Gas-Particle Partitioning and SOA Yields of Organonitrate Products from NO₃-Initiated Oxidation of Isoprene under Varied Chemical Regimes

Abstract

Alkyl nitrate (AN) and secondary organic aerosol (SOA) from the reaction of nitrate radicals (NO 3 ) with isoprene were observed in the Simulation of Atmospheric PHotochemistry In a large Reaction (SAPHIR) chamber during the NO 3 Isop campaign in August 2018. Based on 15 day-long experiments under various reaction conditions, we conclude that the reaction has a nominally unity molar AN yield (observed range 90 ± 40%) and an SOA mass yield of OA + organic nitrate aerosol of 13–15% (with ∼50 μg m –3 inorganic seed aerosol and 2–5 μg m –3 total organic aerosol). Isoprene (5–25 ppb) and oxidant (typically ∼100 ppb O 3 and 5–25 ppb NO 2 ) concentrations and aerosol composition (inorganic and organic coating) were varied while remaining close to ambient conditions, producing similar AN and SOA yields under all regimes. We observe the formation of dinitrates upon oxidation of the second double bond only once the isoprene precursor is fully consumed. We determine the bulk partitioning coefficient for ANs ( K p ∼ 10 –3 m 3 μg –1 ), indicating an average volatility corresponding to a C 5 hydroxy hydroperoxy nitrate.

A Four Carbon Organonitrate as a Significant Product of Secondary Isoprene Chemistry

“…The relative contribution is estimated assuming the same sensitivity for all the measured ONs. Generally, the time‐series of the sum of measured mononitrates, dinitrates and accretion products followed the total gas‐phase alkyl nitrates time evolution measured by a Thermal Dissociation Cavity Ring‐down Spectrometer during the campaign (Brownwood et al., 2021 ), suggesting the CIMS total ONs signal includes the majority of the formed ON. In the dark the primary products increased rapidly after the injection of oxidant precursors and isoprene (Figure 1 ), especially C 5 H 9 NO 5 (INP) , the dominant primary nighttime ON measured by the CIMS in all experiments.…”

“…The comprehensive nighttime isoprene chemistry study was conducted in August 2018 in the atmospheric simulation chamber SAPHIR (Section S1 in Supporting Information ) (Fuchs et al., 2017; Rohrer et al., 2005) at Forschungszentrum Jülich, Germany (Brownwood et al., 2021; Dewald et al., 2020; Vereecken et al., 2021; Wu et al., 2021). One goal of the campaign was to explore different oxidation regimes by applying conditions enhancing the contribution of different reaction pathways (i.e., RO 2 + RO 2 , RO 2 + HO 2 or unimolecular RO 2 ).…”

“…In exp. 2 (Figure 1b), the RO 2 + RO 2 reactions contributed up to 13% and the RO 2 + HO 2 reactions 53% of the loss rate of RO 2 (Brownwood et al, 2021). Under dominant HO 2 conditions (exp.…”

“…It is not clear if produced epoxides and peroxides are unstable and can decompose into C 4 products (C 4 H 7 NO 5 being one of them) in the gas phase or on available surfaces, thus explaining why only low concentration of these were observed. Finally, the possibility of some production by OH oxidation under dark conditions cannot be ruled out, but certainly the contribution was small as isoprene loss due to NO 3 was calculated to be around 90% (Brownwood et al, 2021) and the impact of isoprene + OH on the total yield of C 4 H 7 NO 5 was below 5% for all modeled experiments (more details in Section S1 in Supporting Information S1). However, further exploration and evaluation of these pathways are beyond the scope of this work.…”

Impact of ozone and inlet design on the quantification of isoprene-derived organic nitrates by thermal dissociation cavity ring-down spectroscopy (TD-CRDS)

“…Quartz tubes contain impurities and surface defects that can provide reactive sites (RSs) at elevated temperatures, and the surface-catalysed chemistry of ozone on, for example, mineral silicates is well known (Bulanin et al, 1994;Hanisch and Crowley, 2003a, b;Usher et al, 2003). O 3 can be surfacecatalytically converted to O 2 (Reaction R25) by the formation and loss of reactive, oxygenated surface sites (RSOs) via Reactions (R23) and (R24).…”