Precipitation chemistry remains a major environmental issue in several parts of the world (e.g., eastern North America, south-east Asia, and Europe) due to concerns over acid deposition, eutrophication, trace metal deposition, ecosystem health, biogeochemical cycling, and global climate change. In more recent years, concerns have expanded from wet deposition alone to include such considerations as air concentrations, dry deposition, and surface-air exchange, particularly as they relate to the atmospheric lifetimes of acidifying species, greenhouse gases, and oxidizing species. In spite of these concerns, little has been done to bring these additional factors to the framework of GAW, primarily due to budgetary limitations.
GAW Precipitation Chemistry observations
are focused largely on major ions. Established measurement and analysis methodologies may be found in theMeasurement Guidelines for the GAW Precipitation Chemistry (GAW Report 160). Although trace metals are included in the GAW suite of desirable measurements, they have not been measured at many sites, and the present quality assurance activities for trace metals have been managed through the EMEP program. An assessment of the need for trace metal measurements will be revisited when financial and scientific considerations warrant additional attention. Similar assessments are envisioned for organics, including pesticides, PCBs, and PAHs as well as for dry deposition and droplet (i.e., cloud or fog water) deposition measurements.
Measurements of precipitation chemistry and wet deposition have been made for many years in various regions of the world with varying degrees of success. In general, those areas in which acid deposition has been a major environmental concern have developed and implemented sophisticated, high quality measurement systems. In other areas, however, the number of sites has been insufficient and the measurement methods and programmes remain inadequate and poorly integrated into the GAW program. On a brighter note, while the inconsistency in sampling instrumentation and sampling methods around the world remains a daunting problem, representatives from major networks from Asia, Europe, and North America are in strong, agreement about the major tenets of an acceptable GAW program and have recently issued a comprehensive set of guidelines for the GAW program. The challenge of the present strategic planning period will be to reduce inconsistencies among established programs and to ensure high quality programs in present data sparse regions of the globe.
Cooperative partner networks
The number of official GAW precipitation chemistry measurement sites remains insufficient in South America, Africa, Asia, and Oceania. This is why GAW has established closed cooperation with number of regional precipitation monitoring networks:
Many of the chemical laboratories participate in the GAW annual laboratory inter-comparison studies. The laboratory inter-comparisons will continue in the future under the auspices of QA/SAC Americas. Powerful tools have been established to clearly identify poorly performing laboratories and protocols have been established to work with such laboratories to improve performance. Twinning activities and expert site visits will be required to ensure measurable progress.
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