A simple relationship between cloud drop number concentration and precursor aerosol concentration for the regions of Earth's large marine stratocumulus decks

Hegg, D́ean A.; Covert, D. S.; Jonsson, Haflidi H.; Woods, Roy K.

doi:10.5194/acp-12-1229-2012

Cited by 42 publications

(56 citation statements)

References 59 publications

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“…3.2) showed that the number concentration of particles > 0.1 µm near shore was dominated by particles composed primarily of sulfate. These relatively large, soluble particles are efficient cloud condensation nuclei and explain the strong correlation between accumulation-mode aerosol concentration and droplet number concentration along 20 • S, which has also been observed in other stratocumulus regions (Twohy et al, 2005;Hegg et al, 2012).…”

Section: (A)mentioning

confidence: 91%

“…This is similar to what other VOCALS researchers Allen et al, 2011) have used. Maximum droplet concentration was calculated using a 5-second pre-averaging technique for noise reduction as in Hegg et al (2012). A wing-mounted Particle Measuring Systems (PMS) Passive Cavity Aerosol Spectrometer Probe (PCASP-100) and a cabin mounted Ultra High Sensitivity Aerosol Spectrometer (UHSAS) counted and sized particles from 0.1-3.0 µm and 0.055-1.0 µm dry diameter, respectively.…”

Section: Methods and Objectivesmentioning

confidence: 99%

“…For the PCASP measurements, we have included a logarithmic as well as a linear fit to compare with Hegg et al (2012), who presented sub-cloud PCASP concentrations vs. droplet concentration from the Twin Otter aircraft. The Hegg et al (2012) study yielded a higher slope (0.72 vs. 0.47), but they used maximum measured droplet concentrations. Here we have presented mean droplet concentrations, as they are more representative of the variability of the entire cloud deck.…”

Section: Changes In Aerosol and Cloud Characteristics With Distance Fmentioning

confidence: 99%

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Impacts of aerosol particles on the microphysical and radiative properties of stratocumulus clouds over the southeast Pacific Ocean

et al. 2013

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The southeast Pacific Ocean is covered by the world's largest stratocumulus cloud layer, which has a strong impact on ocean temperatures and climate in the region. The effect of anthropogenic sources of aerosol particles on the stratocumulus deck was investigated during the VOCALS field experiment. Aerosol measurements below and above cloud were made with a ultra-high sensitivity aerosol spectrometer and analytical electron microscopy. In addition to more standard in-cloud measurements, droplets were collected and evaporated using a counterflow virtual impactor (CVI), and the non-volatile residual particles were analyzed.

Many flights focused on the gradient in cloud properties on an E-W track along 20° S from near the Chilean coast to remote areas offshore. Mean statistics, including their significance, from eight flights and many individual legs were compiled. Consistent with a continental source of cloud condensation nuclei, below-cloud accumulation-mode aerosol and droplet number concentration generally decreased from near shore to offshore. Single particle analysis was used to reveal types and sources of the enhanced particle number that influence droplet concentration. While a variety of particle types were found throughout the region, the dominant particles near shore were partially neutralized sulfates. Modeling and chemical analysis indicated that the predominant source of these particles in the marine boundary layer along 20° S was anthropogenic pollution from central Chilean sources, with copper smelters a relatively small contribution.

Cloud droplets were smaller in regions of enhanced particles near shore. However, physically thinner clouds, and not just higher droplet number concentrations from pollution, both contributed to the smaller droplets. Satellite measurements were used to show that cloud albedo was highest 500–1000 km offshore, and actually slightly lower closer to shore due to the generally thinner clouds and lower liquid water paths there. Thus, larger scale forcings that impact cloud macrophysical properties, as well as enhanced aerosol particles, are important in determining cloud droplet size and cloud albedo.

Differences in the size distribution of droplet residual particles and ambient aerosol particles were observed. By progressively excluding small droplets from the CVI sample, we were able to show that the larger drops, some of which may initiate drizzle, contain the largest aerosol particles. Geometric mean diameters of droplet residual particles were larger than those of the below-cloud and above cloud distributions. However, a wide range of particle sizes can act as droplet nuclei in these stratocumulus clouds. A detailed LES microphysical model was used to show that this can occur without invoking differences in chemical composition of cloud-nucleating particles

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Section: (A)mentioning

confidence: 91%

Section: Methods and Objectivesmentioning

confidence: 99%

Section: Changes In Aerosol and Cloud Characteristics With Distance Fmentioning

confidence: 99%

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Impacts of aerosol particles on the microphysical and radiative properties of stratocumulus clouds over the southeast Pacific Ocean

et al. 2013

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“…The coast of California is also important since in this area a persistent stratocumulus deck is found, which means that (1) aerosol retrieval from satellite is more challenging compared with more cloud-free areas, and (2) aerosol-cloud interaction is likely to be important (Hegg et al, 2012;Twohy et al, 2005).…”

Section: Study Domain and Experimental Designmentioning

confidence: 99%

Aerosol optical depth assimilation for a size-resolved sectional model: impacts of observationally constrained, multi-wavelength and fine mode retrievals on regional scale analyses and forecasts

et al. 2013

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Abstract. An aerosol optical depth (AOD) three-dimensional variational data assimilation technique is developed for the Gridpoint Statistical Interpolation (GSI) system for which WRF-Chem forecasts are performed with a detailed sectional model, the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC). Within GSI, forward AOD and adjoint sensitivities are performed using Mie computations from the WRF-Chem optical properties module, providing consistency with the forecast. GSI tools such as recursive filters and weak constraints are used to provide correlation within aerosol size bins and upper and lower bounds for the optimization. The system is used to perform assimilation experiments with fine vertical structure and no data thinning or re-gridding on a 12 km horizontal grid over the region of California, USA, where improvements on analyses and forecasts is demonstrated. A first set of simulations was performed, comparing the assimilation impacts of using the operational MODIS (Moderate Resolution Imaging Spectroradiometer) dark target retrievals to those using observationally constrained ones, i.e., calibrated with AERONET (Aerosol RObotic NETwork) data. It was found that using the observationally constrained retrievals produced the best results when evaluated against ground based monitors, with the error in PM 2.5 predictions reduced at over 90 % of the stations and AOD errors reduced at 100 % of the monitors, along with larger overall error reductions when grouping all sites. A second set of experiments reveals that the use of fine mode fraction AOD and ocean multi-wavelength retrievals can improve the representation of the aerosol size distribution, while assimilating only 550 nm AOD retrievals produces no or at times degraded impact. While assimilation of multi-wavelength AOD shows positive impacts on all analyses performed, future work is needed to generate observationally constrained multi-wavelength retrievals, which when assimilated will generate size distributions more consistent with AERONET data and will provide better aerosol estimates.

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“…Wettable particles with a dry diameter (D dry ) > 0.2 μm will activate at climatically relevant supersaturations (< 1%) almost regardless of their composition, while those containing highly soluble material may act as CCN with D dry as small as 0.05 μm [McFiggans et al, 2006;Andreae and Rosenfeld, 2008]. As such, the composition of aerosol particles, particularly those with D dry < 0.2 μm, can play an important role in determining the number concentration of subcloud CCN, which, combined with the dynamic nature of cloud supersaturation, is thought to ultimately determine the impact of aerosol particles on cloud drop size and number concentration [Hegg et al, 2012].…”

Section: Introductionmentioning

confidence: 99%

Impact of marine biogeochemistry on the chemical mixing state and cloud forming ability of nascent sea spray aerosol

et al. 2013

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[1] The composition and properties of sea spray aerosol, a major component of the atmosphere, are often controlled by marine biological activity; however, the scope of impacts that ocean chemistry has on the ability for sea spray aerosol to act as cloud condensation nuclei (CCN) is not well understood. In this study, we utilize a mesocosm experiment to investigate the impact of marine biogeochemical processes on the composition and mixing state of sea spray aerosol particles with diameters < 0.2 μm produced by controlled breaking waves in a unique ocean-atmosphere facility. An increase in relative abundance of a distinct, insoluble organic particle type was observed after concentrations of heterotrophic bacteria increased in the seawater, leading to an 86 ± 5% reduction in the hygroscopicity parameter (κ) at 0.2% supersaturation. Aerosol size distributions showed very little change and the submicron organic mass fraction increased by less than 15% throughout the experiment; as such, neither of these typical metrics can explain the observed reduction in hygroscopicity. Predictions of the hygroscopicity parameter that make the common assumption that all particles have the same bulk organic volume fractions lead to overpredictions of CCN concentrations by 25% in these experiments. Importantly, key changes in sea spray aerosol mixing state that ultimately influenced CCN activity were driven by bacteria-mediated alterations to the organic composition of seawater.

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A simple relationship between cloud drop number concentration and precursor aerosol concentration for the regions of Earth's large marine stratocumulus decks

Cited by 42 publications

References 59 publications

Impacts of aerosol particles on the microphysical and radiative properties of stratocumulus clouds over the southeast Pacific Ocean

Impacts of aerosol particles on the microphysical and radiative properties of stratocumulus clouds over the southeast Pacific Ocean

Aerosol optical depth assimilation for a size-resolved sectional model: impacts of observationally constrained, multi-wavelength and fine mode retrievals on regional scale analyses and forecasts

Impact of marine biogeochemistry on the chemical mixing state and cloud forming ability of nascent sea spray aerosol

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