New aerosol particle formation via certain ion driven processes

Ions can speed up the formation of aerosol particles. The former studies have mainly concerned on the role of the ion charge itself. We have studied the possible (additional) role of the actual small air ion spectrum shape, and the quantitative role of ion–ion recombination pathway. By means of our ion evolution model, formation of new species (H₂SO₄)_n(NH₃)_m(HNO₃)_k via ion–ion recombination was investigated. The model shows how the generation rate of the new species depends on the concentrations of H₂SO₄ and NH₃, and how it depends on the tropospheric background aerosol situation. The rate can be up to a few new neutral complexes per cubic centimeter and per second. New particle generation via ion–ion recombination provides an extra channel, especially for the clean atmosphere. Former results have shown that such situations are often present in Antarctica. Our aerosol spectrum measurements reveal a number of similar non-Antarctic results. Sometimes, such situations are followed by aerosol bursts, which may be (partly) due to an ion–ion recombination channel.     

New aerosol particle formation via certain ion driven processes

Ions can speed up the formation of aerosol particles. The former studies have mainly concerned on the role of the ion charge itself. We have studied the possible (additional) role of the actual small air ion spectrum shape, and the quantitative role of ion–ion recombination pathway. By means of our ion evolution model, formation of new species (H₂SO₄)_n(NH₃)_m(HNO₃)_k via ion–ion recombination was investigated. The model shows how the generation rate of the new species depends on the concentrations of H₂SO₄ and NH₃, and how it depends on the tropospheric background aerosol situation. The rate can be up to a few new neutral complexes per cubic centimeter and per second. New particle generation via ion–ion recombination provides an extra channel, especially for the clean atmosphere. Former results have shown that such situations are often present in Antarctica. Our aerosol spectrum measurements reveal a number of similar non-Antarctic results. Sometimes, such situations are followed by aerosol bursts, which may be (partly) due to an ion–ion recombination channel.     

210Po in savanna burning plumes

During the FOS-DECAFE experiment at Lamto (Ivory Coast) in January 1991 aerosols samples were collected at ground level above fires in order to investigate the possibility of using²¹⁰Po as a tracer of biomass burning. The concentration of this radionuclide in plants is studied as a function of its content in soils and in the atmospheric background. It is shown that it depends strongly on the atmospheric content in²¹⁰Po, due to dry deposition of the aerosols. The mean concentration of plants at Lamto is found to be about 4.4 pCi of²¹⁰Po/gC during the fire season and falls down to less than 1pCi/gC outside this period. The budget of²¹⁰Po is evaluated taking into account its complete volatilization during the flaming phase, the (²¹⁰Po)_ash/(²¹⁰Po)_plants ratio, which is measured to be about 14% and the percentage of submicron particles in the plume, about 91%. The inferred flux of²¹⁰Po is 3850 Ci/yr for the African savanna, and 5800 Ci/yr for the global savanna. From this flux, fluxes of Ct and Cs are estimated to be 8.4 and 1.1 Tg of C/yr for the worldwide savanna.     

Macro- and micro-scale mixing in chemical reactive plumes

The average dispersion of a plume in the atmospheric boundary layer is strongly influenced by atmospheric turbulence. Atmospheric turbulence determines also concentration fluctuations due to turbulent meandering by large scale turbulent eddies and in-plume fluctuations, due to smaller scale eddies. Conversion of NO to NO₂ in a plume is influenced by micro-scale mixing, due to the concentration fluctuation correlation % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaaca% qGobGaae4tamaaCaaaleqabaGaaeymaaaakiaab+eadaqhaaWcbaGa% ae4maaqaaiaabgdaaaaaaaaa!3AF4!\[\overline {{\rm{NO}}^{\rm{1}} {\rm{O}}_{\rm{3}}^{\rm{1}} } \] and macro-scale mixing, the mixing in of ambient air containing O₃ into the plume.The study of turbulent meandering, in-plume fluctuations, microscale and macro-scale mixing will contribute to a better understanding of concentration fluctuations in general.     

Lidar evidence of thermal plumes in an urban environment

A laser radar (lidar) operating at the ruby line 0.6943 μm has been observing horizontal profiles of particulate concentration in the air some 30 m above the surface within the city of London over a period of one year. Anomalous features in these profiles are interpreted as natural thermal plumes carrying particles upwards from surfaces such as paved roadways, paved parking lots, treed parks, residential construction sites, open ploughed fields, and the river. Insolation of the ground is required for their formation, and neutral thermal stability aloft favours their incidence.     

The vertical profile of concentration fluctuations in near-surface plumes

Field experiments on concentration fluctuations have frequently measured horizontal cross-sections of fluctuation statistics through plumes at fixed heights near the surface, but have not considered the effect of height above the ground in any detail. A set of tracer experiments designed to measure vertical profiles of concentration fluctuations in plumes, with a range of source heights, is described, and profiles of statistics are presented. Considerable variation of the statistics with both source and detector height is found. Near the surface, fluctuation intensity is a minimum and the time and length scales of the fluctuations are greatly increased. Profiles are consistent with the idea that concentration fluctuations near the surface are like those higher up at a greater distance from the source. Lowering the source height reduces the fluctuation intensity at all heights, and also alters the form of the concentration PDF. Results may be explained by the reduced length scale of sheargenerated turbulence near the surface causing enhanced small-scale mixing, which rapidly smooths out much of the fine structure with the plume.     

The effects of source size on concentration fluctuations in plumes

The effects of source size on plume behaviour have been examined in a 1.2 m wind tunnel boundary layer for isokinetic sources with diameters from 3 to 35 mm at source heights of 230 mm and at ground level. Experimental measurements of mean concentration and the variance, intermittency and probability density functions of the concentration fluctuations were obtained. In addition, a fluctuating Gaussian plume model is presented which reproduces many of the observed features of the elevated emission. The mean plume width becomes independent of source size much more rapidly than the instantaneous plume width. Since it is the meandering of the instantaneous plume which generates most of the concentration fluctuations near the source, these are also dependent on source size. The flux of variance in the plume reaches a maximum, whose value is greatest for the smallest source size, close to the source and thereafter is monotonically decreasing. The intermittency factor reaches a minimum, whose value is lowest for the smallest source, and increases back towards one. Concentration fluctuations for the ground-level source are much less dependent on source size due to the effects of the surface.     

The rise of forced plumes in a stably stratified crossflow

Solutions are found describing the rise of turbulent buoyant plumes in a neutral or stably stratified crossflow from a source having finite fluxes of buoyancy, momentum and volume. Plumes from such sources are known as forced plumes. The solutions arise from a unified theory having one disposable parameter, the entrainment constant, and they describe a wide range of behaviour of plumes and jets in a crossflow. Solutions for buoyant plumes indicate that, for any given source, heights of rise are reduced with increased crossflow velocity and increased stratification while for constant environmental parameters, heights of rise are increased with increased fluxes of source buoyancy and momentum and are decreased with source radius. Maximum dilutions occur in plumes from small sources with relatively large buoyancy fluxes in light crossflows with small stratification.     

Income of metals to forest ecosystems with dry aerosol sinks: Influence of weather-forming processes and meteorological factors

Seasonal changes in metal concentrations in acid- and water-soluble fractions of dry aerosols entering forest ecosystems within key sites adjoining the Syktyvkar forest industrial complex are analyzed. Informativity of quantitative characteristics of aerotechnogenic pollution from the ratio of metals in the acid- and water-soluble fractions in the dry aerosol sink and in the surface washout from the coniferous stand is estimated. The influence of weather-forming processes and meteorological factors on the distribution of metals in the surface air layer and vegetation is discussed.     

Thermal plumes and turbulence spectra in the atmospheric boundary layer

Experimental observations on the temperature and wind fields above flat grassy terrain have been obtained with an instrumented 92-m tower during intervals of strong insolation about midday. The turbulence characteristics of the air confirm that free convection prevailed at heights between 16 and 48 m, with some tendency for departure at higher levels. The spectra of temperature and vertical velocity contain gaps at wave numbers in the range 0.01–0.025 m^–1. These are attributed to natural thermal plumes that act as sources of extra energy input to the Kolmogorov-Obukhov-Corrsin scheme of turbulence in or at the low-wave number limit of the inertial subrange. Modified forms of the K-O-C spectral laws for thermally unstable air are derived which agree with the observed spectra over the whole range of wave numbers examined, and which contain the spectral gap at wave numbers corresponding to the thermal plume diameters.     

Vertical profiling of convective dust plumes in southern Morocco during SAMUM

Lifting of dust particles by dust devils and convective plumes may significantly contribute to the global mineral dust budget. During the Saharan Mineral Dust Experiment (SAMUM) in May–June 2006 vertical profiling of dusty plumes was performed for the first time. Polarization lidar observations taken at Ouarzazate (30.9°N, 6.9°W, 1133 m height above sea level) are analyzed. Two cases with typical and vigorous formation of convective plumes and statistical results of 5 d are discussed. The majority of observed convective plumes have diameters on order of 100–400 m. Most of the plumes (typically 50–95%) show top heights <1 km or 0.3DLH with the Saharan dust layer height DLH of typically 3–4 km. Height-to-diameter ratio is mostly 2–10. Maximum plume top height ranges from 1.1 to 2.9 km on the 5 d. 5–26 isolated plumes and clusters of plumes per hour were detected. A low dust optical depth (<0.3) favours plume evolution. Observed surface, 1 and 2–m air temperatures indicate that a difference of 17–20 K between surface and 2-m air temperature and of 0.9–1 K between the 1 and 2-m temperatures are required before convective plumes develop. Favourable horizontal wind speeds are 2–7 m s⁻¹.     

Measurements of bubble plumes and turbulence from a submarine

Abstract

An experiment using turbulence probes and an array of side‐scan and vertically pointing pencil beam sonars mounted on the U.S. submarine Dolphin was carried out to measure turbulence in near‐surface regions of acoustic scattering, in particular, those caused by subsurface bubbles produced by breaking wind waves. The dataset collected during winds of 5–9 m s^?1 reveals the banded patterns of bubbles associated with Langmuir circulation, even though no surface manifestations were visible.

A forward‐pointing side‐scan sonar determined the “age” of bubble clouds after their generation by breaking waves. There is enhanced turbulent dissipation in the bubble clouds, and the dissipation rate close to the surface exceeds that predicted using conventional calculations based on the law of the wall and buoyancy flux. The correspondence between bubbles and turbulence implies a horizontally patchy turbulent structure near the surface. Below the base of the bubble clouds the distance between turbulent patches increases and is much greater than that of the bubble clouds. The submarine provides an excellent platform for multi‐sonar near‐surface studies.     

Application of optical techniques to the study of plumes in stratified fluids

Digital techniques for the correction of signal distortions that arise in planar laser-induced fluorescence (PLIF) measurements (by PC-based video digitizing systems) of jets and plumes and the production of flow statistics in both time and space are reviewed. The entire concentration field is repeatedly imaged in s intervals over hundreds of thousands a points in a plane. By the recognition of signal distortion sources and the employment of corrections, a clearer picture of tracer concentrations may be realized.Fluorescence studies are made with a planar sheet of laser light 430 mm tall and 1.5 mm thick. The fluorescence excitation produced from trace concentrations of Rhodamine 6G is used to visualize and measure the propagation of a jet or plume in a density stratified laboratory tank. The emitted light is collected by a CCD camera in a 512 × 480 pixel format over a 940 × 715 mm field of view. The captured images are corrected for transverse laser sheet intensity distribution; laser beam attenuation; refraction; lens vignette; time varying and spatial noise; digitization aspect ratio; camera response. The measurement and methods of correction are discussed in detail. The resulting image data can then be used to collect tracer concentration statistics for jets and plumes. Instantaneous (i.e. over of a second intervals), average, maximum, minimum, standard deviation, and coefficient of variation are given as introductory examples of image statistics realizable for a buoyant jet.     

Enhancement and suppression of urban heat plumes over Johannesburg

Observations of the atmospheric temperature structure over Johannesburg have revealed the existence of both well-developed and suppressed heat plumes. Suppressed plumes appear to be associated with the incidence in a stable atmosphere of positive wind shear and well-developed katabatic flow away from the city. A localised cool region in the atmosphere above the central city is invariably associated with suppressed plume development. A tentative identification of wind shear and katabatic flow as the major influences on such structures is strengthened by the observation that during conditions of negative windshear and reduced katabatic advection, significant heat plumes are developed. It is at present not possible to assess the extent to which the observations and causative processes are site-specific.Visiting from Bar-Ilan University, Israel.     

The effects of mixing and spreading on density in near-field river plumes

Water mass modification in surface-trapped, near-field river plumes is examined using a 1.5-layer reduced gravity model and a three-dimensional numerical model. Solutions to the layer model are shown to be qualitatively similar to previous observations and three-dimensional simulations of near-field plumes. Analytic analysis of the layer model demonstrates how the near-field plume is controlled by the competing processes of mixing and spreading. The two models are then used to explore the parameter space dependence of density changes within the near-field plume and their associated cross-shore length scales. Both the magnitude of density changes and their length scales are proportional to either estuarine discharge or fresh water discharge; density changes are also inversely proportional to the estuary mouth width. One surprising feature of the parameter space solutions is that the density of water exiting the near-field plume, a measure of the net dilution of the entire near-field plume, is shown to be inversely proportional to local mixing rates. This is because when local mixing is lower, the influence of plume spreading becomes greater; this spreading accelerates the plume, requiring more net mixing to bring the plume back to subcritical flow.     

Conditional concentration statistics for surface plumes in the atmospheric boundary layer

A set of concentration time series from ground-level plumes in the atmosphere has been used to generate conditionally sampled (zeros ignored) plume concentration statistics. These have been compared and contrasted with corresponding unconditionally sampled statistics. It is found that conditional statistics are much less sensitive to the location of the receptor (relative to the mean plume) and to averaging time. Indeed, most of the variation apparent in unconditionally sampled statistics (both explained and unexplained) resides in the intermittency, the fraction of non-zero readings.The data are used to test three commonly used models for the concentration frequency distribution. At the simplest level of modelling, it is assumed that conditional statistics are invariant; then the data are best represented by a clipped-normal distribution. However, an exponential distribution is only slightly conservative and has the advantage of simplicity. A log-normal distribution is clearly not supported by the data. With this simple approach the intermittency remains unspecified and this is a serious deficiency.More advanced modelling must account for the residual variation in conditional statistics, which implies a relationship between these statistics and the intermittency. Although there is evidence for such a relationship in the data, it is not adequately represented by any of the distribution models considered.     

The exponential probability density function and concentration fluctuations in smoke plumes

Observations of 1-s average concentration fluctuations during two trials of a U.S. Army diffusion experiment are presented and compared with model predictions based on an exponential probability density function (pdf). The source is near the surface and concentration monitors are on lines about 30 to 100 m downwind of the source. The observed ratio of the standard deviation to the mean of the concentration fluctuations is about 1.3 on the mean plume axis and 4 to 5 on the mean plume edges. Plume intermittency (fraction of non-zero readings) is about 50%; on the mean plume axis and 10%; on the mean plume edges. A meandering plume model is combined with an exponential pdf assumption to produce predictions of the intermittency and the standard deviation of the concentration fluctuations that are within 20%; of the observations.