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Nov 29, 2016 - Université Paris Diderot (UPD), Institut Pierre Simon Laplace (IPSL), Créteil, France. •S Supporting Information. ABSTRACT: The tem...
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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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Environmental Science & Technology

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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

Environmental Science & Technology

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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