John L. Gras

International Global Atmospheric Chemistry (IGAC) Project's First Aerosol Characterization Experiment (ACE 1): Overview

The southern hemisphere marine Aerosol Characterization Experiment (ACE 1) was the first of a series of experiments that will quantify the chemical and physical processes controlling the evolution and properties of the atmospheric aerosol relevant to radiative forcing and climate. The goals of this series of process studies are to reduce the overall uncertainty in the calculation of climate forcing by aerosols and to understand the multiphase atmospheric chemical system sufficiently to be able to provide a prognostic analysis of future radiative forcing and climate response. ACE 1, which was conducted from November 15 to December 14, 1995, over the southwest Pacific Ocean, south of Australia, quantified the chemical, physical, radiative, and cloud nucleating properties and furthered our understanding of the processes controlling the aerosol properties in this minimally polluted marine atmosphere. The experiment involved the efforts of scientists from 45 research institutes in 11 countries.

The mixture state of individual aerosol particles in the 1997 Indonesian haze episode

Abstract The mixture state of individual aerosol particles collected at altitudes of 1– 5 km on 23 and 25 October 1997, from an aircraft flying over southern Kalimantan during the 1997 Indonesian forest fires, has been examined using the dialyses of water-soluble material with water, and organic material with benzene in conjunction with electron microscopy. Individual aerosol particles in the radius range of 0.1– 2 μm were mainly present as an internal mixture of water-soluble organic material and inorganic salt (mainly ammonium sulfate). Although material comprised of chain aggregations of electron-opaque spherules (elemental carbon) was also found, the proportion of these was small.

Relationships between size segregated mass concentration data and ultrafine particle number concentrations in urban areas

Abstract Mass concentration data derived from samples collected with a micro-orifice uniform deposit impactor (MOUDI) in six Australian urban centers during periods of significant particle loading have been used to investigate the relationships between TSP, PM10, PM2.5, PM1 and ultrafine particles. While PM10 and PM2.5 display a clear relationship, the lack of correlation between PM10 and the coarse fraction of PM10 (PM10–PM2.5) suggests that variation in PM10 is dominated by variance in PM2.5. Given that particles of less than 2.5 μm are suspected to have adverse health effects, increasing the extent of PM2.5 monitoring may improve detection of relationships between air pollution and human health. A lack of correlation between both PM10 and PM2.5 with ultrafine mass concentrations indicates that PM10 and PM2.5 cannot be used as a surrogate for ultrafine mass concentration. Similarly, ultrafine number concentrations cannot be inferred from mass concentration information determined by the MOUDI.

Some optical properties of smoke aerosol in Indonesia and tropical Australia

Aerosol light-scattering coefficient at 530 nm and its hygroscopic growth were determined in biomass-burning smoke in the lower atmosphere over Kalimantan and northern Australia during the 1997 dry-season fires. Both in and away from plumes, light-scattering was considerably greater in the Indonesian region and hygroscopic growth in scattering was also consistently greater. The relative increase in scattering, from 20% to 80% relative humidity, was typically 1.37 in northern Australian and 1.65 in Kalimantan. Limited aerosol light absorption data indicate relatively small absorption in the Indonesian smoke. In part these differences can be explained by different combustion phases, mixed flaming and smoldering in the Australian savanna fires compared with predominantly smoldering in Indonesia, although these and other concurrent measurements suggest that underground peat combustion may have made a significant contribution to the Indonesian smoke.

CN, CCN and particle size in Southern Ocean air at Cape Grim

Abstract Seasonal changes in atmospheric particle size distribution are derived by combining light-scatter particle size spectrometry with particle sizes derived from cloud condensation nucleus (CCN) concentration and sub-micrometre aerosol composition measurements. The distributions obtained are similar to others obtained in the clean marine boundary layer, using different techniques. They show a division of the aerosol “accumulation” mode into two branches, one largely associated with the CCN population and the other with the condensation nucleus (CN) population. A third, and larger, mode which is attributed to seasalt is also observed. Time series of CN and CCN concentrations at Cape Grim are presented and shown to have opposite trends over the past decade. The trend for CN is an increase of around 1.2% per year whilst CCN have decreased at about 3% per year. These trends are shown to be not seasonally uniform, with different dependences for CN and CCN. Finally, existing and possible future global CN and CCN measuring networks are discussed.

Non-sea-salt sulfate, methanesulfonate, and nitrate aerosol concentrations and size distributions at Cape Grim, Tasmania

We collected weekly aerosol samples using high-volume impactors over a period of 20 months (1988–1990) at the Cape Grim baseline station on the northwestern coast of Tasmania, Australia. The samples were analyzed for soluble ionic constituents, including sulfate, methanesulfonate (MS−), ammonium, nitrate, and the major sea-salt ions. The sea-salt component showed only a slight seasonal variation, whereas the non-sea-salt (nss) ions all had pronounced summer maxima. Significant interannual variability was seen between the nss ion concentrations measured during the two summers investigated. Nss sulfate and MS− were present both in the fine and coarse aerosol fractions, in the latter presumably associated with sea-salt particles. During the winter period, there was more nss sulfate in the coarse fraction than in the fine fraction. These observations are consistent with an important role of liquid-phase oxidation in haze and cloud droplets for the production of nss sulfate aerosol. The seasonal behavior of the sulfur and nitrogen species at Cape Grim and their mutual correlations suggest that DMS oxidation is the dominant sulfur source during summer, while nonbiogenic sulfur sources make significant contributions to nss sulfate outside of this season. Correlations of CN and CCN concentrations with nss sulfate, MS−, and wind speed suggest that DMS oxidation and, to a lesser extent, seaspray formation contributes to CN and CCN populations. The contrast between the weak seasonality of the sea-salt component and the pronounced seasonal behavior in both sulfur species and CCN supports the central role of biogenic DMS emissions as precursors of CCN in this region, at least in the biologically productive season.

Ammonia and ammonium concentrations in the antarctic atmosphere

Abstract Concentrations of gaseous ammonia and particulate ammonium ion were determined at the South Pole and near Lake Vanda in the Wright Valley, Victoria Land, during November–December 1980. Ammonia gas concentrations which ranged from less than 0.01 to ~ 0.03 μg m −3 (S.T.P.) represented a small fraction of the total (gas plus partiulate) ammonia. At the South Pole the particulate sulphate was apparently nearly fully neutralized by ammonium whereas in the Wright Valley there was evidence of a greater maritime component and a smaller fraction of ammonium in the aerosol. The presence of ammonia in the South Polar atmosphere indicates tropospheric transport of particles observed at the South Pole in the southern summer.

Southern hemisphere tropospheric aerosol microphysics

Aerosol particle size distribution data have been obtained in the southern hemisphere from approximately 4°S to 44°S and between ground level and 6 km, in the vicinity of eastern Australia. The relative shape of the free-tropospheric size distribution for particles with radii larger than approximately 0.04 μm was found to be remarkably stable with time, altitude, and location for the autumn-winter periods considered. This was despite some large concentration changes which were found to be typical of the southeastern Australian coastal region. The majority of free-troposphere large particles were found to have sulfuric acid or lightly ammoniated sulfate morphology. Large particles in the boundary layer almost exclusively had a sea-salt morphology.

Aircraft measurements of ozone, NOx, CO, and aerosol concentrations in biomass burning smoke over Indonesia and Australia in October 1997: Depleted ozone layer at low altitude over Indonesia

The 1997 El Nino unfolded as one of the most sever El Nino Southern Oscillation (ENSO) events in this century and it coincided with massive biomass burning in the equatorial western Pacific region. To assess the influence on the atmosphere, aircraft observations of trace gases and aerosol were conducted over Kalimantan in Indonesia and Australia. Over Kalimantan in Indonesia, high concentrations of O3, NOx, CO, and aerosols were observed during the flight. Although the aerosol and NOx decreased with altitude, the O3 had the maximum concentration (80.5 ppbv) in the middle layer of the smoke haze and recorded very low concentrations (∼20 ppbv) in the lower smoke layer. This feature was not observed in the Australian smoke. We proposed several hypotheses for the low O3 concentration at low levels over Kalimantan. The most likely are lack of solar radiation and losses at the surface of aerosol particles.

Wind-produced submicron particles in the marine atmosphere

Abstract The mean relationship between wind speed and the concentrations of condensation nuclei (CN) and cloud condensation nuclei (CCN) has been found from shipboard measurements between latitudes 50–54°S in the central south Indian Ocean through two winter months. It took the form: log C = a+bU, where C is the particle concentration (cm−3), U is the wind velocity in m s−1, a was 1.25 for CN (radius r> 1.5 nm) and 0.94 or 0.76 for CCN active at 0.6% or 0.1 % supersaturation respectively, while b was 0.037 for each. This value of b is too low for climatic regulation by wind-produced oceanic CCN to be very important and appears to be more consistent with earlier data than a recently found value of 0.1 for the North Atlantic. Electron microscope photographs of particles collected in very strong winds showed that sea salt could not account for increases in concentrations of particles with radii less than 50 nm although the CN and CCN measurements suggested a continued increase to smaller sizes. Diffusion battery measurements confirmed the presence of these small particles but their nature and origin is unknown.