Atmospheric Chemistry Program Research Articles

An overview of ACE‐Asia: Strategies for quantifying the relationships between Asian aerosols and their climatic impacts

The International Global Atmospheric Chemistry Program (IGAC) has conducted a series of Aerosol Characterization Experiments (ACE) that integrate in situ measurements, satellite observations, and models to reduce the uncertainty in calculations of the climate forcing due to aerosol particles. ACE‐Asia, the fourth in this series of experiments, consisted of two focused components: (1) An intensive field study that sought to quantify the spatial and vertical distribution of aerosol concentrations and properties, the processes controlling their formation, evolution, and fate, and the column‐integrated radiative effect of the aerosol (late March through May 2001). (2) A longer‐term network of ground stations that used in situ and column‐integrated measurements to quantify the chemical, physical, and optical properties of aerosols in the ACE‐Asia study area and to assess their spatial and temporal (seasonal and interannual) variability (2000–2003). The approach of the ACE‐Asia science team was to make simultaneous measurements of aerosol chemical, physical, and optical properties and their radiative impacts in a variety of air masses, often coordinated with satellite overpasses. Three aircraft, two research ships, a network of lidars, and many surface sites gathered data on Asian aerosols. Chemical transport models (CTMs) were integrated into the program from the start, being used in a forecast mode during the intensive observation period to identify promising areas for airborne and ship observations and then later as tools for integrating observations. The testing and improvement of a wide range of aerosol models (including microphysical, radiative transfer, CTM, and global climate models) was one important way in which we assessed our understanding of the properties and controlling processes of Asian aerosols. We describe here the scientific goals and objectives of the ACE‐Asia experiment, its observational strategies, the types of observations made by the mobile platforms and stationary sites, the models that will integrate our understanding of the climatic effect of aerosol particles, and the types of data that have been generated. Eight scientific questions focus the discussion. The intensive observations took place during a season of unusually heavy dust, so we have a large suite of observations of dust and its interaction with air pollutants. Further information about ACE‐Asia can be found on the project Web site at http://saga.pmel.noaa.gov/aceasia/.

Measurement and deduction of emissions of trichloroethene, tetrachloroethene, and trichloromethane (chloroform) in the northeastern United States and southeastern Canada

We present optimal estimates of the emission patterns for trichloroethene (TCE, CHCl = CCl2), tetrachloroethene (perchloroethylene, CCl2 = CCl2), and trichloromethane (chloroform, CHCl3) utilizing hourly gas chromatographie measurements at Nahant, Massachusetts (approximately 10 km northeast of Boston). Our analysis combines the measurements with back trajectory information obtained from the Hybrid Single‐Particle Lagrangian Integrated Trajectory (HYSPLIT‐4) model (National Oceanic and Atmospheric Administration Air Resources Laboratory, Silver Spring, Maryland). Using a Kaiman filter inverse method and an analytical solution of the continuity equation to estimate the effects of eddy diffusion, we calculate the surface emissions on a 1°×1° grid for the selected species necessary to optimally match the observations. These emissions are compared with the estimates determined by the Reactive Chlorine Emissions Inventory (RCEI) working group of the International Global Atmospheric Chemistry Program Global Emissions Inventory Activity (GEIA). The new emissions scenarios computed here provide an observation‐based assessment for comparison with the RCEI emissions inventories for the northeastern United States and southeastern Canada. Results indicate that the RCEI estimates of the anthropogenic emissions of these chemicals in the geographical domain studied differ from our optimal estimates but generally lie within the estimated error of these optimal estimates. Results are sensitive to the assumed vertical distributions and hence to the assumed vertical mixing rates. The current accuracy achievable through this observation‐based technique (±45%) is limited in large part by the uncertainty in the vertical distribution of these compounds in the troposphere over highly emitting regions. Within this accuracy the optimal estimates of emissions presented here indicate that the emissions for many grid cells do not need to be corrected significantly from the 1990 estimates presented in the RCEI. However, for trichloroethene and tetrachloroethene we calculate large statistically significant decreases in emissions for some highly populated urban locations on the East Coast and correspondingly large increases for some less populous grid cells in Pennsylvania and New York State. Only anthropogenic sources of trichloromethane (representing roughly 11% of estimated global emissions) were gridded in the RCEI inventory and included in the initial inventory used here. We find that these anthropogenic emissions are, as expected, too low to explain the observations and that most grid cells require small increases and several cells require substantial increases (∼5–10 nmoles m−2 h−1) to produce emissions estimates that are consistent with observations. This is reasonable given current knowledge of natural land sources (e.g., soil emissions) and a large oceanic source for this compound inferred from previous oceanic observations.

Air’sea fluxes and transfer velocity of CO2 over the North Sea: results from ASGAMAGE

The paper presents an analysis, performed at the Royal Netherlands Meteorological Institute (KNMI), of data obtained at a research platform 9 km off the Dutch coast within the framework of the air’sea gas exchange program ASGAMAGE†. The air’sea transfer velocity of CO2 was determined directly, that is, by observing CO2 fluxes and air’sea concentration differences simultaneously. CO2 fluxes were determined by means of the eddy correlation technique. Special care was taken to avoid the effects water vapour on the CO2 flux measurements. The air and water near the air’sea interface were treated as well-mixed with respect to CO2. The combination of flux and concentration data allowed the computation of the transfer velocity for CO2 without recourse to other gases. Results for two observation periods, one with downward CO2fluxes (May) and one with upward CO2 fluxes (October), are consistent. A relation with UN,10, the wind speed adjusted to a height of 10 m and neutral stratification, was determined for the pooled data from the two experimental phases. The relation found was: k660 = 0.54U2N,102 cm h-1, with k660 the CO2 transfer velocity normalized to salt water (35‰) at a temperature of 20 °C, and UN,10 in m s-1. The 95% confidence interval of the coefficient extends from 0.46 to 0.63. No relations with other geophysical parameters could be found from the present data set.ASGAMAGE is a contraction of ASGASEX (for Air Sea Gas Exchange), an earlier experiment in this field, and MAGE, the Marine Aerosol and Gas Exchange activity of the International Global Atmospheric Chemistry program (IGAC).

Global gridded inventories of anthropogenic emissions of sulfur and nitrogen

Two sets of global inventories of anthropogenic emissions of both oxides of sulfur and oxides of nitrogen for circa 1985 have been produced under the umbrella of the Global Emissions Inventory Activity (GEIA) of the International Global Atmospheric Chemistry Program. The two sets of inventories have different temporal, sectoral, and vertical resolution. Both were compiled using the same data sets; default data sets of global emissions have been refined via the use of more detailed regional data sets. This article reports on the compilation of the annual, one‐vertical‐level inventories, called version 1A; the inventory files are available to the scientific community via anonymous file transform protocol (FTP). Existing global inventories and regional inventories have been updated and combined on a 1° × 1° longitude/latitude grid. The resulting global anthropogenic emissions are 65 Tg S yr−1 and 21 Tg N yr−1; qualitative uncertainty estimates have been assigned on a regional basis. Emissions of both SOx and NOx are strongly localized in the highly populated and industrialized areas of eastern North America and across Europe; other smaller regions of large emissions are associated with densely populated areas with developed industries or in connection with exploitation of fuels or mineral reserves. The molar ratio of nitrogen to sulfur emissions reflects the overall character of sources; its value is generally between 0.33 and 10 for industrialized and heavily populated areas but varies over a wide range for other areas. We suggest that those requiring sulfur or nitrogen emission inventories standardize on the GEIA inventories, which we believe are authoritative and which are freely available to all users by anonymous FTP.

Atmospheric aerosol research in the U.S.: 1991–1994

A common theme underlying many aspects of recent atmospheric aerosol research is the role of particulate matter in climate and climate change. Aerosols influence climate directly, by scattering and absorbing solar radiation, and indirectly via their role as cloud condensation nuclei (CCN), by modifying optical properties and lifetimes of clouds. The atmospheric cycles of two radiatively‐important species—sulfate and soot—have been significantly perturbed by anthropogenic activities, and thus it is expected that anthropogenic aerosols will have significant effects on global mean radiative forcing.There is general agreement that, despite uncertainty in estimates, the averaged global direct effects of aerosols are not negligible and are probably comparable in magnitude but opposite in sign to anthropogenic greenhouse gas forcing. However, all estimates are subject to significant uncertainty, in part because of gaps in understanding of critical factors and relationships. To minimize these uncertainties, a combination of activities must be undertaken (NRC (National Research Council) [1993] and Penner et al. [1993a]): long‐term observations at fixed sites, to establish background conditions and trends; short‐term intensive observations, surface‐based and airborne; laboratory analyses and process studies; model development; and satellite observations to connect local observations to global measurements. There is also a need for new techniques for automated and real‐time measurements of aerosol chemical and physical properties. Components of IGAC (International Global Atmospheric Chemistry Program) and its U.S. component, GTCP (Global Tropospheric Chemistry Program) are focused on obtaining the data needed for quantifying the effects of aerosols on climate (NRC (National Research Council) [1993]), and indeed much of the research reported here reflects this priority.

The importance of atmospheric measurements at Alert, Canada

Since 1986 the Atmospheric Environment Service (AES) of Environment Canada has operated an atmospheric chemistry program at Station Alert, which is located at 82°N on the northern tip of Ellesmere Island.The program provides a detailed record of atmospheric change in the high Arctic that is crucial to both regional and global issues. Plans to reduce the Canadian government's deficit now require Environment Canada to make budgetary cuts and are prompting reevaluation of this and other programs. In light of these circumstances, we would like to take the opportunity to consider the importance of Alert to international atmospheric science.

Production mechanism of C2‐C4 hydrocarbons in seawater: Field measurements and experiments

The production mechanism of light nonmethane hydrocarbons (NMHC) in seawater was investigated during the North Atlantic atmospheric chemistry program (NATAC) in April and May 1991 in the European coastal seas and the North Atlantic. A significant alkene production occurred in the presence of light only. Under conditions of negligible NMHC emissions (low wind velocity) increasing hydrocarbon concentrations were observed during daytime, whereas the concentrations remained constant during night. NMHC formation experiments were carried out with seawater filled in quartz glass bottles and showed the same dependence of light. Experiments with differently pretreated seawater samples indicated that the presence of dissolved organic material (DOM) is also necessary for alkene production. We suggest a two‐step production mechanism for alkenes: first DOM is released, probably from algae, then part of this material is photochemically transformed into alkenes. The production rates in the quartz glass bottles were comparable to the production rates in the ocean surface. This indicates that the processes occurring in the experimental setups represent the processes occurring in the field. Since the production ‐ and emission rates were in the same range it can be concluded that the budget of light alkenes in the remote marine environment is determined by the production in seawater as the dominant source and the flux into the atmosphere as the main loss process.