ALBERT

Description: The concept of geoengineering by marine cloud brightening is based on seeding marine stratocumulus clouds with sub-micrometer sea-salt particles to enhance the cloud droplet number concentration and cloud albedo, thereby producing a climate cooling effect. The efficacy of this as a strategy for global cooling rests on the extent to which aerosol-perturbed marine clouds will respond with increased albedo. Ship tracks, quasi-linear cloud features prevalent in oceanic regions impacted by ship exhaust, are a well-known manifestation of the effect of aerosol injection on marine clouds. We present here an analysis of the albedo responses in ship tracks, based on in situ aircraft measurements and three years of satellite observations of 589 individual ship tracks. It is found that the sign (increase or decrease) and magnitude of the albedo response in ship tracks depends on the mesoscale cloud structure, the free tropospheric humidity, and cloud top height. In a closed cell structure (cloud cells ringed by a perimeter of clear air), nearly 30% of ship tracks exhibited a decreased albedo. Detailed cloud responses must be accounted for in global studies of the potential efficacy of sea-spray geoengineering as a means to counteract global warming.

Description: Anthropogenic emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) may affect concentration levels and composition of biogenic secondary organic aerosols (BSOA) through photochemical reactions with biogenic organic precursors to form organosulfates and nitrooxy organosulfates. We investigated this influence in a field study from 19 May to 22 June, 2011 at two sampling sites in Denmark. Within the study, we identified a substantial number of organic acids, organosulfates and nitrooxy organosulfates in the ambient urban curbside and semi-rural background air. A high degree of correlation in concentrations was found among a group of specific organic acids, organosulfates and nitrooxy organosulfates, which may originate from various precursors, suggesting a common mechanism or factor affecting their concentration levels at the sites. It was proposed that the formation of those species most likely occurred on a larger spatial scale, with the compounds being long-range transported to the sites on the days with the highest concentrations. The origin of the long-range transported aerosols was investigated using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model in addition to modeled emissions of related precursors, including isoprene and monoterpenes using the global Model of Emissions of Gases and Aerosols from Nature (MEGAN) and SO2 emissions using the European Monitoring and Evaluation Program (EMEP) database. The local impacts were also studied by examining the correlation between selected species, which showed significantly enhanced concentrations at the urban curbside site and the local concentrations of various gases, including SO2, ozone (O3), NOx, aerosol acidity and other meteorological conditions. This investigation showed that an inter-play of the local parameters such as the aerosol acidity, NOx, SO2, relative humidity (RH), temperature and global radiation seemed to affect the concentration level of those species, suggesting the influence of aqueous aerosol chemistry. The local impacts, however, seemed minor compared to the regional impacts. The total concentrations of organosulfates and nitrooxy organosulfates, on average, contributed to approximately 0.5–0.8% of PM1 mass at the two sampling sites.

Description: The concept of geoengineering by marine cloud brightening is based on seeding marine stratocumulus clouds with sub-micrometer sea-salt particles to enhance the cloud droplet number concentration and cloud albedo, thereby producing a climate cooling effect. The efficacy of this as a strategy for global cooling rests on the extent to which aerosol-perturbed marine clouds will respond with increased albedo. Ship tracks, cloud regions impacted by ship exhaust, are a well-known manifestation of the effect of aerosol injection on marine clouds. We present here an analysis of the albedo responses in ship tracks, based on in situ aircraft measurements and three years of satellite observations of 589 individual ship tracks. It is found that the sign (increase or decrease) and magnitude of the albedo response in ship tracks depends on the mesoscale cloud structure, the free tropospheric humidity, and cloud top height. In a closed cell structure (cloud cells ringed by a perimeter of clear air), nearly 30% of ship tracks exhibited a decreased albedo. Detailed cloud responses must be accounted for in global studies of the potential efficacy of sea-spray geoengineering as a means to counteract global warming.

Description: Anthropogenic emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) may affect concentration levels and composition of biogenic secondary organic aerosols (BSOA) through photochemical reactions with biogenic organic precursors to form organosulfates and nitrooxy organosulfates. We investigated this influence in a field study from 19 May–22 June 2011 at two sampling sites in Denmark. Within the study, we identified a substantial number of organic acids, organosulfates and nitrooxy organosulfates in the ambient urban curbside and semi-rural background air. A high degree of correlation in concentrations was found among a group of specific organic acids, organosulfates and nitrooxy organosulfates, which may originate from various precursors, suggesting a common mechanism or factor affecting their concentration levels at the sites. It was proposed that the formation of those species most likely occurred on a larger spatial scale with the compounds being long-range transported to the sites on the days with highest concentrations. The origin of the long-range transported aerosols was investigated using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model in addition to modeled emissions of related precursors including isoprene and monoterpenes using the global Model of Emissions of Gases and Aerosols from Nature (MEGAN) and SO2 emissions using the European Monitoring and Evaluation Program (EMEP) database. The local impacts were also studied by examining the correlation between selected species which showed significantly enhanced concentrations at the urban curbside site and the local concentrations of various gases including SO2, ozone (O3), carbon monoxide (CO), NOx, aerosol acidity and other meteorological conditions. This investigation showed that an inter-play of the local parameters such as the aerosol acidity, NOx, relative humidity (RH), temperature and global radiation seemed to influence the concentration level of those species, via such as wet aerosol chemistry. The local impacts however seemed minor on the concentration levels of the studied compounds. The total concentrations of organosulfates and nitrooxy organosulfates contributed to approximately 0.7% of PM1 mass.

Description: Submerged oceanic bubbles, which have a much longer life span than whitecaps or bubble rafts, have been hypothesized to increase the water-leaving radiance and thus affect satellite-based estimates of water-leaving radiance to non-trivial levels. This study explores this effect further to determine whether such bubbles are of sufficient magnitude to impact satellite aerosol optical depth (AOD) retrievals through perturbation of the lower boundary conditions. There has been significant discussion in the community regarding the high positive biases in retrieved AODs in many remote ocean regions. In this study, for the first time, the effects of oceanic bubbles on satellite retrievals of AOD are studied by using a linked Second Simulation of a Satellite Signal in the Solar Spectrum (6S) atmospheric and HydroLight oceanic radiative transfer models. The results suggest an insignificant impact on AOD retrievals in regions with near-surface wind speeds of less than 12 m s−1. However, the impact of bubbles on aerosol retrievals could be on the order of 0.02–0.04 for higher wind conditions within the scope of our simulations (e.g., winds 〈 20 m −1. This bias is propagated to global scales using 1 year of Moderate Resolution Imaging Spectroradiometer (MODIS) and Advanced Microwave Scanning Radiometer EOS (AMSR-E) data to investigate the possible impacts of oceanic bubbles on an enhanced AOD belt observed over the high-latitude southern oceans (also called the enhanced southern oceans anomaly, or ESOA) by some passive satellite sensors. Ultimately, this study is supportive of the null hypothesis: submerged bubbles are not the major contributor to the ESOA feature. This said, as retrievals progress to higher and higher resolutions, such as from airborne platforms, the uniform bubble correction in clean marine conditions should probably be separately accounted for against individual bright whitecaps and bubble rafts.

Description: Submerged oceanic bubbles, which could have a much longer life span than whitecaps or bubble rafts, have been hypothesized to increase the water-leaving radiance and thus affect satellite based estimates of water-leaving radiance to non-trivial levels. This study explores this effect further to determine if such bubbles are of sufficient magnitude to impact satellite Aerosol Optical Depth (AOD) retrievals through perturbation of the lower boundary conditions. Indeed, there has been significant discussion in the community regarding the high positive biases in retrieved AODs in many remote ocean regions. In this study, for the first time, the effects of oceanic bubbles on satellite retrievals of AOD are studied by using a linked Second Simulation of a Satellite Signal in the Solar Spectrum (6S) atmospheric and HydroLight oceanic radiative transfer models. The results suggest an insignificant impact on AOD retrievals in regions with near-surface wind speeds of less than 12 m s−1. However, the impact of bubbles on aerosol retrievals could be on the order of 0.02–0.04 for higher wind conditions within the scope of our simulations (e.g., winds 〈 20 m s−1). This bias is propagated to global scales using one year of Moderate Resolution Imaging Spectroradiometer (MODIS) and Advanced Microwave Scanning Radiometer – Earth (AMSR-E) data to investigate the possible impacts of oceanic bubbles on an enhanced AOD belt observed over the high latitude southern oceans (also called Enhanced Southern Oceans Anomaly, or ESOA) by some passive satellite sensors. Ultimately, this study is supportive of the null hypothesis: submerged bubbles are not the major contributor to the ESOA feature. This said, as retrievals progress to higher and higher resolutions, such as from airborne platforms, in clean marine conditions the uniform bubble correction should probably be separately accounted for against individual bright whitecaps and bubble rafts.