Unraveling the "Pressure-Effect" in Nucleation

The influence of the pressure of a chemically inert carrier-gas on the nucleation rate is one of the biggest puzzles in the research of gas-liquid nucleation. Different experiments can show a positive effect, a negative effect, or no effect at all. The same experiment may show both trends for the same substance depending on temperature, or for different substances at the same temperature. We show how this ambiguous effect naturally arises from the competition of two contributions: nonisothermal effects and pressure-volume work. Our model clarifies seemingly contradictory experimental results and quantifies the variation of the nucleation ability of a substance in the presence of an ambient gas. Our findings are corroborated by results from molecular dynamics simulation and might have important implications since nucleation in experiments, technical applications, and nature practically always occurs in the presence of an ambient gas.Comment: 4 pages, 3 figures. v2: All citations now appear correctly. v3: Updated one point in Fig.

Effect of addition of four base compounds on sulphuric-acid–water new-particle formation : a laboratory study

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Temperature-Dependent Diffusion of H2SO4 in Air at Atmospherically Relevant Conditions : Laboratory Measurements Using Laminar Flow Technique

We report flow tube measurements of the effective sulfuric acid diffusion coefficient at ranges of different relative humidities (from similar to 4 to 70%), temperatures (278, 288 and 298 K) and initial H2SO4 concentrations (from 1 x 10(6) to 1 x 10(8) molecules.cm(-3)). The measurements were carried out under laminar flow of humidified air containing trace amounts of impurities such as amines (few ppt), thus representing typical conditions met in Earth's continental boundary layer. The diffusion coefficients were calculated from the sulfuric acid wall loss rate coefficients that were obtained by measuring H2SO4 concentration continuously at seven different positions along the flow tube with a chemical ionization mass spectrometer (CIMS). The wall loss rate coefficients and laminar flow conditions were verified with additional computational fluid dynamics (CFD) model FLUENT simulations. The determined effective sulfuric acid diffusion coefficients decreased with increasing relative humidity, as also seen in previous experiments, and had a rather strong power dependence with respect to temperature, around proportional to T-5.6, which is in disagreement with the expected temperature dependence of similar to T-1.75 for pure vapours. Further clustering kinetics simulations using quantum chemical data showed that the effective diffusion coefficient is lowered by the increased diffusion volume of H2SO4 molecules via a temperature-dependent attachment of base impurities like amines. Thus, the measurements and simulations suggest that in the atmosphere the attachment of sulfuric acid molecules with base molecules can lead to a lower than expected effective sulfuric acid diffusion coefficient with a higher than expected temperature dependence.Peer reviewe

An extensive data set for in situ microphysical characterization of low-level clouds in a Finnish sub-Arctic site

Continuous, semi-long-term, ground-based in situ cloud measurements were conducted during eight Pallas Cloud Experiments (PaCEs) held in autumn between 2004 and 2019. Those campaigns were carried out in the Finnish sub-Arctic region at the Sammaltunturi station (67 degrees 58'24"N, 24 degrees 06'58"E; 560ma.m.s.l.), the part of the Pallas Atmosphere-Ecosystem Supersite and Global Atmosphere Watch (GAW) program. Two cloud spectrometer ground setups and a weather station were installed on the roof of the station to measure in situ cloud properties and several meteorological variables. Thus, the obtained data sets include the size distribution of cloud droplets as a measured cloud parameter along with the air temperature, dew point temperature, humidity, pressure, horizontal wind speed and direction, (global solar) sun radiation, and visibility at the station. Additionally, the number concentration, effective diameter, median volume diameter, and liquid water content from each instrument were derived. The presented data sets provide a insight into microphysics of low-level clouds in subArctic conditions over a wide range of temperatures (-25.8 to 8.8 degrees C). The data are available in the Finnish Meteorological Institute (FMI) open data repository for each campaign and each cloud spectrometer ground setup individually: https://doi.org/10.23728/FMI-B2SHARE.988739D21B824C709084E88ED6C6D54B (Doulgeris et al., 2021).Peer reviewe

Influence of air mass origin on microphysical properties of low-level cloudsin a subarctic environment

In this work, an analysis was performed to investigate how different long-range transport air masses can affect the microphysical properties of low-level clouds in a clean subarctic environment. The cloud measurements included in situ and remote sensing ground-based techniques and were conducted during eight Pallas Cloud Experiments (PaCEs) held in the autumn between 2004 and 2019. Each PaCE was carried out at the Pallas Atmosphere-Ecosystem Supersite, located in the Finnish subarctic region. Two cloud spectrometer ground setups were installed on the roof of the station to measure cloud microphysical properties: the cloud, aerosol and precipitation spectrometer (CAPS) and the forward-scattering spectrometer probe (FSSP). Air mass histories were analyzed using the Lagrangian FLEXible PARTicle dispersion model (FLEXPART) in order to investigate the differences between five distinct source regions ( "Arctic ", "Eastern ", "Southern ", "Western " and "Local "). We observed clear differences in the cloud microphysical properties for the air mass source regions. Arctic air masses were characterized by low liquid water content (LWC), low cloud droplet number concentration (Nc) and comparatively large median volume and effective droplet diameter. The Western region (marine North Atlantic) differed from the Arctic by both higher Nc and LWC. The Eastern region (continental Eurasia) only had a little higher LWC than the Arctic but substantially higher Nc and a smaller droplet diameter. The Southern region (continental Europe) had high Nc and LWC and a very similar droplet diameter to the Eastern region. Finally, the relationship between Nc and droplet size (i.e., the Twomey effect) was characterized for the different source regions, indicating that all region clouds were sensitive to increases in Nc.Peer reviewe

In situ cloud ground-based measurements in the Finnish sub-Arctic : intercomparison of three cloud spectrometer setups

Continuous, semi-long-term, ground-based in situ cloud measurements were conducted during the Pallas Cloud Experiment (PaCE) in 2013. The measurements were carried out in Finnish sub-Arctic region at Sammaltunturi station (67 degrees 58 ' N, 24 degrees 07 ' E; 560m a.s.l.), part of Pallas Atmosphere - Ecosystem Supersite and Global Atmosphere Watch (GAW) program. The main motivation of the campaign was to conduct in situ cloud measurements with three different cloud spectrometer probes and perform an evaluation of their ground-based setups. Therefore, we mutually compared the performance of the cloud and aerosol spectrometer (CAS), the cloud droplet probe (CDP) and the forward-scattering spectrometer probe (FSSP-100) (DMT; Boulder, CO, USA). We investigated how different meteorological parameters affect each instrument's ground-based setup operation and quantified possible biases and discrepancies of different microphysical cloud properties. Based on the obtained results we suggested limitations for further use of the instrument setups in campaigns where the focus is on investigating aerosol-cloud interactions. Measurements in this study were made by instruments owned by the Finnish Meteorological Institute and results concern their operation in sub-Arctic conditions with frequently occurring supercooled clouds. The measured parameter from each instrument was the size distribution, and additionally we derived the number concentration, the effective diameter, the median volume diameter and the liquid water content. A complete intercomparison between the CAS probe and the FSSP-100 ground setups and additionally between the FSSP-100 and the CDP probe ground setups was made and presented. Unfortunately, there was not a sufficient amount of common data to compare all three probes together due to operational problems of the CDP ground setup in sub-zero conditions. The CAS probe that was fixed to one direction lost a significant number of cloud droplets when the wind direction was out of wind iso-axial conditions in comparison with the FSSP-100 and the CDP, which were both placed on a rotating platform. We revealed that CAS and FSSP-100 had good agreement in deriving sizing parameters (effective diameter and median volume diameter from 5 to 35 mu m) even though CAS was losing a significant amount of cloud droplets. The most sensitive derived parameter was liquid water content, which was strongly connected to the wind direction and temperature.Peer reviewe

Slowdown or Crisis? Editorial Introduction

SUMMARY This introduction poses the dilemma of how to characterise the current state of the world economy — is it a mere slowdown, subject to correction by the implementation of better?designed market?orientated strategies, or does it represent a more historically significant crisis, whose resolution depends upon changes in the broader pattern of social relations? The discussion is pursued in three parts. In the first section the dimensions of the current slowdown/crisis are presented. Thereafter a bifurcation is drawn between these two broad framework of analysis. And finally, each of the individual articles in this Bulletin is briefly introduced in the context of the debate between the slowdown and crisis approaches. RESUMEN Introducción editorial Esta introducción plantea el dilema de cómo caracterizar el actual estado de la economía mundial. ¿Se trata de una mera recesión, susceptible de ser rectificada mediante la implementación de estrategias orientadas al mercado, mejor diseñadas o representa una crisis históricamente más significativa, cuya solución depende de cambios en el modelo más amplio de las relaciones sociales? El análisis se realiza en tres partes. En la primera sección se presentan las dimensiones de la crisis/recesión actual. A continuación, se traza una línea divisoria entre ambos marcos de análisis. Finalmente, cada artículo incursiona brevemente en el debate sobre ambos enfoques. RESUME Introduction editoriale Cette introduction pose le problème de savoir comment caractériser l'état actuel de l'économie mondiale — c'est un ralentissement tout simple, sujet à des changements dûs à la mise en pratique de stratégies mieux conçues et orientées vers les marchés, ou est ce que cela représente une crise beaucoup plus importante du point de vue historique, dont la résolution dépend des changements dans la grande structure des relations sociales? Le débat se poursuit en trois parties. Dans la premiere partie est présentée l'étendue du ralentissement/crise actuels. Ensuite une séparation est faite entre ces deux larges charpentes d'analyse. Et pour finir, chaque article dans ce bulletin est introduit brièvement dans le contexe du débat entre les points de vue du ralentissement et de Ja crise

Design and field campaign validation of a multi-rotor unmanned aerial vehicle and optical particle counter

© Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License (https://creativecommons.org/licenses/by/4.0/).Small unmanned aircraft (SUA) have the potential to be used as platforms for the measurement of atmospheric particulates. The use of an SUA platform for these measurements provides benefits such as high manoeuvrability, re-usability, and low-cost when compared with traditional techniques. However, the complex aerodynamics of an SUA (particularly for multirotor airframes), combined with the miniaturisation of particle instruments poses difficulties for accurate and representative sampling of particulates. The work presented here relies on computational fluid dynamics with Lagrangian particle tracking (CFD-LPT) simulations to influence the design of a bespoke meteorological sampling system: the UH-AeroSAM. This consists of a custom built airframe, designed to reduce sampling artefacts due to the propellers, and a purpose built open-path optical particle counter–the Ruggedised Cloud and Aerosol Sounding System (RCASS). OPC size distribution measurements from the UH-AeroSAM are compared with the Cloud and Aerosol Precipitation Spectrometer (CAPS) for measurements of Stratus cloud during the Pallas Cloud Experiment (PaCE) in 2019. Good agreement is demonstrated between the two instruments. The integrated dN/dlog(Dp) is shown to have a coefficient of determination of 0.8, and a regression slope of 0.9 when plotted 1:1.Peer reviewe