Measurement of Hydroxyl Concentrations in Air Using a Tunable uv Laser Beam

Abstract

We report the first measurement of hydroxyl (OH) concentrations in air by detecting the resonance fluorescence excited by tunable radiation near 2825.8 A, On a particular day, the OH concentrations deduced from these measurements ranged from a high of 1.5x lo 8 OH/cm 3 in the early afternoon to a level below 5x lo 6 OH/cm 3 at night. This Letter reports the first measurement of hydroxyl (OH) concentrations in air by detecting the resonance fluorescence excited by a tunable laser source in the ultraviolet. 1 Weinstock 2 has pointed out that OH plays a central role in smog formation and in controlling the global concentrations of carbon monoxide and methane. The ability to measure ambient concentrations of OH should afford an accurate determination of numerous photochemical cycles operative in either polluted or natural atmosphere. However, the ambient concentration of OH has never been established experimentally, and various estimates 2,3 yield results ranging from 7xl0 4 to 5xl0 8 OH/ cm 3 .In performing the experiments in the laboratory, it was observed that the OH concentration in air varied during the course of the day. On a particular day, it ranged from a value of 1.5xl0 8 OH/cm 3 in the early afternoon (-2:00 p.m.), to 1.6xio 7 OH/cm 3 at around 6:00 p.m., and to a level below our present detection limit of 5xl0 6 OH/cm 3 (0.2 part per 10 12 in air) at night (-10:30 p.m.). The OH concentration also dropped to a level below the detection limit when it was raining or overcast outside, or when the ventilation system was shut off to prevent the fresh air outside from getting into the laboratory. It was estimated that a significant portion of the air near the focal region came from outside the laboratory with a transport time of about 6 sec. We are currently improving the collection efficiency of our optics by a factor of 10, and expect to monitor the diurnal variation of OH concentration in air both inside the laboratory and outside.The experiments were performed with a tunable source near the P x (2) line (2825.8 A) in the 2 n (v = 0) -2 S + (v = 1) transitions 4 of OH. The light source near 2825.8 A was derived from the second harmonic of the output from a dye laser system; the latter consisted of one oscillator and two amplifier stages, 5 and were operated with rhodamine S dissolved in methanol and at a rep-etition rate of one pulse every 10 sec. Frequency narrowing was accomplished using a Littrow grating as an end reflector and an intracavity Fabry-Perot etalon. Fine tuning was done by varying the mixture of helium and propylene which filled the etalon spacing. The output pulse from the laser typically measured 50 mJ in energy and 0.4 cm" 1 in spectral width, and lasted about 0.5 (Jsec. The second harmonic of the output was generated in a 5-cm crystal of ammonium dihydrogen phosphate oriented for index matching. With optimal focusing, an output energy of up to 6 mJ per pulse at the second harmonic frequency was obtained. Figure 1 depicts a schematic of the experimental setup. The second harmonic beam near ...

Simulations of atmospheric OH, O<sub>3</sub> and NO<sub>3</sub> reactivities within and above the boreal forest

“…Being highly reactive, OH has a short lifetime (depending on the conditions, but usually much less than 1 s; Jacob, 1999) and is capable of reacting with most functional groups. The concentration of OH was first measured in the 1970s (Wang and Davis, 1974), but even with great advances in instrument development, it is still difficult to detect such low concentrations of such a reactive compound (Mao et al, 2012;Novelli et al, 2014a, b). The measurement is therefore still associated with large uncertainties.…”

Fluorescence Methods

“…The false OH concentration is proportional to the square of the excitation power and linear in ozone and water vapour number density. Early attempts (Wang & Davis, 1974;Wang et al, 1975Wang et al, , 1976Davis et al, 1976) to measure tropospheric OH by LIF detected only laser-generated OH.…”

“…This is superior to excitation at 282 nm because the product of the ozone absorption cross section and the photolysis quantum yield of O 1 D is reduced by a factor of ∼ 30 at 308 nm relative to 282 nm. Early instruments for tropospheric OH detection Wang and Davis, 1974;Wang et al, 1975Wang et al, , 1976) failed due to lasergenerated OH. Hard and co-workers (1984) reduced laser-generated OH using FAGE, as described in Section 2.2.5, and their early success combining FAGE with excitation at 308 nm using high repetition-rate lasers has been adopted as a model by most research groups using LIF to observe tropospheric OH (Creasey et al, 1997;Faloona et al, 2004;Hard et al, 1995;Holland et al, 1995;Kanaya et al, 2001).…”

Radiocarbon tracer measurements of atmospheric hydroxyl radical concentrations

“…The other approach to the measurement of atmospheric OH concentrations has been basically spectroscopic, typified by the laser-induced fluorescence methods developed by Wang et al (1974) and Davis et al (1976), and the direct absorption measurements of Perner et al (1976). The radiochemical technique was first described in Campbell et al (1979).…”