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Gaseous air pollutants
Published in Abhishek Tiwary, Ian Williams, Air Pollution, 2018
In the simplest type of diffusion tube, a linear molecular diffusion gradient is set up between the atmospheric concentration at the lower end of an inert plastic tube (7 cm long and 1 cm diameter) and zero concentration at an absorbent-coated substrate at the upper end (Figure 2.12). The gas molecules diffuse down the gradient through the air molecules down the gradient and are absorbed on the substrate. The rate of diffusion depends only on the near-constant molecular diffusivity of the gas, as the long thin tube inhibits turbulent transport on to the substrate. However, high wind speeds can cause turbulent transport up the tube to be a significant source of error. The tube is left in position for a period of days to weeks, capped to prevent further absorption, and returned to the laboratory for analysis. A quantity Q mol of the gas is transferred onto the substrate in time t, where Q is given by: Q=D12At(Ca−C0)/z
Measurement of gases and particles
Published in Abhishek Tiwary, Jeremy Colls, Air Pollution, 2017
In the simplest type of diffusion tube, a linear molecular diffusion gradient is set up, between the atmospheric concentration at the lower end of an inert plastic tube (7 cm long and 1 cm diameter) and zero concentration at an absorbent-coated substrate at the upper end (Figure 4.2). The gas molecules diffuse down the gradient through the air molecules down the gradient and are absorbed on the substrate. The rate of diffusion depends only on the near-constant molecular diffusivity of the gas, since the long thin tube inhibits turbulent transport on to the substrate. However, high wind speeds can cause turbulent transport up the tube to be a significant source of error. The tube is left in position for a period of days to weeks, capped to prevent further absorption, and returned to the laboratory for analysis. A quantity Q mol of the gas is transferred onto the substrate in time t, where Q is given by:
Air and air quality
Published in Stephen Battersby, Clay's Handbook of Environmental Health, 2016
The most widely used diffusion tubes are for the monitoring of nitrogen dioxide (NO2) because the NO2 objectives include an annual mean concentration and the extent of NO2 pollution is widespread. Each diffusion tube is made of clear plastic and about 70mm long. The tubes are fixed vertically with the open end at the bottom. The tubes need to be sited on a bracket at least 50mm from the nearest surface. A cap is removed from one end and at the closed end there is an absorbent, which for NO2 is triethanolamine (TEA) in a preparation (of water or acetic acid). The tubes following exposure of two to four weeks are sent to an analysing laboratory. Hence they can only ever be used to produce longer-term averages and any study should be at least six months long.
A novel high-volume Photochemical Emission Aging flow tube Reactor (PEAR)
Published in Aerosol Science and Technology, 2019
Mika Ihalainen, Petri Tiitta, Hendryk Czech, Pasi Yli-Pirilä, Anni Hartikainen, Miika Kortelainen, Jarkko Tissari, Benjamin Stengel, Martin Sklorz, Heikki Suhonen, Heikki Lamberg, Ari Leskinen, Astrid Kiendler-Scharr, Horst Harndorf, Ralf Zimmermann, Jorma Jokiniemi, Olli Sippula
Toluene is a SOA precursor compound, which is frequently used in aerosol aging experiments. Experiments were carried out with toluene to compare features of PEAR with those of previously reported OFR. Toluene vapor was fed into the PEAR without seed aerosol via a diffusion tube at initial concentrations of 94 and 230 pbb, together with ozone, purified air, and water vapor (relative humidity of 50%). The particles measured downstream of the reactor resulted from nucleation and subsequent condensation of oxidation products of the toluene. Toluene gas phase concentrations and particulate organic mass at the outflow of the PEAR were analyzed by a high-resolution proton-transfer-reaction time-of-flight mass spectrometer (HR-PTR-ToF-MS, Ionicon, Austria) and a high-resolution aerosol mass spectrometer (HR-AMS, Aerodyne Research Inc., MA, USA), respectively. The photochemical age of the resulting OA was estimated from a reaction constant between toluene and OH of 5.7 × 10−12 cm3 s−1 (Barmet et al. 2012), while SOA yields were calculated by dividing the formed OA mass by the reacted mass of toluene (Odum et al. 1996). OA from aging of toluene, as well as from the combustion emissions from sources described in the following section, were characterized from AMS mass spectra using the fragmentation table for elemental analysis of OA AMS spectra (Canagaratna et al. 2015) and OSC calculation (Kroll et al. 2011).
Adsorption characteristics of the carbonaceous adsorbents for organic compounds in a model exhaust gas from thermal treatment processing
Published in Journal of the Air & Waste Management Association, 2022
The liquid reagents of benzene and chlorobenzene were fed into the diffusion tube, which was kept at a constant temperature in the range of 30–50°C. The diffusion tube was selected by the design consideration on the volatilization rates of the target compounds and the outlet gas concentrations required. The structure of the diffusion tube is shown in Figure S2. The diffusive rate of the liquid compound can be calculated by the next equation.
Interfacial geometry and its effect on the estimation of binary gas diffusivities in an isothermal Stefan column
Published in Chemical Engineering Communications, 2021
Marimar Moreno, Isamaris Moreno, María del Sol Jaime, Shayra G. Maisonet, Carlos A. Ramírez
Several previous reports have attempted to quantify the relationship between interfacial curvature and DAB estimates using the Stefan column method. For example:Lee and Wilke (1954) tested 11 liquid-gas pairs in a carefully constructed metallic diffusion tube assembly operated at 25 °C and atmospheric pressure. Diffusion path length corrections were applied at both ends of the column to the vertical distance measured from top to interface. They reported interfacial curvature corrections of 0.1 - 0.2 cm in a 1.75 cm-diameter column (AR unspecified). No correlation between these path length corrections to the system’s operational settings or to the estimated diffusivities could be found.Ben Aim et al. (1967) attempted to quantify some of the end effects identified in the diffusion column literature by performing experiments with benzene/air at 25 °C. Since the meniscus was located ∼1 cm from the top of their 3 cm-long glass tubes (either 0.10, 0.49, or 0.59 cm inside diameter), they attempted to correct the diffusion path length along the curved interface by calculating the latter’s average displacement from the horizontal using photographic enlargements of the meniscus profile. Surface area and path length corrections such as those described in the Theoretical Background section were not made, and it is unclear how the reported DAB’s were affected by the corrections. Therefore, the results of Ben Aim et al. (1967) cannot be compared with the present findings.Pommersheim and Ranck (1973) studied the transport of pentane gas in dry nitrogen in a glass diffusion tube at 31.4 °C and atmospheric pressure. The temporal descent of the liquid-gas interface was tracked with a cathetometer. These data allowed calculation of the binary gas diffusivity-pressure product for the system. Diffusion path length corrections were calculated following the procedure of Lee and Wilke (1954), but only at the top where the gas phase interacts with the external environment. History repeated itself in this case: the corrections could not be correlated with the operational settings or the DAB estimates. Path length corrections due to interfacial curvature were not considered.