The original development and deployment of the flowing chemical reaction chamber was performed and described in detail in Tackett (2008). Therefore, only a brief description of the flowing chemical reaction chamber will be given here.
The 750 cm long quartz flowtube (Figure 2.1) is equipped with a 20 mm ID opening at one end to serve as the inlet into the flowtube for sampling ambient air. Quartz was used to enable photochemical processes for method development. The opposite end of the flowtube has a ¼ʺ OD vacuum port that is attached to a diaphragm pump to draw the ambient air through the flowtube at a rate of 1.5 slpm. The flowtube has four ¼ʺ OD glass side ports for introducing the reagent gases into the flowtube along with sampling the final stable halogenated product. The two side ports (left side) located near the inlet each connect to a quartz gas dispersion frit for introducing the reagent gases into the flowtube. The frits were designed to disperse the reagents at the head of the flowtube in countercurrent fashion. Turbulence within the flowtube enhances mixing of
the ambient radicals with the reagent gases. An image of the flowtube is shown in Figure 2.1.
Figure 2.1 Photo of the quartz flowtube used in the flowing chemical reaction method. The reagent gas inlet frit is located on the far left and the sampling inlet is located on the far right side of the flowtube.
The dimensions and flow rate for the flowtube were based on minimizing wall loss of the sampled halogen radicals. Within the limit of laminar flow, molecular
diffusion can be used to calculate the time scale (τMD) for which the radicals will diffuse
to the walls of the flowtube and be irreversibly absorbed. The τMD is influenced by the
size of the diffusing gases, the temperature and pressure within the vessel, along with the medium in which the molecule is traveling. First, laminar flow within the flowtube must be ensured so that the only cause of mass transfer to the walls will be from molecular diffusion. Laminar flow is defined as smooth, undistributed flow where the flow layers
run parallel to one another. To determine if a flow is laminar the Reynolds number (Re)
can be calculated (Equation 2.I).
(2.I)
Ambient Sample Inlet
Frit Sample
The Reynolds number characterizes the flow and is calculated using the fluid linear
velocity (u) in m∙s-1, the inner diameter of the flowtube (d), and the kinematic viscosity of
the fluid (ν) in m2∙s-1
. Laminar flow is defined as a Reynolds number of <2300. The fluid linear velocity is calculated using Equation 2.II.
For the flowtube method a volumetric flow rate of 1.5 slpm is used, that along with the
flowtube’s radius (r) of 1 cm, a linear velocity of 0.08 m∙s-1
is achieved. At a temperature
of 300K, air has a kinematic velocity of 1.6x10-5 m2∙s-1, as calculated using the online
Engineering Toolbox (http://www.engineeringtoolbox.com/). This yields a Reynolds number of 100 which is within the laminar flow regime.
The molecular diffusion time scale can be calculated using Equation 2.III, where r
represents the radius of the flowtube in meters and Dg represents the gas-phase diffusion
coefficient for the specie of interest (Roberts and Webster, 2002).
Of the four species of interest, (ClO, Cl, BrO and Br) Cl atoms have the largest diffusion
coefficient of 0.183 cm2∙s-1. The gas-phase diffusion coefficient was calculated using
Equation (2.IV), where k is the boltzmann constant (1.38x10-23 m2∙kg∙s-1∙K-1), T is the
temperature (273K), and f is the frictional velocity.
(2.II)
(2.III)
Equation 2.III yields a time scale of ~2.7 seconds for Cl atoms to diffuse to the walls of the flowtube. Therefore, the flowtube reaction mechanism must occur within a time scale much less than 2.7 seconds to minimize the loss of halogen radicals to the flowtube walls. Based on the chlorine atom’s timescale for molecular diffusion (2.7 secs), the entire
flowtube reaction mechanism should occur within ~0.027 seconds. This is two orders of magnitude faster than the molecular diffusion timescale, which will minimize wall loss. The first step toward achieving the desired time scale of ~0.027 seconds is to determine the NO concentration needed to convert all the XO to X.
Given that kClO+NO is 2.04x10-11 cm3∙molec-1∙s-1, we calculate an NO concentration of 72
ppb using equation 2.V, which represents the necessary concentration needed to convert all the XO to X on the desired timescale for the entire flowtube mechanism. However, a higher concentration of NO (1 ppm) was used during deployment based on NO
calibration standard availability. This would only decrease the time scale for the
conversion of XO to X. Using Equation 2.V, XO will have a lifetime of < 0.002 seconds, one order of magnitude less than the target time frame for the flowtube reaction
mechanism.
Following the conversion of all the XO to X, trans-2-butene must then scavenge all the available halogen atoms. Using Equation 2.VI, an initial concentration of 52ppb was calculated for trans-2-butene, to ensure that all the halogen radicals will react with trans- 2-butene within our desired flowtube reaction time frame.
(2.VI) (2.V)
However, halogen atoms are extremely reactive and within the flowtube can be
scavenged by various compounds present in the ambient Arctic air. The two main sinks for bromine and chlorine atoms in ambient air are ozone and methane, respectively. To guarantee that trans-2-butene reacts with >99% of the halogen atoms Equation 2.VII and 2.VIII were used to determine the necessary trans-2-butene concentration. For the bromine atom calculation the typical Arctic ambient ozone concentration of 40 ppb
(1x1012 molec∙cm-3) was used and the BrO + O3 rate constant was 7x10-13 cm3∙molec-1∙s-1.
The solution from Equation 2.VII produced a trans-2-butene concentration of 370 ppb.
A similar calculation for the competing reaction of Cl + CH4 was performed, using a
methane concentration of 1.98 ppm (5x1013 molec∙cm-3) and rate constant of 4.33x10-14
cm3∙molec-1∙s-1. This yielded a concentration of 26 ppb for trans-2-butene.
Based on the two calculations a concentration of 370 ppb of trans-2-butene will be sufficient to react with 99% of both chlorine and bromine atoms. During the field deployment a concentration of 1.5 ppm was used for both NO and trans-2-butene, as excess of both had no effect on the chromatography during laboratory studies. This assured a >99% efficiency for trans-2-butene to react with both bromine and chlorine atoms, while allowing the flowtube reactions to complete on a time scale that is orders of magnitude faster than the molecular diffusion rate of Cl or Br atoms to the walls of the flowtube.
(2.VII)