a problem in some laboratory and source studies, but can be avoided in future studies by experimental planning based on our findings. Some biogenic VOCs can have substantial contributions of reaction with O3 under some operating conditions, especially for OFR254. NO3 may have played an unexpectedly significant role in some past laboratory studies. RO2 fate is similar to that in the atmosphere under low-NOx conditions. A comparison of OFRs with typical environmental chamber studies using UV blacklights and with the atmosphere is presented. OFRs' key advantages are their short experimental time scales, portability to field sites, enabling a direct connection of field and laboratory studies, and controllable and predictable radical chemistry. This study further establishes the usefulness of these reactors and enables better experiment planning and interpretation.