Figure 4 - uploaded by Cheryl Ernest
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8 Photodiode response curve. Shows the change in photodiode response () to a constant laser intensity as the photolysis laser is scanned over the measured wavelength range. The polynomial fit (-) was used to correct the composite action spectrum.
Source publication
Formaldehyde (HCHO) plays a primary role in tropospheric chemistry. Its photochemical activity is an important source of radical species such as HCO, H, and subsequently HO2 as well as molecular hydrogen and carbon monoxide. As a source of hydrogen radicals (HOx = OH + HO2), HCHO plays a significant role in the oxidative capacity of the atmosphere,...
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The atmospheric role of photochemical processes involving NO2 beyond its dissociation limit (398 nm) is controversial. Recent experiments have confirmed that excited NO2* beyond 420 nm reacts with water according to NO2*+H2O→HONO+OH. However, the estimated kinetic constant for this process in the gas phase is quite small (k≈10⁻¹⁵–3.4×10⁻¹⁴ cm³ mole...
Citations
... Various experimental techniques have been applied in the gas phase to either confirm the molecular structure or provide information to combustion models for the oxidation of CH 2 O. Absorption spectroscopy (AS) and four-wave mixing have been employed to investigate the rotational structure of the electronic ground [8,9] and excited states [10][11][12], while Fourier transform infrared spectroscopy (FTIR) has been used to measure CH 2 O concentrations in gas samples collected from a dimethyl ether (DME) flame [13]. CH 2 O planar laser-induced fluorescence (PLIF) has allowed determination of the 2D distribution of formaldehyde in applications such as turbulent flames [14][15][16][17][18][19], high-pressure environments [20], across shock waves [21], and to mark the ignition zone in a scramjet engine. ...
... Demonstration of qualitative CH 2 O PLIF has been accomplished at repetition rates as high as 100 kHz [22], and the intensity change in the PLIF signal has also been used as a semiquantitative technique in DME flames [6]. Many of the described techniques for measuring concentration suffer from spatial resolution limitations, lineof-sight averaging (AS [8,10], FTIR [13]), or sensitivities to the molecular quenching environment (PLIF [15,[17][18][19][20][21]). ...
... The mole fractions of N 2 and CH 2 O were set by the following procedure, similar to that used by Walser et al. [32,33]: a 37% CH 2 O aqueous solution (Fischer Scientific) was first placed in a windowless stainless-steel evaporation chamber; the evaporation chamber was heated to 475 K with heating tape, and the pressure vessel was evacuated and heated to 450 K; after 20 min, the valve between the chambers was opened, and the gas passed through a colder section to condense out some of the water before entering the windowed vessel; after a few minutes, the test chamber was again isolated from the evaporation chamber with a manual valve; heated N 2 was then slowly added from a separate port to attain the desired pressure. As previously reported in [10,32], every time the vessel was cooled down to room temperature, a white film of polymerized CH 2 O formed on the inner walls, requiring cleaning. To avoid polymerization, the pressure vessel was covered by heat tape, and hot air jets were directed at the windows. ...
Nanosecond electronic-resonance-enhanced coherent anti-Stokes Raman scattering (ERE-CARS) is evaluated for the measurement of formaldehyde ( ${{\rm{CH}}_2}{\rm{O}}$ C H 2 O ) concentrations in reacting and nonreacting conditions. The three-color scheme utilizes a 532 nm pump beam and a scanned Stokes beam near 624 nm for Raman excitation of the C–H symmetric stretch ( ${\nu _1}$ ν 1 ) vibrational mode; further, a 342 nm resonant probe is tuned to produce the outgoing CARS signal via the $1_0^14_0^3$ 1 0 1 4 0 3 vibronic transition between the ground ( ${\tilde X}{^{{1}}{{\rm{A}}_1}}$ X ~ 1 A 1 ) and first excited ( ${\tilde A}{^{{1}}{{\rm{A}}_2}}$ A ~ 1 A 2 ) electronic states. This allows detection of ${{\rm{CH}}_2}{\rm{O}}$ C H 2 O at concentrations as low as ${{9}} \times {{10}^{14}}\;{\rm{molecules}}/{\rm{cm}}^3$ 9 × 10 14 m o l e c u l e s / c m 3 (55 parts per million) in a calibration cell with ${{\rm{CH}}_2}{\rm{O}}$ C H 2 O and ${{\rm{N}}_2}$ N 2 at 1 bar and 450 K with 3% uncertainty. The measurements show a quadratic dependence of the signal with ${{\rm{CH}}_2}{\rm{O}}$ C H 2 O number density. Pressure scaling experiments up to 11 bar in the calibration cell show an increase in signal up to 8 bar. We study pressure dependence up to 11 bar and further apply the technique to characterize the ${{\rm{CH}}_2}{\rm{O}}$ C H 2 O concentration in an atmospheric premixed dimethyl ether/air McKenna burner flame, with a maximum concentration uncertainty of 11%. This approach demonstrates the feasibility for spatially resolved measurements of minor species such as ${{\rm{CH}}_2}{\rm{O}}$ C H 2 O in reactive environments and shows promise for application in high-pressure combustors.
