Chamber experiments to study gas phase chemical mechanisms and to evaluate instrumental capabilities to measure volatile organic compounds (VOCs) in both low and high NOx regimes

Experiments conducted in atmospheric simulation chambers have played a large role in elucidating chemical processes that occur in the atmosphere by simplifying the matrix yet retaining the important reactants and co-factors that influence the outcomes of photochemical reactions in the real world. ACOM has a dedicated 10-m3 environmental chamber which is used both for this purpose and to test and compare instruments side-by-side to evaluate their analytical capabilities and limitations. The majority of atmospheric chemistry experiments conducted using chambers have been done at relatively high values of atmospheric nitrogen oxide(s) (NO_x) concentrations. There is a great deal of current interest in lower NO_x regimes where NO_x < 100 ppt because emissions technologies are driving ambient NO_x levels downward at a high rate. Atmospheric chemical reactions can take different pathways depending on the amount of NO_x available. We recently investigated the oxidation pathways of n-butane (nC₄H₁₀) and 1-butene (1-C₄H₈), as well as isoprene (C₅H₈) in high -and low-NO_x environments using the NCAR Trace Organic Gas Analyzer (TOGA) and a Proton Transfer Reaction Mass Spectrometer PTR-MS. Our data suggest that C₄ alkene and alkane oxidation through the HO₂ pathway is analogous to isoprene, and that C₄ hydroperoxide oxidation products are also susceptible to decomposition within commercial instruments. A conclusion from this study is that any instrument deployed in low NO_x environments should be tested and evaluated for hydroperoxide artifact interference with OVOC measurements prior to deployment. We show that, in contrast to PTR-MS instruments, TOGA has minimal instrument bias with respect to OVOCs in low-NO_x environments due to the design of its introduction system and choice of wetted material. This feature of the TOGA instrument is allowing us to study the distributions of oxidation products in the low-NO_x environments encountered during the NASA Atmospheric Tomography (ATom) mission and other field campaigns involving low-NO_x regimes. Figure 1 shows results from the isoprene portion of the experiment that demonstrate these effects.

Figure 1. Depiction of VOC reactant isoprene's photochemical loss throughout a chamber experiment under low-NO_x conditions, and concurrent accumulation of OVOC products methacrolein (MACR), MVK, and formaldehyde (HCHO). The large excursion of the PTR-MS data from the box-modeled and TOGA MVK+MACR data in (b) indicate that the PTR-MS has a large artifact when measuring isoprene oxidation products in low-NO_x environments whereas TOGA does not. Missing data in the TOGA time series are from times that TOGA sampled non-chamber air.