As discussed by Penner et al. [1993a], long-term monitoring of physical, chemical and optical properties of aerosol is an important part of the strategy for reducing uncertainty in aerosol forcing estimates. A number of long-term monitoring networks have now been in place for several years. The Atmosphere / Ocean Chemistry Experiment (AEROCE) network in the North Atlantic was started in 1987. Significant recent findings include evidence for long-range transport of Saharan dust to the western North Atlantic ( Arimoto et al. [1992]) and the role of mineral aerosol transport as a source of nutrients to the ocean ( Duce et al. [1991]). Observations of variability of trace metal concentrations that are attributed to fluctuating transport ranges ( Arimoto et al. [1992] and Veron et al. [1992]) assist in the characterization of pathways of aerosol transport to the North Atlantic. The studies also demonstrated that metals that are enriched in the atmosphere due to anthropogenic influences may be used as tracers for evaluating the relative importance of natural and anthropogenic sources of species such as sulfate ( Arimoto et al. [1992]).
Hofmann [1993] reports twenty years of balloon-borne
tropospheric aerosol measurements at Laramie, Wyoming, a background
continental site. Seasonal variations were observed in all particle
size ranges, with summer maxima in particle number throughout the
troposphere. A long-term decreasing trend (1.6--1.8% per year) in
number concentrations of particles with diameter greater than 0.3
m was observed, and it was suggested that this trend is related
to reductions in SO
emissions in the U.S. during this period.
Bodhaine and Dutton [1993] report long-term decreases in Arctic Haze
measured at the National Oceanic and Atmospheric Administration's
Climate Diagnostics and Monitoring Laboratory (CMDL) monitoring
station near Pt. Barrow, operating since 1972; the decreases are also
attributed to decreased anthropogenic pollution emissions from the
U.S. and the former Soviet Union.
The Interagency Monitoring of Protected Visual Environments (IMPROVE) program was initiated in 1988 as a cooperative effort by the National Park Service, the Fish and Wildlife Service, the Bureau of Land Management, the Forest Service, and the Environmental Protection Agency. The network routinely monitors light scattering and extinction and the major visibility-reducing aerosol species: sulfates, nitrates, organics, light-absorbing carbon, and dust. Although the primary objectives of the program are visibility-related, the measurements may also be useful in addressing some aerosol-climate issues. A summary of findings from the first three years of operation ( Malm et al. [1994]) show distinct regional differences in the fine-particle mass budgets. For example, nitrate is the largest mass component in southern California only, whereas sulfate is the largest single component in six of the regions studied (primarily in the eastern U.S.) and organics are the largest in nine regions, including the Colorado plateau, the northern and central Rockies, and the Sierra Nevada. Even in instances where it is not the dominant species by mass, the contribution of sulfate to light extinction is generally most important, due to its hygroscopicity. The Department of Energy's Atmospheric Radiation Measurement (ARM) Project ( Penner et al. [1992a]) and the National Oceanic and Atmospheric Administration's Climate Diagnostics and Monitoring Laboratory are presently instrumenting new sites which will measure simultaneous chemical, physical and optical properties of aerosol and will relate these to local radiation measurements.
Harriss et al. [1994] present an overview of the Arctic Boundary Layer Expedition (ABLE) 3B, held during July--August 1990 in subarctic and Arctic regions of Canada and along the east coast of North America. The study characterized tropospheric trace gas and aerosol composition and combined ground, aircraft, and satellite measurements to study effects of both local biogenic sources and long-range transport upon observed concentrations. The lowest observed aerosol concentrations in the boundary layer were associated with Arctic sources; clean conditions were observed in the free troposphere for airmasses originating from the tropical Pacific. The Atmospheric Environment Special Issue on Arctic Air, Snow and Ice Chemistry ( Davidson and Schnell [1993]) summarizes findings from the Dye 3 Gas and Aerosol Sampling Program (DGASP), the Arctic Gas and Aerosol Sampling Program (AGASP), and monitoring programs.
The Chemical Instrumentation Test and Evaluation (CITE 3) mission ( Hoell Jr. et al. [1993]) was designed to test airborne instrumentation for the detection of sulfur species for tropospheric sulfur budget studies. Plumes containing elevated number concentrations of submicron aerosol particles from biomass burning in Africa and South America were detected over the equatorial and tropical South Atlantic during the experiment ( Andreae et al. [1994]). The Biomass Burning Airborne and Spaceborne Experiment in the Amazonias (BASE-A) and in Brazil (BASE-B) ( Kaufman et al. [1992] and Ward et al. [1992]) studied emissions from biomass burning in the tropics.
An overview of results from a joint Soviet-American experiment for the study of Asian desert dust is given by Golitsyn and Gillette [1993]; the study included characterization of optical properties of dust, needed to assess climate impacts. The third joint Soviet-American Gases and Aerosols experiment (SAGA 3) was designed to study trace gases and particles, including sulfur, in the remote marine boundary layer ( Huebert et al. [1993]). The Atlantic Stratocumulus Transition Experiment / Marine Aerosol and Gas Exchange (ASTEX/MAGE) experiment, held in the summer of 1992, attempted a new approach in the study of marine trace gas and aerosol cycles by employing a Lagrangian experiment strategy ( Huebert et al. [1994]).
Although not specific to single campaigns, summaries of observations of several aerosol types have appeared in the literature during 1991--1994. Hudson [1993] reviews the progession of knowledge of cloud condensation nuclei. Fitzgerald [1991] presents a summary of observations and of present understanding of marine aerosols. The text edited by Levine [1991] compiles characteristics and emissions estimates for aerosols generated from biomass burning.