A Novel Flow Reactor for Studying Reactions on Liquid Surfaces Coated by Organic Monolayers:  Methods, Validation, and Initial Results

Abstract

A new flow reactor has been developed that allows the study of heterogeneous kinetics on an aqueous surface coated by an organic monolayer. Computational fluid dynamics simulations have been used to determine the flow characteristics for various experimental conditions. In addition a mathematical framework has been developed to derive the true first-order wall loss rate coefficient, k(1st)(w), from the experimentally observed wall loss rate, k(obs). Validation of the new flow reactor is performed by measuring the uptake of O(3) by canola oil as a function of pressure and flow velocity and the reactive uptake coefficients of N(2)O(5) by aqueous 60 wt % and 80 wt % H(2)SO(4). Using this new flow reactor, we also determined the reactive uptake coefficient of N(2)O(5) on aqueous 80 wt % H(2)SO(4) solution coated with an 1-octadecanol (C(18)H(37)OH) monolayer. The uptake coefficient was determined as (8.1 +/- 3.2) x 10-4, which is about 2 orders of magnitude lower compared to the reactive uptake coefficient on a pure aqueous 80 wt % H(2)SO(4) solution. Our measured reactive uptake coefficient can be considered as a lower limit for the reactive uptake coefficient of aqueous aerosols coated with organic monolayers in the atmosphere, because in the atmosphere organic monolayers will likely also consist of surfactants with shorter lengths and branched structures which will have a smaller overall effect.

Detailed heterogeneous chemistry in an urban plume box model: reversible co-adsorption of O<sub>3</sub>, NO<sub>2</sub>, and H<sub>2</sub>O on soot coated with benzo[a]pyrene

“…As such, the parametrization of N 2 O 5 hydrolysis yields gas-phase concentrations in good agreement with climatological observations (Evans and Jacob, 2005). This may be due to the fact that the N 2 O 5 hydrolysis in aqueous aerosol particles follows an absorption type reaction mechanism (Hanson and Ravishankara, 1991;Knopf et al, 2007;Cosman et al, 2008), i.e. the uptake can be dominated by dissolution of the gas-phase species into the particle governed by Henry's law constant and by subsequent reaction in the bulk (Finlayson-Pitts and Pitts, 2000).…”

The effect of fatty acid surfactants on the uptake of ozone to aqueous halogenide particles

“…The effect was most pronounced with pentadecanoic acid and stearic acid, which they ascribed to the ability of these fatty acids to spontaneously form relatively well ordered, dense films. This indicates the importance of the surface phase state of surfactants in determining the phase transfer properties, an observation also made in conjunction with N 2 O 5 uptake (Bertram and Thornton, 2009;Knopf et al, 2007) and with acetic acid uptake .…”

“…It turns out that an important aspect of organic coatings may be that they reduce the mass transfer between the gas and particle phases. Organic coatings on aqueous aerosol particles can also affect the process of CCN formation and growth (Andrews and Larson, 1993;Chuang,the ozone and halogen budgets in the marine boundary layer, several studies have focused on the reactive uptake of N 2 O 5 and have shown a decrease of the reactive uptake of N 2 O 5 in presence of monolayers surfactants (Badger et al, 2006;Knopf et al, 2007;McNeill et al, 2006;McNeill et al, 2007;Park et al, 2007;Riemer et al, 2009;Stewart et al, 2004). For example Thornton and Abbatt (2005) demonstrated that the presence of a monolayer of hexanoic acid inhibits N 2 O 5 uptake by a factor of 3 to 4 on deliquesced sea salt aerosol.…”

Toward a general parameterization of N<sub>2</sub>O<sub>5</sub> reactivity on aqueous particles: the competing effects of particle liquid water, nitrate and chloride

“…However, in contrast to the uniform chemical composition of stratospheric particles, tropospheric particles are chemically diverse and strong variations in ambient relative humidity (RH) lead to varying levels of particle liquid water. As a result, strong variations in γ (N 2 O 5 ) are expected and have been confirmed by laboratory investigations (Anttila et al, 2006;Folkers et al, 2003;Hallquist et al, 2003;Kane et al, 2001;Knopf et al, 2007;McNeill et al, 2006;Mozurkewich and Calvert, 1988;Park et al, 2007;Thornton and Abbatt, 2005;Thornton et al, 2003) and measurements of the steady-state lifetime of N 2 O 5 (Brown et al, 2006). Results from these studies have been used to refine the parameterization of γ (N 2 O 5 ) in the current generation of regional and global models (Davis et al, 2008;Evans and Jacob, 2005;Riemer et al, 2003).…”

“…The proposed reaction mechanism qualitatively explains the dependence of γ (N 2 O 5 ) on: i) H 2 O(l) (Thornton et al, 2003), ii) particle NO − 3 , which inhibits N 2 O 5 hydrolysis (R2) (Mentel et al, 1999;Wahner et al, 1998), and iii) organic coatings, which have been theorized to either suppress N 2 O 5 accommodation Reaction (R1) or decrease water availability at the surface (Anttila et al, 2006;Folkers et al, 2003;Knopf et al, 2007;McNeill et al, 2006;Park et al, 2007). Highly acidic particles, such as those found in the stratosphere or in ammonia-deficient regions of the troposphere, appear to promote an additional acid catalyzed channel, with γ (N 2 O 5 ) measured on such particles to be between 0.03-0.1 and thus not necessarily consistent with the above mechanism (Fried et al, 1994;Hu and Abbatt, 1997;Mozurkewich and Calvert, 1988;Robinson et al, 1997).…”