John C. Sheppard

The age of glacier peak tephra in west-central Montana

At Sheep Mountain Bog, near Missoula, Montana, a late-glacial tephra, that probably fell in late summer, is preserved as an 8-mm-thick graded bed overlain by another 8 mm of redeposited ash mixed with lake deposits. Sediment surrounding the ash was 14C dated to about 11,200 yr B.P. Electron-microprobe analyses of the volcanic glass and hornblende phenocrysts from this ash layer indicate that they are similar in major-element chemistry to those of Glacier Peak layer G previously considered to be about 12,000 yr old or older.

Contrasting climatic histories for the Snake River Plain, Idaho, resulting from multiple thermal maxima

Abstract Ten sites near the Snake River Plain have consistent differences in their climatic histories. Sites at low elevation reflect the “early Holocene xerothermic” of the Pacific Northwest, whereas most climatic chronologies at high elevation indicate maximum warmth or aridity somewhat later, ca. 6000 yr ago. This elevational contrast in climatic histories is duplicated at three sites from the central Snake River Plain. For sites in such close proximity, the different chronologies cannot be explained by changes in atmospheric circulation during the late Quaternary. Rather, the differences are best explained by the autecology of the plants involved and the changing seasonal climate. The seasonal climatic sequence predicted by multiple thermal maxima explains the high- and low-elevation chronologies. During the early Holocene, maximum insolation and intensified summer drought in July forced low-elevation vegetation upward. However, moisture was not a limiting factor at high elevation, where vegetation moved upward in response to increased length of growing season coincident with maximum September insolation 6000 yr ago.

Radiocarbon tracer measurements of atmospheric hydroxyl radical concentrations

Radiochemical techiques have many untapped applications in atmospheric chemistry, especially when great sensitivity is required. We describe the application of these techniques to the measurement of hydroxyl radical concentrations in the troposphere.

Intercomparison of Local Hydroxyl Measurements by Radiocarbon and FAGE Techniques

Abstract A direct intercomparison of near-surface tropospheric HO concentration measurements by two different techniques was made in October–November 1992 at a rural site near Pullman, Washington. The atmosphere at the site is believed to contain low levels of anthropogenic pollution. The instruments inlets were located at the same height (3.5 m) above the ground and were separated by 10 m along a line normal to the prevailing wind. Readings of the FAGE3 and radiocarbon instruments showed a high correlation (r2 = 0.74) despite HO concentrations that were frequently near the detection limit of the instruments. An unweighted least squares regression shows a slope significantly different from unity, indicating different calibration scales of the two instruments.

Measurements of the diurnal OH cycle by a 14C-tracer method

The hydroxyl free radical, OH, is generally recognized as the primary oxidant for the removal of pollutants from the Earths atmosphere. Measurements of OH concentrations are needed to test models of atmospheric photochemistry. We have used the 14C-tracer technique to measure five ambient diurnal OH concentration cycles in relatively pure and in polluted air. The early-to mid-October maximum midday OH concentrations for pure and polluted air were found to be, respectively, 2.4 × 106 and 9.5 × 106 radicals cm–3 (10–13–4 × 10–13 relative to air). Night-time OH concentrations of less than 2 × 105 radicals cm–3 were measured. Estimates of OH concentrations from photochemical models and trace gas lifetimes are consistent with our observations.

Radiochemically ultrapure 14CO

Abstract Preparation of ultra high-purity 14CO tracers by the reduction of 14CO2 by Zn at 395°C are described. Purity levels of 10 −8 14 CO 2 14 CO have been obtained by successive freeze-outs of 12CO2 carrier gas at liquid-nitrogen temperature. Radiochemical purity of the 14CO is limited by radiolysis and surface catalysis. The radiolysis rate of 14CO corresponds to a G(CO2) of 2.5 ± 0.9. Techniques are discussed to reduce these problems.

Precision of the radiochemical OH measurement method

Abstract Twenty-eight radiochemical 14 C tracer measurement of tropospheric hydroxyl radical (OH) concentrations were obtained at a rural site near Washington State University, Pullman, WA (117°W, 47°N). Diurnal OH concentration variations were observed for the five days between 9 August and 14 August 1990. These data made it possible to estimate the midday precision and detection limit of the radiochemical OH measurement method. Experiments performed at a peak O 3 photolysis rate J (O( 1 D)) of (3.0±0.2) × 10 −5 s −1 yielded a mean midday OH concentration of (5.6±0.1 (1 σ )) × 10 6 cm −3 . Other data put an upper bound of 16% on fluctuations of instrument sensitivity. Low-light or nighttime background OH concentrations were less than (2.6±2) × 10 5 cm −3 . A lower detection limit of 10 5 cm −3 was obtained when extra care was taken with the low-level 14 C counting procedure.

The catalytic oxidation of 14CO in a flow reactor used to measure atmospheric hydroxyl radical concentrations

Measurements of ultra-low CO oxidation rates of materials suitable for construction of a hydroxyl radical monitor are described. The procedure consists of passing an ultrapure 14CO tracer over the test material and collecting the resultant 14CO2. Oxidation rates are measured for stainless steel, aluminum, alumina, titanium, and Teflon. Results show the oxidation rate with aluminum is at least an order of magnitude less than any other material (<104 cm−2s−1). These rates are three to four orders of magnitude lower than reported previously in the literature. Application to a ground-based hydroxyl monitor is discussed.