scholarly journals Cloud condensation Nuclei over the Southern Ocean: wind dependence and seasonal cycles

2016 ◽  
Author(s):  
John L. Gras ◽  
Melita Keywood
Keyword(s):  
Southern Ocean ◽  
Marine Boundary Layer ◽  
Numerical Models ◽  
Cloud Condensation Nuclei ◽  
Sea Salt ◽  
Aerosol Size Distribution ◽  
Biological Source ◽  
Microphysical Properties ◽  
Coarse Mode ◽  
Aitken Mode

Abstract. Multi-decadal observations of aerosol microphysical properties from regionally representative sites can be used to challenge regional or global numerical models that simulate atmospheric aerosol. Presented here is an analysis of multi-decadal observations at Cape Grim (Australia) that characterise production and removal of the background marine aerosol in Southern Ocean marine boundary layer (MBL) on both short-term weather-related and underlying seasonal scales. A trimodal aerosol distribution comprises Aitken nuclei ( 350 nm) modes, with the Aitken mode dominating number concentration. While the integrated particle number in the MBL over the clean Southern Ocean is only weakly dependent on wind speed the different modes in the aerosol size distribution vary in their relationship with windspeed. The balance between a positive wind dependence in the coarse mode and negative dependence in the accumulation/CCN mode leads to a relatively flat wind dependence in summer and moderately strong positive wind dependence in winter. The change-over in wind dependence of these two modes occurs in a very small size range at the mode intersection, indicative of differences in the balance of production and removal in the coarse and accumulation/CCN modes. While a marine biological source of reduced sulfur appears to dominate CCN concentration over the summer months (December to February) other components contribute to CCN over the full annual cycle. Wind-generated coarse mode sea-salt is an important CCN component year round and is the second most important contributor to CCN from autumn through to mid-spring (March to November). A portion of the non-seasonal dependent contributor to CCN can clearly be attributed to wind generated sea-salt with the remaining part potentially being attributed to long range transported material. Under conditions of greater supersaturation, as expected in more convective cyclonic systems and their associated fronts, Aitken mode particles become increasingly important as CCN.

scholarly journals Cloud condensation nuclei over the Southern Ocean: wind dependence and seasonal cycles

2017 ◽  
Vol 17 (7) ◽  
pp. 4419-4432 ◽  
Author(s):  
John L. Gras ◽  
Melita Keywood
Keyword(s):  
Wind Speed ◽  
Southern Ocean ◽  
Marine Boundary Layer ◽  
Numerical Models ◽  
Cloud Condensation Nuclei ◽  
Sea Salt ◽  
Aerosol Size Distribution ◽  
Condensation Nuclei ◽  
Coarse Mode ◽  
Aitken Mode

Abstract. Multi-decadal observations of aerosol microphysical properties from regionally representative sites can be used to challenge regional or global numerical models that simulate atmospheric aerosol. Presented here is an analysis of multi-decadal observations at Cape Grim (Australia) that characterise production and removal of the background marine aerosol in the Southern Ocean marine boundary layer (MBL) on both short-term weather-related and underlying seasonal scales.A trimodal aerosol distribution comprises Aitken nuclei (< 100 nm), cloud condensation nuclei (CCN)/accumulation (100–350 nm) and coarse-particle (> 350 nm) modes, with the Aitken mode dominating number concentration. Whilst the integrated particle number in the MBL over the clean Southern Ocean is only weakly dependent on wind speed, the different modes in the aerosol size distribution vary in their relationship with wind speed. The balance between a positive wind dependence in the coarse mode and negative dependence in the accumulation/CCN mode leads to a relatively flat wind dependence in summer and moderately strong positive wind dependence in winter. The changeover in wind dependence of these two modes occurs in a very small size range at the mode intersection, indicative of differences in the balance of production and removal in the coarse and accumulation/CCN modes.Whilst a marine biological source of reduced sulfur appears to dominate CCN concentration over the summer months (December to February), other components contribute to CCN over the full annual cycle. Wind-generated coarse-mode sea salt is an important CCN component year round and is the second-most-important contributor to CCN from autumn through to mid-spring (March to November). A portion of the non-seasonally dependent contributor to CCN can clearly be attributed to wind-generated sea salt, with the remaining part potentially being attributed to long-range-transported material. Under conditions of greater supersaturation, as expected in more convective cyclonic systems and their associated fronts, Aitken mode particles become increasingly important as CCN.

An Evaluation of WRF Simulations of Clouds over the Southern Ocean with A-Train Observations

2014 ◽  
Vol 142 (2) ◽  
pp. 647-667 ◽  
Author(s):  
Yi Huang ◽  
Steven T. Siems ◽  
Michael J. Manton ◽  
Gregory Thompson
Keyword(s):  
Boundary Layer ◽  
Southern Ocean ◽  
Large Scale ◽  
Marine Boundary Layer ◽  
Cloud Condensation Nuclei ◽  
Surface Fluxes ◽  
Shortwave Radiation ◽  
Microphysical Properties ◽  
Formidable Challenge ◽  
Weak Surface

Abstract The representation of the marine boundary layer (BL) clouds remains a formidable challenge for state-of-the-art simulations. A recent study by Bodas-Salcedo et al. using the Met Office Unified Model highlights that the underprediction of the low/midlevel postfrontal clouds contributes to the largest bias of the surface downwelling shortwave radiation over the Southern Ocean (SO). A-Train observations and limited in situ measurements have been used to evaluate the Weather Research and Forecasting Model, version 3.3.1 (WRFV3.3.1), in simulating the postfrontal clouds over Tasmania and the SO. The simulated cloud macro/microphysical properties are compared against the observations. Experiments are also undertaken to test the sensitivity of model resolution, microphysical (MP) schemes, planetary boundary layer (PBL) schemes, and cloud condensation nuclei (CCN) concentration. The simulations demonstrate a considerable level of skill in representing the clouds during the frontal passages and, to a lesser extent, in the postfrontal environment. The simulations, however, have great difficulties in portraying the widespread marine BL clouds that are not immediately associated with fronts. This shortcoming is persistent to the changes of model configuration and physical parameterization. The representation of large-scale conditions and their connections with the BL clouds are discussed. A lack of BL moisture is the most obvious explanation for the shortcoming, which may be a consequence of either strong entrainment or weak surface fluxes. It is speculated that the BL wind shear/turbulence may be an issue over the SO. More comprehensive observations are necessary to fully investigate the deficiency of the simulations.

Arctic fog chemistry induces the unexpected growth of Aitken mode particles to CCN-active particles

2021 ◽  
Author(s):  
Erik H. Hoffmann ◽  
Andreas Tilgner ◽  
Simonas Kecorius ◽  
Hartmut Herrmann
Keyword(s):  
Growth Mechanism ◽  
Marine Boundary Layer ◽  
Cloud Condensation Nuclei ◽  
Methanesulfonic Acid ◽  
Particle Growth ◽  
High Water ◽  
The Arctic ◽  
Radiative Balance ◽  
Active Particles ◽  
Aitken Mode

&lt;p&gt;New particle formation (NPF) and early growth are efficient processes producing high concentrations of cloud condensation nuclei (CCNs) precursors in the Arctic marine boundary layer (AMBL). However, due to short lifetime and lack of condensable vapors, newly formed particles do often not grow beyond 50 nm and cause low CCN particle concentrations in the AMBL. Thus, even the smallest amount of Aitken mode particle growth is capable to significantly increase the CCN budget. However, the growth mechanism of Aitken-mode particles from NPF into CCN range in the Arctic is still rather unclear and was therefore investigated during the cruise campaign PASCAL in 2017.&lt;/p&gt; &lt;p&gt;During PASCAL, aerosol particles measurements were performed and an unexpected rapid growth of Aitken mode particles was observed right after fog episodes. Combined field data analyses and detailed multiphase chemistry box model simulations with the CAPRAM mechanism were performed to study the underlying processes. Resulting, a new mechanism is proposed explaining how particles with d &lt; 50 nm are able to grow into CCN size range in the Arctic without requiring high water vapor supersaturation (SS). The investigations demonstrated that the rapid post-fog particle growth of Aitken mode is related to chemical processes within the Arctic fog. The redistribution of semi-volatile acidic (e.g., methanesulfonic acid) and basic (e.g., ammonia) compounds from processed CCN-active particles to smaller CCN-inactive particles can cause a rapid particle growth of Aitken mode particles after fog evaporation enabling them to grow towards CCN size. Comparisons of the model results with Berner impactor measurements supports the proposed growth mechanism.&lt;/p&gt; &lt;p&gt;Overall, this study provided new insights on how the increasing frequency of NPF and fog-related particle processing can increase in the number of CCNs and cloud droplets leading to an increased albedo of Arctic clouds and thus affect the radiative balance in the Arctic. Since fogs will occur more frequently in the Arctic as a result of climate change, this growth mechanism and a deeper knowledge on its feedbacks can be essential to understand Arctic warming.&lt;/p&gt;

scholarly journals Modelling the contribution of sea salt and dimethyl sulfide derived aerosol to marine CCN

2002 ◽  
Vol 2 (1) ◽  
pp. 17-30 ◽  
Author(s):  
Y. J. Yoon ◽  
P. Brimblecombe
Keyword(s):  
Sulfuric Acid ◽  
Dimethyl Sulfide ◽  
Marine Boundary Layer ◽  
Cloud Condensation Nuclei ◽  
Accommodation Coefficient ◽  
Sea Salt ◽  
Wind Speeds ◽  
Transfer Processes ◽  
Clean Air ◽  
The Relationship

Abstract. The concentration of cloud condensation nuclei (CCN) in the marine boundary layer (MBL) was estimated from dimethyl sulfide (DMS) flux, sea salt (SS) emission, and aerosols entrained from the free troposphere (FT). Only under clean air conditions, did the nucleation of DMS derived sulfur (DMS CCN) contribute significantly to the MBL CCN. The accommodation coefficient for sulfuric acid mass transfer was found to be a very important parameter in the modeling the contribution of DMS to MBL CCN. The relationship between seawater DMS and MBL CCN was found to be non-linear mainly due to the transfer processes of sulfuric acid onto aerosols. In addition, sea salt derived CCN (SS CCN) and entrained aerosol from the FT (FT CCN) affected the MBL CCN directly, by supplying CCN, and indirectly, by behaving as an efficient sink for sulfuric acid. The SS CCN explained more than 50% of the total predicted MBL CCN when wind speeds were moderate and high. Sea salt and FT aerosol may often be more efficient sources of MBL CCN than DMS.

scholarly journals Biomass Burning Aerosol as a Modulator of Droplet Number in the Southeast Atlantic Region

2019 ◽  
Author(s):  
Mary Kacarab ◽  
K. Lee Thornhill ◽  
Amie Dobracki ◽  
Steven G. Howell ◽  
Joseph R. O'Brien ◽  
...  
Keyword(s):  
Biomass Burning ◽  
Marine Boundary Layer ◽  
Cloud Condensation Nuclei ◽  
Central Africa ◽  
Aerosol Size Distribution ◽  
Atlantic Region ◽  
Radiative Impact ◽  
Biomass Burning Aerosol ◽  
Aerosol Cloud Interactions ◽  
Southeastern Atlantic

Abstract. The southeastern Atlantic (SEA) and its associated cloud deck, off the west coast of central Africa, is an area where aerosol-cloud interactions can have a strong radiative impact. Seasonally, extensive biomass burning (BB) aerosol plumes from southern Africa reach this area. The NASA ObseRvations of Aerosols above Clouds and their intEractionS (ORACLES) study focused on quantitatively understanding these interactions and their importance. Here we present measurements of cloud condensation nuclei (CCN) concentration, aerosol size distribution, and vertical updraft velocity in and around the marine boundary layer (MBL) collected by the NASA P-3B aircraft during the August 2017 ORACLES deployment. BB aerosol levels vary considerably but systematically with time; high aerosol concentrations were observed in the MBL (800–1000 cm−3) early on, decreasing mid-campaign to concentrations between 500–800 cm−3. By late August and early September, relatively clean MBL conditions were sampled (

scholarly journals Marine boundary layer aerosol in the eastern North Atlantic: seasonal variations and key controlling processes

2018 ◽  
Vol 18 (23) ◽  
pp. 17615-17635 ◽  
Author(s):  
Guangjie Zheng ◽  
Yang Wang ◽  
Allison C. Aiken ◽  
Francesca Gallo ◽  
Michael P. Jensen ◽  
...  
Keyword(s):  
Boundary Layer ◽  
North Atlantic ◽  
Seasonal Variations ◽  
Dimethyl Sulfide ◽  
Marine Boundary Layer ◽  
Sulfide Oxidation ◽  
Aerosol Size Distribution ◽  
Eastern North Atlantic ◽  
Aitken Mode ◽  
Aerosol Properties

Abstract. The response of marine low cloud systems to changes in aerosol concentration represents one of the largest uncertainties in climate simulations. Major contributions to this uncertainty are derived from poor understanding of aerosol under natural conditions and the perturbation by anthropogenic emissions. The eastern North Atlantic (ENA) is a region of persistent but diverse marine boundary layer (MBL) clouds, whose albedo and precipitation are highly susceptible to perturbations in aerosol properties. In this study, we examine MBL aerosol properties, trace gas mixing ratios, and meteorological parameters measured at the Atmospheric Radiation Measurement Climate Research Facility's ENA site on Graciosa Island, Azores, Portugal, during a 3-year period from 2015 to 2017. Measurements impacted by local pollution on Graciosa Island and during occasional intense biomass burning and dust events are excluded from this study. Submicron aerosol size distribution typically consists of three modes: Aitken (At, diameter Dp<∼100 nm), accumulation (Ac, Dp within ∼100 to ∼300 nm), and larger accumulation (LA, Dp>∼300 nm) modes, with average number concentrations (denoted as NAt, NAc, and NLA below) of 330, 114, and 14 cm−3, respectively. NAt, NAc, and NLA show contrasting seasonal variations, suggesting different sources and removal processes. NLA is dominated by sea spray aerosol (SSA) and is higher in winter and lower in summer. This is due to the seasonal variations of SSA production, in-cloud coalescence scavenging, and dilution by entrained free troposphere (FT) air. In comparison, SSA typically contributes a relatively minor fraction to NAt (10 %) and NAc (21 %) on an annual basis. In addition to SSA, sources of Ac-mode particles include entrainment of FT aerosols and condensation growth of Aitken-mode particles inside the MBL, while in-cloud coalescence scavenging is the major sink of NAc. The observed seasonal variation of NAc, being higher in summer and lower in winter, generally agrees with the steady-state concentration estimated from major sources and sinks. NAt is mainly controlled by entrainment of FT aerosol, coagulation loss, and growth of Aitken-mode particles into the Ac-mode size range. Our calculation suggests that besides the direct contribution from entrained FT Ac-mode particles, growth of entrained FT Aitken-mode particles in the MBL also represent a substantial source of cloud condensation nuclei (CCN), with the highest contribution potentially reaching 60 % during summer. The growth of Aitken-mode particles to CCN size is an expected result of the condensation of sulfuric acid, a product from dimethyl sulfide oxidation, suggesting that ocean ecosystems may have a substantial influence on MBL CCN populations in the ENA.

scholarly journals Spatial and temporal distribution of atmospheric aerosols in the lowermost troposphere over the Amazonian tropical rainforest

2005 ◽  
Vol 5 (6) ◽  
pp. 1527-1543 ◽  
Author(s):  
R. Krejci ◽  
J. Ström ◽  
M. de Reus ◽  
J. Williams ◽  
H. Fischer ◽  
...  
Keyword(s):  
Size Distribution ◽  
Mixed Layer ◽  
Rain Forest ◽  
Marine Boundary Layer ◽  
Aerosol Size Distribution ◽  
Average Particle ◽  
Number Density ◽  
Modal Shape ◽  
Accumulation Mode ◽  
Aitken Mode

Abstract. We present measurements of aerosol physico-chemical properties below 5 km altitude over the tropical rain forest and the marine boundary layer (MBL) obtained during the LBA-CLAIRE 1998 project. The MBL aerosol size distribution some 50-100km of the coast of French Guyana and Suriname showed a bi-modal shape typical of aged and cloud processed aerosol. The average particle number density in the MBL was 383cm-3. The daytime mixed layer height over the rain forest for undisturbed conditions was estimated to be between 1200-1500m. During the morning hours the height of the mixed layer increased by 144-180mh-1. The median daytime aerosol number density in the mixed layer increased from 450cm-3 in the morning to almost 800cm-3 in the late afternoon. The evolution of the aerosol size distribution in the daytime mixed layer over the rain forest showed two distinct patterns. Between dawn and midday, the Aitken mode particle concentrations increased, whereas later during the day, a sharp increase of the accumulation mode aerosol number densities was observed, resulting in a doubling of the morning accumulation mode concentrations from 150cm-3 to 300cm-3. Potential sources of the Aitken mode particles are discussed here including the rapid growth of ultrafine aerosol particles formed aloft and subsequently entrained into the mixed layer, as well as the contribution of emissions from the tropical vegetation to Aitken mode number densities. The observed increase of the accumulation mode aerosol number densities is attributed to the combined effect of: the direct emissions of primary biogenic particles from the rain forest and aerosol in-cloud processing by shallow convective clouds. Based on the similarities among the number densities, the size distributions and the composition of the aerosol in the MBL and the nocturnal residual layer we propose that the air originating in the MBL is transported above the nocturnal mixed layer up to 300-400km inland over the rain forest by night without significant processing.

scholarly journals Effect of primary organic sea spray emissions on cloud condensation nuclei concentrations

2012 ◽  
Vol 12 (1) ◽  
pp. 89-101 ◽  
Author(s):  
D. M. Westervelt ◽  
R. H. Moore ◽  
A. Nenes ◽  
P. J. Adams
Keyword(s):  
Surface Tension ◽  
Southern Ocean ◽  
Source Function ◽  
Cloud Condensation Nuclei ◽  
Organic Aerosol ◽  
Sea Salt ◽  
Sea Spray ◽  
Aerosol Emission ◽  
Sea Spray Aerosol ◽  
Surfactant Effects

Abstract. This work estimates the primary marine organic aerosol global emission source and its impact on cloud condensation nuclei (CCN) concentrations by implementing an organic sea spray source function into a series of global aerosol simulations. The source function assumes that a fraction of the sea spray emissions, depending on the local chlorophyll concentration, is organic matter in place of sea salt. Effect on CCN concentrations (at 0.2% supersaturation) is modeled using the Two-Moment Aerosol Sectional (TOMAS) microphysics algorithm coupled to the GISS II-prime general circulation model. The presence of organics affects CCN activity in competing ways: by reducing the amount of solute available in the particle and decreasing surface tension of CCN. To model surfactant effects, surface tension depression data from seawater samples taken near the Georgia coast were applied as a function of carbon concentrations. A global marine organic aerosol emission rate of 17.7 Tg C yr−1 is estimated from the simulations. Marine organics exert a localized influence on CCN(0.2%) concentrations, decreasing regional concentrations by no more than 5% and by less than 0.5% over most of the globe, assuming direct replacement of sea salt aerosol with organic aerosol. The decrease in CCN concentrations results from the fact that the decrease in particle solute concentration outweighs the organic surfactant effects. The low sensitivity of CCN(0.2%) to the marine organic emissions is likely due to the small compositional changes: the mass fraction of OA in accumulation mode aerosol increases by only ~15% in a biologically active region of the Southern Ocean. To test the sensitivity to uncertainty in the sea spray emissions process, we relax the assumption that sea spray aerosol number and mass remain fixed and instead can add to sea spray emissions rather than replace existing sea salt. In these simulations, we find that marine organic aerosol can increase CCN by up to 50% in the Southern Ocean and 3.7% globally during the austral summer. This vast difference in CCN impact highlights the need for further observational exploration of the sea spray aerosol emission process as well as evaluation and development of model parameterizations.

scholarly journals A global off-line model of size-resolved aerosol microphysics: II. Identification of key uncertainties

2005 ◽  
Vol 5 (12) ◽  
pp. 3233-3250 ◽  
Author(s):  
D. V. Spracklen ◽  
K. J. Pringle ◽  
K. S. Carslaw ◽  
M. P. Chipperfield ◽  
G. W. Mann
Keyword(s):  
Nucleation Rate ◽  
Marine Boundary Layer ◽  
Cloud Condensation Nuclei ◽  
Accommodation Coefficient ◽  
Sulfate Aerosol ◽  
Sea Salt ◽  
Condensation Nuclei ◽  
Microphysical Processes ◽  
Global Mean ◽  
Aerosol Properties

Abstract. We use the new GLOMAP model of global aerosol microphysics to investigate the sensitivity of modelled sulfate and sea salt aerosol properties to uncertainties in the driving microphysical processes and compare these uncertainties with those associated with aerosol and precursor gas emissions. Overall, we conclude that uncertainties in microphysical processes have a larger effect on global sulfate and sea salt derived condensation nuclei (CN) and cloud condensation nuclei (CCN) concentrations than uncertainties in present-day sulfur emissions. Our simulations suggest that uncertainties in predicted sulfate and sea salt CCN abundances due to poorly constrained microphysical processes are likely to be of a similar magnitude to long-term changes in sulfate and sea salt CCN due to changes in anthropogenic emissions. A microphysical treatment of the global sulfate aerosol allows the uncertainty in climate-relevant aerosol properties to be attributed to specific processes in a way that has not been possible with simpler aerosol schemes. In particular we conclude that: (1) changes in the binary H2SO4-H2O nucleation rate and condensation rate of gaseous H2SO4 cause a shift in the vertical location of the upper tropospheric CN layer by as much as 3 km, while the shape of the CN profile is essentially pre-served (2) uncertainties in the binary H2SO4-H2O nucleation rate have a relatively insignificant effect on marine boundary layer (MBL) aerosol properties; (3) emitting a fraction of anthropogenic SO2 as particulates (to represent production of sulfate particles in power plant plumes below the scale of the model grid (which is of the order of 300 km)) has the potential to change the global mean MBL sulfate-derived CN concentrations by up to 72%, and changes of up to a factor 20 can occur in polluted continental regions; (4) predicted global mean MBL sulfate and sea salt CCN concentrations change by 10 to 60% when several microphysical processes are changed within reasonable uncertainty ranges; (5) sulfate and sea salt derived CCN concentrations are particularly sensitive to primary particle emissions, with global mean MBL sulfate and sea salt CCN changing by up to 27% and local concentrations over continental regions changing by more than 100% when the percentage of anthropogenic SO2 emitted as particulates is changed from 0 to 5%; (6) large changes in sea spray flux have insignificant effects on global sulfate aerosol except when the mass accommodation coefficient of sulfuric acid on the salt particles is set unrealistically low.

scholarly journals Consistent simulation of bromine chemistry from the marine boundary layer to the stratosphere – Part 1: Model description, sea salt aerosols and pH

2008 ◽  
Vol 8 (19) ◽  
pp. 5899-5917 ◽  
Author(s):  
A. Kerkweg ◽  
P. Jöckel ◽  
A. Pozzer ◽  
H. Tost ◽  
R. Sander ◽  
...  
Keyword(s):  
Size Distribution ◽  
Atmospheric Chemistry ◽  
General Circulation ◽  
Circulation Model ◽  
Reaction Rates ◽  
Sea Salt ◽  
Aerosol Size Distribution ◽  
Model Description ◽  
Aerosol Chemistry ◽  
Coarse Mode

Abstract. This is the first article of a series presenting a detailed analysis of bromine chemistry simulated with the atmospheric chemistry general circulation model ECHAM5/MESSy. Release from sea salt is an important bromine source, hence the model explicitly calculates aerosol chemistry and phase partitioning for coarse mode aerosol particles. Many processes including chemical reaction rates are influenced by the particle size distribution, and aerosol associated water strongly affects the aerosol pH. Knowledge of the aerosol pH is important as it determines the aerosol chemistry, e.g., the efficiency of sulphur oxidation and bromine release. Here, we focus on the simulated sea salt aerosol size distribution and the coarse mode aerosol pH. A comparison with available field data shows that the simulated aerosol distributions agree reasonably well within the range of measurements. In spite of the small number of aerosol pH measurements and the uncertainty in its experimental determination, the simulated aerosol pH compares well with the observations. The aerosol pH ranges from alkaline aerosol in areas of strong production down to pH-values of 1 over regions of medium sea salt production and high levels of gas phase acids, mostly polluted regions over the oceans in the Northern Hemisphere.

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