Temporal evolution of stable water isotopologues in cloud droplets in a hill cap cloud in central Europe (HCCT-2010)

In this work, we present the first study resolving the temporal evolution of δ2H and δ18O values in cloud droplets during 13 different cloud events. The cloud events were probed on a 937 m high mountain chain in Germany in the framework of the Hill Cap Cloud Thuringia 2010 campaign (HCCT-2010) in September and October 2010. The δ values of cloud droplets ranged from −77‰ to −15‰ (δ2H) and from −12.1‰ to −3.9‰ (δ18O) over the whole campaign. The cloud water line of the measured δ values was δ2H=7.8×δ18O+13×10−3, which is of similar slope, but with higher deuterium excess than other Central European Meteoric Water Lines. Decreasing δ values in the course of the campaign agree with seasonal trends observed in rain in central Europe. The deuterium excess was higher in clouds developing after recent precipitation revealing episodes of regional moisture recycling. The variations in δ values during one cloud event could either result from changes in meteorological conditions during condensation or from variations in the δ values of the water vapor feeding the cloud. To test which of both aspects dominated during the investigated cloud events, we modeled the variation in δ values in cloud water using a closed box model. We could show that the variation in δ values of two cloud events was mainly due to changes in local temperature conditions. For the other eleven cloud events, the variation was most likely caused by changes in the isotopic composition of the advected and entrained vapor. Frontal passages during two of the latter cloud events led to the strongest temporal changes in both δ2H (≈ 6‰ per hour) and δ18O (≈ 0.6‰ per hour). Moreover, a detailed trajectory analysis for the two longest cloud events revealed that variations in the entrained vapor were most likely related to rain out or changes in relative humidity and temperature at the moisture source region or both. This study illustrates the sensitivity of stable isotope composition of cloud water to changes in large scale air mass properties and regional recycling of moisture

Airborne Observations of Strong Biogenic NOx Emissions from the Namibian Savanna at the End of the Dry Season

We report the observation of substantial emissions of NO x from a several-hundred-square-kilometer region of savanna in northern Namibia in September 1992. The estimated emission rates lie near the high end of the range of values reported from flux chamber studies on various tropical savanna soils and appear to be associated with a light rainfall event which occurred 4 days prior to the observations. If our measurements are typical for this situation, then biogenic emission of NO x following sporadic rainfall events during and at the end of the dry season may be an important regional source rivaling that arising from savanna burning and may have important consequences for regional scale ozone formation

Fast Response tunable diode laser spectroscopy for trace gas flux measurements

A fast tunable diode laser spectrometer was applied for measurements of N2O emissions from a harvested wheat field located in Sealand, Denmark. The dual-channel instrument uses two tone frequency modulation and signal detection at 11 MHz in conjunction with rapid (1 ms) scanning of the laser. A dichroic beam combiner and a mechanical chopper allow time multiplexing. The absorption signals are recorded and analysed on-line. The combination of a software, current and temperature controlled line locking scheme results in high stability of the instrument. The time response (200 ms) of the spectrometer is limited by the measurement-cell gas exchange. For eddy correlation measurements, the trace gas mixing ratio and wind data were sampled synchronously at a rate of 10 Hz using an interrupt-driven algorithm. Mean N2O flux detection limits were 6 pptv m s−, corresponding to a deposition velocity of 2 × 10−5 m s− for a half hour measurement time. In addition, concentration gradients were determined from measurements at four different inlet heights at 0.1 ppbv precision in a shared integration time of 30 min. These measurements resulted in a flux detection limit of 7 pptv m s−1 ror the boundary layer mixing conditions encountered

Micrometeorological measurement and source region analysis of nitrous oxide emissions from an agricultural soil

Nitrous oxide (N2O) fluxes were measured by eddy correlation during a field experiment in August 1993. The measurements took place on a harvested wheat field located in northwest Sealand (Denmark). A. tunable diode laser absorption spectrometer (TDLAS) was used for N2O detection. The N2O mixing ratio was sampled at a rate of 10 Hz synchronously with the three-dimensional wind data, air temperature, water vapour and carbon dioxide (CO2) concentrations. Fast Fourier transform methods were applied to analyse spectral and co-spectral power distributions and to compute the fluxes from the covariance functions. Balance of the energy budget indicated adequate frequency coverage of the flux contributions. Continuous measurements were carried out in half hour intervals during daytime and fluxes in the range of 40–120 ng N m−2 s−1 were obtained. The footprint regions for the individual measurements were estimated by Markovian trajectory simulations taking into account the atmospheric stability conditions and wind distributions inferred from the micrometeorological measurements. The influence of adjoining fields with different crops was evident in the Bowen ratio and CO2 flux, but not noticeable in the N2O flux. This observation is consistent with supporting analysis of the soil depth dependence of the nitrous oxide production, which indicates that the major contributions arise from levels below 0.9 m

Measurements of N2O fluxes from Fertilized Grassland using a fast response tunable diode laser spectrometer

A fast response tunable diode laser spectrometer was used to make N2O flux measurements by both eddy correlation and concentration gradient techniques during a methods intercomparison field program in April 1992 at a site in Stirling, Scotland. A description of the site and the results of the intercomparison are presented in companion papers. Sufficient instrumental precision and time resolution for N2O flux determination using both techniques were obtained by application of the recently developed two-tone frequency modulation coupled with fast scanning of the laser. The use of a dedicated digital signal processor allowed zero-overhead on-line data handling at a rate of 10 Hz such that the time response of the system was only limited by the gas exchange time in the multipass sample cell (200 ms). Vertical concentration gradients that lead to a difference of ≤ 1 part per billion by volume in the N2O mixing ratio at 0.06- and 1.05-m elevation were statistically resolved within 1 min. Eddy correlation measurements with intake heights of 2.25 m and 2.75 m were made in conjunction with two different sonic anemometers. The software developed for reduction and analysis of the 10-Hz eddy correlation data was based on time efficient FFT methods and performed time-base matching of the data set, drift correction, coordinate rotation, and evaluation of the covariances and the frequency power distributions. N2O fluxes determined with this technique were in the range of 38–113 ng N m−2 s−1. An unusual shape of the N2O concentration covariance function can be ascribed to fetch inhomogeneities, and we conclude that the eddy correlation data set contains information that can be used to characterize the spatial variability in N2O emission in addition to its spatial integral

FM Spectroscopy at 1.3 microns using InGaAsP Lasers: A Prototype Field Instrument for Atmospheric Chemistry Research

Two-tone frequency modulation spectroscopy has been used in conjunction with InGaAsP lasers in the 1.3¬,um region to monitor weak water vapor absorptions in a long path White cell. Detection electronics that reduce the effect of Johnson noise are described. The system was capable of detecting optical densities corresponding to <1.7 X 10-6 in a 1-Hz bandwidth. Factors limiting the difference between observed and shot noise limited performance for these types of laser and condition are discussed