Aerosol residence time

Poet, S. E., H. E. Moore and E. A. Martell, Lead-210, Bi-210, and Polonium-210 in the Atmosphere Accurate Ratio and Application to Aerosol Residence Time Determination, J. Geophys. Res.,... 

Mechanisms and rates of transport of nuclear test debris in the upper and lower atmosphere are considered. For the lower thermosphere vertical eddy diffusion coefficients of 3-6 X 106 cm.2 sec. 1 are estimated from twilight lithium enhancement observations. Radiochemical evidence for samples from 23 to 37 km. altitude at 31° N indicate pole-ward mean motion in this layer. Large increases in stratospheric debris in the southern hemisphere in 1963 and 1964 are attributed to debris from Soviet tests, transported via the mesosphere and the Antarctic stratosphere. Most of the carbon-14 remains behind in the Arctic stratosphere. 210Bi/ 210Pb ratios indicate aerosol residence times of only a few days at tropospheric levels and only several weeks in the lower stratosphere. Implications for the inventory and distribution of radioactive fallout are discussed. 

Rn, 210Pb, 210Bi and 210Po profiles and aerosol residence times versus altitude. Journal of Geophysical Research, 78, 7065-75. 

Concentrations of Po were measured in the North Atlantic troposphere. Concentrations of Po were measured in rain water, surface seawater, and the marine microlayer. An excess of Po activity was measured in the aerosol relative to what was expected on the basis of Pb and aerosol residence times. In surface seawaters, deficiencies of Po were observed. The mechanism of Po enrichment in the atmospheric aerosol was attributed to possible enrichments from the organic components of the marine microlayer or air sea exchange of organic polonium species. However, no organic compounds of polonium were actually isolated or characterized. 

Weickmann H.K. and Pueschel R.F., Atmospheric aerosols-, residence times, retain-ment factor and climatic effects. Beitr. Phys. Atmos. , Bd. 46, s. 112-118 (1973). 

Martell E. A. and Moore H. E. (1974) Tropospheric aerosol residence times a critical review. J. Rech. Atmos. 8, 903-910. 

Furthermore, measurement of the gross sea-salt aerosol concentration is not sufficient. Because the aerosol residence time depends on the particle size, the magnitude of the hysteresis effect will be sensitive to the particle size. Thus, the instantaneous particle-size distribution and the history of this distribution must be known. 

Contributions from local sources are presumably important, but aerosol residence-time considerations and the general flow of air masses across the continent suggest that there is a very extensive areal contribution to the atmospheric burden of trace metals at the study site. Mean-residence time in the atmosphere depends on particle size and climatic regime. Esmen and Com (1971) found that the mean-residence time of submicron-size particles in urban air in the absence of wet precipitation was roughly 10 -10 hr and decreased to 10-100 hr for 1-10 pm particles. Moore et al. (1973) recently reevaluated the mean-residence time of the tropospheric aerosol and concluded that 4-6 days is the most probable value. 

Schemes that one may apply to deduce aerosol residence times from various radioactive elements have been reviewed by Junge (1963), Martell and Moore (1974), and Turekian et al. (1977). The published data admit residence times in the range 4-72 days, but crowd into two groups of values averaging 6 and 35 days, respectively. From the evidence available to him, Junge (1963) concluded that the higher value was appropriate to the troposphere as a whole and that the lower values were applicable only to the boundary layer near the Earth surface. Martell and Moore (1974), after having critically reviewed older and newer data, came to the opposite conclusion, namely, that the high values are due to the contribution of stratospheric aerosols, apart from misinterpretations of some data, while the lower values represent the true tropospheric residence time essentially independent of altitude.

Aerosol residence times in the troposphere are roughly 1-2 weeks if all S02 sources were shut off today, anthropogenic sulfate aerosols would disappear from the planet in 2 weeks. By contrast, not only are GHG residence times measured in decades to centuries, but because of the great inertia of the climate system, as noted in the previous chapter, the effect of GHG forcing takes decades to be fully transformed into equilibrium climate warming. As a result, if both C02 and aerosol emissions were to cease today, the Earth would continue to warm as the climate system continues to respond to the accumulated amount of C02 already in the atmosphere. 

A few of the U/Th nuclides are supplied to the sea via atmospheric deposition and diffusion through sediment pore waters. Decay of Rn in the atmosphere to °Pb and its subsequent removal by wet and dry deposition is an important source of dissolved °Pb to the sea. As the bulk of the Rn in the atmosphere is of continental origin, the flux of °Pb via this route depends on factors such as distance from land and aerosol residence times. °Po is also deposited on the sea surface through this source, but its flux is <10% of that of °Pb. Leaching of atmospheric dust by sea water can also contribute to nuclide fluxes near the air-sea interface, this mechanism has been suggested as a source for dissolved Th. 

Coarse mode particles (>2.5 pm diameter) tend to result from mechanical processes such as construction, traffic, combustion, and the soil lifted and dispersed by wind action. The main competing mechanisms determining the stability of this size range of particles are turbulent mixing and sedimentation. For typical coarse particles in the atmosphere, the aerosol residence times range from several hours to about a day [121]. 

The subject of this particular volume relates to aerosol particle physics including aerosol characterisation, the formation mechanism, the aerodynamic size distribution of the activity and aerosol residence time, instrumentation techniques, aerosol collection and sampling, various kinds of environmental (atmospheric aerosols), particularly radioactive aerosols and the special case of radon decay product aerosols (indoors and outdoors) and the unattached fl ac-tion, thoron decay product aerosols, the deposition patterns of aerosol particles in the lung and the subsequent uptake into human subjects and risk assessment. 

Shapiro and Forbes-Resha (1976) much earlier estimated a mean tropospheric aerosol residence time for Be-bearing aerosols of 35.4 days, significantly higher, i.e. more than four times higher, at Fullerton, California (33°52 N, 117°55 W), also at mid-latitude, over an almost 2-year period, with relatively light precipitation. 

Koch et al. (1996) using a three-dimensional chemical tracer model similar to that of Balkanski et al. (1993) calculated a tropospheric aerosol residence time of 9 days for aerosols at different latitudes from 80° S to 80°N. The respective residence time of Be aerosols was 21 days reflecting the high altitude versus low altitude source regions of these two tracers. They also found that the tropospheric residence time is a function of latitude (Balkanski et al., 1993) according to Equation (4.2). 

Papastefanou and Bondietti (1991) estimated tropospheric aerosol residence times ranging from 4.8 to 15.3 days (average 8.2 days) based on the Bi/ Pb activity ratios for 21 measurements of aerosol samplings carried out during an annual period at Oak Ridge, Tennessee (35°58 N, 84°17 W) at temperate latitude with high precipitation (wet climate). 

Poet et al. (1972) estimated tropospheric aerosol residence times ranging from 1.59 to 13 days (average 5.4 days) when based on the Bi/ °Pb activity ratios and from 11 to 77 days (average 24 days) when based on the 2i°Po/ °Pb activity ratios for 20 measurements of aerosol samplings carried out in surface air during a 4 1/2-year period at Boulder, Colorado (40°01 N, 105°17 W). They concluded that a mean tropospheric residence time of about 4 days could be applied for aerosol particles in the lower troposphere and about a week for aerosol particles in precipitation. They also found that the mean aerosol residence time increases with altitude within the troposphere by less than a factor of 3 (Moore et al., 1973). 

Very early on, Lehmann and Sittkus (1959) estimated aerosol residence times of 20 days from °Po/ °Pb activity ratios in air at Freiburg, Germany (47°59 N, 7°51 E), while Peirson et al. (1966) estimated aerosol residence times of 40 days from Po/ Pb activity ratios in air at Milford Haven, Wales (51°40 N, 5°02 E). 

Lambert et al. (1982) estimated aerosol residence times varying from 8.8 to 10.5 days based on the °Bi/ °Pb activity ratios in air, while later in another work they estimated aerosol residence times of 7 to 9 days (average 8.58 days) based on the Bi/ Pb activity ratios in air near Paris over a 6-year period (Lambert et al., 1983). 

Marley, N.A., Gaffney, O.S., Drayton, P.J., Cunningham, M.M., Orlandini, K.A., Paode, R. (2000). Measurement of Pb, Po and Bi in size-fractionated atmospheric aerosols An estimate of fine-aerosol residence times. Aerosol Sci. Technol. 32, 569-583. 

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