ALBERT

Description: Meteorology and microphysics affect cloud formation, cloud droplet distributions and shortwave reflectance. The Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE) and the Stratocumulus Observations of Los-Angeles Emissions Derived Aerosol-Droplets (SOLEDAD) studies provided measurements in six case studies of cloud thermodynamic properties, initial particle number distribution and composition, and cloud drop distribution. In this study, we use simulations from a chemical and microphysical aerosol-cloud parcel (ACP) model with explicit kinetic drop activation to reproduce observed cloud droplet distributions of the case studies. Four cases had sub-adiabatic lapse rates, resulting in fewer activated droplets, lower liquid water content (LWC) and higher cloud base height than an adiabatic lapse rate. A weighted ensemble of simulations that reflect measured variation in updraft velocity and cloud base height was used to reproduce observed droplet distributions. Simulations show organic hygroscopicity in internally mixed cases causes small effects on cloud reflectivity (CR) (〈0.01), except for cargo ship and smoke plumes, which increased CR by 0.02 and 0.07, respectively, owing to their high organic mass fraction. Organic hygroscopicity had larger effects on droplet concentrations for cases with higher aerosol concentrations near the critical diameter (namely polluted cases with a modal peak near 0.1 µm). Differences in simulated droplet spectral widths (k) caused larger differences in CR than organic hygroscopicity in cases with organic mass fractions of 60% or less for the cases shown. Finally, simulations from a numerical parameterization of cloud droplet activation suitable for GCMs compared well with the ACP model, except under high organic mass fraction.

Description: Primary marine aerosol (PMA)-cloud interactions off the coast of California were investigated using observations of marine aerosol, CCN and stratocumulus clouds during the Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE) and the Stratocumulus Observations of Los-Angeles Emissions Derived Aerosol-Droplets (SOLEDAD) studies. Based on recently reported measurements of PMA size distributions, a constrained lognormal mode fitting procedure was devised to isolate PMA number size distributions from total aerosol size distributions and applied to E-PEACE measurements. During the 12-day E-PEACE cruise on the R/V Point Sur , PMA typically contributed less than 15% of total particle concentrations. PMA number concentrations averaged 12 cm -3 during a relatively calmer period (average wind speed 12 m/s 1 ) lasting 8 days, and 71 cm -3 during a period of higher wind speeds (average 16 m/s 1 ) lasting 5 days. On average, PMA contributed less than 10% of total CCN at supersaturations up to 0.9% during the calmer period; however, during the higher wind speed period, PMA comprised 5 – 63% of CCN (average 16 – 28%) at supersaturations less than 0.3%. Sea salt was measured directly in the dried residuals of cloud droplets during the SOLEDAD study. The mass fractions of sea salt in the residuals averaged 12 to 24% during three cloud events. Comparing the marine stratocumulus clouds sampled in the two campaigns, measured peak supersaturations were 0.2 ± 0.04% during E-PEACE and 0.05 – 0.1% during SOLEDAD. The available measurements show that CDNC increased with 〉100 nm particles in E-PEACE but decreased in the three SOLEDAD cloud events.

Description: About half of present-day cloud condensation nuclei originate from atmospheric nucleation, frequently appearing as a burst of new particles near midday. Atmospheric observations show that the growth rate of new particles often accelerates when the diameter of the particles is between one and ten nanometres. In this critical size range, new particles are most likely to be lost by coagulation with pre-existing particles, thereby failing to form new cloud condensation nuclei that are typically 50 to 100 nanometres across. Sulfuric acid vapour is often involved in nucleation but is too scarce to explain most subsequent growth, leaving organic vapours as the most plausible alternative, at least in the planetary boundary layer. Although recent studies predict that low-volatility organic vapours contribute during initial growth, direct evidence has been lacking. The accelerating growth may result from increased photolytic production of condensable organic species in the afternoon, and the presence of a possible Kelvin (curvature) effect, which inhibits organic vapour condensation on the smallest particles (the nano-Kohler theory), has so far remained ambiguous. Here we present experiments performed in a large chamber under atmospheric conditions that investigate the role of organic vapours in the initial growth of nucleated organic particles in the absence of inorganic acids and bases such as sulfuric acid or ammonia and amines, respectively. Using data from the same set of experiments, it has been shown that organic vapours alone can drive nucleation. We focus on the growth of nucleated particles and find that the organic vapours that drive initial growth have extremely low volatilities (saturation concentration less than 10(-4.5) micrograms per cubic metre). As the particles increase in size and the Kelvin barrier falls, subsequent growth is primarily due to more abundant organic vapours of slightly higher volatility (saturation concentrations of 10(-4.5) to 10(-0.5) micrograms per cubic metre). We present a particle growth model that quantitatively reproduces our measurements. Furthermore, we implement a parameterization of the first steps of growth in a global aerosol model and find that concentrations of atmospheric cloud concentration nuclei can change substantially in response, that is, by up to 50 per cent in comparison with previously assumed growth rate parameterizations.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Trostl, Jasmin -- Chuang, Wayne K -- Gordon, Hamish -- Heinritzi, Martin -- Yan, Chao -- Molteni, Ugo -- Ahlm, Lars -- Frege, Carla -- Bianchi, Federico -- Wagner, Robert -- Simon, Mario -- Lehtipalo, Katrianne -- Williamson, Christina -- Craven, Jill S -- Duplissy, Jonathan -- Adamov, Alexey -- Almeida, Joao -- Bernhammer, Anne-Kathrin -- Breitenlechner, Martin -- Brilke, Sophia -- Dias, Antonio -- Ehrhart, Sebastian -- Flagan, Richard C -- Franchin, Alessandro -- Fuchs, Claudia -- Guida, Roberto -- Gysel, Martin -- Hansel, Armin -- Hoyle, Christopher R -- Jokinen, Tuija -- Junninen, Heikki -- Kangasluoma, Juha -- Keskinen, Helmi -- Kim, Jaeseok -- Krapf, Manuel -- Kurten, Andreas -- Laaksonen, Ari -- Lawler, Michael -- Leiminger, Markus -- Mathot, Serge -- Mohler, Ottmar -- Nieminen, Tuomo -- Onnela, Antti -- Petaja, Tuukka -- Piel, Felix M -- Miettinen, Pasi -- Rissanen, Matti P -- Rondo, Linda -- Sarnela, Nina -- Schobesberger, Siegfried -- Sengupta, Kamalika -- Sipila, Mikko -- Smith, James N -- Steiner, Gerhard -- Tome, Antonio -- Virtanen, Annele -- Wagner, Andrea C -- Weingartner, Ernest -- Wimmer, Daniela -- Winkler, Paul M -- Ye, Penglin -- Carslaw, Kenneth S -- Curtius, Joachim -- Dommen, Josef -- Kirkby, Jasper -- Kulmala, Markku -- Riipinen, Ilona -- Worsnop, Douglas R -- Donahue, Neil M -- Baltensperger, Urs -- England -- Nature. 2016 May 25;533(7604):527-31. doi: 10.1038/nature18271.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Paul Scherrer Institute, Laboratory of Atmospheric Chemistry, CH-5232 Villigen, Switzerland. ; Carnegie Mellon University, Center for Atmospheric Particle Studies, Pittsburgh, Pennsylvania 15213, USA. ; CERN, CH-1211 Geneva, Switzerland. ; Goethe University Frankfurt, Institute for Atmospheric and Environmental Sciences, 60438 Frankfurt am Main, Germany. ; Department of Physics, University of Helsinki, PO Box 64, FI-00014 Helsinki, Finland. ; Department of Applied Environmental Science, University of Stockholm, SE-10961 Stockholm, Sweden. ; Institute for Atmospheric and Climate Science, ETH Zurich, 8092 Zurich, Switzerland. ; Chemical Sciences Division, Earth System Research Laboratory, NOAA, Boulder, Colorado, USA. ; Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, USA. ; Helsinki Institute of Physics, University of Helsinki, PO Box 64, FI-00014 Helsinki, Finland. ; Institute for Ion and Applied Physics, University of Innsbruck, 6020 Innsbruck, Austria. ; Ionicon Analytik GmbH, 6020 Innsbruck, Austria. ; WSL Institute for Snow and Avalanche Research SLF, 7260 Davos, Switzerland. ; University of Eastern Finland, 70211 Kuopio, Finland. ; Finnish Meteorological Institute, 00101 Helsinki, Finland. ; National Center for Atmospheric Research, Atmospheric Chemistry Observations and Modeling Laboratory, Boulder, Colorado 80301, USA. ; Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, Karlsruhe, Germany. ; School of Earth and Environment, University of Leeds, LS2 9JT Leeds, UK. ; Department of Chemistry, University of California, Irvine, California 92697, USA. ; Faculty of Physics, University of Vienna, 1090 Vienna, Austria. ; SIM, University of Lisbon and University of Beira Interior, 1849-016 Lisbon, Portugal. ; Aerodyne Research, Inc., Billerica, Massachusetts 01821, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/27225126" target="_blank"〉PubMed〈/a〉

Description: Vertical number fluxes of aerosol particles and vertical fluxes of CO2 were measured with the eddy covariance method at the top of a 53 m high tower in the Amazon rain forest as part of the LBA (The Large Scale Biosphere Atmosphere Experiment in Amazonia) experiment. The observed aerosol number fluxes included particles with sizes down to 10 nm in diameter. The measurements were carried out during the wet and dry season in 2008. In this study focus is on the dry season aerosol fluxes, with significant influence from biomass burning, and these are compared with aerosol fluxes measured during the wet season. The primary goal is to quantify the dry deposition sink and to investigate whether particle deposition velocities change when going from the clean wet season into the more polluted dry season. Furthermore, it is tested whether the rain forest is always a net sink of particles in terms of number concentrations, or if particle emission from the surface under certain circumstances may dominate over the dry deposition sink. The particle deposition velocity vd increased linearly with increasing friction velocity in both seasons and the relations are described by vdd=(2.7 u* −0.2)×10−3 (dry season) and vdw=2.5 u*×10−3 (wet season), where u* is the friction velocity. The fact that the two relations are very similar to each other indicates that the seasonal change in aerosol number size distribution is not enough for causing any significant change in deposition velocity. In general, particle deposition velocities in this study are low compared to studies over boreal forests. The reason is probably domination of accumulation mode particles in the Amazon boundary layer, both in the dry and wet season, and low wind speeds in the tropics compared to the midlatitudes. Net particle deposition fluxes prevailed in daytime in both seasons and the deposition flux was considerably larger in the dry season due to the much higher dry season particle concentration. In the dry season, nocturnal particle fluxes behaved very similar to the nocturnal CO2 fluxes. Throughout the night, the measured particle flux at the top of the tower was close to zero, but early in the morning there was an upward particle flux peak that is not likely a result of entrainment or local pollution. It is possible that these morning upward particle fluxes are associated with emission of natural biogenic particles from the rain forest. Emitted particles may be stored within the canopy during stable conditions at nighttime, similarly to CO2, and being released from the canopy when conditions become more turbulent in the morning.

Description: Size-resolved vertical aerosol number fluxes of particles in the diameter range 0.25–2.5 μm were measured with the eddy covariance method from a 53 m high tower over the Amazon rain forest, 60 km NNW of Manaus, Brazil. This study focuses on data measured during the relatively clean wet season, but a shorter measurement period from the more polluted dry season is used as a comparison. Size-resolved net particle fluxes of the five lowest size bins, representing 0.25–0.45 μm in diameter, pointed downward in more or less all wind sectors in the wet season. This is an indication that the source of primary biogenic aerosol particles may be small in this particle size range. In the diameter range 0.5–2.5 μm, vertical particle fluxes were highly dependent on wind direction. In wind sectors where anthropogenic influence was low, net emission fluxes dominated. However, in wind sectors associated with higher anthropogenic influence, net deposition fluxes dominated. The net emission fluxes were interpreted as primary biogenic aerosol emission, but deposition of anthropogenic particles seems to have masked this emission in wind sectors with higher anthropogenic influence. The emission fluxes were at maximum in the afternoon when the mixed layer is well developed, and these emissions were best correlated with horizontal wind speed by the equation log10F=0.47·U+2.26 where F is the emission number flux of 0.5–2.5 μm particles [m−2s−1] and U is the horizontal wind speed [ms−1] at the top of the tower.

Description: Number fluxes of particles with diameter larger than 10 nm were measured with the eddy covariance method over the Amazon rain forest during the wet season as part of the LBA (The Large Scale Biosphere Atmosphere Experiment in Amazonia) campaign 2008. The primary goal was to investigate whether sources or sinks dominate the aerosol number flux in the tropical rain forest-atmosphere system. During the measurement campaign, from 12 March to 18 May, 60% of the particle fluxes pointed downward, which is a similar fraction to what has been observed over boreal forests. The net deposition flux prevailed even in the absolute cleanest atmospheric conditions during the campaign and therefore cannot be explained only by deposition of anthropogenic particles. The particle transfer velocity vt increased with increasing friction velocity and the relation is described by the equation vt = 2.4×10−3×u* where u* is the friction velocity. Upward particle fluxes often appeared in the morning hours and seem to a large extent to be an effect of entrainment fluxes into a growing mixed layer rather than primary aerosol emission. In general, the number source of primary aerosol particles within the footprint area of the measurements was small, possibly because the measured particle number fluxes reflect mostly particles less than approximately 200 nm. This is an indication that the contribution of primary biogenic aerosol particles to the aerosol population in the Amazon boundary layer may be low in terms of number concentrations. However, the possibility of horizontal variations in primary aerosol emission over the Amazon rain forest cannot be ruled out.

Description: Vertical number fluxes of aerosol particles and vertical fluxes of CO2 were measured with the eddy covariance method at the top of a 53 m high tower in the Amazon rain forest as part of the LBA (The Large Scale Biosphere Atmosphere Experiment in Amazonia) experiment. The observed aerosol number fluxes included particles with sizes down to 10 nm in diameter. The measurements were carried out during the wet and dry season in 2008. In this study focus is on the dry season aerosol fluxes, with significant influence from biomass burning, and these are compared with aerosol fluxes measured during the wet season. Net particle deposition fluxes dominated in daytime in both seasons and the deposition flux was considerably larger in the dry season due to the much higher dry season particle concentration. The particle transfer velocity increased linearly with increasing friction velocity in both seasons. The difference in transfer velocity between the two seasons was small, indicating that the seasonal change in aerosol number size distribution is not enough for causing any significant change in deposition velocity. In general, particle transfer velocities in this study are low compared to studies over boreal forests. The reasons are probably the high percentage of accumulation mode particles and the low percentage of nucleation mode particles in the Amazon boundary layer, both in the dry and wet season, and low wind speeds in the tropics compared to the midlatitudes. In the dry season, nocturnal particle fluxes behaved very similar to the nocturnal CO2 fluxes. Throughout the night, the measured particle flux at the top of the tower was close to zero, but early in the morning there was an upward particle flux peak that is not likely a result of entrainment or local pollution. It is possible that these morning upward particle fluxes are associated with emission of primary biogenic particles from the rain forest. Emitted particles may be stored within the canopy during stable conditions at nighttime, similarly to CO2, and being released from the canopy when conditions become more turbulent in the morning.

Description: From May 2008 to March 2009 aerosol emissions were measured using the eddy covariance method covering the size range 0.25 to 2.5 μm diameter (Dp) from a 105 m tower, in central Stockholm, Sweden. Supporting chemical aerosol data were collected at roof and street level. Results show that the inorganic fraction of sulfate, nitrate, ammonium and sea salt accounts for approximately 15% of the total aerosol mass 〈 1 μm Dp (PM1) with water soluble soil contributing 11% and water insoluble soil 47%. Carbonaceous compounds were at the most 27% of PM1 mass. It was found that heating the air from the tower to 200 °C resulted in the loss of approximately 60% of the aerosol volume at 0.25 μm Dp whereas only 40% of the aerosol volume was removed at 0.6 μm Dp. Further heating to 300 °C caused very little additional losses

Description: Size-resolved vertical aerosol number fluxes of particles in the diameter range 0.25–2.5 μm were measured with the eddy covariance method from a 53 m high tower over the Amazon rain forest, 60 km NNW of Manaus, Brazil. This study focuses on data measured during the relatively clean wet season, but a shorter measurement period from the more polluted dry season is used as a comparison. Size-resolved net particle fluxes of the five lowest size bins, representing 0.25–0.45 μm in diameter, were in general dominated by deposition in more or less all wind sectors in the wet season. This is an indication that the source of primary biogenic aerosol particles may be small in this particle size range. Transfer velocities within this particle size range were observed to increase linearly with increasing friction velocity and increasing particle diameter. In the diameter range 0.5–2.5 μm, vertical particle fluxes were highly dependent on wind direction. In wind sectors where anthropogenic influence was low, net upward fluxes were observed. However, in wind sectors associated with higher anthropogenic influence, deposition fluxes dominated. The net upward fluxes were interpreted as a result of primary biogenic aerosol emission, but deposition of anthropogenic particles seems to have masked this emission in wind sectors with higher anthropogenic influence. The net emission fluxes were at maximum in the afternoon when the mixed layer is well developed, and were best correlated with horizontal wind speed according to the equation log10 F=0.48 · U+2.21 where F is the net emission number flux of 0.5–2.5 μm particles [m−2 s−1] and U is the horizontal wind speed [ms−1] at the top of the tower.