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

Abstract. A GLObal Model of Aerosol Processes (GLOMAP) has been developed as an extension to the TOMCAT 3-D Eulerian off-line chemical transport model. GLOMAP simulates the evolution of the global aerosol size distribution using a sectional two-moment scheme and includes the processes of aerosol nucleation, condensation, growth, coagulation, wet and dry deposition and cloud processing. We describe the results of a global simulation of sulfuric acid and sea spray aerosol. The model captures features of the aerosol size distribution that are well established from observations in the marine boundary layer and free troposphere. Modelled condensation nuclei (CN>3nm) vary between about 250–500 cm-3 in remote marine boundary layer regions and are generally in good agreement with observations. Modelled continental CN concentrations are lower than observed, which may be due to lack of some primary aerosol sources or the neglect of nucleation mechanisms other than binary homogeneous nucleation of sulfuric acid-water particles. Remote marine CN concentrations increase to around 2000–10 000 cm

Download Full-text

Assessing the potential efficacy of marine cloud brightening for cooling Earth using a simple heuristic model

Atmospheric Chemistry and Physics ◽

10.5194/acp-21-14507-2021 ◽

2021 ◽

Vol 21 (19) ◽

pp. 14507-14533

Author(s):

Robert Wood

Keyword(s):

Boundary Layer ◽

Size Distribution ◽

Radiative Forcing ◽

Marine Boundary Layer ◽

Cloud Droplet ◽

Aerosol Size Distribution ◽

Heuristic Model ◽

Droplet Concentration ◽

Salt Mass ◽

Large Eddy

Abstract. A simple heuristic model is described to assess the potential for increasing solar reflection by augmenting the aerosol population below marine low clouds, which nominally leads to increased cloud droplet concentration and albedo. The model estimates the collective impact of many point source particle sprayers, each of which generates a plume of injected particles that affects clouds over a limited area. A look-up table derived from simulations of an explicit aerosol activation scheme is used to derive cloud droplet concentration as a function of the sub-cloud aerosol size distribution and updraft speed, and a modified version of Twomey's formulation is used to estimate radiative forcing. Plume overlap is accounted for using a Poisson distribution, assuming idealized elongated cuboid plumes that have a length driven by aerosol lifetime and wind speed, a width consistent with satellite observations of ship track broadening, and a depth equal to an assumed boundary layer depth. The model is found to perform favorably against estimates of brightening from large eddy simulation studies that explicitly model cloud responses to aerosol injections over a range of conditions. Although the heuristic model does not account for cloud condensate or coverage adjustments to aerosol, in most realistic ambient remote marine conditions these tend to augment the Twomey effect in the large eddy simulations, with the result being a modest underprediction of brightening in the heuristic model. The heuristic model is used to evaluate the potential for global radiative forcing from marine cloud brightening as a function of the quantity, size, and lifetime of salt particles injected per sprayer and the number of sprayers deployed. Radiative forcing is sensitive to both the background aerosol size distribution in the marine boundary layer into which particles are injected and the assumed updraft speed. Given representative values from the literature, radiative forcing sufficient to offset a doubling of carbon dioxide ΔF2×CO2 is possible but would require spraying 50 % or more of the ocean area. This is likely to require at least 104 sprayers to avoid major losses of particles due to near-sprayer coagulation. The optimal dry diameter of injected particles, for a given salt mass injection rate, is 30–60 nm. A major consequence is that the total salt emission rate (50–70 Tg yr−1) required to offset ΔF2×CO2 is a factor of five lower than the emissions rates required to generate significant forcing in previous studies with climate models, which have mostly assumed dry diameters for injected particles in excess of 200 nm. With the lower required emissions, the salt mass loading in the marine boundary layer for ΔF2×CO2 is dominated by natural salt aerosol, with injected particles only contributing ∼ 10 %. When using particle sizes optimized for cloud brightening, the aerosol direct radiative forcing is shown to make a minimal contribution to the overall radiative forcing.

Download Full-text

Marine boundary layer measurements of new particle formation and the effects nonprecipitating clouds have on aerosol size distribution

Journal of Geophysical Research Atmospheres ◽

10.1029/94jd00797 ◽

1994 ◽

Vol 99 (D7) ◽

pp. 14443 ◽

Cited By ~ 256

Author(s):

W. A. Hoppel ◽

G. M. Frick ◽

J. W. Fitzgerald ◽

R. E. Larson

Keyword(s):

Boundary Layer ◽

Size Distribution ◽

Marine Boundary Layer ◽

Particle Formation ◽

Aerosol Size Distribution ◽

New Particle Formation

Download Full-text

Assessing the potential efficacy of marine cloud brightening for cooling Earth using a simple heuristic model

10.5194/acp-2021-327 ◽

2021 ◽

Author(s):

Robert Wood

Keyword(s):

Boundary Layer ◽

Size Distribution ◽

Radiative Forcing ◽

Marine Boundary Layer ◽

Cloud Droplet ◽

Aerosol Size Distribution ◽

Heuristic Model ◽

Droplet Concentration ◽

Salt Mass ◽

Large Eddy

Abstract. A simple heuristic model is described to assess the potential for increasing solar reflection by augmenting the aerosol population below marine low clouds, which nominally leads to increased cloud droplet concentration and albedo. The model estimates the collective impact of many point-source particle sprayers, each of which generates a plume of injected particles that affects clouds over a limited area. A widely-used aerosol activation scheme is used to derive cloud droplet concentration as a function of the sub-cloud aerosol size distribution and updraft speed, and a modified version of Twomey's formulation is used to estimate radiative forcing. Plume overlap is accounted for using a Poisson distribution assuming idealized elongated cuboid plumes that have a length driven by aerosol lifetime and wind speed, a width consistent with satellite observations of ship track broadening, and a depth equal to an assumed boundary layer depth. The model is found to perform favorably against estimates of brightening from large eddy simulation studies that explicitly model cloud responses to aerosol injections over a range of conditions. Although the heuristic model does not account for cloud condensate or coverage adjustments to aerosol, in most realistic ambient remote marine conditions these tend to augment the Twomey effect in the large eddy simulations, with the resulting being a modest underprediction of brightening in the heuristic model. The heuristic model is used to evaluate the potential for global radiative forcing from marine cloud brightening as a function of the quantity, size, and lifetime of salt particles injected per sprayer and the number of sprayers deployed. Radiative forcing is sensitive to both the background aerosol size distribution in the marine boundary layer into which particles are injected, and the assumed updraft speed. Given representative values from the literature, radiative forcing sufficient to offset a doubling of carbon dioxide ΔF2xCO2 is possible but would require spraying over 50% or more of the ocean area. This is likely to require at least 104 sprayers to avoid major losses of particles due to near-sprayer coagulation. The optimal dry diameter of injected particles, for a given salt mass injection rate, is 30–60 nm. A major consequence is that the total salt emission rate (50–70 Tg/yr) required to offset F2xCO2 is a factor of five lower than the emissions rates required to generate significant forcing in previous studies with climate models, which have mostly assumed dry diameters for injected particles in excess of 200 nm. With the lower required emissions, the salt mass loading in the marine boundary layer for F2xCO2 is dominated by natural salt aerosol, with injected particles only contributing ~ 10%. When using particle sizes optimized for cloud brightening, the aerosol direct radiative forcing is shown to make a minimal contribution to the overall radiative forcing.

Download Full-text

A global off-line model of size-resolved aerosol microphysics: I. Model development and prediction of aerosol properties

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-5-179-2005 ◽

2005 ◽

Vol 5 (1) ◽

pp. 179-215 ◽

Cited By ~ 12

Author(s):

D. V. Spracklen ◽

K. J. Pringle ◽

K. S. Carslaw ◽

M. P. Chipperfield ◽

G. W. Mann

Keyword(s):

Boundary Layer ◽

Sulfuric Acid ◽

Size Distribution ◽

Marine Boundary Layer ◽

Transport Model ◽

Cloud Condensation Nuclei ◽

Aerosol Size Distribution ◽

Sea Spray ◽

Condensation Nuclei ◽

Upper Troposphere

Abstract. A GLObal Model of Aerosol Processes (GLOMAP) has been developed as an extension to the TOMCAT 3-D Eulerian off-line chemical transport model. GLOMAP simulates the evolution of the global aerosol size distribution using a sectional two-moment scheme and includes the processes of aerosol nucleation, condensation, growth, coagulation, wet and dry deposition and cloud processing. We describe the results of a global simulation of sulfuric acid and sea spray aerosol. The model captures features of the aerosol size distribution that are well established from observations in the marine boundary layer and free troposphere. Modelled condensation nuclei (CN>3 nm) vary between about 250–500 cm-3 in remote marine boundary layer regions and between 2000 and 10 000 cm-3 (at standard temperature and pressure) in the upper troposphere. Cloud condensation nuclei (CCN) at 0.2% supersaturation vary between about 1000 cm-3 in polluted regions and between 10 and 500 cm-3 in the remote marine boundary layer. New particle formation through sulfuric acid-water binary nucleation occurs predominantly in the upper troposphere, but the model results show that these particles contribute greatly to aerosol concentrations in the marine boundary layer. It is estimated that sea spray emissions account for only ~10% of CCN in the tropical marine boundary layer, but between 20 and 75% in the mid-latitude Southern Ocean.

Download Full-text

Combining airborne in situ and ground-based lidar measurements for attribution of aerosol layers

10.5194/acp-2017-1085 ◽

2018 ◽

Author(s):

Anna Nikandrova ◽

Ksenia Tabakova ◽

Antti Manninen ◽

Riikka Väänänen ◽

Tuukka Petäjä ◽

...

Keyword(s):

Boundary Layer ◽

Size Distribution ◽

Aerosol Size Distribution ◽

Long Range Transport ◽

Back Trajectories ◽

Clear Sky ◽

Temporal And Spatial Variability ◽

Range Transport ◽

Temporal And Spatial

Abstract. Understanding the distribution of aerosol layers is important for determining long range transport and aerosol radiative forcing. In this study we combine airborne in situ measurements of aerosol with data obtained by a ground-based High Spectral Resolution Lidar (HSRL) and radiosonde profiles to investigate the temporal and vertical variability of aerosol properties in the lower troposphere. The HSRL was deployed in Hyytiälä, Southern Finland, from January to September 2014 as a part of the US DoE ARM (Atmospheric Radiation Measurement) mobile facility during the BAECC (Biogenic Aerosols – Effects on Cloud and Climate) Campaign. Two flight campaigns took place in April and August 2014 with instruments measuring the aerosol size distribution from 10 nm to 10 µm at altitudes up to 3800 m. Two case studies from the flight campaigns, when several aerosol layers were identified, were selected for further investigation: one clear sky case, and one partly cloudy case. During the clear sky case, turbulent mixing ensured low temporal and spatial variability in the measured aerosol size distribution in the boundary layer whereas mixing was not as homogeneous in the boundary layer during the partly cloudy case. The elevated layers exhibited greater temporal and spatial variability in aerosol size distribution, indicating a lack of mixing. New particle formation was observed in the boundary layer during the clear sky case, and nucleation mode particles were also seen in the elevated layers that were not mixing with the boundary layer. Interpreting local measurements of elevated layers in terms of long-range transport can be achieved using back trajectories from Lagrangian models, but care should be taken in selecting appropriate arrival heights, since the modelled and observed layer heights did not always coincide. We conclude that higher confidence in attributing elevated aerosol layers with their air mass origin is attained when back trajectories are combined with lidar and radiosonde profiles.

Download Full-text

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

Atmospheric Chemistry and Physics ◽

10.5194/acp-18-17615-2018 ◽

2018 ◽

Vol 18 (23) ◽

pp. 17615-17635 ◽

Cited By ~ 10

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.

Download Full-text

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

Atmospheric Chemistry and Physics ◽

10.5194/acp-5-1527-2005 ◽

2005 ◽

Vol 5 (6) ◽

pp. 1527-1543 ◽

Cited By ~ 28

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.

Download Full-text

Marine boundary layer aerosol in Eastern North Atlantic: seasonal variations and key controlling processes

10.5194/acp-2018-574 ◽

2018 ◽

Author(s):

Guangjie Zheng ◽

Yang Wang ◽

Allison C. Aiken ◽

Francesca Gallo ◽

Mike Jensen ◽

...

Keyword(s):

Boundary Layer ◽

North Atlantic ◽

Seasonal Variations ◽

Dimethyl Sulfide ◽

Marine Boundary Layer ◽

Cloud Condensation Nuclei ◽

Sulfide Oxidation ◽

Aerosol Size Distribution ◽

Eastern North Atlantic ◽

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 derive 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 from 2015 to 2017. Measurements impacted by local pollutions on Graciosa Island and during occasional intense biomass burning and dust events are excluded from this analysis. Submicron aerosol size distribution typically consists of three modes: Aitken (At), Accumulation (Ac), and Larger Accumulation (LA) 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, 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 At mode particles inside MBL, while 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 estimate based on the major sources and sink. NAt is mainly controlled by entrainment of FT aerosol, coagulation loss, and growth of At mode particles into Ac mode size range. Our calculation suggests besides the direct contribution from entrained FT Ac mode particles, growth of entrained FT At mode particles in the MBL also represent a substantial source of cloud condensation nuclei (CCN), with the highest contribution potentially reaching nearly 60 % during summer. The growth of At mode particles to CCN size is expected a result of the condensation of sulfuric acid from dimethyl sulfide oxidation, suggesting that ocean ecosystems may have a substantial influence on MBL CCN populations in ENA.

Download Full-text