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High-NOx photooxidation of n-dodecane: Temperature dependence of SOA formation. Houssni Lamkaddam, Aline Gratien, Edouard Pangui, Mathieu Cazaunau, Bénédicte Picquet-Varrault, and Jean-Francois Doussin Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03821 • Publication Date (Web): 29 Nov 2016 Downloaded from http://pubs.acs.org on December 7, 2016
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High-NOx
photooxidation
of
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dependence of SOA formation.
n-dodecane:
Temperature
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Houssni Lamkaddam*, Aline Gratien*, Edouard Pangui, Mathieu Cazaunau,
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Bénédicte Picquet-Varrault and Jean-François Doussin.
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Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), UMR7583, CNRS,
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Université Paris-Est-Créteil (UPEC) et Université Paris Diderot (UPD), Institut Pierre Simon
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Laplace (IPSL), Créteil, France
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*Correponding author :
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Address: LISA (UPEC), 61 avenue du Général de Gaulle, Créteil (France).
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+ 33 (0)1 45 17 15.
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[email protected]
Fax: + 33 (0)1 45 17 15 64.
Mail:
Phone:
[email protected],
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Abstract
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The temperature and concentration dependence of secondary organic aerosol (SOA) yields has
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been investigated for the first time for the photooxidation of n-dodecane (C12H26) in the presence 1 ACS Paragon Plus Environment
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of NOx in the CESAM chamber (French acronym for “Chamber for Atmospheric Multiphase
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Experimental Simulation”). Experiments were performed with and without seed aerosol between
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283 and 304.5 K. In order to quantify the SOA yields, a new parametrization is proposed to
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account for organic vapor loss to the chamber walls. Deposition processes were found to impact
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the aerosol yields by a factor from 1.3 to 1.8 between the lowest and the highest value. As with
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other photooxidation systems, experiments performed without seed and at low concentration of
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oxidant showed a lower SOA yield than other seeded experiments. Temperature did not
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significantly influence SOA formation in this study. This unforeseen behavior indicates that the
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SOA is dominated by sufficiently low volatility products for which a change in their partitioning
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due to temperature would not significantly affect the condensed quantities.
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1. Introduction
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Ultrafine particulate matter in the atmosphere is ubiquitous and known to impact human health1
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and global climate2. Organic material represents a large fraction (20-50%) of submicron
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particulate mass, and secondary organic aerosol (SOA) can contribute up to 90% of that fraction3-
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SOA is produced by the oxidation of volatile organic compound (VOC) leading to the formation
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of low volatility products which partition between the gas and particle phase.
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However, the degradation mechanisms of the VOC involved in SOA formation are very complex
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and remain poorly understood, limiting the accuracy of chemical transport model (CTM)
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predictions of organic aerosol (OA) mass in the atmosphere5-7. Even though CTM models use
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parametrizations derived from atmospheric simulation chamber studies, model output is impacted
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by uncertainties such as SOA precursor budgets, oxidation mechanisms of oxygenated products
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and a neglected complexity of the system8, resulting in a gap between measured and modeled OA
. Unlike primary organic aerosol (POA), i.e. organic particles directly emitted in the atmosphere,
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of 1–2 orders of magnitude5-7. It is therefore essential to improve our knowledge of processes
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involved in the SOA formation.
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Alkanes are a class of VOCs mostly emitted from human activities including combustion sources,
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vehicle exhaust, and evaporation and can represent up to 40 – 50% of the anthropogenic VOC in
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urban areas9-10. In the atmosphere, alkanes react mainly with OH radicals in daytime to form
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alkyl peroxy radicals (RO2), which in the presence of NO will give either an alkyl nitrate or an
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alkoxy radical which could react with O2, decompose or isomerize11. A substantial fraction of the
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unresolved complex mixture (UCM) of fossil fuels is composed of long-chain alkane (C>10)
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constituting the intermediate-VOC (IVOC)10, 12, known to be a potential SOA precursor13. Beside
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the fact that it represents a relevant class of compound to the atmosphere, long-chain alkanes are
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also good targets for investigating the sensitivity of SOA formation to different reaction
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pathways, i.e. fragmentation, functionalization or oligomerization, during atmospheric oxidation.
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Indeed, considering their relatively simple structure, their atmospheric chemistry is reasonably
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well known which makes them good model species to test explicit models14-16. Consequently, a
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number of studies have been carried out on SOA formation from long-chain alkanes to identify
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environmental conditions and factors that influence the production and the molecular
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composition of SOA17-28.
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The diversity in structures and lengths of the alkanes found in the UCM has been shown to
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impact SOA yields18-23, 25. It has been demonstrated that for linear structures increasing carbon
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chain length increases the SOA production, and for structures with the same carbon number,
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cyclic, linear and branched alkane show respectively a decreasing SOA yield. In addition, the
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level of nitrogen oxide (NOx) is an important environmental factor which determines the
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oxidation regime of the system and consequently the chemical composition of the oxidation
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products, i.e. the carbon atom number, the nature and distribution of functional groups on the 3 ACS Paragon Plus Environment
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molecule. Studies under both high and low-NOx regimes have been performed, giving a
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framework on degradation mechanisms of alkanes, the composition of the gas and particle
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phases, and the SOA yields17, 22-27. In particular, under high-NOx condition a key pathway of the
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alkoxy radical fate is isomerization which produces 1,4-hydroxycarbonyls (1,4-HC). These 1,4-
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HC follow successive multiphase reactions depending on the relative humidity, leading to the
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formation of a dihydrofuran (DHF) by the loss of a water molecule from a cyclic hemiacetal29-33.
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This process leads to the formation of a new double bound which makes the products very
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reactive toward OH but also toward O326, 29.
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Significant work has been carried out on the SOA formation from long-chain alkanes17-27, but
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these studies were all performed at room temperature. Temperature is an important environmental
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factor affecting both the vapor pressure of the semi-volatile organic compounds (SVOC) and the
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rate constants of the oxidation processes, and hence alters the SOA formation. To our knowledge,
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only one study explored the temperature sensitivity on SOA production from a long-chain alkane.
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Takekawa et al.28, using the Odum formalism34has shown that the n-undecane SOA yield taken at
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100 µg/m3 was 1.48 fold lower at 303 K than 283 K. However, the Odum parametrization used
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was not very efficient in representing the rather sparse data.
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Furthermore, among the studies cited above, n-dodecane constituted the most studied long-chain
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alkane as SOA precursor19, 22, 25. However, initial alkane mixing ratios used in these studies were
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very sparse. Presto, et al.25 and Loza, et al.22 have reported SOA yields with relatively low
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concentration of precursor from 9.2 to 63.6 ppbv while Lim and Ziemann19 used 1 ppmv resulting
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in much higher concentration of organic aerosol. Consequently, SOA yields reported in these
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studies are difficult to decouple due to their differences in partitioning.
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In this work, the influence of temperature, seed and precursor concentration on the
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photooxidation of n-dodecane was investigated under high-NOx conditions. In addition, particle 4 ACS Paragon Plus Environment
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and vapor loss to the wall were estimated in order to account for them when calculating SOA
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yields.
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2. Material and Methods
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2.1. CESAM chamber. The photooxidation experiments were performed in the CESAM
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chamber which has been described in detail elsewhere35. In short, the facility consists of 4.2 m3
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stainless steel cylindrical vessel. Above the chamber, three high-pressure xenon arc lamps (4 kW,
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MH Diffusion®, MacBeam™ 4000) equipped with 6.5 mm thick Pyrex filters provide irradiation
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with a spectrum very similar to the solar spectrum. When all the lamps were switched on, the
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NO2 photolysis frequency (jNO2) was (2.49 ± 0.21) ⅹ 10-3 s-1. In most experiments, only two
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lamps were used with a jNO2 of (1.10 ± 0.04) ⅹ 10-3 s-1. The chamber’s double walls allowed the
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circulation of a coolant liquid connected to a thermostat (LAUDA Integral T 10000 W), enabling
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temperature control to ±1 K at 283 and 293 K, and ±1.5 K at 304.5 K during the experiments. At
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the bottom of the chamber a stainless steel fan allowed fast mixing35 within 60 s of the
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introduction of gas species and seed particles.
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2.2. Chamber conditioning and experimental procedure. At the beginning of each
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measurement campaign, the chamber was cleaned manually using ultrapure water (18.2 MΩ,
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ELGA Maxima) and lint free wipes (SpecWipe® 3). The chamber walls were then heated at
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323 K and the chamber was evacuated down to secondary vacuum. Between each experiment, the
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chamber was also baked at 323 K, evacuated down to secondary vacuum, and maintained under
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vacuum overnight at 4 ⅹ 10-4 mbar.
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All experiments began by filling the chamber with clean dry air to 10 mbar above the
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atmospheric pressure by mixing approximately 800 mbar of nitrogen produced from the
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evaporation of a pressurized liquid nitrogen tank (Messer, purity > 99.995 %, H2O < 5 ppmv) and 5 ACS Paragon Plus Environment
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200 mbar of oxygen (Air Liquide, ALPHAGAZ™ class 1, purity 99.9 %) and were carried out at
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a relative humidity
