In‐situ detection of tropospheric OH radicals by folded long‐path laser absorption. Results from the POPCORN Field Campaign in August 1994

Abstract

Ground based in‐situ measurements of tropospheric hydroxyl radicals were conducted by folded long‐path laser absorption as part of the field campaign POPCORN in August 1994. The OH instrument used an open optical multiple‐reflection cell of 38.5 m base length through which the laser beam was passed up to 80 times. The broadband emission of a short‐pulse UV laser together with a multichannel detection system allowed the simultaneous observation of six OH absorption lines in a spectral interval of Δλ≃0.24 nm at 308.1nm (A²Σ+,υ′ = 0← X²Π,υ″ = 0 transition). Along with the OH radicals, the trace gases SO2, HCHO, and naphthalene were measured by this technique. The large spectral detection range covered a multitude of rotational absorption lines of these trace gases which were all used for multicomponent analysis, thus allowing a specific and sensitive detection of tropospheric OH radicals. An average 2σ detection limit of 1.5 × 106 OH/cm³ for an integration time of 200 seconds and an absorption light path length of 1848 m was determined from the field measurements. In total, 392 OH data were obtained by long‐path absorption during 16 days of field measurements. The observed OH concentrations reached peak values of 13 × 106 cm−3 at noon.

In-situ Measurements of Tropospheric Hydroxyl Radicals by Folded Long-Path Laser Absorption During the Field Campaign POPCORN

“…The residual (trace e) is dominated by noise (1.5 ·10 −5 RMS). This spectrum is an excellent example for the case where the OH absorption lines around 308.16 nm, although strong with an optical density of about 1.3 ·10 −4 (see trace c), are almost completely masked by partly anticorrelated HCHO absorption lines of comparable strength (reference spectra can be found in Dorn et al, 1996). This again demonstrates the advantage of a broad spectral detection interval for the reliable evaluation of complex atmospheric absorption spectra.…”

“…The simultaneous fit of all components is essential for a precise concentration measurement since some trace gases, in particular OH and formaldehyde, possess inverse absorption structures (see Dorn et al, 1996). Thus, if HCHO is present in ambient air in a certain concentration the OH absorption lines can appear significantly reduced (an example is shown in Figure 3(II) below).…”

“…Each spectrum was finally divided by an air spectrum measured under the same ambient conditions immediately before or after but without the reference gas in the laser beam, in order to correct for absorption of atmospheric components along the light path. Examples of reference spectra can be found in Dorn et al (1996).…”

OH clock determination by proton transfer reaction mass spectrometry at an environmental chamber

“…Long-term monitoring of OH concentrations has only been demonstrated using CIMS (Rohrer and Berresheim, 2006). OH concentration measurements using DOAS are currently only operated by researchers at the Jülich Forschungszentrum in field and chamber campaigns (Dorn et al, 1996;Brauers et al, 2001;Schlosser et al, 2007). According to Schlosser et al (2009) all three techniques (DOAS, LIF, CIMS) involve elaborate, expensive, custom-made experimental setups.…”

UV‐Visible Differential Optical Absorption Spectroscopy (DOAS)

“…Panel (F) records the absorption band of an unknown absorber. Very precise calibrations of the reference spectra are needed because of the very narrow wavelength interval used (307.96-308.19 nm) (Dorn et al, 1996;Mount et al, 1997;Brauers et al, 2001). Figure 3.10(A) shows an atmospheric absorption spectrum at 308 nm, and the corresponding superimposed fitted reference spectra together with a ninth-order polynomial.…”