Dean A. Hegg

Chemical apportionment of aerosol column optical depth off the mid‐Atlantic coast of the United States

Aerosol column optical depths derived from airborne Sun photometer and in situ measurements of aerosol properties in 14 vertical profiles off the mid-Atlantic coast of the United States in June show excellent agreement. Simultaneous measurements of the chemical compositions of the aerosol allows an assessment of the chemical apportionment of the aerosol column optical depths. The optical depths had essentially three chemical components, which, in order of descending average contributions, were condensed water, carbonaceous species, and sulfate. These results do not support the common assumption that sulfate dominates aerosol optical depths in polluted regions.

Airborne Measurements of Carbonaceous Aerosols on the East Coast of the United States

In this paper we report results of aircraft measurements of mass concentrations of carbonaceous aerosols, total aerosol mass, and simultaneously determined light scattering and absorption coefficients onshore and offshore of the eastern coast of the United States. We describe the sampling and analytical methodology used to derive spatially resolved aerosol carbon concentrations and discuss these in the context of other concurrently collected data. The carbon mass was, on average, 50% of the total dry aerosol mass. The carbon mass fraction tended to increase with altitude, suggesting that ground-based measurements can significantly underestimate the importance of carbon species in the column aerosol mass budget. The aerosol carbon mass was significantly correlated with aerosol light absorption, suggesting that the black and organic carbon components have, at least in part, common combustion sources. A regression of the light absorption coefficient onto carbon mass suggests that ∼10% of the carbon mass is in the form of black (light-absorbing) carbon.

Humidification factors for atmospheric aerosols off the mid‐Atlantic coast of the United States

The light-scattering coefficients of aerosols as a function of relative humidity (RH) and wavelength were measured in flights off the mid-Atlantic coast of the United States during July 1996. At a wavelength of 550 nm the ratio of the total light-scattering coefficient at RH = 80% to that at RH = 30% in westerly airflows had a mean value of 2.30 with a standard deviation of 0.24; in northerly and southerly airflows the mean value was 1.81 with a standard deviation of 0.37. This ratio generally decreased with increasing altitude and with increasing mass ratio of aerosol carbon to sulfate, and it increased with increasing wavelength. The aerosol hemispheric backscatter ratio decreased by 30 to 40% as the RH increased from 30 to 80%. Direct radiative forcing by aerosols advected off the mid-Atlantic coast of the United States is strongly dependent on RH. At a RH of 80%, direct radiative forcing by the aerosol is roughly twice that of the dry aerosol.

Measurements of Aitken nuclei and cloud condensation nuclei in the marine atmosphere and their relation to the DMS-cloud-climate hypothesis

New airborne measurements provide support for the hypothesis that layers of high concentrations of Aitken nuclei near the tops of marine clouds are due to photochemical nucleation. They also reveal a significant correlation between cloud condensation nucleus (CCN) concentrations in the boundary layer and mean cloud droplet concentration in stratus clouds topping a marine boundary layer. Nonsea salt sulfate mass and the concentration of CCN active at 1% supersaturation are also significantly correlated. These results provide quantitative support for some facets of the DMS-cloud-climate hypothesis.

A preliminary study of the effect of ammonia on particle nucleation in the marine boundary layer

A ternary nucleation model for the H2SO4-NH3-H2O system is presented in an effort to examine the effect of NH3 on heteromolecular homogeneous nucleation in the marine boundary layer (MBL). The results from this nucleation model suggest that ammonia could, in fact, enhance the nucleation rate over that of the binary system, H2SO4-H2O. The magnitude of this enhancement is introduced as an enhancement ratio, which, in principle, is applicable to any binary nucleation rate for H2SO4-H2O. Also presented are preliminary results from a simple aerosol model using this enhancement ratio. These results suggest that under conditions typical of the marine environment it may be possible to produce enough particles to balance the various particle sinks characteristic of the MBL.

Emissions from Ships with respect to Their Effects on Clouds

Emissions of particles, gases, heat, and water vapor from ships are discussed with respect to their potential for changing the microstructure of marine stratiform clouds and producing the phenomenon known as ‘‘ship tracks.’’ Airborne measurements are used to derive emission factors of SO 2 and NO from diesel-powered and steam turbine-powered ships, burning low-grade marine fuel oil (MFO); they were ;15‐89 and ;2‐25 g kg21 of fuel burned, respectively. By contrast a steam turbine‐powered ship burning high-grade navy distillate fuel had an SO2 emission factor of ; 6gk g 21. Various types of ships, burning both MFO and navy distillate fuel, emitted from ;4 3 1015 to 2 3 1016 total particles per kilogram of fuel burned (;4 3 1015‐1.5 3 1016 particles per second). However, diesel-powered ships burning MFO emitted particles with a larger mode radius (;0.03‐0.05 mm) and larger maximum sizes than those powered by steam turbines burning navy distillate fuel (mode radius ;0.02 mm). Consequently, if the particles have similar chemical compositions, those emitted by diesel ships burning MFO will serve as cloud condensation nuclei (CCN) at lower supersaturations (and will therefore be more likely to produce ship tracks) than the particles emitted by steam turbine ships burning distillate fuel. Since steam turbine‐powered ships fueled by MFO emit particles with a mode radius similar to that of diesel-powered ships fueled by MFO, it appears that, for given ambient conditions, the type of fuel burned by a ship is more important than the type of ship engine in determining whether or not a ship will produce a ship track. However, more measurements are needed to test this hypothesis. The particles emitted from ships appear to be primarily organics, possibly combined with sulfuric acid produced by gas-to-particle conversion of SO 2. Comparison of model results with measurements in ship tracks suggests that the particles from ships contain only about 10% water-soluble materials. Measurements of the total particles entering marine stratiform clouds from diesel-powered ships fueled by MFO, and increases in droplet concentrations produced by these particles, show that only about 12% of the particles serve as CCN. The fluxes of heat and water vapor from ships are estimated to be ;2‐22 MW and;0.5‐1.5 kg s21, respectively. These emissions rarely produced measurable temperature perturbations, and never produced detectable perturbations in water vapor, in the plumes from ships. Nuclear-powered ships, which emit heat but negligible particles, do not produce ship tracks. Therefore, it is concluded that heat and water vapor emissions do not play a significant role in ship track formation and that particle emissions, particularly from those burning low-grade fuel oil, are responsible for ship track formation. Subsequent papers in this special issue discuss and test these hypotheses.

Measurements of sulfate production in natural clouds

Abstract Measurements are presented on sulfate production in wave clouds. Twenty eight cases are analyzed and an empirical rate equation for sulfate production in these clouds is derived. Some of the measured rates suggest higher oxidation rates in clouds than have previously been suggested. If the rate is assumed to be proportional to the concentration of sulfite in the drops, regression analysis of the data set shows the oxidation rate to be proportional to the first power of the explicit hydrogen ion concentration in the drops and to be relatively insensitive to temperature. Possible mechanisms for the sulfate production are discussed in light of the field measurements

Influence of Humidity On the Aerosol Scattering Coefficient and Its Effect on the Upwelling Radiance During ACE-2

Aerosol scattering coefficients (σsp) have been measured over the ocean at different relative humidities (RH) as a function of altitude in the region surrounding the Canary Islands during the Second Aerosol Characterization Experiment (ACE-2) in June and July 1997. The data were collected by the University of Washington passive humidigraph (UWPH) mounted on the Pelican research aircraft. Concurrently, particle size distributions, absorption coefficients and aerosol optical depth were measured throughout 17 flights. A parameterization of σsp as a function of RH was utilized to assess the impact of aerosol hydration on the upwelling radiance (normalized to the solar constant and cosine of zenith angle). The top of the atmosphere radiance signal was simulated at wavelengths corresponding to visible and near-infrared bands of the EOS-AM )“Terra” (detectors, MODIS and MISR. The UWPH measured σsp at 2 RHs, one below and the other above ambient conditions. Ambient σsp was obtained by interpolation of these 2 measurements. The data were stratified in terms of 3 types of aerosols: Saharan dust, clean marine (marine boundary layer background) and polluted marine aerosols (i.e., 2- or 1-day old polluted aerosols advected from Europe). An empirical relation for the dependence of σsp on RH, defined by σsp(RH)=k. (1−RH/100)−γ, was used with the hygroscopic exponent γ derived from the data. The following γ values were obtained for the 3 aerosol types: γ(dust)=0.23±0.05, γ(clean marine)= 0.69±0.06 and γ(polluted marine)=0.57±0.06. Based on the measured γs, the above equation was utilized to derive aerosol models with different hygroscopicities. The satellite simulation signal code 6S was used to compute the upwelling radiance corresponding to each of those aerosol models at several ambient humidities. For the pre-launch estimated precision of the sensors and the assumed viewing geometry of the instrument, the simulations suggest that the spectral and angular dependence of the reflectance measured by MISR is not sufficient to distinguish aerosol models with various different combinations of values for dry composition, γ and ambient RH. A similar behavior is observed for MODIS at visible wavelengths. However, the 2100 nm band of MODIS appears to be able to differentiate between at least same aerosol models with different aerosol hygroscopicity given the MODIS calibration error requirements. This result suggests the possibility of retrieval of aerosol hygroscopicity by MODIS.

OXIDATION OF SULFUR DIOXIDE IN AQUEOUS SYSTEMS WITH PARTICULAR REFERENCE TO THE ATMOSPHERE

Laboratory studies of the uncatalyzed liquid phase oxidation of SO2 by oxygen are reviewed; significant discrepancies exist between the derived pseudo first order rate coefficients. The production rate is independent of the concentration of oxygen. For pH < 7 the activation energy for this reaction is 7.1 × 103Jmol−1 and for 7

Cloud water chemistry and the production of sulfates in clouds

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