Hans-Peter Dorn

Comparison of measured OH concentrations with model calculations

The influence of chemical precursors and sunlight on the atmospheric OH abundance is investigated by a comparison of locally measured tropospheric OH with model calculations. The latter are based on the gas phase reaction mechanism of the regional acid deposition model (RADM2) which incorporates an explicit inorganic and a comprehensive organic chemistry. The experimental data were obtained in the planetary boundary layer during two sets of campaigns. In Deuselbach (1983) and Schauinsland (1984), rural conditions were encountered with NOx concentrations on the average of 2.2 and 0.9 ppb, respectively. This data set was already compared with model calculations based upon an older and less detailed chemical reaction scheme (Perner et al., 1987). Since then the experimental data were reanalyzed leading to modified measured OH concentrations and also to modified precursor concentrations. For a consistent comparison with the more recent campaigns in Julich (1987 and 1988) we have redone the calculations. The modeled and measured OH concentrations of the campaigns in 1983 and 1984 correlate well with a coefficient of correlation of r = 0.73. The model overpredicts OH by about 20%. Under more polluted conditions in Julich with average NOx concentrations of 4 ppb the correlation coefficient between experimental and modeled data are significantly smaller (r = 0.61). Possible reasons are the influence of not measured precursors, for example isoprene, and the inapplicability of a quasi-steady state model under the spatially inhomogeneous conditions in Julich. Again the model overpredicts the OH concentration by about 15%, which is somewhat smaller than for the rural case. The precision of the comparison is limited by the uncertainties of the chemical reaction rate constants.

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.

Kinetic Study of the OH-isoprene and O3-isoprene reaction in the atmosphere simulation chamber, SAPHIR

Kinetic studies conducted in the new atmosphere simulation chamber SAPHIR at the Research Center Julich allow a thorough investigation of oxidation of isoprene induced by O 3 and the OH radical under atmospheric conditions. Rate coefficients for the 03-isoprene and OH-isoprene reaction are determined from measured concentration-time profiles. For the reaction of O 3 with isoprene the rate coefficient is determined to be (9.6 ± 0.7) x 10 -18 cm 3 molecule -1 s -1 at 286 K. The rate coefficient for the reaction OH + isoprene is (10.0 ± 1.2) x 10 -11 cm 3 molecule -1 s -1 at 294 K. The kinetic parameters determined in SAPHIR at atmospheric concentrations agree well with recent recommendations.

Investigation of OH absorption cross sections of rotational transitions in the band under atmospheric conditions: Implications for tropospheric long‐path absorption measurements

The accuracy of tropospheric hydroxyl radical measurements by long-path absorption spectroscopy is ultimately limited by the uncertainty of the effective OH absorption cross sections. The latter were determined from calculated spectra for the Q1(2), Q1(3), and P1(1) rotational lines of the OH A2∑+, (υ′=0)←X2Π, (υ″=0) transition at 308 nm. The calculations took into account Doppler broadening, measured data of the collisional broadening of OH by air molecules, and instrumental line broadening effects. The calculated spectra were compared with OH absorption spectra measured in a flow reactor near room temperature at 1013 hPa. Excellent agreement between calculated and measured OH spectra was achieved using the collision-broadening parameters determined recently by Leonard (1990). The effective absorption cross sections at the line center (peak absorption cross sections) calculated for the Voigt line shape at 300 K and 1013hPa are: σeff(v˜0)[ Q1(2) ]=(1.67±0.1)×10-16 cm2, σeff(v˜0)[ Q1(3) ]=(1.4±0.1)×10-16 cm2, σeff(v˜0)[ P1(1) ]=(1.41±0.1)×10-16 cm2. The full width at half maximum of the spectral lines is 2.5 pm, and the shape factor a is 1.41. Calculations of the effective absorption cross section under different atmospheric conditions demonstrate a strong pressure dependence for all lines. At 2800 m altitude (720 hPa) the peak absorption cross section of the OH Voigt lines is about 35% higher than at sea level. The temperature dependence in an interval of ±20 K around room temperature (at 1013 hPa) is only small (±3% for the Q lines and ±6% for the P1(1) line). The estimated total uncertainty of the effective absorption cross section is 8%.

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.

Field Data and Model Calculations for the Hydroxyl Radical

Abstract Locally measured tropospheric OH concentrations are compared with model calculations to study the influence of chemical precursors and sunlight. The chemical scheme was taken from the gas phase reaction mechanism of the Regional Acid Deposition Model, which includes an explicit inorganic and a comprehensive organic chemistry. The experimental investigations probed planetary boundary layer air in two sets of campaigns. In Deuselbach (1983) and Schauinsland (1984) rural conditions were encountered with NOx concentrations on the average of 2.2 ppb and 0.9 ppb, respectively. This dataset was already compared with model results based upon an older and less detailed chemical reaction scheme (Perner et al.). Since then, the experimental data were reanalyzed leading to modified measured OH concentrations and also to modified precursor concentrations. For a consistent comparison with the more recent campaigns in Julich (1987 and 1988) the calculations have been redone. The modeled and experimental OH conc...