Μm Radius Range Research Articles

Atmospheric Sea Spray Modeling in the North‐East Atlantic Ocean Using Tunnel‐Derived Generation Functions and the SUMOS Cruise Data Set

AbstractThis study contributes to the communal effort to improve understanding of sea spray generation and transport. For the first time, laboratory‐derived sea spray generation functions (SSGFs) are parameterized in the Meso‐NH mesoscale atmospheric model and are field tested. Formulated from the MATE19 laboratory experiments (Bruch et al., 2021, https://doi.org/10.1007/s10546-021-00636-y) the two SSGFs are driven by the upwind component of the wave‐slope variance (herein B21A), or both and the wind friction velocity cubed (herein B21B). In this first attempt to incorporate the SSGFs in Meso‐NH, the simulations are run without a wave model, and the wave‐wind SSGFs are assumed wind‐dependent. Model evaluation is achieved with a new set of sea spray and meteorological measurements acquired over the 0.1–22.75 μm radius range and U10 1–20 m s−1 wind speeds onboard R/V Atalante during the 25 day SUMOS field campaign in the Bay of Biscay. The B21B SSGF offers particularly good sensitivity to a wide range of environmental conditions over the size range, with an average overestimation by a factor 1.5 compared with measurements, well below the deviations reported elsewhere. B21A also performs well for larger droplets at wind speeds above 15 m s−1. Associated with airflow separation and wave breaking, wave‐slope variance allows to represent multiple wave scales and to scale sea spray generation in the laboratory and the field. Using Meso‐NH simulations we find that sea spray may be transported inland and to altitudes well above the marine atmospheric boundary layer.

Cloud droplet growth in shallow cumulus clouds considering 1-D and 3-D thermal radiative effects

Abstract. The effect of 1-D and 3-D thermal radiation on cloud droplet growth in shallow cumulus clouds is investigated using large eddy simulations with size-resolved cloud microphysics. A two-step approach is used for separating microphysical effects from dynamical feedbacks. In step one, an offline parcel model is used to describe the onset of rain. The growth of cloud droplets to raindrops is simulated with bin-resolved microphysics along previously recorded Lagrangian trajectories. It is shown that thermal heating and cooling rates can enhance droplet growth and raindrop production. Droplets grow to larger size bins in the 10–30 µm radius range. The main effect in terms of raindrop production arises from recirculating parcels, where a small number of droplets are exposed to strong thermal cooling at cloud edge. These recirculating parcels, comprising about 6 %–7 % of all parcels investigated, make up 45 % of the rain for the no-radiation simulation and up to 60 % when 3-D radiative effects are considered. The effect of 3-D thermal radiation on rain production is stronger than that of 1-D thermal radiation. Three-dimensional thermal radiation can enhance the rain amount up to 40 % compared to standard droplet growth without radiative effects in this idealized framework. In the second stage, fully coupled large eddy simulations show that dynamical effects are stronger than microphysical effects, as far as the production of rain is concerned. Three-dimensional thermal radiative effects again exceed one-dimensional thermal radiative effects. Small amounts of rain are produced in more clouds (over a larger area of the domain) when thermal radiation is applied to microphysics. The dynamical feedback is shown to be an enhanced cloud circulation with stronger subsiding shells at the cloud edges due to thermal cooling and stronger updraft velocities in the cloud center. It is shown that an evaporation–circulation feedback reduces the amount of rain produced in simulations where 3-D thermal radiation is applied to microphysics and dynamics, in comparison to where 3-D thermal radiation is only applied to dynamics.

Vertical coarse aerosol fluxes in the atmospheric surface layer over the North Polar Waters of the Atlantic

Sea spray emission fluxes were calculated on the basis of vertical gradients of the aerosol concentration data collected with a laser particle counter during four polar scientific cruises carried out in the summers of 2000, 2001, 2002, and 2003. The calculated flux values are the first obtained with a gradient method from data measured on the open ocean; such a method has a much better physical basis than any involving calculating fluxes from concentrations measured at one level only. The sea spray generation function we obtained was compared with other functions from the literature. We show that most literature sea spray generation functions are underestimated for the 1–8 μm radius range, some of them by a factor of 6.

Changes in scavenging of particles by droplets due to weak electrification in clouds

Electrical charges on aerosol particles and droplets modify the droplet–particle collision efficiencies involved in scavenging, and the droplet–droplet and particle–particle collision efficiencies involved in coalescence of droplets and particles, even in only weakly electrified clouds and aerosol layers. This work places electrically enhanced scavenging, and the electrical inhibition of scavenging in the context of the microphysics of weakly electrified clouds. Collision efficiencies are calculated by numerical integration to obtain particle trajectories, that are determined by the complex interplay of electrical, gravitational and phoretic forces together with inertia. These modify the trajectory of a particle as it is carried by flow around the falling droplet. Conversely, the flow around the particle also modifies the trajectory of the droplet. The flows are specified analytically, using a hybrid of the Proudman–Pearson stream function for that region close to the droplet or particle, where it is accurate, merging into the exact Oseen stream function for larger distances, where that becomes accurate. The effect of the flow around the particle on the motion of the droplet was simulated using Langmuir's superposition technique on the hybrid stream functions. The treatment of inertia in the present calculations allows an extension of the scope of our previous work by a factor of 10 larger in particle size (10 3 in mass). The coverage is extended to a wide range of atmospheric conditions and particle densities. The pressures and temperatures used in the models ranged from a representation of the lower troposphere at ∼ 1 km altitude (900 hPa, 10 °C) to that of the middle stratosphere at ∼ 30 km altitude (12 hPa, − 47 °C). The particles considered range from 0.1 μm to 10 μm radius; the droplet radii range from 4 μm to 50 μm; particle densities range from 300 kg m − 3 to 2500 kg m − 3; particle charges range from 2e to 100e with droplet charges of like sign of 100e; and relative humidities range from 10% to 100%. For the larger particles (radii greater than about 3 μm) interacting with the larger droplets (radii greater than about 15 μm) the effects of inertia increase with particle density and dominate at the larger densities. For particles with radii in the range 1–3 μm the ‘Greenfield Gap’ of very low collision efficiencies was found, and was determined to be due to the effects of the gravitational force causing a reduction of collisions of particles with the front of the droplet, and the effect of inertia overcoming the tendency for the weight to produce a collision in the slow velocity region in the rear. When the electrical or phoretic forces are sufficiently large the Greenfield Gap is closed. When the particles have radii < 3 μm inertial effects no longer dominate the collisions, although inertia modifies the weight effects for particles with radii down to about 0.5 μm. For charged aerosol particles with radii smaller than about 0.1 μm interacting with droplets or background aerosol particles smaller than a radius of about 15 μm, the long range electrical repulsive force is effective in opposing the phoretic forces and keeping the particle out of range of the short range attractive image force. Thus ‘electroscavenging’ gives way to ‘electroprotection’ against the scavenging or coagulation processes otherwise caused by Browninan diffusion or phoretic forces. In an atmosphere of temperature 10 °C and pressure 900 hPa the net phoretic force reduces to zero and becomes repulsive for particles with radii above about 2 μm (depending on particle conductivity). This enhances the development of the Greenfield Gap. However, the value of this radius (at which the net phoretic force is zero) increases strongly with decreasing temperature and pressure (increasing altitude) as expected from theory, and is about 5 μm in the middle troposphere and more than 10 μm in the stratosphere. Thus a net attractive phoretic force on particles extends into the 1–3 μm radius range in the upper troposphere; however, the weight and inertial effects can ensure the presence of the Greenfield Gap in that range for 2000 kg m − 3 particles up to the middle stratosphere.

The mid-infrared spectrum of the zodiacal and exozodiacal light

The zodiacal light is the dominant source of the mid-infrared sky brightness seen from Earth, and exozodiacal light is the dominant emission from planetary and debris systems around other stars. We observed the zodiacal light spectrum with the mid-infrared camera ISOCAM over the wavelength range 5–16 μm and a wide range of orientations relative to the Sun (solar elongations 68°–113°) and the ecliptic (plane to pole). The temperature in the ecliptic ranged from 269 K at solar elongation 68° to 244 K at 113°, and the polar temperature, characteristic of dust 1 AU from the Sun, is 274 K. The observed temperature is exactly as expected for large (>10 μm radius), low-albedo (<0.08), rapidly-rotating, gray particles 1 AU from the Sun. Smaller particles (<10 μm radius) radiate inefficiently in the infrared and are warmer than observed. We present theoretical models for a wide range of particle size distributions and compositions; it is evident that the zodiacal light is produced by particles in the 10–100 μm radius range. In addition to the continuum, we detect a weak excess in the 9–11 μm range, with an amplitude of 6% of the continuum. The shape of the feature can be matched by a mixture of silicates: amorphous forsterite/olivine provides most of the continuum and some of the 9–11 μm silicate feature, dirty crystalline olivine provides the red wing of the silicate feature (and a bump at 11.35 μm), and a hydrous silicate (montmorillonite) provides the blue wing of the silicate feature. The presence of hydrous silicate suggests the parent bodies of those particles were formed in the inner solar nebula. Large particles dominate the size distribution, but at least some small particles (radii ∼1 μm) are required to produce the silicate emission feature. The strength of the feature may vary spatially, with the strongest features being at the lowest solar elongations as well as at high ecliptic latitudes; if confirmed, this would imply that the dust properties change such that dust further from the Sun has a weaker silicate feature. To compare the properties of zodiacal dust to dust around other main sequence stars, we reanalyzed the exozodiacal light spectrum for β Pic to derive the shape of its silicate feature. The zodiacal and exozodiacal spectra are very different. The exozodiacal spectra are dominated by cold dust, with emission peaking in the far-infrared, while the zodiacal spectrum peaks around 20 μm. We removed the debris disk continuum from the spectra by fitting a blackbody with a different temperature for each aperture (ranging from 3.7″ to 27″); the resulting silicate spectra for β Pic are identical for all apertures, indicating that the silicate feature arises close to the star. The shape of the silicate feature from β Pic is nearly identical to that derived from the ISO spectrum of 51 Oph; both exozodiacal features are very different from that of the zodiacal light. The exozodiacal features are roughly triangular, peaking at 10.3 μm, while the zodiacal feature is more boxy, indicating a different mineralogy.

Comparison of aerosol size distributions, radiative properties, and optical depths determined by aircraft observations and Sun photometers during SAFARI 2000

The Meteorological Office C‐130 aircraft performed a dedicated flight over the Etosha Pan surface‐based Aerosol Robotic Network (AERONET) Sun photometer site on 13 September 2000 during the Southern African Aerosol Regional Science Initiative (SAFARI 2000) intensive measurement campaign. Aerosol optical depths at different wavelengths, τaerλ, are derived from in situ measurements of the scattering and absorption coefficients and from various radiometric measurements and compared to those derived from the Sun photometer site. The estimates of τaerλ from the various measurements are shown to be in good agreement. The exception to this is when τaerλ is derived from the Passive Cavity Aerosol Spectrometer Probe (PCASP), as this method is shown to be extremely sensitive to the pitch angle of the aircraft; therefore, τaerλ differs for profile ascents and profile descents. However, the aerosol size distribution measured by the PCASP and derived from the AERONET site are in excellent agreement over the 0.05–1.0 μm radius range, which contains the majority of the optically active particles. C‐130‐derived refractive indices and single scattering albedos are also shown to be in excellent agreement with those derived from the AERONET site. The consistency between in situ and remotely sensed data suggests that, for aerosol well mixed in the vertical, data from AERONET may be used with confidence in validating satellite measurements and modeling studies of the radiative properties and effects of aerosols.

Parameterization of drop effective radius for drizzling marine stratocumulus

We describe a parameterization of cloud drop effective radius Re developed for drizzling marine stratocumulus clouds. The parameterization was derived using data from the CIMMS LES model with explicit formulation of drop size resolving microphysics. We showed that in the case of moderate drizzle an accurate parameterization of Re in the 8 ÷ 20 μm radius range can be obtained on the basis of two microphysical variables: liquid water content and drop concentration. For heavy drizzle, only a three‐variable parameterization proves to be sufficiently accurate. We developed two versions of the parameterization. The first one is based on traditional cloud microphysical prognostic variables representing partial moments of the drop distribution function: cloud water content, drizzle water content, and cloud drop concentration. The second one is based on the full moments of the drop size distribution function: drop concentration, total liquid water content, and radar reflectivity. The first version of the parameterization is rather sensitive to the value of the threshold radius that divides cloud and drizzle water. As a result, the errors are larger than in the second full moment version. It is concluded that a three‐variable full moment parameterization provides the most accurate representation of effective radius in precipitating stratocumulus cloud layers.

Retrieval of columnar aerosol size distributions and radiative-forcing evaluations from sun-photometric measurements taken during the CLEARCOLUMN (ACE 2) experiment

Spectral measurements of direct solar irradiance were taken within 13 narrow-band channels in the 401–3676 nm wavelength range, using the IR–RAD sun-radiometer at the Sagres station in southern Portugal during the CLEARCOLUMN (ACE 2) experiment, from June 16 to 25 July 1997. The measurements performed on 21 clear-sky days were examined in terms of the Bouguer–Lambert–Beer law, following realistic correction procedures for Rayleigh scattering and absorption by minor atmospheric gases, to determine more than 2100 spectral series of aerosol optical depth. All these spectral series were examined with the King inversion method to retrieve the columnar aerosol particle size distributions over the 0.07–10 μm radius range, for values of the real part of the particulate refractive index ranging between 1.43 and 1.50 and values of the imaginary part between 0.003 and 0.010. Using the well-known 6S computer code for all the columnar aerosol size-distribution curves determined on ten “golden days” and both refractive index parts varying with wavelength, evaluations of the change Δ F↑ caused by aerosol particles in the outgoing solar radiation flux were made at solar zenith angles θ ranging between 15 and 76° and for spectral albedo features of both clear water and green vegetation surfaces. The results show that the radiative forcing Δ F↑ assumes positive values (associated with cooling effects) for the clear water surface on all the measurement days and at all the solar zenith angles, and mostly negative values (warming effects) for the green vegetation surface in the range θ<60°. The present evaluations also give clear evidence of the close dependence of Δ F↑ not only on the surface albedo, solar zenith angle and aerosol optical depth but also on the mean single scattering albedo of the columnar aerosols.

Interannual variation of ambient aerosol characteristics on the east coast of the United States

We investigate the interannual variability of the seasonally recurring aerosol haze of the United States East Coast. The data consist of observations collected by five Sun/sky radiometer stations. We compare the data collected in 1996 in conjunction with the TARFOX experiment to similar data collected at the same locations in 1993. The regional mean optical thickness remains essentially constant in the two years. The Mount Pinatubo stratospheric mode decreases spectral dependence in 1993, especially for optical thickness below 0.20. The volume size distributions inverted from the sky measurements show an increase in accumulation mode particle size with increasing aerosol optical thickness in both years. At moderate to high optical thickness (τ670 &gt; 0.20) the particles in the fine particle mode, 0.1–0.6 μm radius range, from both years have the same lognormal parameters (rm = 0.21, σ = 0.45–0.50). The volume of these particles increases linearly with τ. These size particles dominate the optical properties of the aerosol in the visible spectrum and cause remarkable interannual consistency in the aerosol optical characteristics. The 1993 and 1996 phase functions agree to within 7–10% for optical thicknesses greater than 0.2.

Predictions of CMA utilization for in-situ SO 2 removal in utility boilers

Highly porous grains of solid CMA have been generated by Levendis (1990). Since CMA is water soluble, various CMA/H 2O solutions may be atomized to various size droplets and subjected to various temperature histories to achieve sorbents with virtually any size and porosity. To assist in determining the optimum size and porosity, the studies of Simons (1988) were extended. Particles in the 5 to 10 μm radius range with porosities (θ) greater than 90% clearly yield utilizations which are favorable to in-situ SO 2 removal. These particles are considerably larger than those required for limestone (1 to 3 μm) injection and are readily achievable with high pressure liquid atomizers.

An experimental investigation on the collision behaviour of hydrocarbon droplets

The collisional dynamics of equal-sized water and normal-alkane droplets, in the 150 μm radius range, have been experimentally studied for situations involving 0(1) droplet Weber numbers and head-on to grazing impact parameters. Results show that in the parametric range investigated the behaviour of hydrocarbon droplets is significantly more complex than that of water droplets. For head-on collisions, while permanent coalescence always results for water droplets, the outcome is quite nonmonotonic for the hydrocarbon droplets in that, with increasing droplet Weber number, the collision can result in permanent coalescence, bouncing, permanent coalescence again, and coalescence followed by separation with or without production of satellite droplets. Similar complexities exist for off-centre collisions. Phenomenological explanations are offered for these observations based on the material properties of the fluids, the relative influences of the normal and shearing aspects of the collision, and the nature and extent of energy dissipation due to droplet deformation during collision.

Effect of nonprecipitating clouds on the aerosol size distribution in the marine boundary layer

The size distribution of particles smaller than 0.5 µm has been measured over the tropical Atlantic and Pacific oceans with a differential mobility analyzer. In regions remote from continental influences the size distribution generally has peaks at about .02‐.03 µm and at .08‐.15 µm with a minimum in the .05‐.08 µm radius range. The data provides strong evidence that nonprecipitating clouds play an important role in transferring material from the gas phase and from smaller particles into the 0.08 to .15 µm radius range, and that they are responsible for the doubly peaked size distributions frequently observed over the oceans.

The number concentration and size distribution of the soot particles in the 0.02–0.5 μm radius range at sites of different pollution levels

The concentration and size distribution of soot particles were determined in the 0.02–0.5 μm radius range on the basis of morphological identification at the following sites: suburban Budapest, a rural station in Hungary and from a ship on the South Atlantic Ocean. The average concentration values obtained are 1500, 260, 4.8 (tropical zone) and 0.34 cm −3 ( φ>40° S latitude), respectively. The size distributions can be approximated by logarithmic normal distribution and do not differ significantly from each other. The ratio of the concentration of internally mixed particles to the total soot concentration varies with the particle size and sampling site.

Annual variation of the surface and volume concentration of the aerosol particles in rural air

The surface and volume concentrations of aerosol particles have been calculated from number size distributions obtained by microscopic evaluation of membrane filter samples in the 0.02–50 μm radius range. The annual variations of the monthly averages are presented. The values of the linear correlation coefficients indicate a weak relationship between the individual surface and volume concentrations for the whole size range considered (except for the winter months) and a very strong correlation in the fine size range. The average ratio of the surface concentration of the sulfate particles to the total surface is equal to 36 and 60% in winter and summer, respectively.

A stratospheric aerosol increase

Large disturbances were noted in the stratospheric aerosol content in the midlatitude northern hemisphere commencing about 7 months after the eruption of La Soufriere and less than 1 month after the eruption of Sierra Negra. The aerosol was characterized by a very steep size distribution in the 0.15 to 0.25 µm radius range and contained a volatile component. Measurements near the equator and at the South Pole indicate that the disturbance was widespread. These observations were made before the May 18 eruption of Mt. St. Helens.

Response characteristics of knollenberg light-scattering aerosol counters

Response calculations are presented for four commercially available “Knollenberg” light-scattering aerosol counters: the classical scattering aerosol spectrometer probe (CSASP), the active scattering aerosol spectrometer probe (ASASP), the forward scattering spectrometer probe (FSSP) and the axially scattering spectrometer probe (ASSP). The results show sensitivity of the response to aerosol refractive index for values of indexes characteristic of atmospheric aerosols, and for a particular index, multivalued response for particles in the 0.5–4 μm radius range. The response calculations have been validated for two of these instruments (the CSASP and ASASP) by measurement of monodisperse spherical particles. The size resolution of these two instruments is significantly less than advertised by the manufacturer and measurement of irregular particles causes additional loss of resolution.

On the concentration and size distribution of atmospheric sulfate particles under rural conditions

The concentration and size distribution of ammonium sulfate particles were studied at a rural sampling site by using morphological identification. The size distribution of pure and mixed ammonium sulfate particles as well as the sulfate fraction of total aerosol and its size dependence were determined in the 0.02–0.3 μm radius range. The averaged results are presented for the summer and winter months.

Aerosol particle size distributions in the 0.25–5.0 μm radius range in Northern England

An analysis has been made of continuous measurements on the particle size distribution of the atmospheric aerosol in five particle radius categories between 0.25 and 5.0 μm. An automatic Royco model 225 optical particle counter together with a digital line printer was used to obtain the measurements at a 10 min sampling frequency from 21 July to 8 September 1975 at Durham Observatory, 2 km SSW of Durham City in the North East of England. Measurements were also taken every 10 min from 15 December to 6 January 1976 on the summit of Great Dun Fell, 842 msl, on the Northern Pennine Range, England. Diurnal variation associated with the particle number concentration shows maximum concentration from about 02:00–08:00 h BST and a minimum over the period 14:00–20:00 h BST. The particle size distribution for both the Durham and Great Dun Fell site follows the shape of a Junge log-radius type distribution with slopes β equal to 3.04 and 2.74 respectively. The distributions show that the number concentration over the five radius intervals is an order of magnitude lower at the mountain site of Great Dun Fell than at Durham Observatory. An analysis of the sampling frequency of particle concentrations showed that the measurements could be made less frequently by factors up to 20, without loss of accurate information.

The Life Cycle of Valley Fog. Part II: Fog Microphysics

Extensive measurements were made of the microphysics of valley fog in the Chemung River Valley near Elmira, New York. This paper discusses data on drop size distributions, drop concentrations, liquid water contents, and haze and cloud nucleus concentrations obtained on eight fog nights. The behavior patterns of the microphysical variables were found to be extremely consistent. Shallow ground fog usually occurs prior to the formation of deep valley fog. The data show that ground fog is characterized by droplet concentrations of 100 to 200 per cubic centimeter in the 1 to 10 μm radius range with mean radii of 2 to 4 μm. As deep fog forms aloft, droplet concentration near the surface decreases to less than 2 cm−3 and the mean radius increases from 6 to 12 μm. Droplets of radii <3 μm disappear. Thereafter, droplet concentration and liquid water content increase gradually until the first visibility minimum at the surface when typical values range from 12 to 25 cm−3 and 50 to 150 mg cm−3, respectively. The small droplets reappear at first visibility minimum. Subsequently, bimodal drop size distributions occur in approximately half of the fogs with one mode at 2–3 μm radius and a second mode between 6 and 12 μm. Aloft, drop size distributions become narrower and the mean radius decreases with both increasing altitude and increasing age of the fog. The cloud nucleus concentration active at S = 3.0% is usually between 800 and 1000 cm−3 near the surface and decreases to 500–800 cm−3 at 300 m. It is argued from the data that supersaturation in the thin ground fog exceeds that in deep fog. The initial surface obscuration in deep fog appears to be due to droplets that form aloft and are transported downward into unsaturated air by turbulent diffusion. New droplets are apparently not generated near the surface until after the first visibility minimum.

Size distribution of atmospheric giant particles

The size distribution of atmospheric giant particles in the range 5–100 μm radius was measured at three locations in Washington State using a rotary inertial impactor. The size distribution was continuous at the different locations. A significant change in the slope of the dN dlog r vs. r plot of the size distribution generally occurred between 30 and 40 μm radius. The slope B ( dN dlog r = Ar −B ) in the 5–25 μm radius range averaged 2.3 while in the 40 to 100 μm radius range B averaged 6.7. A complete atmospheric aerosol size distribution for the 0.01–100 μm radius range was obtained by combining thermal precipitator (0.01–1 μm) and inertial impaction (5–100 μm) data. It followed from the calculation of the atmospheric aerosol mass distribution (assuming spherical particles of unit density) that approximately 20 per cent of the total mass was in particles of less than 1 μm radius, 35 per cent in the 1–10μm range, 45 per cent in the 10–50 μm range, and less than 1 per cent in the greater than 50 μm radius range.