Detailed chemical analysis of wintertime PM₁₀ collected at a rural village site in Germany showed the presence of a series of compounds that correlated very well with levoglucosan, a known biomass burning tracer compound. Nitrated aromatic compounds with molecular formula C₇H₇NO₄ (M(w) 169) correlated particularly well with levoglucosan, indicating that they originated from biomass burning as well. These compounds were identified as a series of methyl-nitrocatechol isomers (4-methyl-5-nitrocatechol, 3-methyl-5-nitrocatechol, and 3-methyl-6-nitrocatechol) based on the comparison of their chromatographic and mass spectrometric behaviors to those from reference compounds.Aerosol chamber experiments suggest that m-cresol, which is emitted from biomass burning at significant levels, is a precursor for the detected methyl-nitrocatechols. The total concentrations of these compounds in the wintertime PM₁₀were as high as 29 ng m⁻³, indicating the secondary organic aerosol (SOA) originating from the oxidation of biomass burning VOCs contributed non-negligible amounts to the regional organic aerosol loading.
The atmospheric oxidation of isoprene by OH radicals under low NO x conditions primarily leads to hydroxy hydroperoxides (ISOPOOH), and further, to isoprene epoxy diols (IEPOX), which have been identified as important SOA precursors. Recent studies indicate that an additional class of highly oxidized ISOPOOH oxidation products might contribute equally to SOA. Nonetheless, kinetic investigations of the phase transfer of ISOPOOH and of its oxidation products are largely missing, resulting in large uncertainties in understanding the respective atmospheric chemistry and its implications. In the present work, the partitioning behavior of synthetic 1,2-ISOPOOH and its OH oxidation products was investigated in chamber experiments with a (NO3 –)-CI-APi-ToFMS under low NO x conditions for sulfate seed particles under variation of relative humidity and particle acidity conditions. For acidic sulfate particles, a reactive uptake coefficient of γ(1,2‑ISOPOOH)(pH=0) = (9 ± 4) × 10–3 was determined. For monomeric oxidation products, a parametrization of measured uptake coefficients based on estimated vapor pressures was obtained with γ(C5 product) = −1.302 × 10–2 × ln(vap.press.·atm–1) – 0.1662, n = 15, R 2 = 0.727. Dimeric RO2 accretion products were observed in the gas phase for the first time. In a model study, an unexpectedly large proportion of these products was used to constrain the rate constant of intramolecular hydrogen shift reactions of C5O6H11 radicals to an upper limit of k = 0.002 s–1. To evaluate the atmospheric relevance, the results of this study were implemented in a remote-case model study performed with F0AM, which resulted in an increased SOA mass formation of 31% by way of the investigated ISOPOOH oxidation and phase transfer pathways.
<p><span><span>We have shown in previous work that Proton-Transfer-Reaction Time-of-Flight Mass Spectrometry (PTR-ToF-MS) in combination with the &#8220;Chemical Analysis of Aerosol Online" (CHARON) inlet is a powerful tool for direct and online analysis of sub-&#181;m particulate organic matter in the urban atmosphere. Herein, we report on the first CHARON PTR-ToF-MS measurements in the continental background environment, at the TROPOS Research Station in Melpitz near Leipzig (Germany) during a three week period in February 2019.</span></span></p><p><span><span>A state-of-the-art CHARON PTR-TOF 6000X2 instrument (IONICON Analytik GmbH, Austria) was used for measuring particulate organic compounds online (i.e., without filter pre-collection) and in real-time (< 1-min time resolution), at sub-ng m<sup>-3</sup> mass concentrations, and on an elementary composition level. Periodic switching between the standard PTR-MS gas-phase inlet and the CHARON particle inlet made it possible to comprehensively measure atmospheric organic matter in both the gaseous and particulate state at a time resolution of 10 minutes. In addition, an aerosol mass spectrometer (HR-TOF-AMS) and an aerosol chemical speciation monitor (ACSM; both Aerodyne Inc., USA) were deployed for monitoring the composition of non-refractory particulate matter. A Dual Mobility Particle Size Spectrometer (TROPOS-type T-MPSS) was used for determining the total particle size distribution. In addition, particles were collected on filters once per day and analysed offline in the laboratory.</span></span></p><p><span><span>The CHARON PTR-TOF 6000X2 instrument operated stably and reliably over the three week measurement period. Our data show that a single instrument can be used for characterizing both gaseous and particle-bound organic matter in the atmosphere at 10 minute time resolution. The obtained data agree well with ACSM, HR-TOF-AMS and T-MPSS results. A comparison with the offline results obtained from the filter samples confirmed that the CHARON PTR-ToF-MS technique accurately measures the atmospheric concentrations of selected anhydrosugars and polycyclic aromatic hydrocarbons (PAHs). We also show that CHARON PTR-ToF-MS data are useful for improving the source apportionment of particles via positive matrix factorization (PMF).</span></span></p><p><span><span>This work is part of a project that has received funding from the European Union&#8217;s Horizon 2020 research and innovation programme under grant agreement No 654109 (ACTRIS-2). F. P. has received funding through the EU's Horizon 2020 programme under grant agreement N&#186;674911 (IMPACT).&#160;</span></span></p>
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