Residence Time Of Air Research Articles

Corrigendum to: “Real-time measurements of ammonia, acidic trace gases and water-soluble inorganic aerosol species at a rural site in the Amazon Basin" published in Atmos. Chem. Phys., 4, 967–987, 2004

A detailed description of the WAD/SJAC and the analytical procedures is given elsewhere (Slanina et al., 2001; Wyers et al., 1993). A simplified sketch of the sampling system is shown in Fig. 2. The air flow through the instrument (∼17 l min−1, STP) was generated by a scroll pump outside the wooden house and could be adjusted with a needle valve. The flow was measured continuously (1 min time resolution) with a mass flow meter (Bronkhorst, F-112ACHA-55-V). After the sample air passed the steel elbow and/or pre-impactor and the PFA Teflon tubing, it entered a horizontally aligned WAD that scavenges soluble gaseous species. Trace gases (such as NH3, HNO3, HNO2, HCl and SO2) were collected in a 10−4 M NaHCO3 absorption solution. The liquid input was controlled automatically by an infrared sensor and a switching valve and the liquid was continuously pumped out of the denuder at a flow rate of 0.5– 0.6 ml min−1 by a peristaltic pump. The liquid effluent was collected in a sample reservoir (“gas sample”, see Fig. 2). Artifacts due to evaporation of aerosol phase species in the WAD can be excluded because the characteristic time for formation/evaporation of NH4NO3 is >10 s (Dlugi, 1993), while the mean residence time of the sample air in the WAD is ∼0.002 s (annulus volume: 0.0018 l, flow rate: ∼17 l min−1). After the WAD, the air entered a reservoir where it was mixed with steam of highly purified water. The supersaturation causes aerosol particles to grow rapidly (within 0.1 s) into droplets of at least 2μm diameter. These droplets, containing the dissolved aerosol species were then collected in a cyclone (Khlystov et al., 1995). The cyclone effluent (“aerosol sample”) was transferred into the sample reservoir by a peristaltic pump at a flow rate of 0.5–0.6 ml min−1.

AIR SCRUBBING TECHNIQUES FOR AMMONIA AND ODOR REDUCTION AT LIVESTOCK OPERATIONS: REVIEW OF ON-FARM RESEARCH IN THE NETHERLANDS

Acid scrubbers and biotrickling filters have been developed for ammonia (NH3) removal at pig and poultry housesin The Netherlands over the last 20 years to prevent acidification and eutrophication of soils. Because of growingsuburbanization, odor removal is increasingly considered important as well. In this review, we report the results of the on-farmresearch on full-scale operated scrubbers for treatment of exhaust air from animal houses with regard to NH3 and odorremoval in The Netherlands. The NH3 removal of acid scrubbers ranged from 40% to 100% with an overall average of 96%.The NH3 removal of biotrickling filters ranged from -8% to +100% with an overall average of 70%. Minimum empty bed airresidence times (EBRTs) were 0.4 to 1.1 s. For acid scrubbers, process control with pH measurement and automatic waterdischarge is sufficient to guarantee sufficient NH3 removal. For biotrickling filters, however, improvement of process controlis necessary to guarantee sufficient NH3 removal. The odor removal of acid scrubbers ranged from 3% to 51% with an overallaverage of 27%. The odor removal of biotrickling filters ranged from -29% to +87% with an overall average of 51%.Minimum EBRTs were 0.5 to 2.3 s. Further research is necessary to explain this variation and to improve the odor removalefficiency of both acid scrubbers and biotrickling filters.

Radon fluxes in tropical forest ecosystems of Brazilian Amazonia: night‐time CO2 net ecosystem exchange derived from radon and eddy covariance methods

AbstractRadon‐222 (Rn‐222) is used as a transport tracer of forest canopy–atmosphere CO2 exchange in an old‐growth, tropical rain forest site near km 67 of the Tapajós National Forest, Pará, Brazil. Initial results, from month‐long periods at the end of the wet season (June–July) and the end of the dry season (November–December) in 2001, demonstrate the potential of new Rn measurement instruments and methods to quantify mass transport processes between forest canopies and the atmosphere. Gas exchange rates yield mean canopy air residence times ranging from minutes during turbulent daytime hours to greater than 12 h during calm nights. Rn is an effective tracer for net ecosystem exchange of CO2 (CO2 NEE) during calm, night‐time hours when eddy covariance‐based NEE measurements are less certain because of low atmospheric turbulence. Rn‐derived night‐time CO2 NEE (9.00±0.99 μmol m−2 s−1 in the wet season, 6.39±0.59 in the dry season) was significantly higher than raw uncorrected, eddy covariance‐derived CO2 NEE (5.96±0.51 wet season, 5.57±0.53 dry season), but agrees with corrected eddy covariance results (8.65±1.07 wet season, 6.56±0.73 dry season) derived by filtering out lower NEE values obtained during calm periods using independent meteorological criteria. The Rn CO2 results suggest that uncorrected eddy covariance values underestimate night‐time CO2 loss at this site. If generalizable to other sites, these observations indicate that previous reports of strong net CO2 uptake in Amazonian terra firme forest may be overestimated.

Hydrogen Halides on Jupiter and Saturn

The quest to detect gaseous HCl, HBr, and HF in the atmospheres of Jupiter and Saturn has led to a tentative detection of 1 ppb HCl near Saturn's cloud deck. The detection is puzzling because, while these hydrogen halides may be present several scale heights below the clouds, they are expected to react with ammonia to form solid ammonium halide salts in the upper troposphere. I show that the loss timescale for condensation of gaseous hydrogen halides onto particles is ∼10 3–10 5 s for realistic cloud densities and particle sizes, which is much less than the ∼10 8 s residence time of upper tropospheric air. The hydrogen halides can only survive transport up to the cloud layer if less than 1 in 10 6 of their collisions with particle surfaces leads to condensation, which is unlikely. Even in the absence of foreign particles, homogeneous nucleation would probably prevent supersaturations in excess of a few hundred, which is ∼10 20–10 40 times too low to explain the observation. These calculations therefore suggest that hydrogen halides cannot exist at part-per-billion levels in the upper troposphere. The interplanetary source of halogens is also too low to produce detectable quantities of hydrogen halides except perhaps at pressures less than 1 mbar. A possible detection of chlorine by the Galileo probe at pressures exceeding 9 bars on Jupiter may be consistent with the equilibrium abundance of gaseous HCl or NH 4Cl.

Estimating Chemical Errors in Large-scale Simulations of Aircraft Emissions

Calculations of a relatively comprehensive chemical reaction mechanism applied to individual parcels of aircraft emissions under upper tropospheric conditions show that over a typical residence time of air in the up. per troposphere, each aircraft-emitted NOx molecule produces about 2-3 molecules of O3 per day as long as the reactive NOy remains in the upper troposphere. In the upper tropospheric environment characterized by low ambient turbulence and appreciable vertical wind shear, aircraft plumes grow slowly during initial stages, and take several days to diffuse to a size comparable to the grid volume used by global-scale Eulerian models. By assuming aircraft emissions are immediately diluted into a larger grid cell volume, larger-scale models will overestimate the calculated O3 production by 20 to 30 %. This overestimate depends on the vertical diffusion efficiency, vertical wind shear, and NOx concentrations in the ambient environment. This overestimate can be compensated for in larger scale models by reducing the reaction coefficient of the NO + HO2 reaction by less than 3% under typical upper tropospheric conditions.

SCRAM: A scoring and ranking system for persistent, bioaccumulative, and toxic substances for the North American Great Lakes

Hundreds of chemical contaminants have been identified in the Great Lakes System of North America. Depending on the agency or organization, various subset lists of these contaminants have been identified as chemicals of potential concern. However, there is no agreement on the method that should be used to make management decisions. Except for consensus on approximately 40 chemicals that most North American agencies agree can cause deleterious effects if released into the environment, no agreement has been reached regarding the priority that contaminants should receive for further action. That leaves hundreds of chemicals that have been, are being, or potentially could be released into the environment that have not been evaluated yet. A profile for potential chemicals of concern is generally thought to include persistence in the environment, potential to bioaccumulate, and ability to cause toxic effects at environmentally relevant concentrations. Except for the International Joint Commission's definition of persistence (> 8 weeks residence time in air, water, soil or sediment), there is little concurrence about what defines these characteristics. For instance, the State of Michigan currently has no established definitions or profiles of persistent, bioaccumulative, toxic substances. Furthermore, there is no standard process to rank chemicals relative to these characteristics. The Chemical Scoring and Ranking Assessment Model (SCRAM) has been developed to provide a process to rank-order chemicals based on these characteristics. The SCRAM system was developed primarily for use in the Great Lakes region of North America and particularly in Michigan, but it is not site-specific. Use of this system may assist in pollution prevention activities and other future chemical control efforts, allowing attention to be focused first on those chemicals likely to present the greatest hazard.

Characterization of Indoor Particle Sources: A Study Conducted in the Metropolitan Boston Area

An intensive particle monitoring study was conducted in homes in the Boston, Massachusetts, area during the winter and summer of 1996 in an effort to characterize sources of indoor particles. As part of this study, continuous particle size and mass concentration data were collected in four single-family homes, with each home monitored for one or two 6-day periods. Additionally, housing activity and air exchange rate data were collected. Cooking, cleaning, and the movement of people were identified as the most important indoor particle sources in these homes. These sources contributed significantly both to indoor concentrations (indoor-outdoor ratios varied between 2 and 33) and to altered indoor particle size distributions. Cooking, including broiling/baking, toasting, and barbecuing contributed primarily to particulate matter with physical diameters between 0.02 and 0.5 microm [PM((0.02-0.5))], with volume median diameters of between 0.13 and 0.25 microm. Sources of particulate matter with aerodynamic diameters between 0.7 and 10 microm [PM((0.7-10))] included sautéing, cleaning (vacuuming, dusting, and sweeping), and movement of people, with volume median diameters of between 3 and 4.3 microm. Frying was associated with particles from both PM((0.02-0.5)) and PM((0.7-10)). Air exchange rates ranged between 0.12 and 24.3 exchanges/hr and had significant impact on indoor particle levels and size distributions. Low air exchange rates (< 1 exchange/hr) resulted in longer air residence times and more time for particle concentrations from indoor sources to increase. When air exchange rates were higher (> 1 exchange/hr), the impact of indoor sources was less pronounced, as indoor particle concentrations tracked outdoor levels more closely.

Characterization of indoor particle sources: A study conducted in the metropolitan Boston area.

An intensive particle monitoring study was conducted in homes in the Boston, Massachusetts, area during the winter and summer of 1996 in an effort to characterize sources of indoor particles. As part of this study, continuous particle size and mass concentration data were collected in four single-family homes, with each home monitored for one or two 6-day periods. Additionally, housing activity and air exchange rate data were collected. Cooking, cleaning, and the movement of people were identified as the most important indoor particle sources in these homes. These sources contributed significantly both to indoor concentrations (indoor-outdoor ratios varied between 2 and 33) and to altered indoor particle size distributions. Cooking, including broiling/baking, toasting, and barbecuing contributed primarily to particulate matter with physical diameters between 0.02 and 0.5 microm [PM((0.02-0.5))], with volume median diameters of between 0.13 and 0.25 microm. Sources of particulate matter with aerodynamic diameters between 0.7 and 10 microm [PM((0.7-10))] included sautéing, cleaning (vacuuming, dusting, and sweeping), and movement of people, with volume median diameters of between 3 and 4.3 microm. Frying was associated with particles from both PM((0.02-0.5)) and PM((0.7-10)). Air exchange rates ranged between 0.12 and 24.3 exchanges/hr and had significant impact on indoor particle levels and size distributions. Low air exchange rates (< 1 exchange/hr) resulted in longer air residence times and more time for particle concentrations from indoor sources to increase. When air exchange rates were higher (> 1 exchange/hr), the impact of indoor sources was less pronounced, as indoor particle concentrations tracked outdoor levels more closely.

Effect of aqueous phase cloud chemistry on tropospheric ozone

The sensitivity of tropospheric O3 to aqueous HOx (OH + HO2) phase chemistry in clouds is examined using photochemical model calculations of the O3 production efficiency per unit NOx and the chemical lifetime of O3, combined with estimates for the residence time of air in clouds. It is found that the maximum perturbation to O3 from cloud chemistry in the tropics and midlatitudes summer is less than 3%. This result is supported by calculations using a three‐dimensional, continental‐scale model for North America.

Planetary boundary layer structure and air mass transport during the International Arctic Ocean Expedition 1991

During the International Arctic Ocean Expedition 1991 north of latitudes 70° N from August 1 to October 6 (late summer and early autumn), the dominant air mass origin was from the seas that surround the central Arctic Ocean, while land was a much less important source. This resulted in air temperatures near 0 °C, fogs and extensive stratus layers. In the autumn, long residence time of the air over the pack ice and subsidence became more common, accompanied by decreasing cloudiness, foginess and air temperature. The residence time of air over the pack ice and the time since land contact had medians of 52 and 91 h, respectively. The structure and evolution of the marine boundary layer above the pack ice is described. A stable layer, 300–1700 m high, grew in height proportionally to the square root of the distance from the ice edge, the fetch, up to 3000 km. After 400 km, a mixed layer formed, typically 100–250 m thick, related to the presence of low level jets in 60–90% of the profiles. Layers of subcritical Richardson number were present in 65–80% of the profiles above and below the jet centres. Cloud streets and roll vortices were observed above the pack ice with wavelengths of 4–12 km. The high aspect ratios of 7–9 of the few earlier roll observations above ice were confirmed. Cold air advection and weak convection caused the roll vortices in aged air that had been advected usually 2000 km or more over the ice. Breaking of Kelvin-Helmholtz waves, gravity waves or other reasons for intermittent planetary boundary layer breakdown combined with vertical gradients in concentration are suggested to have caused the observed sudden changes, in an hour or less, in aerosol and tracer concentrations and to have influenced fog formation. Roll vortices are suggested as the cause of periodic changes in aerosol concentrations and fog formation. DOI: 10.1034/j.1600-0889.1996.t01-1-00004.x

Determination of the dry deposition flux of SO2 using cosmogenic 35S and 7Be measurements

The radionuclides 35S (half‐life = 87 days) and 7Be (half‐life = 53 days) are produced in the atmosphere by cosmic rays. The former quickly becomes gaseous 35SO2 while 7Be is directly associated with aerosols. The production rates of both nuclides increase virtually exponentially with increasing altitude. Conversion of 35SO2 to particulate 35SO4 is effected by oxidation after which, like 7Be, it is subject to removal by precipitation in the troposphere. The 35SO2 is also removed from the gaseous state by solution and oxidation in water droplets (clouds) and, at the planetary surface, through “dry deposition” (i.e., oxidation on plant and other surfaces). The kinetics controlling removal of 35SO2 also operate on SO2 in the atmosphere from natural and pollution sources. Through measurements of 35SO2,35SO4, 7Be, SO2 and SO4 in air samples in the boundary layer and the wet precipitation fluxes of the 35SO4, 7Be, and SO4 integrated over monthly intervals, we have established the following parameters at New Haven, Connecticut for 1992. Based on the 7Be balance the average effective height of the troposphere, Ht, varies from 13 km for the winter months to 17 through 21 km for the spring and summer months. The height of the surface reactive “boundary” layer, Hb, is about 0.5 km for all but the month of August. By optimization procedures, best solutions were obtained for scavenging coefficients for both the free troposphere and the surface reactive layer together with the dry deposition coefficient and the mean residence time of air in the surface reactive layer. The highest fraction of dry deposition flux to total S flux occurred in the fall and winter. The weighted average for the entire year is 0.25.

The use of tracer gases to determine dust dispersion patterns and ventilation parameters in a mineral processing plant.

A study was conducted in a fluorspar milling plant to assess the effectiveness of tracer gases as a reliable supplement to conventional air-monitoring and ventilation measurements. In the course of this study, a tracer gas was used as a surrogate substance to analyze the direction and the rate of spread of contaminants from various potential dust production points in the plant. Time-weighted average and continuous mineral dust concentrations were measured in several areas of the plant; these results were compared and correlated with steady-state tracer gas concentrations in the mill. Time-weighted average dust concentrations varied between 0.18 and 0.57 mg/m3 for total dust and 0.04 and 0.20 mg/m3 for quartz respirable dust, depending on the location. Correlation of these values with steady-state tracer gas concentrations yielded linear relationships with correlation coefficients (R2) of 0.95 and 0.87, respectively, for total and quartz dust. Results from this study, therefore, indicate that tracer gases may help model the spread of airborne respirable dust from point sources. These tracer gas releases also allowed the simultaneous quantitative determination of air residence times and contaminant clearance times from the building. Hence, tracer gases will help industrial hygienists obtain useful data with respect to building ventilation.

COMPARISON OF THREE TRACER GASES FOR DETERMINING VENTILATION EFFECTIVENESS AND CAPTURE EFFICIENCY

Sulphur hexafluoride, nitrous oxide and helium were compared as tracer gases for determining the mean residence time of air in a test room and the capture efficiency of a local exhaust hood. The mixtures of tracer gases were fed into the inlet air and two sites in the test room with and without mixing of room air. We determined the capture efficiency by releasing the tracer gases through the cylindrical injection system in front of the exhaust hood at different capture velocities and under various cross-draught conditions. The results of the measurements in the test room showed good agreement between the different tracer gases except in the stagnant zone. The first capture efficiency measurements indicated that SF6, the heaviest gas, gave smaller values obviously due to incomplete mixing of tracer gases. However, the difference disappeared when mixing of the tracer gas was speeded up by pumping extra air or by modifying the injection system.

Lead 210 in surface air and soils from California: Implications for the behavior of trace constituents in the planetary boundary layer

Measurements are reported of 210Pb activities in aerosols collected during the winter of 1985–1986 at Davis, located in California's Central Valley, and of 210Pb and 137Cs inventories in soils collected from the nearby Sierra Nevada mountains and Sierra Nevada‐Central Valley foothills. The results show that Central Valley surface air 210Pb activities increase (up to 98 disintegrations per min (dpm) 1000 m−3 STP) while high‐pressure systems are stationed over the western United States. During these periods the processes that control 210Pb activities in the boundary layer are the emanation of 222Rn from soils and, most likely, the exchange of air between the Central Valley boundary layer and the free troposphere. Model calculations indicate that the mean residence time of air in the Central Valley boundary layer during these periods ranges from 1 day to 4.5 days, depending upon the intensity of the regional high. Furthermore, because 210Pb is not removed efficiently from the boundary layer by other processes, these results indicate that as much as 90% of the 210Pb produced as a result of the decay of 222Rn emanating from Central Valley soils eventually escapes from the Central Valley to be deposited elsewhere, even though the boundary layer is relatively stagnant. Thus it is expected that a similar percentage of other species characterized by gas‐to‐particle conversion rates of about 3 days or longer (e.g., SO4 = from SO2) will also eventually escape from the Central Valley boundary layer to be deposited elsewhere. During the passage of cold fronts, surface air 210Pb activities decrease dramatically (to as low as 6 dpm 1000 m−3 STP). The study of 210Pb and 137Cs inventories in Central Valley‐Sierra Nevada foothill soils and in coastal soils indicates that this decrease is a result of marine air displacing the stagnant air in the Central Valley boundary layer and not a result of local washout of 210Pb. In the Sierra Nevada the ratio of the soil inventories of atmospherically derived 210Pb to 137Cs increases with elevation. This increase indicates that 210Pb derived recently from the continental boundary layer is preferentially deposited at higher elevations in the Sierra Nevada. A simple mass balance calculation indicates that between 28 and 53% of the 210Pb deposited in the Sierra Nevada is derived recently from the continental boundary layer. Together, the surface air and soil studies suggest that a large fraction of this 210Pb was transported from the Central Valley boundary layer during the passage of precipitating fronts.

Enhancement of exposure to radon progeny as a consequence of passive smoking

Among indoor air pollutants, radon and tobacco smoke take dominant positions. Because radon decay products have a relatively short residence time in air, the extent of the equilibrium between radon and its daughter products is linearly proportional to the carcinogenic risk, at least at low exposure levels. The relevant factor is the equilibrium factor F. This paper discusses the enhancement of radon exposure as a result of the presence of particulate matter originating from tobacco smoke. The presence of tobacco smoke provides a mechanism for radon progeny to be attached to inhalable particles and to remain in indoor air for a prolonged time. The results of our study indicate a significant increase in F as a consequence of passive smoking. These modelling efforts are consistent with the experimental data reported previously.

Uranium miner bioassay systems: lead-210 in whiskers.

The relationship of a 210Pb bioassay system utilizing whiskers and the effects of the residence time of uranium mine ventilation air is presented. Inhalations of 210Pb and 222Rn were considered initially to be of minimal effect on the correlations between 210Pb in biological samples and exposures from short-lived radon daughters. However, dependent on the residence time of mine ventilation air, the short-lived daughters can contribute 6–75% of the total 210Pb intake per working level month, 210Pb in the ventilation air 0–94% and 222Rn 0–75%. Leakage of old mine air into the ventilation air streams increases the percentage of 210Pb in the ventilation air at the expense of the other two sources. The percentage of 210Pb atoms per working level month to 218Po atoms per working level month is constant for ventilation air of less than approximately 15-min residence time and for ventilation conditions where a minimum of old mine air blends into the ventilation air streams, 210Pb bioassay systems would overestimate the hazards associated with 218Po atoms under other ventilation conditions.

Lead-212 in the urban boundary layer of New York City.

The radioactive emanation product lead-212 is useful in estimating rates of air exchange within the urban boundary layer. The concentration of lead-212 is negligible in air of oceanic origin as well as in air above continental areas under snow cover. On several days when conditions were such that one of these types of air mass approached New York City, measurements were made which show that the source strength of lead-212 within the city is relatively constant. On two such days vertical profiles of the concentration of lead-212 were measured from the Empire State Building, which served as a sampling tower. From the data of these profiles and a two-layer model of the urban boundary layer, we estimate the vertical eddy diffusivity to be of the order of tens of square meters per second and the residence time of air within the street layer to be of the order of S minutes. These results are consistent with the observed distribution of stable lead and with an independent estimate of the eddy viscosity from a wind profile. Under moderate wind conditions and with a mixing depth of hundreds of meters, virtually all the horizontal transport of lead-212 and other tracers with street-level sources takes place in the advective layer.

Stratospheric circulation studies based on natural and artificial radioactive tracer elements

The nature of large scale stratospheric circulation is studied using the cosmic ray produced isotopes P 32 , Be 7 , S 35 and Na 22 as tracers. Supplementary information obtained from observations of the distribution of the bomb-produced Na22 and radongenic Pb 210 is taken into account. The activities of these tracer elements have been measured in the stratospheric air, up to altitudes of 20 km, during 1960-64. Data are fairly extensive for studying the characteristics of the mean circulation in the stratosphere as well as seasonal changes in patterns of mixing/transport of air in certain regions of the stratosphere. The interpretation of the data on cosmic ray tracers is based on a comparison of their observed activities with the expected production rates due to cosmic rays. For this purpose, the work of Lal & Peters is extended to evaluate the variations in the relative production rates of the isotopes P 32 , Be 7 , S 35 and Na 22 in the atmosphere. These have to be taken into account when isotope data are compared for different altitudes and latitudes in the stratosphere where relative isotope production rates are different because of the markedly different prevailing energy spectrum of nucleons. The analysis allows us to distinguish three zones in the lower stratosphere (below 20 km), well separated from the tropopause, having distinct circulation patterns. These regions are separately well mixed either vertically or horizontally; the mean time of residence of aerosols in these regions differs appreciably too. The most stable region in the stratosphere is found to be 18–20 km region at 0–30° latitude, where apparent residence times are of the order of twenty months. Polar regions are observed to exhibit an enhanced vertical mixing during November-February. Combining these results with the observations of dispersion of bomb-produced Na 22 , which appeared in significant amounts from early 1962 onwards all over the stratosphere, we deduce that in the polar regions, vertical mixing occurs rapidly during November-February so that any activity injected in this region at 20 km or so mixes downwards at the rate of about 1.5 km month −1 . It is concluded that the observed spring peaks in the troposphere are merely the consequence of this phenomena which is triggered in upper levels (above 20 km) of the stratosphere during October-November. The observations of concentrations of Pb 210 in the stratosphere are discussed. The analysis reveals that an appreciable gravitational settling of Pb 210 seems to have occurred, at least during the period over which data were collected, from the stratospheric air between 55°-75° latitude. These observations imply that the residence time of air in this region of the atmosphere is appreciably higher than that deduced from tracers which attach themselves to aerosols. Lastly, the Pb 210 data indicate that appreciable amounts of tropospheric radon presumably enter the equatorial stratosphere; this conelusion rests on the observation that Pb 210 concentrations are higher in this region compared to that in the surrounding air. DOI: 10.1111/j.2153-3490.1966.tb00250.x

The influence of the weather on the concentration of nuclear bomb debris in air

Daily measurements of the concentration of nuclear bomb debris in air in terms of gross β radioactivity have been made by the Norwegian Defense Research Establishment at a number of places in Norway, The weather developments were examined for samples collected during the period July 1, 1957, to June 30, 1959. It was found that low radioactivity was predominantly associated with centers of low pressure, with influx of arctic air, or with rainy weather. High radioactivity was predominantly associated with centers of high pressure, with influx of warm subtropical air, or with marked frontal passages. This indicates that concentrations of nuclear bomb debris become adjusted to weather conditions fairly rapidly. The statistics seem to be satisfactorily explained if the principal removal agent is precipitation and the principal ways of replenishment are vertical downward transport in high-pressure centers and overturning of air in frontal zones. The low concentration in arctic air may well be due to a short residence time of air in the Arctic. It need not reflect a lack of downward flux in this area.