Impact of NO_<i>x</i> on secondary organic aerosol (SOA) formation from <i>α</i>-pinene and <i>β</i>-pinene photooxidation: the role of highly oxygenated organic nitrates

• Main Authors: I. Pullinen, S. Schmitt, S. Kang, M. Sarrafzadeh, P. Schlag, S. Andres, E. Kleist, T. F. Mentel, F. Rohrer, M. Springer, R. Tillmann, J. Wildt, C. Wu, D. Zhao, A. Wahner, A. Kiendler-Scharr
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-09-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://acp.copernicus.org/articles/20/10125/2020/acp-20-10125-2020.pdf
• ISSN: 1680-7316; 1680-7324

<p>The formation of organic nitrates (ONs) in the gas phase and their impact on mass formation of secondary organic aerosol (SOA) was investigated in a laboratory study for <span class="inline-formula"><i>α</i></span>-pinene and <span class="inline-formula"><i>β</i></span>-pinene photooxidation. Focus was the elucidation of those mechanisms that cause the often observed suppression of SOA mass formation by <span class="inline-formula">NO_<i>x</i></span>, and therein the role of highly oxygenated multifunctional molecules (HOMs). We observed that with increasing <span class="inline-formula">NO_<i>x</i></span> concentration (a) the portion of HOM organic nitrates (HOM-ONs) increased, (b) the fraction of accretion products (HOM-ACCs) decreased, and (c) HOM-ACCs contained on average smaller carbon numbers.</p> <p>Specifically, we investigated HOM organic nitrates (HOM-ONs), arising from the termination reactions of HOM peroxy radicals with <span class="inline-formula">NO_<i>x</i></span>, and HOM permutation products (HOM-PPs), such as ketones, alcohols, or hydroperoxides, formed by other termination reactions. Effective uptake coefficients <span class="inline-formula"><i>γ</i>_eff</span> of HOMs on particles were determined. HOMs with more than six O atoms efficiently condensed on particles (<span class="inline-formula"><i>γ</i>_eff&gt;0.5</span> on average), and for HOMs containing more than eight O atoms, every collision led to loss. There was no systematic difference in <span class="inline-formula"><i>γ</i>_eff</span> for HOM-ONs and HOM-PPs arising from the same HOM peroxy radicals. This similarity is attributed to the multifunctional character of the HOMs: as functional groups in HOMs arising from the same precursor HOM peroxy radical are identical, vapor pressures should not strongly depend on the character of the final termination group. As a consequence, the suppressing effect of <span class="inline-formula">NO_<i>x</i></span> on SOA formation cannot be simply explained by replacement of terminal functional groups by organic nitrate groups.</p> <p><span id="page10126"/>According to their <span class="inline-formula"><i>γ</i>_eff</span> all HOM-ONs with more than six O atoms will contribute to organic bound nitrate (<span class="inline-formula">OrgNO₃</span>) in the particulate phase. However, the fraction of <span class="inline-formula">OrgNO₃</span> stored in condensable HOMs with molecular masses&thinsp;<span class="inline-formula">&gt;</span>&thinsp;230&thinsp;Da appeared to be substantially higher than the fraction of particulate <span class="inline-formula">OrgNO₃</span> observed by aerosol mass spectrometry. This result suggests losses of <span class="inline-formula">OrgNO₃</span> for organic nitrates in particles, probably due to hydrolysis of <span class="inline-formula">OrgNO₃</span> that releases <span class="inline-formula">HNO₃</span> into the gas phase but leaves behind the organic rest in the particulate phase. However, the loss of <span class="inline-formula">HNO₃</span> alone could not explain the observed suppressing effect of <span class="inline-formula">NO_<i>x</i></span> on particle mass formation from <span class="inline-formula"><i>α</i></span>-pinene and <span class="inline-formula"><i>β</i></span>-pinene.</p> <p>Instead we can attribute most of the reduction in SOA mass yields with increasing <span class="inline-formula">NO_<i>x</i></span> to the significant suppression of gas phase HOM-ACCs, which have high molecular mass and are potentially important for SOA mass formation at low-<span class="inline-formula">NO_<i>x</i></span> conditions.</p>