William C. Graustein

Transport and residence times of tropospheric aerosols inferred from a global three‐dimensional simulation of 210Pb

A global three-dimensional model is used to investigate the transport and tropospheric residence time of 210Pb, an aerosol tracer produced in the atmosphere by radioactive decay of 222Rn emitted from soils. The model uses meteorological input with 4°×5° horizontal resolution and 4-hour temporal resolution from the Goddard Institute for Space Studies general circulation model (GCM). It computes aerosol scavenging by convective precipitation as part of the wet convective mass transport operator in order to capture the coupling between vertical transport and rainout. Scavenging in convective precipitation accounts for 74% of the global 210Pb sink in the model; scavenging in large-scale precipitation accounts for 12%, and scavenging in dry deposition accounts for 14%. The model captures 63% of the variance of yearly mean 210Pb concentrations measured at 85 sites around the world with negligible mean bias, lending support to the computation of aerosol scavenging. There are, however, a number of regional and seasonal discrepancies that reflect in part anomalies in GCM precipitation. Computed residence times with respect to deposition for 210Pb aerosol in the tropospheric column are about 5 days at southern midlatitudes and 10–15 days in the tropics; values at northern midlatitudes vary from about 5 days in winter to 10 days in summer. The residence time of 210Pb produced in the lowest 0.5 km of atmosphere is on average four times shorter than that of 210Pb produced in the upper atmosphere. Both model and observations indicate a weaker decrease of 210Pb concentrations between the continental mixed layer and the free troposphere than is observed for total aerosol concentrations; an explanation is that 222Rn is transported to high altitudes in wet convective updrafts, while aerosols and soluble precursors of aerosols are scavenged by precipitation in the updrafts. Thus 210Pb is not simply a tracer of aerosols produced in the continental boundary layer, but also of aerosols derived from insoluble precursors emitted from the surface of continents. One may draw an analogy between 210Pb and nitrate, whose precursor NOx is sparingly soluble, and explain in this manner the strong correlation observed between nitrate and 210Pb concentrations over the oceans.

Calcium oxalate accumulation and soil weathering in mats of the hypogeous fungus Hysterangium crassum

Abstract Fungal mats of Hysterangium crassum (Tul. and Tul.) Fischer occupied a mean of 9.6% of the upper 10 cm of soil developed under a 40–65 yr old stand of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) in Oregon. This hypogeous basidiomycete exudes large amounts of oxalic acid, some of which precipitates with Ca in microscopic crystals of calcium oxalate, resulting in a mean CaC2O4 content of 82g m−2 for the entire soil. Soil oxalate concentration was significantly greater within fungal mats (P

Vertical transport of tropospheric aerosols as indicated by 7Be and 210Pb in a chemical tracer model

We use the natural radionuclides 7 Be and 210 Pb as aerosol tracers in a three-dimensional chemical tracer model (based on the Goddard Institute for Space Studies general circulation model (GCM) 2) in order to study aerosol transport and removal in the troposphere. Beryllium 7, produced in the upper troposphere and stratosphere by cosmic rays, and 210 Pb, a decay product of soil-derived 222 Rn, are tracers of upper and lower tropospheric aerosols, respectively. Their source regions make them particularly suitable for the study of vertical transport processes. Both tracers are removed from the troposphere primarily by precipitation and are useful for testing scavenging parameterizations. In particular, model convection must properly transport and scavenge both ascending 210 Pb and descending 7 Be. The ratio 7 Be/ 210 Pb cancels most model errors associated with precipitation and serves as an indicator of vertical transport. We show that over land the annual average 7 Be/ 210 Pb ratio for surface concentrations and deposition fluxes vary little globally. In contrast, the seasonal variability of the 7 Be/ 210 Pb concentration ratio over continents is quite large ; the ratio peaks in summer when convective activity is maximum. The model overestimates 7 Be in the tropics, a problem which we relate to flaws in the GCM parameterization of wet convection (excessive convective mass fluxes and no allowance for entrainment). The residence time of tropospheric 7 Be calculated by the model is 23 days, in contrast with a value of about 9 days calculated for 210 Pb, reflecting the high-altitude versus low-altitude source regions of these two tracers.

The Use of Strontium-87/Strontium-86 Ratios to Measure Atmospheric Transport into Forested Watersheds

Strontium-87/strontium-86 ratios indicate the sources of strontium in samples of natural waters, vegetation, and soil material taken from watersheds in the Sangre de Cristo Mountains of New Mexico. More than 75 percent of the strontium in the vegetation is ultimately derived from atmospheric transport and less than 25 percent from the weathering of the underlying rock. Much of the airborne strontium enters the watersheds by impacting on coniferous foliage, but deciduous foliage apparently traps little, if any, strontium-bearing aerosol. The strontium and presumably other nutrients are continuously recycled in a nearly closed system consisting of upper soil horizons, forest litter, and the standing crop of vegetation.

Temporal variability of summer-time ozone and aerosols in the free troposphere over the eastern North Atlantic

In the free troposphere over Tenerife in the summer, O3 concentrations are anti-correlated with major pollutant aerosols (nss-SO = 4 and NO−3) and with 210Pb, a tracer for boundary layer sources. In contrast, O3 is highly correlated with 7Be, a product of cosmic ray interactions in the upper troposphere and stratosphere. This suggests that natural O3 sources (i.e. the stratosphere) might be playing an important role. Nonetheless our results do not preclude the possibility that substantial amounts of pollution-related O3 could be transported in the free troposphere. However, to be consistent with our results, the transport mechanisms would have to incorporate efficient processes for the removal of pollutant aerosol species and 210Pb.

Sources of nitrate and ozone in the marine boundary layer of the tropical north Atlantic

During the period April 1989 through December 1990, O3 concentrations in the marine boundary layer at Barbados, West Indies, show a pronounced seasonal cycle. Daily averaged values in the winter and spring often fall in the range of 25–35 ppbv for periods of several days, and they seldom fall below 20 ppbv. In contrast, during the summer, values typically fall in the range of 10–20 ppbv. During the winter-spring period, there is a very strong negative correlation between O3 and a number of aerosol species, including NO3−. These anticorrelations appear to be driven by changing transport patterns over the North Atlantic as opposed to chemical reactions involving O3 and nitrogen species in the atmosphere. Analyses of isentropic trajectories clearly show that high O3 and low NO3− are associated with transport from higher latitudes and high altitudes. Conversely, high NO3− and relatively low O3 are associated with transport from Africa. Our study suggests that North America and the middle troposphere (and stratosphere) are not strong sources for NO3− over the tropical North Atlantic. The strong correlation of NO3− with 210Pb and the weaker correlation with Saharan dust indicates that NO3− is derived principally from continental surface sources, probably in Europe and North Africa, but not from the Saharan soil material itself. During several extended periods, NO3− and 210Pb were strongly correlated and their concentrations were high relative to nss SO4=; these factors, coupled with trajectories originating in Africa, suggest that African biomass burning was a significant source at these times. In contrast, biomass burning appears to be a minor source for O3 as measured at Barbados, perhaps accounting for an enhancement of about 5 ppbv at most during these periods.

7Be and 210Pb Indicate an upper troposphere source for elevated ozone in the summertime subtropical free troposphere of the eastern North Atlantic

The relative abundance of 7Be and 210Pb in aerosols, represented as the “normalized fraction”, 7Be/(7Be + n210Pb), is unaffected by aerosol scavenging. This normalized fraction is an index of the relative influence on an air parcel of stratospheric and upper tropospheric processes versus those in the continental boundary layer (CBL). The high correlation of ozone concentration with the normalized fraction observed during the summer at Izana, Canary Islands, indicates that 1) low altitude transport of air from the CBL is associated with low ozone concentrations; 2) high altitude transport through the troposphere from the CBL is associated with net ozone production; and 3) the stratosphere is not a direct source of a significant fraction of the ozone observed at Izana during the summer.

An Examination of the Atmospheric Chemistry of Mercury Using 210Pb and 7Be

Measurements of Hg (total gas-phase, precipitation-phase andparticulate-phase), aerosol mass, particulate 210Pb and7Be and precipitation 210Pb were made at an atmosphericcollection station located in a near remote area of northcentral Wisconsin,U.S.A. (46°10′N, 89°50′W) during the summers of 1993, 1994and 1995. Total Hg and 210Pb were observed to correlate strongly(slope = 0.06 ± 0.03 ng mBq-1; r2 =0.72) in rainwater. Mercury to 210Pb ratios in particulate matter(0.03 ± 0.02 ng mBq-1; r2 = 0.06) wereconsistent with the ratio in rain. Enrichment of the Hg/mass ratio (approx.5–50×) relative to soil and primary pollutant aerosols indicatedthat gas-to-particle conversion had taken place during transport. Comparisonof these results with models for the incorporation of Hg into precipitationindicates that atmospheric particles deliver more Hg to precipitation than canbe explained by the presence of soot. A lack of correlation between totalgas-phase Hg (TGM) and a 7Be/210Pb function suggests novertical concentration gradient within the troposphere, and allows an estimateof TGM residence time of 1.5 ± 0.6 yr be made based on surface airsamples.

Semiannual cycles of pollution at Bermuda

To identify the sources and determine the transport pathways for aerosol during the Atmosphere/Ocean Chemistry Experiment (AEROCE), we examined the temporal variations of trace elements in daily aerosol samples collected at Bermuda from 1988 to 1994. Crustal (e.g., Al) and marine (e.g., Na) elements showed annual cycles with summer and winter maxima, respectively. In contrast, pollution-derived elements (e.g., Sb) showed unusual semiannual cycles with strong spring maxima and weaker fall maxima, which to the best of our knowledge, have not been previously documented. The seasonality in trace element concentrations was mainly transport-driven: The spring maxima of pollutants were caused by rapid westerly transport from North America, and the fall maxima were caused by North American air slowly transported to Bermuda by large high-pressure systems that stagnated over the lower mid-Atlantic states. Low concentrations of pollution elements in winter resulted from the southwestward extension of the Bermuda-Azores high-pressure system that brought marine air to Bermuda from the east or northeast while hindering transport from North America and Africa. The summer minima in pollutants were associated with air transported from the eastern Atlantic and Africa. The variations of the trace gases O3 and CO and two naturally occurring radionuclides, 210Pb and 7Be, showed semiannual cycles similar to those of the pollution-derived trace elements.

Mica Granites of the Kern Mountains Pluton, Eastern White Pine County, Nevada: Remobilized Basement of the Cordilleran Miogeosyncline?

Among the many granitic plutons within the Basin and Range province of western North America, those in the Kern Mountains of eastern Nevada and western Utah are unusual structurally, mineralogically, and chemically. The two largest intrusions, which together are exposed over an area of 130 sq km (51 sq mi), have well-defined border facies of leucocratic and generally aplitic rocks, or of protoclastic rock developed from coarser core-facies material. Within the core facies are abundant aplitic, leucocratic dikes. The core facies of the larger intrusion (Tungstonia Granite) is a two-mica granite with conspicuous phenocrystic books of muscovite up to 5 cm across. The smaller intrusion (Skinner Canyon Granite) lacks muscovite, and Fe-Ti oxides and sphene are constant accessory phases in it. Rb-Sr and K-Ar isotopic analyses suggest a wide range of initial Sr 87 /Sr 86 ratios, which for the most part are unusually high—to nearly 0.7246 for the Tungstonia Granite—and a complex, protracted period of magmatism and isotopic adjustment that extended from possibly Mesozoic into mid-Cenozoic time. The simplest interpretation is that the final emplacement of the Tungstonia body occurred 60 m.y. ago and that the Skinner Canyon intrusion occurred between 30 and 45 m.y. ago. The data prompt the conclusion that the magmas represent remobilized sialic basement whose model dates (assuming an initial ratio typical for mantle-derived magmas) are in excess of 1.5 b.y. The occurrence of such anatectic intrusions within the relatively intensely metamorphosed parts of the infrastructure of the Sevier orogenic belt, rather than elsewhere in the Basin and Range province, is not unexpected, and it possibly represents a more advanced evolutionary stage than do the gneiss domes previously documented within this same belt.