Effect of NO<sub><i>x</i></sub> on 1,3,5-trimethylbenzene (TMB) oxidation product distribution and particle formation

Abstract

Secondary organic aerosol (SOA) represents a significant fraction of the tropospheric aerosol and its precursors are volatile organic compounds (VOCs). Anthropogenic VOCs (AVOC) dominate the VOC budget in many urban areas with 1,3,5-trimethylbenzene (TMB) being among the most reactive aromatic AVOCs. TMB formed highly oxygenated organic molecules (HOMs) in an NO x -free environment, which could contribute to new particle formation (NPF) depending on oxidation conditions where elevated OH oxidation enhanced particle formation. The experiments were performed in an oxidation flow reactor, the Go:PAM unit, under controlled OH oxidation conditions. By addition of NO x to the system we investigated the effect of NO x on particle formation and on the product distribution. We show that the formation of HOMs, and especially HOM accretion products, strongly varies with NO x conditions. We observe a suppression of HOM and particle formation with increasing NO x / TMB ratio and an increase in the formation of organonitrates (ONs) mostly at the expense of HOM accretion products. We propose reaction mechanisms and pathways that explain the formation and observed product distributions with respect to oxidation conditions. We hypothesise that, based on our findings from TMB oxidation studies, aromatic AVOCs may not contribute significantly to NPF under typical NO x /AVOC conditions found in urban atmospheres.

Oxygenated products formed from OH-initiated reactions of trimethylbenzene: Autoxidation and accretion

“…On account of their extraordinarily low vapor pressure, HOM dimers contribute more significantly to the formation and growth of atmospheric new particles than HOM monomers. Our C18 oxidation products have similar ion formulas to the dimer products in recent 1,3,5-TMB oxidation experiments (Molteni et al, 2018;Tsiligiannis et al, 2019). In our experiments, the formation of C18H26O8-15, C18H28O9-15, and C18H30O12-15 can be explained by reactions of two C9H13Ox• , one C9H13Ox• and one C9H15Ox• , and two C9H15Ox• respectively.…”

“…On the other hand, this observation could also be due to the much higher RO2 concentrations in our experiments. The amount of reacted 1,3,5-TMB in our experiment is around 74.1 ppb (~ 1.8 × 10 12 molecules cm -3 ), whereas in the experiments of Tsiligiannis et al (2019) and Molteni et al (2018), the numbers are 26 ppb (~ 6.5 ×10 11 molecules cm -3 ) and 22.3 ppb (~5.6 × 10 11 molecules cm -3 ), respectively. Figure 6 describes the distribution of C9 products detected by Nitrate CI-APi-TOF in the absence of NOx (Exp.…”

“…reported by Molteni et al (2018t). The lack of a m/z-transmission correction in the former two studies could partially explain the discrepancy (Molteni et al, 2018;Tsiligiannis et al, 2019). On the other hand, this observation could also be due to the much higher RO2 concentrations in our experiments.…”

“…and C18 signals by Tsiligiannis et al (2019), 43.3%-52.4% modelled by Tsiligiannis et al (2019), and 39%…”

“…radicals have been shown to be fast enough to compete with other bimolecular reactions even under NO concentrations as high as in urban environment (Tsiligiannis et al, 2019).…”