Molecular composition and volatility of multi-generation products formed from isoprene oxidation by nitrate radical

• Main Authors: R. Wu, L. Vereecken, E. Tsiligiannis, S. Kang, S. R. Albrecht, L. Hantschke, D. Zhao, A. Novelli, H. Fuchs, R. Tillmann, T. Hohaus, P. T. M. Carlsson, J. Shenolikar, F. Bernard, J. N. Crowley, J. L. Fry, B. Brownwood, J. A. Thornton, S. S. Brown, A. Kiendler-Scharr, A. Wahner, M. Hallquist, T. F. Mentel
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-07-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://acp.copernicus.org/articles/21/10799/2021/acp-21-10799-2021.pdf

<p>Isoprene oxidation by nitrate radical (NO<span class="inline-formula">₃</span>) is a potentially important source of secondary organic aerosol (SOA). It is suggested that the second or later-generation products are the more substantial contributors to SOA. However, there are few studies investigating the multi-generation chemistry of isoprene-NO<span class="inline-formula">₃</span> reaction, and information about the volatility of different isoprene nitrates, which is essential to evaluate their potential to form SOA and determine their atmospheric fate, is rare. In this work, we studied the reaction between isoprene and NO<span class="inline-formula">₃</span> in the SAPHIR chamber (Jülich) under near-atmospheric conditions. Various oxidation products were measured by a high-resolution time-of-flight chemical ionization mass spectrometer using Br<span class="inline-formula">⁻</span> as the reagent ion. Most of the products detected are organic nitrates, and they are grouped into monomers (C<span class="inline-formula">₄</span> and C<span class="inline-formula">₅</span> products) and dimers (C<span class="inline-formula">₁₀</span> products) with 1–3 nitrate groups according to their chemical composition. Most of the observed products match expected termination products observed in previous studies, but some compounds such as monomers and dimers with three nitrogen atoms were rarely reported in the literature as gas-phase products from isoprene oxidation by NO<span class="inline-formula">₃</span>. Possible formation mechanisms for these compounds are proposed. The multi-generation chemistry of isoprene and NO<span class="inline-formula">₃</span> is characterized by taking advantage of the time behavior of different products. In addition, the vapor pressures of diverse isoprene nitrates are calculated by different parametrization methods. An estimation of the vapor pressure is also derived from their condensation behavior. According to our results, isoprene monomers belong to intermediate-volatility or semi-volatile organic compounds and thus have little effect on SOA formation. In contrast, the dimers are expected to have low or extremely low volatility, indicating that they are potentially substantial contributors to SOA. However, the monomers constitute 80 % of the total<span id="page10800"/> explained signals on average, while the dimers contribute less than 2 %, suggesting that the contribution of isoprene NO<span class="inline-formula">₃</span> oxidation to SOA by condensation should be low under atmospheric conditions. We expect a SOA mass yield of about 5 % from the wall-loss- and dilution-corrected mass concentrations, assuming that all of the isoprene dimers in the low- or extremely low-volatility organic compound (LVOC or ELVOC) range will condense completely.</p>