A model for particle formation and growth in the atmosphere with molecular resolution in size

International audienceThe formation and growth of atmospheric aerosol particles is considered using an exact discrete method with molecular resolution in size space. The method is immune to numerical diffusion problems that are a nuisance for typical simulation methods using a sectional representation for the particle size distribution. For condensational growth, a slight modification is proposed for the Fuchs-Sutugin expression, which improves the prediction of the growth rate of nano-sized particles by as much as a factor of two. The presented method is applied to particle formation in a Finnish Boreal forest and is shown to capture the essential features of the dynamics quite nicely. Furthermore, it is shown that the growth of the particles is roughly linear, which means that the amount of condensable vapour is constant (of the order 1013 1/m3)

C18O (3-2) observations of the Cometary Globule CG 12: a cold core and a C18O hot spot

The feasibility of observing the C18O (3-2) spectral line in cold clouds with the APEX telescope has been tested. As the line at 329.330 GHz lies in the wing of a strong atmospheric H2O absorption it can be observed only at high altitude observatories. Using the three lowest rotational levels instead of only two helps to narrow down the physical properties of dark clouds and globules. The centres of two C18O maxima in the high latitude low mass star forming region CG 12 were mapped in C18O (3-2) and the data were analyzed together with spectral line data from the SEST. The T_MB(3-2)/T_MB(2-1) ratio in the northern C18O maximum, CG 12 N, is 0.8, and in the southern maximum, CG 12 S, ~2. CG 12 N is modelled as a 120'' diameter (0.4pc) cold core with a mass of 27 Msun. A small size maximum with a narrow, 0.8 kms-1, C18O (3-2) spectral line with a peak temperature of T_MB ~11 K was detected in CG 12 S. This maximum is modelled as a 60'' to 80'' diameter (~0.2pc) hot (80 K < Tex < 200 K) ~1.6 Msun clump. The source lies on the axis of a highly collimated bipolar molecular outflow near its driving source. This is the first detection of such a compact, warm object in a low mass star forming region.Comment: APEX A&A special issue, accepte

Multicomponent aerosol dynamics model UHMA: model development and validation

A size-segregated aerosol dynamics model UHMA (University of Helsinki Multicomponent Aerosol model) was developed for studies of multicomponent tropospheric aerosol particles. The model includes major aerosol microphysical processes in the atmosphere with a focus on new particle formation and growth; thus it incorporates particle coagulation and multicomponent condensation, applying a revised treatment of condensation flux onto free molecular regime particles and the activation of nanosized clusters by organic vapours (Nano-K&#246;hler theory), as well as recent parameterizations for binary H₂SO₄-H₂O and ternary H₂SO₄-NH₃-H₂O homogeneous nucleation and dry deposition. The representation of particle size distribution can be chosen from three sectional methods: the hybrid method, the moving center method, and the retracking method in which moving sections are retracked to a fixed grid after a certain time interval. All these methods can treat particle emissions and atmospheric transport consistently, and are therefore suitable for use in large scale atmospheric models. In a test simulation against an accurate high resolution solution, all the methods showed reasonable treatment of new particle formation with 20 size sections although the hybrid and the retracking methods suffered from artificial widening of the distribution. The moving center approach, on the other hand, showed extra dents in the particle size distribution and failed to predict the onset of detectable particle formation. In a separate test simulation of an observed nucleation event, the model captured the key qualitative behaviour of the system well. Furthermore, its prediction of the organic volume fraction in newly formed particles, suggesting values as high as 0.5 for 3&ndash;4 nm particles and approximately 0.8 for 10 nm particles, agrees with recent indirect composition measurements

Cluster activation theory as an explanation of the linear dependence between formation rate of 3nm particles and sulphuric acid concentration

International audienceAccording to atmospheric observations new particle formation seems to be a function of sulphuric acid concentration to the power from one to two. The nucleation theorem then predicts that the critical cluster contains one to two sulphuric acid molecules. However, existing nucleation theories predicts that the power is more (or equal) than 2. Here we present an activation theory, which can explain the observed slope. In cluster activation the clusters containing one sulphuric acid molecule will activate for further growth due to heterogeneous nucleation, heterogeneous chemical reactions including polymerization or activation of soluble clusters. In the activation process organic vapours are typically needed as condensing agents

Why formation rate of 3 nm particles depends linearly on sulphuric acid concentration?

International audienceAccording to atmospheric observations new particle formation seems to be a function of sulphuric acid concentration to the power from one to two. The nucleation theorem then predicts that the critical cluster contains one to two sulphuric acid molecules. However, existing nucleation theories predicts that the power is more (or equal) than 2. Here we present an activation theory, which can explain the observed slope. In cluster activation the clusters containing one sulphuric acid molecule will activate for further growth due to heterogeneous nucleation, heterogeneous chemical reactions including polymerization or activation of soluble clusters. In the activation process organic vapours are typically needed as condensing agents

ASTEROID SIZING BY RADIOGALAXY OCCULTATION AT 5 GHZ

Stellar occultations by asteroids observed at visual wavelengths have been an important tool for studying the size and shape of asteroids and for revising the orbital parameters of asteroids. At radio frequencies, a shadow of an asteroid on the Earth is dominated by diffraction effects. Here, we show, for the first time, that a single observation of an occultation of a compact radio source at a frequency of 5 GHz can be used to derive the effective size of the occulting object and to derive the distance between the observer and the center of the occultation path on the Earth. The derived diameter of the occulting object, asteroid (115) Thyra, is 75 +/- 6 km. The observed occultation profile shows features that cannot be explained by diffraction of a single asteroid.Peer reviewe

Light and colour of cirrus, translucent and opaque dust in the high-latitude area of LDN 1642

We have performed a 5-colour surface photometric study of the high-galactic-latitude area of dark nebula LDN 1642. Scattered light properties are presented of diffuse, translucent and opaque dust over the range of 3500 -- 5500 A. Far infrared absolute photometry at 200 um improves the precision of and provides a zero point to the extinction. The intensity of the scattered light depends on dust column density in a characteristic way: for optically thin dust the intensity first increases linearly, then turns to a saturation value; at still larger extinctions the intensity turns down to a slow decrease. The $A_V$ value of the saturated intensity maximum shifts in a systematic way, from $A_V\approx$ 1.5 mag at 3500 A, to $\sim 3$ mag at 5500 A. The intensity curves offer a straight-forward explanation for the behaviour of the scattered-light colours. At the intensity peak the colour agrees with the integrated starlight colour, while it is bluer at the low- and redder at the high-column-density side of the peak, respectively. These colour changes are a direct consequence of the wavelength dependence of the extinction. We have compared the colours of the LDN 1642 area with other relevant observational studies: high-latitude diffuse/translucent clouds, wide-field cirrus dust; and externally illuminated AGB-star envelopes. For extragalactic low-surface-brightness sources cirrus is an unwanted foreground contaminant. Our results for cirrus colours can help to distinguish cases where a diffuse plume or stream, apparently associated with a galaxy or a group or cluster, is more likely a local cirrus structure. Keywords: ISM: dust, extinction -- ISM: clouds, individual LDN 1642 -- Galaxy: solar neighbourhood -- Astronomical instruments, methods and techniques: methods -- Physical data and processes: scatteringComment: 20 pages, 10 figures, accepted for publication in MNRAS, published as MNRAS advance article on June 27 202

X-ray Halos and Large Grains in the Diffuse Interstellar Medium

Recent observations with dust detectors on board the interplanetary spacecraft Ulysses and Galileo have recorded a substantial flux of large interstellar grains with radii between 0.25 and 2.0 mu entering the solar system from the local interstellar cloud. The most commonly used interstellar grain size distribution is characterized by a a^-3.5 power law in grain radii a, and extends to a maximum grain radius of 0.25 mu. The extension of the interstellar grain size distribution to such large radii will have a major effect on the median grain size, and on the amount of mass needed to be tied up in dust for a given visual optical depth. It is therefore important to investigate whether this population of larger dust particles prevails in the general interstellar medium, or if it is merely a local phenomenon. The presence of large interstellar grains can be mainly inferred from their effect on the intensity and radial profiles of scattering halos around X-ray sources. In this paper we examine the grain size distribution that gives rise to the X-ray halo around Nova Cygni 1992. The results of our study confirm the need to extend the interstellar grain size distribution in the direction of this source to and possibly beyond 2.0 mu. The model that gives the best fit to the halo data is characterized by: (1) a grain size distribution that follows an a^-3.5 power law up to 0.50 mu, followed by an a^-4.0 extension from 0.50 mu to 2.0 mu; and (2) silicate and graphite (carbon) dust-to-gas mass ratios of 0.0044 and 0.0022, respectively, consistent with solar abundances constraints. Additional observations of X-ray halos probing other spatial directions are badly needed to test the general validity of this result.Comment: 17 pages, incl. 1 figure, accepted for publ. by ApJ Letter