Martin Hausmann

OH radicals in the boundary layer of the Atlantic Ocean. 1. Measurements by long-path laser absorption spectroscopy

Knowing the concentration of hydroxyl (OH) radicals is most important for the understanding of the chemical processes in the troposphere. This paper describes the first direct measurements of OH in the boundary layer of the tropical Atlantic Ocean. The use of Differential Optical Absorption Spectroscopy provided direct measurements of OH with a calibration uncertainty of 6%. The 1-σ precision of the OH data was in the range of (1–4) × 106 cm−3 because of the exceptional experimental conditions encountered on the ship. On 10 measurement days we collected a total of 483 OH concentration data between 5°N and 40°S. Careful analysis was applied to select data not affected by the ship and its exhaust. The selected data (N = 238) exhibit diurnal profiles with maxima around 7×106 cm−3 for overhead Sun and clean air conditions. On average the measured OH concentrations are 16% higher than corresponding box model calculations based on simultaneously measured trace gas concentrations and photolysis frequencies. The deviation from the 1:1 relation, however, is covered by the combined calibration errors of OH, CO, and the photolysis frequencies.

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

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.

Intercomparison of tropospheric OH radical measurements by multiple folded long‐path laser absorption and laser induced fluorescence

An intercomparison of in-situ OH measurements by differential optical absorption spectroscopy (DOAS) and laser-induced fluorescence spectroscopy (LIF) was carried out in August 1994 in a clean rural environment in North-East Germany. A large data set of temporally overlapping OH measurements with well defined measurement errors was obtained and compared. Both instruments encountered the same air masses, except when the wind came from NNW and caused a perturbation of the DOAS measurements. Excluding that wind sector, the weighted regression analysis of 137 data pairs (70% of all available data pairs) yields a linear relationship between the DOAS and LIF measurements with a correlation coefficientr = 0.90. The unity slope (1.01±0.04) and the non-significant intercept (0.28±0.15) × 106 cm−3 demonstrate that both OH instruments agreed excellently in their calibrations and accurately measured OH.

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

Absolutely calibrated in-situ measurements of tropospheric hydroxyl radicals, formaldehyde, sulfur dioxide, and naphthalene (C10H8) were performed by long-path laser absorption spectroscopy during the field campaign POPCORN. The absorption light path was folded into an open optical multiple reflection cell with a mirror separation of 38.5 m. Using a light path length of 1848 m and an integration time of 200 s, the average 1σ-detection limits of OH, HCHO, SO2 and C10H8 during POPCORN were 8.7 · 105 cm−3, 8.3 · 109 cm−3, 2.4 · 109 cm−3, 1.5 · 108 cm−3, respectively. In total, 392 identifications of OH in air spectra were made in a rural environment between August 5 and August 23, 1994. We present and discuss OH absorption spectra and diurnal OH concentration profiles of three days which are representative for measurements under different pollution conditions during POPCORN. The observed maximum and median OH radical concentrations are 1.3 · 107 OH/cm3 and 4.0 · 106 OH/cm3, respectively. The measured diurnal variation of the OH concentration shows a good correlation with the primary formation reaction of OH radicals which is the photolysis of ambient ozone. Deviations from this correlation in the morning and evening hours, when the OH concentration is higher than expected from the ozone photolysis, demonstrate the importance of other photochemical HOx production pathways during POPCORN.

A New In Situ Laser Long-Path Absorption Instrument for the Measurement of Tropospheric OH Radicals

Abstract The authors describe a high-resolution long-path differential optical absorption spectrometer developed for the measurement of tropospheric hydroxyl radical concentrations. The instrument uses an atmospheric absorption light path of up to 3000-m total length that is folded into an optical multiple-reflection cell of 20-m base length. A frequency-doubled picosecond laser system serves as a light source, which has a large spectral bandwidth of 0.41 nm (FWHM) centered around 308 nm. In combination with a high-resolution Echelle spectrograph and an optical multichannel detector, a spectral range of 0.26 nm which comprises six OH absorption lines of the A2∑+, (ν′ = 0)← X2Π, (ν″=0) transition, can be simultaneously recorded. This large spectral detection range facilitates the subtraction of interfering absorption signals of other atmospheric trace gases (SO2, HCHO, naphthalene) and significantly improves the detection specificity for hydroxyl radicals. A 2σ OH detection limit of 1.5× 106 OH cm−3 for an...

Improvement of differential optical absorption spectroscopy with a multichannel scanning technique

Differential optical absorption spectroscopy (DOAS) of atmospheric trace gases requires the detection of optical densities below 0.1%. Photodiode arrays are used more and more as detectors for DOAS because they allow one to record larger spectral intervals simultaneously. This type of optical multichannel analyzer (OMA), however, shows sensitivity differences among the individual photodiodes (pixels), which are of the order of 1%. To correct for this a sensitivity reference spectrum is usually recorded separately from the trace-gas measurements. Because of atmospheric turbulence the illumination of the detector while an atmospheric absorption spectrum is being recorded is different from the conditions during the reference measurement. As a result the sensitivity patterns do not exactly match, and the corrected spectra still show a residual structure that is due to the sensitivity difference. This effect usually limits the detection of optical densities to approximately 3 × 10(-4). A new method for the removal of the sensitivity pattern is presented in this paper: Scanning the spectrometer by small wavelength increments after each readout of the OMA allows one to separate the OMA-fixed pattern and the wavelength-fixed structures (absorption lines). The properties of the new method and its applicability are demonstrated with simulated spectra. Finally, first atmospheric measurements with a laser long-path instrument demonstrate a detection limit of 3 × 10(-5) of a DOAS experiment.

Intercomparison of Tropospheric OH Measurements by Different Laser Techniques during the POPCORN Campaign 1994

In-situ OH measurements by laser-induced fluorescence (LIF) spectroscopy and folded long-path differential optical absorption spectroscopy (DOAS) were carried out in a rural environment in North-East Germany as part of the field experiment POPCORN in August 1994. The large set of OH data obtained allowed an intercomparison of both techniques based on relative diurnal profiles and simultaneously measured absolute concentrations. Most of the time the two OH instruments encountered the same air and agreed well in the measured relative diurnal variations. Only on a few occasions the measurements significantly disagreed due to a perturbation of the DOAS measurements by a local OH source in the north-western wind sector. Excluding data from this wind direction, the statistical analysis of 137 data pairs yields a correlation coefficient of r = 0.90 and a weighted linear fit with a slope of 1.09 ± 0.12. The correlations are carefully analyzed. The comparison of both instruments is discussed in the light of newly published effective absorption cross-sections for H2O and O2 that affect the calibration of LIF.

Detection of tropospheric OH radicals by long-path differential-optical-absorption spectroscopy: Experimental setup, accuracy, and precision

This paper describes a newly developed long-path differential-optical-absorption-spectroscopy instrument used for the measurement of tropospheric OH radicals. The instrument consists of a high resolution echelle spectrometer in conjunction with a multiple-reflection cell of 38.5 m base length and a UV laser light source that provides a spectral line width of 0.41 nm. Local in situ absorption measurements at total path lengths of either 1.85 or 3.1 km can be performed. The simultaneous observation of six atmospheric OH rotational absorption lines (Q1(2), Q21(2), R2(2), Q1(3), Q21(3), and P1(1)) around 308 nm allows OH measurements with high specificity. A new method to accurately determine the precision and the detection limit of each individual OH measurement data point is presented. Presently, a 2σ-detection limit of 1.5×106 OH cm−3 is achieved (based on 1.85 km absorption path length and about 6 min integration time), which corresponds to a minimum detectable optical density of 2.5×10−5. The absolute instrumental accuracy was calculated to be better than 6.5%, which emphasizes the qualification of the longpath absorption technique as an absolute method. Examples of field experiments are reported to illustrate the present performance.

Simple Monte Carlo methods to estimate the spectra evaluation error in differential-optical-absorption spectroscopy.

Differential-optical-absorption spectroscopy (DOAS) permits the sensitive measurement of concentrations of trace gases in the atmosphere. DOAS is a technique of well-defined accuracy; however, the calculation of a statistically sound measurement precision is still an unsolved problem. Usually one evaluates DOAS spectra by performing least-squares fits of reference absorption spectra to the measured atmospheric absorption spectra. Inasmuch as the absorbance from atmospheric trace gases is usually very weak, with optical densities in the range from 10(-5) to 10(-3), interference caused by the occurrence of nonreproducible spectral artifacts often determines the detection limit and the measurement precision. These spectral artifacts bias the least-squares fitting result in two respects. First, spectral artifacts to some extent are falsely interpreted as real absorption, and second, spectral artifacts add nonstatistical noise to spectral residuals, which results in a significant misestimation of the least-squares fitting error. We introduce two new approaches to investigate the evaluation errors of DOAS spectra accurately. The first method, residual inspection by cyclic displacement, estimates the effect of false interpretation of the artifact structures. The second method applies a statistical bootstrap algorithm to estimate properly the error of fitting, even in cases when the condition of random and independent scatter of the residual signal is not fulfilled. Evaluation of simulated atmospheric measurement spectra shows that a combination of the results of both methods yields a good estimate of the spectra evaluation error to within an uncertainty of ~10%.

The minihole event on 6. Feb. 1990 : influence of Mie-scattering on the evaluation of spectroscopic measurements

Groundbased differential optical absorption spectroscopic measurements of stratospheric trace gases are sensitive to stratospheric aerosol (PSC) layers as measured at Feb. 6, 1990, over northern Scandinavia. Mie extinction alters the intensity distribution of zenith scattered sunlight, in an extreme case presented here this can lead to 30%–40% changes of the airmass factor (AMF) for ozone. NO2 and OClO AMFs change less, O2 AMFs change dramatically. Ozone measurements at Kiruna, Sweden, for the time Feb. 4–8, 1990 are reported. No significant change in the O3 total column was observed during that period, when the effect of PSCs on the AMF was taken into account.