Donald H. Stedman

An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux

Eddy-correlation measurements with a newly developed fast-response NOx sensor indicate that the deposition velocity at a height of about 6m above a soybean field has a maximum value near 0.6cms-1 for NOx and is usually about 2/3 ofthat found for ozone. In these studies, over 90% of the NOx is NO2. The corresponding minimum surface resistance for NOx calculated as the quantity remaining after atmospheric resistances are subtracted is about 1.3 s cm−1, which is larger than expected on the basis of leaf stomatal resistance alone. Emission of NO from sites in the plant canopy and soil where NO2 is deposited and reduced to NO or release of NOx as a result of biological activity may have lessened the downward fluxes of NOx as measured. During windy conditions at night, surface resistances are found to have values of about 15scm−1 for NOx (again, greater than 90% NO2) and 1.8scm−1 for O3, corresponding to deposition velocities of 0.05cms−1 and 0.3cms−1, respectively.

Measurements of SOx, NOx and aerosol species on Bermuda

During August 1982 and January and February 1983, General Motors Research Laboratories operated an air monitoring site on the southwest coast of Bermuda. The data show that the levels of the NOx and SOx species reaching Bermuda are determined by the direction of the air flow. The highest levels of sulfate (mean = 4.0 μg m−3), nitric acid (126 ppt) and other species are observed when air masses arrive from the northeastern United States while the lowest levels (sulfate = 1.1 μg m−3; nitric acid = 41 ppt) occur during air flow from the SE direction. With westerly air flow, increases in many anthropogenic constituents such as particulate sulfate, lead, elemental carbon, sulfur dioxide, nitrogen dioxide, nitric acid and ozone are observed. These species are generally the lowest during SE winds which bring high concentrations of soil- and crustal-related aerosol species. The source of this crustal material appears to be the Sahara Desert. On the average, the levels of anthropogenic constituents are higher in winter because of frequent intrusions of N American air masses. Conversely, the levels of crustal materials are higher in summer when the SE flow is more prevalent.

Airborne measurements of the vertical flux of ozone in the boundary layer

A fast-response chemiluminescent ozone sensor was mounted in an aircraft instrumented for air motion and temperature measurements. Measurements of the vertical flux of ozone by the eddy correlation technique were obtained after correcting for time delay and pressure sensitivity in the ozone sensor output. The observations were taken over eastern Colorado for two days in April, one a morning and the other an afternoon flight. Since the correlation coefficient of ozone and vertical velocity is small compared to, for example, temperature and vertical velocity in the lower part of the convective boundary layer, an averaging length of the order of 100 km was required to obtain a reasonably accurate estimate of the ozone flux. The measured variance of ozone appeared to be too large, probably mainly due to random noise in the sensor output, although the possibility of the production of ozone fluctuations by chemical reactions cannot be dismissed entirely. Terms in the budget equation for ozone were estimated from the aircraft measurements and the divergence of the ozone flux was found to be large compared to the flux at the surface divided by the boundary-layer height.

An experimental study of sulfur and NOx fluxes over grassland

Three independent sulfur sensors were used in a study of sulfur eddy fluxes to a field of wheat stubble and mixed grasses, conducted in Southern Ohio in September, 1979. Two of these sensors were modified commercial instruments; one operated with a prefilter to measure gaseous sulfur compounds and the other with a denuder system to provide submicron particulate sulfur data. The third sensor was a prototype system, used to measure total sulfur fluxes. The data obtained indicated that the deposition velocity for gaseous sulfur almost always exceeded that for particulate sulfur; average surface conductances were about 1.0 cm s−1 for gaseous sulfur in the daytime and about 0.4 cm s−1 for particulate sulfur. The data indicate that nighttime values were probably much lower. The total sulfur sensor provided support for these conclusions. The boundary-layer quantity ln(z0/zH)was found to be 2.75 ± 0.55, in close agreement with expectations and thus providing some assurance that the site was adequate for eddy flux studies. However, fluxes derived using a prototype NOx sensor were widely scattered, partially as a consequence of sensor noise but also possibly because of the effects of nearby sources of natural nitrogen compounds.

Determination of the Absolute Rate of Solar Photolysis of NO2

Improvements made on the direct NO2 photolysis actinometer developed by J. O. Jackson have produced more precise data. A plot of measured values of the photolysis rate J1 vs Eppley uv photometer readings produces a curved rather than the straight line correlation previously reported. This curvature arises from the Eppley and NO2 absorption spectrum overlap, backround surface albedoes, the Ep-pleys cosine response and inherent errors in the chemical equation used. New J1 measurement vs Eppley data is shown, and a procedure for calculating instantaneous J-i values from an Eppley output is suggested.

Molecular structure of nitrogen trichloride as determined by electron diffraction

Abstract Nitrogen trichloride was found to have a bond length of r g = 1.759 ± 0.002 A and a Cl-N-Cl angle of 107.1 ± 0.5°. The bond angle is larger than that found in NF 3 , consistent with the (recently revised) trends displayed by the trihalides of phosphorus and arsenic, but much lower than the 120° angle reported for the isoelectronic molecule N(SiH 3 ) 3 . Moreover, a comparison between selected compounds reveals that the N-Cl bond length is appreciably greater, relatively, than the N-Si bond length. Accordingly, the bond angles and bond lengths suggest a greater reluctance of the nitrogen lone pairs to delocalize onto Cl than onto SiH 3 groups. Mean amplitudes of vibration of NCl 3 were derived both from the diffraction data and from recently published infrared and Raman frequencies. The values agree within the estimated uncertainties.

The NO/NO2/O3 Photostationary State in Claremont, California

Seventeen days of detailed measurements of NO, NO2, O3, HNO3 and the frequency of NO2 photolysis—j(NO2)—were carried out in Claremont, CA, in September 1980. Under conditions when the rate of change of NO concentration is small, there must be a balance between formation and loss processes. In the classical photostationary state this balance is between NO2 photolysis: NO2 + hv → NO + O and reaction with ozone: NO + O3 → NO2 + O2. The results show that the latter reaction with ozone is inadequate to balance the formation step; a significant contribution is required from another NO oxidation process, possibly peroxy radical oxidation. If so, the inferred concentration of peroxy radicals shows a diurnal variation, peaking around solar noon.

A FAST RESPONSE SENSOR FOR OZONE EDDY-CORRELATION FLUX MEASUREMENTS*

Abstract The measurement of fluxes of gaseous pollutants to the ground may be made using eddy-correlation techniques. The application of a chemiluminescent and other fast response detectors is discussed. As an example, a specific study of ozone fluxes is described.

Chemiluminescent Measurement of Parts-Per-Billion Levels of Nickel Carbonyl in Air

Abstract We have demonstrated the chemiluminescent detection of Ni(CO)4 in air using a reaction with O2/O3 and purified CO. The detection limit on our prototype detector is 0.01 ppb, with linear response into the ppm range. Decay of Ni(CO)4 in air has been studied in the ppb range and is approximately first order with a half life of about 30 minutes.

Variations in the ambient ozone concentration during the 26 February 1979 solar eclipse

Abstract The ambient ozone concentration was measured during the partial solar eclipse on 26 February 1979, at Argonne National Laboratory. A 50% reduction in the ozone concentration occurred during the eclipse as a result of photochemical reactions. The observed lag time between the minima of the solar radiation and the ozone concentration is consistent with the computed lag time. The labile nature of ozone is self-evident in what may be the first measurement of the ambient ozone concentration during a solar eclipse.