Assessing the degree of plug flow in oxidation flow reactors (OFRs): a study on a potential aerosol mass (PAM) reactor

Oxidation flow reactors (OFRs) have been developed to achieve high degrees of oxidant exposures over relatively short space times (defined as the ratio of reactor volume to the volumetric flow rate). While, due to their increased use, attention has been paid to their ability to replicate realistic tropospheric reactions by modeling the chemistry inside the reactor, there is a desire to customize flow patterns. This work demonstrates the importance of decoupling tracer signal of the reactor from that of the tubing when experimentally obtaining these flow patterns. We modeled the residence time distributions (RTDs) inside the Washington University Potential Aerosol Mass (WU-PAM) reactor, an OFR, for a simple set of configurations by applying the tank-in-series (TIS) model, a one-parameter model, to a deconvolution algorithm. The value of the parameter, N, is close to unity for every case except one having the highest space time. Combined, the results suggest that volumetric flow rate affects mixing patterns more than use of our internals. We selected results from the simplest case, at 78?s space time with one inlet and one outlet, absent of baffles and spargers, and compared the experimental F curve to that of a computational fluid dynamics (CFD) simulation. The F curves, which represent the cumulative time spent in the reactor by flowing material, match reasonably well. We value that the use of a small aspect ratio reactor such as the WU-PAM reduces wall interactions; however sudden apertures introduce disturbances in the flow, and suggest applying the methodology of tracer testing described in this work to investigate RTDs in OFRs to observe the effect of modified inlets, outlets and use of internals prior to application (e.g., field deployment vs. laboratory study).by Dhruv Mitroo, Yujian Sun, Daniel P. Combest, Purushottam Kumar and Brent J. William

Bulk and molecular-level characterization of laboratory-aged biomass burning organic aerosol from oak leaf and heartwood fuels

The chemical complexity of biomass burning organic aerosol (BBOA) greatly increases with photochemical aging in the atmosphere, necessitating controlled laboratory studies to inform field observations. In these experiments, BBOA from American white oak (Quercus alba) leaf and heartwood samples was generated in a custom-built emissions and combustion chamber and photochemically aged in a potential aerosol mass (PAM) flow reactor. A thermal desorption aerosol gas chromatograph (TAG) was used in parallel with a high-resolution time-of-flight aerosol mass spectrometer (AMS) to analyze BBOA chemical composition at different levels of photochemical aging. Individual compounds were identified and integrated to obtain relative decay rates for key molecules. A recently developed chromatogram binning positive matrix factorization (PMF) technique was used to obtain mass spectral profiles for factors in TAG BBOA chromatograms, improving analysis efficiency and providing a more complete determination of unresolved complex mixture (UCM) components. Additionally, the recently characterized TAG decomposition window was used to track molecular fragments created by the decomposition of thermally labile BBOA during sample desorption. We demonstrate that although most primary (freshly emitted) BBOA compounds deplete with photochemical aging, certain components eluting within the TAG thermal decomposition window are instead enhanced. Specifically, the increasing trend in the decomposition m∕z 44 signal (CO2+) indicates formation of secondary organic aerosol (SOA) in the PAM reactor. Sources of m∕z 60 (C2H4O2+), typically attributed to freshly emitted BBOA in AMS field measurements, were also investigated. From the TAG chemical speciation and decomposition window data, we observed a decrease in m∕z 60 with photochemical aging due to the decay of anhydrosugars (including levoglucosan) and other compounds, as well as an increase in m∕z 60 due to the formation of thermally labile organic acids within the PAM reactor, which decompose during TAG sample desorption. When aging both types of BBOA (leaf and heartwood), the AMS data exhibit a combination of these two contributing effects, causing limited change to the overall m∕z 60 signal. Our observations demonstrate the importance of chemically speciated data in fully understanding bulk aerosol measurements provided by the AMS in both laboratory and field studies

Measuring acetic and formic acid by proton-transfer-reaction mass spectrometry: sensitivity, humidity dependence, and quantifying interferences

We present a detailed investigation of the factors governing the quantification of formic acid (FA), acetic acid (AA), and their relevant mass analogues by proton-transfer-reaction mass spectrometry (PTR-MS), assess the underlying fragmentation pathways and humidity dependencies, and present a new method for separating FA and AA from their main isobaric interferences. PTR-MS sensitivities towards glycolaldehyde, ethyl acetate, and peroxyacetic acid at <i>m/z</i> 61 are comparable to that for AA; when present, these species will interfere with ambient AA measurements by PTR-MS. Likewise, when it is present, dimethyl ether can interfere with FA measurements. For a reduced electric field (<i>E/N</i>) of 125 Townsend (Td), the PTR-MS sensitivity towards ethanol at <i>m/z</i> 47 is 5–20 times lower than for FA; ethanol will then only be an important interference when present in much higher abundance than FA. Sensitivity towards 2-propanol is <1% of that for AA, so that propanols will not in general represent a significant interference for AA. Hydrated product ions of AA, glycolaldehyde, and propanols occur at <i>m/z</i> 79, which is also commonly used to measure benzene. However, the resulting interference for benzene is only significant when <i>E/N</i> is low (&lesssim;100 Td). Addition of water vapor affects the PTR-MS response to a given compound by (i) changing the yield for fragmentation reactions and (ii) increasing the importance of ligand switching reactions. In the case of AA, sensitivity to the molecular ion increases with humidity at low <i>E/N</i> but decreases with humidity at high <i>E/N</i> due to water-driven fragmentation. Sensitivity towards FA decreases with humidity throughout the full range of <i>E/N</i>. For glycolaldehyde and the alcohols, the sensitivity increases with humidity due to ligand switching reactions (at low <i>E/N</i>) and reduced fragmentation in the presence of water (at high <i>E/N</i>). Their role as interferences will typically be greatest at high humidity. For compounds such as AA where the humidity effect depends strongly on the collisional energy in the drift tube, simple humidity correction factors (<i>X</i>_R) will only be relevant for a specific instrumental configuration. We recommend <i>E/N</i> ~ 125 Td as an effective condition for AA and FA measurements by PTR-MS, as it optimizes between the competing <i>E/N</i>-dependent mechanisms controlling their sensitivities and those of the interfering species. Finally, we present the design and evaluation of an online acid trap for separating AA and FA from their interfering species at <i>m/z</i> 61 and 47, and we demonstrate its performance during a field deployment to St. Louis, USA, during August–September of 2013

Atmospheric Reactivity of Fullerene (C₆₀) Aerosols

Rapid growth and adoption of nanomaterial-based technologies underpin a risk for unaccounted material release to the environment. Carbon-based materials, in particular fullerenes, have been widely proposed for a variety of applications. A quantitative understanding of how they behave is critical for accurate environmental impact assessment. While their aqueous phase reactivity, fate, and transport have been studied for over a decade, aerosol phase reactivity remains unexplored. Here, the transformation of C₆₀, as nanocrystal (<i>n</i>C₆₀) aerosols, is evaluated over a range of simulated atmospheric conditions. Upon exposure to UV light, gas-phase O₃, and ^•OH, <i>n</i>C₆₀ is readily oxidized. This reaction pathway is likely limited by diffusion of oxidants within/through the <i>n</i>C₆₀ aerosol. Further, gas-phase oxidation induces disorder in the crystal structure without affecting aerosol (aggregate) size. Loss of crystallinity suggests aged <i>n</i>C₆₀ aerosols will be less effective ice nuclei, but an increase in surface oxidation will improve their cloud condensation nuclei ability