Impact of NO<sub>x</sub> on secondary organic aerosol (SOA) formation from <i>α</i>-pinene and <i>β</i>-pinene photo-oxidation: the role of highly oxygenated organic nitrates

Abstract

<p><strong>Abstract.</strong> The formation of organic nitrates (ON) in the gas phase and their impact on mass formation of Secondary Organic Aerosol (SOA) was investigated in a laboratory study for <i>α</i>-pinene and <i>β</i>-pinene photo-oxidation. Focus was the elucidation of those mechanisms that cause the often observed suppression of SOA mass formation by NO<sub>x</sub>, and therein the role of highly oxygenated multifunctional molecules (HOM). We observed that with increasing NO<sub>x</sub> (a) the portion of HOM organic nitrates (HOM-ON) increased, (b) the fraction of accretion products (HOM-ACC) decreased and (c) HOM-ACC contained on average smaller carbon numbers.</p> <p>Specifically, we investigated HOM organic nitrates (HOM-ON), arising from the termination reactions of HOM peroxy radicals with NO<sub>x</sub>, and HOM permutation products (HOM-PP), such as ketones, alcohols or hydroperoxides, formed by other termination reactions. Effective uptake coefficients γeff of HOM on particles were determined. HOM with more than 6 O-atoms efficiently condensed on particles (<i>γ</i></sub><sub>eff</sub> > 0.5 in average) and for HOM containing more than 8 O-atoms, every collision led to loss. There was no systematic difference in <i>γ</i></sub><sub>eff</sub> for HOM-ON and HOM-PP arising from the same HOM peroxy radicals. This similarity is attributed to the multifunctional character of the HOM: as functional groups in HOM arising from the same precursor HOM peroxy radical are identical, vapor pressures should not strongly depend on the character the final termination group. As a consequence, the suppressing effect of NO<sub>x</sub> on SOA formation cannot be simply explained by replacement of terminal functional groups by organic nitrate groups.</p> <p>The fraction of organic bound nitrate (OrgNO<sub>3</sub>) stored in gas-phase HOM-ON appeared to be substantially higher than the fraction of particulate OrgNO<sub>3</sub> observed by aerosol mass spectrometry. This result suggests losses of OrgNO<sub>3</sub> for organic nitrates in particles, probably due to hydrolysis of OrgNO<sub>3</sub> that releases HNO<sub>3</sub> into the gas phase but leaves behind the organic rest in the particulate phase. However, the loss of HNO<sub>3</sub> alone, could not explain the observed suppressing effect of NO<sub>x</sub> on particle mass formation from <i>α</i>-pinene and <i>β</i>-pinene.</p> <p>We therefore attributed most of the reduction in SOA mass yields with increasing NO<sub>x</sub> to the significant suppression of gas-phase HOM-ACC which have high molecular mass and are potentially important for SOA mass formation at low NO<sub>x</sub> conditions.</p>

Highly Oxygenated Organic Nitrates Formed from NO3 Radical-Initiated Oxidation of β-Pinene

“…For the formation of C 10 H 16 NO 2 n •, an “alkoxy-peroxy” pathway provides a plausible mechanism. , For example, the nitrooxyalkoxy radical C 10 H 16 NO 4 • can be competitively formed from the initially formed NO 3 -RO 2 • (C 10 H 16 NO 5 •) and then undergo H-migration and O 2 addition forming C 10 H 16 NO 6 •. , This peroxy radical can then again undergo further autoxidation; such an autoxidation path through an alkoxy radical stage is called an alkoxy-peroxy pathway and yields the progressive series of C 10 H 16 NO 2 n • products (with an even number of O atoms). In this study, both C 10 H 16 NO 2 n +1 • and C 10 H 16 NO 2 n • radicals were observed, with time series of the most abundant radicals C 10 H 16 NO 9 • and C 10 H 16 NO 8 • shown in Figure a.…”

“…This indicates that reactions with NO 3 radicals lead to higher mass oxidation products, as expected, and contribute more to SOA than ozonolysis reactions. The converted mass yields of HOM-ON from β-pinene + NO 3 were averaged to 12.5% (−6.8%/+14.6%), assuming that the volatility of HOM-ON was low enough to condense irreversibly onto the particle phase. , Considering the SOA yields of β-pinene + NO 3 , − ,,, these HOM-ON should contribute a significant fraction, especially at low organic aerosol loadings in an ambient atmosphere. Note that the volatility of HOM-ON was not determined in this study.…”

“…The peak area of ions identified using the mass spectrometer was normalized to the total ion signal and then converted to concentrations using a calibration coefficient (C) of 2.5 × 10 10 molecule cm −3 nc −1 (normalized count), determined using H 2 SO 4 as described by Pullinen et al 45 A mass-independent transmission efficiency equal to H 2 SO 4 (with an uncertainty of −0%/+14%) was used when applying the calibration coefficient to HOM, as determined in our previous study. 45 The determination of this calibration value and its application to HOM are described in detail in the Supporting Information (SI) (Section S1). The concentrations of the identified ON with more than 6 O atoms were summed to the total concentrations of HOM-ON after a wall loss rate of 6 × 10 −4 s −1 , and a dilution rate of 1.5 × 10 −5 s −1 were accounted for.…”

“…Considering how the RO 2 • radical chain is terminated to form closed-shell products (see the SI), 33 For the formation of C 10 H 16 NO 2n •, an "alkoxy-peroxy" pathway provides a plausible mechanism. 33,45 For example, the nitrooxyalkoxy radical C 10 H 16 NO 4 • can be competitively formed from the initially formed NO 3 -RO 2 • (C 10 H 16 NO 5 •) NO 9 here are mixtures containing both hydroxynitrates and hydroperoxynitrates, whereas these corresponding termination products demonstrated a typical time series of first-generation products, with a quick peak appearance after each β-pinene addition and subsequent decay. Notably, the carbonylnitrates C 10 H 15 NO 8 showed a much higher signal than the other two termination products.…”

Simulation of Monoterpene SOA Formation by Multiphase Reactions Using Explicit Mechanisms

“…The NO x regime can also influence terpene SOA formation. In general, the reaction of RO 2 with HO 2 radicals leads to relatively lower volatility products than the RO 2 with NO reaction. ,, In addition, HOM, a product from autoxidation, more efficiently forms at the lower NO x level . The contribution of HOM to SOA mass can be increased with decreasing NO x , particularly in urban environments.…”