An absolute calibration for gas-phase hydroxyl measurements

We describe a new method of calibrating tropospheric hydroxyl (OH) instruments. Ozone-alkene mixtures produce steady-state OH radical concentrations. The steady state is governed by competition between OH production in the reaction of ozone with the alkene and OH removal by reactions with the alkene, ozone, and the reactor wall. In a flowtube reactor transporting an ozone-alkene mixture, the OH wall loss rate can be measured by varying the alkene concentration. Delivery of the reaction mixture to the sampling entry of an atmospheric OH measurement instrument provides an absolute calibration of the instrument's response to OH. The present precision of calibration is +/-8% (1-sigma), based on reproducibility over a wide range of ozone concentrations. The accuracy (+/-43%) is limited by uncertainties in kinetic rate coefficients and OH yield, which can be improved. The calibration requires no photon flux measurements or lamp-dependent absorption coefficients, which have inherent, variable, systematic uncertainties, and it has been tested in field experiments.

Atmospheric ammonia measurement using a VUV/photo-fragmentation laser-induced fluorescence technique

Vacuum ultraviolet/photofragmentation laser-induced fluorescence has been demonstrated to be a highly specific and sensitive method for the quantitative measurement of atmospheric ammonia (NH(3)). The fluorescence detected in this approach results from the two 193-nm photon photofragmentation step NH(3)?NH(2)? NH(b(1)Sigma(+)) followed by the excitation of the NH(b(1)Sigma(+)) NH(c(1)Pi) transition via a 450-nm photon with final emission being observed from the NH(c(1) Pi) NH(a(1)Delta) transition at 325 nm. Limits of detection for the instrumentpresented here are < 10 pptv and < 4 pptv for 1- and 5-min integration periods, respectively, in ambient sampling conditions. The technique is free from interferences and system performance does not significantly degrade in adverse sampling conditions (i.e., rain, fog, clouds, haze, etc.). Spectroscopic selectivity in the NH(b(1)Sigma(+))?NH(c(1)Pi) transition is sufficient to resolve (15)NH(3) and (14)NH(3) contributions for use in atmospheric tracer studies. Average ammonia measurements at Stone Mountain, GA, ranged from approximately 110 pptv for air temperatures <5 degrees C to approximately 240 pptv for air temperatures >/=<5 degrees C over the period from Dec. 1987 to the end of Apr. 1988.

Instrumental Requirements for Global Atmospheric Chemistry

The field of atmospheric chemistry is data-limited, primarily because of the challenge of measuring the key chemical constituents in the global environment. Several recent advances, however, in rugged, portable, remotesensing, ground-based instrumentation and accurate, fast-response airborne instrumentation have provided powerful tools for the understanding of stratospheric ozone, particularly in polar regions. Current discoveries of the role of heterogeneous chemical processes point to the need for better techniques for characterization of stratospheric aerosols. In the troposphere, advances in in situ, sensitive methods for detecting reactive nitrogen compounds have demonstrated the role that these compounds have in controlling global oxidation processes, but better measurements of the reservoir species by which the long-ranged transport of pollutant-reactive nitrogen compounds is thought to occur are urgently needed. The role of hydrocarbons, particularly those of natural origin, in ozone formation in rural areas has focused attention on the requirement for better speciation of these ubiquitous compounds. Lastly, rigorous instrument intercomparison experiments have provided unbiased estimates of measurement capabilities.

Detection of atmospheric OH radicals

The detection of atmospheric OH radicals by laser long path absorption spectroscopy is described. This technique is specific to OH and easily calibrated. In various field measurements from below the detection limit (5*10(5)/cm3) up to 8.7*10(6) OH radicals/cm3 have been observed. Measurements of meteorologic parameters and mixing ratios of other trace gases simultaneously with OH allow comparison of observed OH levels with reaction kinetic model calculations.