Patricia K. Quinn

The case against climate regulation via oceanic phytoplankton sulphur emissions

More than twenty years ago, a biological regulation of climate was proposed whereby emissions of dimethyl sulphide from oceanic phytoplankton resulted in the formation of aerosol particles that acted as cloud condensation nuclei in the marine boundary layer. In this hypothesis—referred to as CLAW—the increase in cloud condensation nuclei led to an increase in cloud albedo with the resulting changes in temperature and radiation initiating a climate feedback altering dimethyl sulphide emissions from phytoplankton. Over the past two decades, observations in the marine boundary layer, laboratory studies and modelling efforts have been conducted seeking evidence for the CLAW hypothesis. The results indicate that a dimethyl sulphide biological control over cloud condensation nuclei probably does not exist and that sources of these nuclei to the marine boundary layer and the response of clouds to changes in aerosol are much more complex than was recognized twenty years ago. These results indicate that it is time to retire the CLAW hypothesis.

Carbohydrate-like composition of submicron atmospheric particles and their production from ocean bubble bursting

Oceans cover over two-thirds of the Earth’s surface, and the particles emitted to the atmosphere by waves breaking on sea surfaces provide an important contribution to the planetary albedo. During the International Chemistry Experiment in the Arctic LOwer Troposphere (ICEALOT) cruise on the R/V Knorr in March and April of 2008, organic mass accounted for 15–47% of the submicron particle mass in the air masses sampled over the North Atlantic and Arctic Oceans. A majority of this organic component (0.1 - 0.4 μ m-3) consisted of organic hydroxyl (including polyol and other alcohol) groups characteristic of saccharides, similar to biogenic carbohydrates found in seawater. The large fraction of organic hydroxyl groups measured during ICEALOT in submicron atmospheric aerosol exceeded those measured in most previous campaigns but were similar to particles in marine air masses in the open ocean (Southeast Pacific Ocean) and coastal sites at northern Alaska (Barrow) and northeastern North America (Appledore Island and Chebogue Point). The ocean-derived organic hydroxyl mass concentration during ICEALOT correlated strongly to submicron Na concentration and wind speed. The observed submicron particle ratios of marine organic mass to Na were enriched by factors of ∼102–∼103 over reported sea surface organic to Na ratios, suggesting that the surface-controlled process of film bursting is influenced by the dissolved organic components present in the sea surface microlayer. Both marine organic components and Na increased with increasing number mean diameter of the accumulation mode, suggesting a possible link between organic components in the ocean surface and aerosol–cloud interactions.

Variations in the methanesulfonate to sulfate molar ratio in submicrometer marine aerosol particles over the south Pacific Ocean

Seawater concentrations of dimethylsulfide (DMS) and atmospheric concentrations of DMS, sulfur dioxide, methanesulfonate (MSA), and non-sea-salt (nss) sulfate were measured over the eastern Pacific Ocean between 105° and 110°W from 20°N to 60°S during February and March 1989. Although the samples collected in the southern hemisphere appear to be of marine origin, no significant correlation was found between the latitudinal distributions of DMS, SO2, MSA, and nss SO4=. However, an inverse correlation (r2 = 0.87) was found between atmospheric temperature and the MSA to nss SO4= molar ratio in submicrometer aerosol particles with a decrease in temperature corresponding to an increase in the molar ratio. Although this trend is consistent with laboratory results indicating the favored production of MSA at lower temperatures, it is contrary to southern hemisphere baseline station data. This suggests either a decrease in the supply of DMS relative to nonmarine sources of nss SO4= at the baseline stations in winter or additional mechanisms that affect the relative production of MSA and nss SO4=.

Aerosol optical properties in the marine boundary layer during the First Aerosol Characterization Experiment (ACE 1) and the underlying chemical and physical aerosol properties

Measurements were made onboard the NOAA R/V Discoverer during the First Aerosol Characterization Experiment (ACE 1) to understand the optical properties of a minimally perturbed natural aerosol system in terms of its chemical and physical properties. ACE 1 took place during November and December of 1995 in the Southern Ocean region south of Australia. Reported here are observations at a wavelength of 550 nm of the submicron and supermicron aerosol scattering coefficient, σsp; single scattering albedo, ω0; and the hemispheric backscattered fraction and mass scattering efficiencies of non-sea-salt sulfate, sea salt, and the total aerosol. Also presented is the Angstrom exponent, a, for the 450 and 700 nm wavelength pair. Variations in these parameters were found to be a strong function of the relative concentrations and size distributions of the dominant aerosol chemical components. Both the submicron and supermicron aerosol mass were composed primarily of water-soluble ionic species. This is in agreement with an experiment-average single scattering albedo of 0.99 (−0.4, +1.0%). Of the submicron ionic mass, 80±10% was sea salt, 16±8% was non-sea-salt sulfate, and 4±3% was methanesulfonate. Sea salt composed 99±0.7% of the supermicron ionic mass. The magnitude of scattering by both submicron and supermicron aerosol was controlled by sea salt. The backscattered fraction for the submicron aerosol averaged 0.11±0.02 and was controlled by the tailing of coarse mode sea-salt mass into the submicron size range. Calculated mass scattering efficiencies for submicron non-sea-salt sulfate ion averaged 1.5±0.74 m2 g−1 (at 30 to 45% relative humidity) with the highest values corresponding to continentally influenced air masses where sulfate aerosol surface mean diameters and surface area concentrations were the largest. Mass scattering efficiencies for submicron sea salt were higher (averaging 4.2±0.96 m2 g−1) due to the tailing of coarse mode sea salt into the particle size range most efficient for light scattering. Given the similar lifetimes of submicron non-sea-salt sulfate and sea salt in the marine boundary layer, it is evident that sea salt controls the aerosol optical properties in this Southern Ocean region.

Processes controlling the distribution of aerosol particles in the lower marine boundary layer during the First Aerosol Characterization Experiment (ACE 1)

The goals of the International Global Atmospheric Chemistry (IGAC) Programs First Aerosol Characterization Experiment (ACE 1) are to determine and understand the properties and controlling factors of the aerosol in the remote marine atmosphere that are relevant to radiative forcing and climate. A key question in terms of this goal and the overall biogeochemical sulfur cycle is what factors control the formation, growth, and evolution of particles in the marine boundary layer (MBL). To address this question, simultaneous measurements of dimethylsulfide (DMS), sulfur dioxide (SO2), the aerosol chemical mass size distribution, and the aerosol number size distribution from 5 to 10,000 nm diameter were made on the National Oceanic and Atmospheric Administration (NOAA) ship Discoverer. From these data we conclude that the background MBL aerosol during ACE 1 often was composed of four distinct modes: an ultrafine (UF) mode (Dp = 5–20 nm), an Aitken mode (Dp = 20–80 nm), an accumulation mode (Dp = 80–300 nm), and a coarse mode (Dp > 300 nm). The presence of UF mode particles in the MBL could be explained by convective mixing between the free troposphere (FT) and the MBL associated with cloud pumping and subsidence following cold frontal passages. There was no evidence of major new particle production in the MBL. Oceanic emissions of DMS appeared to contribute to the growth of Aitken and accumulation mode particles. Coarse mode particles were comprised primarily of sea salt. Although these particles result from turbulence at the air-sea interface, the instantaneous wind speed accounted for only one third of the variance in the coarse mode number concentration in this region.

Physical properties of marine boundary layer aerosol particles of the mid-Pacific in relation to sources and meteorological transport

Aerosol measurements were made on three cruises in the mid-Pacific along longitude 140°W from 55°N to 70°S for a total of about 90 days in 1992 and 1993. The three data sets document the aerosol concentration and general features of its number-size distribution in the marine boundary layer (MBL) and their variation with latitude and meteorological conditions. Mean concentration varied from 300 cm−3 in the tropics to 500 cm−3 in the midlatitudes outside of continental air masses. Infrequent short-term spikes in concentration ranged up to 2000 cm−3. Two dominant modes were observed, the Aitken and accumulation, with mean diameters of 25 to 60 nm and 150 to 200 nm, respectively. An intermittent ultrafine mode was noted at diameters less than 25 nm. The concentration and dominance of one mode over another depended on the relative strength of the entrainment of ultrafine and Aitken particles from the free troposphere (FT) into the MBL compared to the rate of growth of Aitken mode into accumulation mode particles and removal rate of the accumulation mode. In general, aging times were shorter in the subtropics, longer in the tropics, and variable in the midlatitudes. The rate of new particle formation within the MBL itself was either low and did not contribute significantly to the observed number concentration or, if the rate was high, it occurred infrequently and was not observed in these experiments.

Maritime Aerosol Network as a component of Aerosol Robotic Network

The paper presents the current status of the Maritime Aerosol Network (MAN), which has been developed as a component of the Aerosol Robotic Network (AERONET). MAN deploys Microtops handheld Sun photometers and utilizes the calibration procedure and data processing (Version 2) traceable to AERONET. A web site dedicated to the MAN activity is described. A brief historical perspective is given to aerosol optical depth (AOD) measurements over the oceans. A short summary of the existing data, collected on board ships of opportunity during the NASA Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies (SIMBIOS) Project is presented. Globally averaged oceanic aerosol optical depth (derived from island-based AERONET measurements) at 500 nm is similar to 0.11 and Angstrom parameter (computed within spectral range 440-870 nm) is calculated to be similar to 0.6. First results from the cruises contributing to the Maritime Aerosol Network are shown. MAN ship-based aerosol optical depth compares well to simultaneous island and near-coastal AERONET site AOD.

Submicron aerosol composition at Trinidad Head, California, during ITCT 2K2: Its relationship with gas phase volatile organic carbon and assessment of instrument performance

[1] Two Aerodyne aerosol mass spectrometers (AMSs) were deployed at Trinidad Head on the north Californian coast during the National Oceanographic and Atmospheric Administration Intercontinental Transport and Chemical Transformation 2002 (ITCT 2K2) experiment, to study the physiochemical properties of submicron aerosol particles within the Pacific marine boundary layer. One AMS was modified to allow the study of sea salt-based particles, while the other used a temperature cycling system on its inlet. The reported loadings increased by a factor of 2 when the temperature approached the dew point, which is due to the inlet performance and has implications for other AMS experiments and applications. The processed data were compared with those of a particle into liquid sampler-ion chromatograph and showed that the ammonium, sulfate and organic fractions of the particles were consistently found within a single, normally acidic, accumulation mode at around 300 - 400 nm. However, when influenced by land-based sources, vehicle emissions and increased ammonium loadings were seen. The concentrations of nitrate in the accumulation mode were low, but it was also found within sea salt particles in the coarse mode and can be linked to the displacement of chloride. The organic fraction showed a high degree of chemical ageing and evidence of nitrogen-bearing organics was also observed. The particulate organic data were compared to the volatile organic carbon data derived from an in-situ gas chromatograph-mass spectrometer-flame ionization detector and relationships were found between the gas and particle phase chemicals in both the overall concentrations and the levels of oxidation.

Measurements of chloride depletion and sulfur enrichment in individual sea-salt particles collected from the remote marine boundary layer

Changes in the elemental ratios of Cl/Na and S/Na in sea-salt particles are expected from the atmospheric reactions of sulfuric and nitric acids with these particles. Chloride depletion is expected to occur upon the liberation of HCl to the gas phase, with the particles remaining enriched in sulfate or nitrate. The elemental ratios of Ca/Na, Mg/Na and K/Na should remain constant during this process. Analysis of chloride depletion and sulfur enrichment was obtained for individual sodium-containing particles from the remote marine Pacific atmosphere in both the accumulation mode (0.06 ≤ Dp ≤ 1.0 μm, where Dp is the particle diameter) and the coarse mode (Dp > 1.0 μm) size range. Sodium-containing particles comprised close to 100% of the coarse mode and 11 to 31% of the accumulation mode by number. Aerosols were collected with a low-perssure impactor and examined with a transmission electron microscope (TEM) coupled with an energy-dispersive X ray (EDX) detector. The elemental ratios obtained from the atmospheric particles were determined by comparison with values obtained from laboratory-generated sea-salt, sodium chloride, and sodium sulfate particles of known size and chemical composition, which served as a calibration set. The elemental ratios of Ca/Na, Mg/Na, and K/Na were found to remain fairly constant between individual sea-salt particles of various sizes for more than 85% of the particles examined. Deviations in the ratio of Cl/Na and S/Na from that of reference seawater values were observed most commonly for the submicrometer sea-salt aerosol. The Cl/Na ratio was significantly (Students t test, 99.9%) lower than that of reference seawater for 89% of the particles examined, while the S/Na ratios were higher for 100% of the particles. The Cl/Na ratio measured in 48% of the coarse sea-salt particles (1.0 < Dp ≤ 2.5 μm) reflected the ratio in bulk seawater, while the remaining particles had statistically lower ratios and qualitatively different morphologies. All but 3% of these coarse particles had enhanced S/Na ratios over that of bulk seawater. Estimates of non-sea-salt (nss) sulfate mass ranged from 216 to 1422 fg for particles of 0.50 μm in diameter to 861 and 5235 fg for particles of 0.80 μm in diameter, corresponding to 74 to 96% of the sea-salt particle mass. These values are compared with the recent measurements of Mouri and Okada [1993] as well as predictions from the atmospheric chemistry models of in-cloud sulfate production of Hegg et al., [1992] and estimations of S(IV) oxidation in sea-salt aerosol water by Chameides and Stelson [1992].

Dimethylsulfide/cloud condensation nuclei/climate system - Relevant size-resolved measurements of the chemical and physical properties of atmospheric aerosol particles

Atmospheric aerosol particles resulting from the oxidation of dimethylsulfide (DMS) may have an impact on global climate if they result in an enhancement in the cloud condensation nuclei (CCN) number concentration and shortwave cloud albedo. To characterize and quantify relationships within the DMS/CCN/climate system, simultaneous measurements were made over the northeastern Pacific Ocean in April and May 1991 of particulate non-sea-salt sulfate, methanesulfonate, and ammonium mass size distributions, number size distributions of particles having diameters between 0.02 and 9.6 μm, CCN concentrations at 0.3% supersaturation, relative humidity, and temperature. Comparisons between particle mass and surface area indicate that non-sea-salt sulfate, methanesulfonate, and ammonium were not involved in new particle production on the 12- to 24-hour time scale of the measurements. Instead, high levels of available particulate surface area resulted in the condensation of the gas phase precursors onto existing aerosol. A doubling of non-sea-salt sulfate, methanesulfonate, and ammonium mass corresponded to a 40 to 50% increase in number in the accumulation mode size range. Likewise, a doubling of the non-sea-salt sulfate mass corresponded to a 40% increase in the CCN number concentration. As methanesulfonate made up a very small fraction of the soluble particulate mass, no correlations were found between methanesulfonate mass and CCN number. In a separate experiment, measurements were made of particulate non-sea-salt sulfate, methanesulfonate, and ammonium mass size distributions over the central Pacific Ocean during February 1991. The percent of methanesulfonate in the supermicrometer particle size fraction was greater in these samples than in those collected over coastal waters of the northeastern Pacific. In both regions the non-sea-salt sulfate mass size distributions were bimodal, while ammonium was found to be concentrated in larger accumulation mode particles.