Kevin J. Noone

A safe operating space for humanity

Identifying and quantifying planetary boundaries that must not be transgressed could help prevent human activities from causing unacceptable environmental change, argue Johan Rockstrom and colleagues.

Design and Calibration of a Counterflow Virtual Impactor for Sampling of Atmospheric Fog and Cloud Droplets

An instrument is described that samples cloud droplets by removing them from the surrounding air and small unactivated particles through inertial impaction. The sampled droplets are then evaporated, leaving behind the material dissolved or suspended in the droplets as residue particles or gases. The instrument is capable of sampling droplets as a function of their size; it has an adjustable cut size in the range between about 9 and 30 μm in diameter, rejects droplets and particles smaller than the cut size, and captures droplets larger than the cut size. Details of the instrumental design and construction are discussed, as well as a relative calibration of the collection efficiency. Results from the calibration experiments indicate that the counterflow virtual impactor probe behaves in accordance with theoretical predictions using Stokes number calculations. A complete description of the calibration methodology is presented.

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.

Hygroscopic growth of aerosol particles and its influence on nucleation scavenging in cloud: Experimental results from Kleiner Feldberg

The hygroscopic growth of individual aerosol particles has been measured with a Tandem Differential Mobility Analyser. The hygroscopic growth spectra were analysed in terms of diameter change with increasing RH from ≤20% to 85%. The measurements were carried out during the GCE cloud experiment at Kleiner Feldberg, Taunus, Germany in October and November 1990.Two groups of particles with different hygroscopic growth were observed. The less-hygroscopic group had average growth factors of 1.11, 1.04 and 1.02 for particle diameters of 50, 150 and 300 nm, respectively. The more-hygroscopic group had average growth factors of 1.34, 1.34, and 1.37 for the same particle diameters. The average fraction of less-hygroscopic particles was about 50%. Estimates of the soluble fractions of the particles belonging to the two groups are reported.Hygroscopic growth spectra for total aerosol, interstitial aerosol and cloud drop residuals were measured. A comparison of these hygroscopic growths of individual aerosol particles provides clear evidence for the importance of hygroscopic growth in nucleation scavenging. The measured scavenged fraction of particles as a function of diameter can be explained by the hygroscopic growth spectra.

In situ aerosol-size distributions and clear-column radiative closure during ACE-2

As part of the second Aerosol Characterization Experiment (ACE-2) during June and July of 1997, aerosol-size distributions were measured on board the CIRPAS Pelican aircraft through the use of a DMA and 2 OPCs. During the campaign, the boundary-layer aerosol typically possessed characteristics representative of a background marine aerosol or a continentally influenced aerosol, while the free-tropospheric aerosol was characterized by the presence or absence of a Saharan dust layer. A range of radiative closure comparisons were made using the data obtained during vertical profiles flown on 4 missions. Of particular interest here are the comparisons made between the optical properties as determined through the use of measured aerosol-size distributions and those measured directly by an airborne 14-wavelength sunphotometer and 3 nephelometers. Variations in the relative humidity associated with each of the direct measurements required consideration of the hygroscopic properties of the aerosol for size-distribution-based calculations. Simultaneous comparison with such a wide range of directly-measured optical parameters not only offers evidence of the validity of the physicochemical description of the aerosol when closure is achieved, but also provides insight into potential sources of error when some or all of the comparisons result in disagreement. Agreement between the derived and directly-measured optical properties varied for different measurements and for different cases. Averaged over the 4 case studies, the derived extinction coefficient at 525 nm exceeded that measured by the sunphotometer by 2.5% in the clean boundary layer, but underestimated measurements by 13% during pollution events. For measurements within the free troposphere, the mean derived extinction coefficient was 3.3% and 17% less than that measured by the sunphotometer during dusty and non-dusty conditions, respectively. Likewise, averaged discrepancies between the derived and measured scattering coefficient were −9.6%, +4.7%, +17%, and −41% for measurements within the clean boundary layer, polluted boundary layer, free troposphere with a dust layer, and free troposphere without a dust layer, respectively. Each of these quantities, as well as the majority of the >100 individual comparisons from which they were averaged, were within estimated uncertainties.

The Monterey Area Ship Track Experiment

In June 1994 the Monterey Area Ship Track (MAST) experiment was conducted off the coast of California to investigate the processes behind anthropogenic modification of cloud albedo. The motivation for the MAST experiment is described here, as well as details of the experimental design. Measurement platforms and strategies are explained, and a summary of experiment operations is presented. The experiment produced the largest dataset to date of direct measurements of the effects of ships on the microphysics and radiative properties of marine stratocumulus clouds as an analog for the indirect effects of anthropogenic pollution on cloud albedo.

Sulfate, nitrate, methanesulfonate, chloride, ammonium, and sodium measurements from ship, island, and aircraft during the Atlantic Stratocumulus Transition Experiment/Marine Aerosol Gas Exchange

We measured aerosol methanesulfonic acid (MSA), Cl−, NO3−, SO4=, Na+, and NH4+ concentrations and size distributions and gaseous ammonia and nitric acid concentrations from Santa Maria, Azores during the Atlantic Stratocumulus Transition Experiment/Marine Aerosol and Gas Exchange (ASTEX/MAGE) experiment in June 1992. We also sampled some of these species in the free troposphere and marine boundary layer (MBL) from the National Center for Atmospheric Researchs (NCARs) Electra aircraft and at the surface from the R/V Oceanus. In relatively clean Atlantic marine air masses the mean (and standard error of the mean) observed MSA concentrations (27 ± 2 parts per trillion molar mixing ratio, ppt) were smaller than in continental polluted air masses (36 ± 6 ppt), reflecting either longer aerosol lifetimes in air with higher cloud condensation nuclei (CCN) concentrations or more active photochemical production in polluted air. Mean NH4+/non-sea-salt (nss) SO4= molar ratios in marine and continental air masses were 0.65 ± 0.05 and 1.3 ± 0.2, respectively, indicating that continental sulfate was neutralized by ammonia to a greater extent than marine sulfate was. Most of the NH4+ and nss SO4= was contained in submicrometer modes; Na+, Cl−, and NO3− were in supermicron modes; and MSA showed bimodal behavior, with major and minor peaks at 0.3–0.4 μm and 1.5 μm, respectively. The molar ratio NH4+/nss SO4= relative to size confirms that most ammonia vapor condensed on submicron nss SO4=. The average dry deposition fluxes for aerosol nss SO4=, MSA, NH4+ and NO3− during ASTEX/MAGE were 1.49, 0.09, 0.18, and 3.25 μmol m−2 d−1, respectively.

The Impact of Ship-Produced Aerosols on the Microstructure and Albedo of Warm Marine Stratocumulus Clouds: A Test of MAST Hypotheses 1i and 1ii

Anomalously high reflectivity tracks in stratus and stratocumulus sheets associated with ships (known as ship tracks) are commonly seen in visible and near-infrared satellite imagery. Until now there have been only a limited number of in situ measurements made in ship tracks. The Monterey Area Ship Track (MAST) experiment, which was conducted off the coast of California in June 1994, provided a substantial dataset on ship emissions and their effects on boundary layer clouds. Several platforms, including the University of Washington C-131A aircraft, the Meteorological Research Flight C-130 aircraft, the National Aeronautics and Space Administration ER-2 aircraft, the Naval Research Laboratory airship, the Research Vessel Glorita, and dedicated U.S. Navy ships, participated in MAST in order to study processes governing the formation and maintenance of ship tracks. This paper tests the hypotheses that the cloud microphysical changes that produce ship tracks are due to (a) particulate emission from the ship’s stack and/or (b) sea-salt particles from the ship’s wake. It was found that ships powered by diesel propulsion units that emitted high concentrations of aerosols in the accumulation mode produced ship tracks. Ships that produced few particles (such as nuclear ships), or ships that produced high concentrations of particles but at sizes too small to be activated as cloud drops in typical stratocumulus (such as gas turbine and some steam-powered ships), did not produce ship tracks. Statistics and case studies, combined with model simulations, show that provided a cloud layer is susceptible to an aerosol perturbation, and the atmospheric stability enables aerosol to be mixed throughout the boundary layer, the direct emissions of cloud condensation nuclei from the stack of a diesel-powered ship is the most likely, if not the only, cause of the formation of ship tracks. There was no evidence that salt particles from ship wakes cause ship tracks.

The Kleiner Feldberg Cloud Experiment 1990. An overview

An overview is given of the Kleiner Feldberg cloud experiment performed from 27 October until 13 November 1990. The experiment was carried out by numerous European research groups as a joint effort within the EUROTRAC-GCE project in order to study the interaction of cloud droplets with atmospheric trace constituents. After a description of the observational site and the measurements which were performed, the general cloud formation mechanisms encountered during the experiment are discussed. Special attention is given here to the process of moist adiabatic lifting. Furthermore, an overview is given regarding the pollutant levels in the gas phase, the particulate and the liquid phase, and some major findings are presented with respect to the experimental objectives. Finally, a first comparison attempts to put the results obtained during this campaign into perspective with the previous GCE field campaign in the Po Valley.

An overview of the Lagrangian experiments undertaken during the North Atlantic regional Aerosol Characterisation Experiment (ACE‐2)

One of the primary aims of the North Atlantic regional Aerosol Characterisation Experiment (ACE-2) was to quantify the physical and chemical processes affecting the evolution of the major aerosol types over the North Atlantic. The best, practical way of