The dynamics of the ultrafast excited-state multiple intermolecular proton transfer (PT) reactions in gas-phase complexes of 1H-pyrrolo[3,2-h]quinoline with water and methanol (PQ(H2O)n and PQ(MeOH)n , where n = 1, 2) is modeled using quantum-chemical simulations. The minimum energy ground-state structures of the complexes are determined. Molecular dynamics simulations in the first excited state are employed to determine reaction mechanisms and the time evolution of the PT processes. Excited-state dynamics results for all complexes reveal synchronous excited-state multiple proton transfer via solvent-assisted mechanisms along an intermolecular hydrogen-bonded network. In particular, excited-state double proton transfer is the most effective, occurring with the highest probability in the PQ(MeOH) cluster. The PT character of the reactions is suggested by nonexistence of crossings between ππ* and πσ* states
The stable carbon kinetic isotope effect (KIE) of ethane photooxidation by OH radicals was deduced by employing both laboratory measurements and theoretical calculations. The investigations were designed to elucidate the temperature dependence of KIE within atmospherically relevant temperature range. The experimental KIE was derived from laboratory compound‐specific isotope analyses of ethane with natural isotopic abundance exposed to OH at constant temperature, showing ε values of 7.16 ± 0.54‰ (303 K), 7.45 ± 0.48‰ (288 K), 7.36 ± 0.28‰ (273 K), 7.61 ± 0.28‰ (263 K), 8.89 ± 0.90‰ (253 K), and 9.42 ± 2.19‰ (243 K). Compared to previous studies, a significant improvement of the measurement precision was reached at the high end of the investigated temperature range. The KIE was theoretically determined as well, in the temperature range of 150 K to 400 K, by calculating the reaction rate coefficients of 12C and singly 13C substituted ethane isotopologues applying chemical quantum mechanics together with transition state theory. Tunneling effect and internal rotations were also considered. The agreement between experimental and theoretical results for rate coefficients and KIE in an atmospherically relevant temperature range is discussed. However, both laboratory observations and computational predictions show no significant temperature dependence of the KIE for the ethane oxidation by OH radicals.
Abstract. We report the first-time use of the Lagrangian particle
dispersion model (LPDM) FLEXPART to simulate isotope ratios of the biomass
burning tracer levoglucosan. Here, we combine the model results with
observed levoglucosan concentrations and δ13C to assess the
contribution of local vs. remote emissions from firewood domestic heating to the particulate matter sampled during the cold season at two measurements stations of the Environmental Agency of North Rhine-Westphalia, Germany. For the investigated samples, the simulations indicate that the largest part of the sampled aerosol is 1 to 2 d old and thus originates from local to regional sources. Consequently, ageing, also limited by the reduced
photochemical activity in the dark cold season, has a minor influence on
the observed levoglucosan concentration and δ13C. The retro
plume ages agree well with those derived from observed δ13C
(the “isotopic” ages), demonstrating that the limitation of backwards
calculations to 7 d for this study does not introduce any significant
bias. A linear regression analysis applied to the experimental levoglucosan
δ13C vs. the inverse concentration confirms the young age of
aerosol. The high variability in the observed δ13C implies that
the local levoglucosan emissions are characterized by different isotopic
ratios in the range of −26.3 ‰ to −21.3 ‰. These values
are in good agreement with previous studies on levoglucosan source-specific
isotopic composition in biomass burning aerosol. Comparison between measured
and estimated levoglucosan concentrations suggests that emissions are
underestimated by a factor of 2 on average. These findings demonstrate
that the aerosol burden from home heating in residential areas is not of
remote origin. In this work we show that combining Lagrangian modelling with
isotope ratios is valuable to obtain additional insight into source
apportionment. Error analysis shows that the largest source of uncertainty
is limited information on isotope ratios of levoglucosan emissions. Based on
the observed low extent of photochemical processing during the cold season,
levoglucosan can be used under similar conditions as a conservative tracer
without introducing substantial bias.
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