Atmospheric deposition is the process by which manmade and naturally occurring gases and particles are removed from the atmosphere and deposited on the Earth’s surfaces (e.g., vegetation, soils, surface waters, and man-made structures) via dry or wet (e.g, precipitation, dew, fog) processes. It remains a major environmental issue in several parts of the world due to concerns over acidification and eutrophication of natural ecosystems, bioaccumulation of toxics and metals, impacts on biogeochemical cycling and biodiversity, human health, and global climate change. Quantifying the chemical composition and spatial distribution of atmospheric deposition regionally and globally via all physical processes (total atmospheric deposition) is important for understanding the transport and fate of chemical species in the atmosphere as well as the associated environmental and socio-economic impacts.
Observations and Analyses of Precipitation Chemistry and Wet Deposition
Measurements of common chemicals found in precipitation have been performed by national and regional monitoring networks for many years in North America and Europe and increasingly in East Asia and Africa, as well as at some GAW global and regional stations. Among the commonly measured ions (referred to as major ions) are sulphate, nitrate, ammonium, chloride, sodium, potassium, calcium, magnesium, phosphate, fluoride, pH and acidity. Standard protocols and data quality objectives described in the Manual for the GAW Precipitation Chemistry Programme: Guidelines, Data Quality Objectives and Standard Operating Procedures (GAW Report No. 160, currently being updated) as well as the GAW Inter-laboratory Comparison Studies, have helped reduce uncertainties and inconsistencies in sampling and analytical methods by improving the quality, comparability, and representativeness of these measurements.
Available regionally-representative high quality precipitation chemistry and precipitation depth measurements from monitoring stations around the world, in combination with global model-ensemble results, formed the basis for a scientific assessment entitled “A global assessment of precipitation chemistry and deposition of sulfur, nitrogen, sea salt, base cations, organic acids, acidity and pH, and phosphorus” (Vet et al., 2014, Atmospheric Environment Vol. 93), produced by the Scientific Advisory Group for Total Atmospheric Deposition and contributing experts. The compiled and quality-assured dataset as well as maps produced as part of the assessment are fully accessible to the wider scientific community for further applications (e.g., model validation/evaluation, critical loads and exceedance determinations, ecosystem health and services studies, climate services, education and policy assessment) and can be downloaded from the World Data Centre for Precipitation Chemistry.
Vet et al. (2014) concluded that although some inorganic ions (namely sulfate, nitrate and ammonia) are reasonably well characterized globally, major regions of the world remain insufficiently monitored for important ions in precipitation. These regions include South America, parts of North America, much of Asia, Africa, Oceania, the polar regions and the oceans. Even in regions where sampling is relatively abundant, little information exists for phosphorus, organic forms of nitrogen, organic acids, and other important forms of carbon in atmospheric deposition. Measurements of these species are required to improve our understanding of biogeochemical cycling, the effects of deposition on ecosystem health, and the role of atmospheric deposition on climate change.
Dry Deposition Estimates
Measurement-based estimates of total deposition require not only high quality precipitation concentrations and wet deposition measurements of major ions, but also routine estimates of the corresponding chemicals that are dry deposited. Direct measurements of dry deposition are not currently operationally feasible, thus, there is a need for a robust method for inferring dry deposition fluxes using concentration measurements of gaseous and aerosol species and model-estimated deposition velocities. Currently, measurement-based inferential estimates of dry deposition are available only for a few countries (e.g. limited areas in Africa, Australia, Canada, Japan and the U.S.) and are limited to sulfur, some nitrogen species, and a few cations. Efforts are presently under consideration to develop laboratory intercomparison methods paralleling existing programs for assessing data quality for wet precipitation measurement laboratories.
Emerging technologies, such as satellite measurements and other remote sensing instruments, offer the potential to augment these surface measurements. Chemical transport models are also critical tools, since in-situ measurements have limited geographic and temporal coverage and will never provide adequate spatial or temporal coverage to estimate total atmospheric deposition on a global scale.
Measurement-model Fusion for Total Atmospheric Deposition
The World Meteorological Organization’s (WMO) Global Atmosphere Watch (GAW) Programme coordinates high-quality observations of atmospheric composition from global to local scales with the aim to drive high-quality and impact science while co-producing a new generation of products and services. In line with this vision, GAW’s Scientific Advisory Group for Total Atmospheric Deposition (SAG-TAD) has a mandate to produce global maps of wet, dry and total atmospheric deposition for important atmospheric chemicals to enable research into biogeochemical cycles and assessment of ecosystem and human health effects.
The most suitable scientific approach for this activity is the emerging technique of measurement-model fusion for total atmospheric deposition. This technique requires global-scale measurements of atmospheric trace gases, particles, precipitation composition and precipitation depth, as well as predictions of the same from global/regional chemical transport models. The fusion of measurement and model results requires data assimilation and mapping techniques.
Contributing and/or Partner Networks
In order to enhance geographical coverage, GAW works in close cooperation with major regional precipitation and air monitoring networks:
For a list of GAW sites measuring precipitation chemistry and wet deposition please visit the GAW Station Information System.
GAW Laboratory Inter-comparison Studies
An important key aspect to the success of any measurement programme is the accuracy of sample chemical analysis. Over 80 chemical laboratories around the world participate in the bi-annual laboratory inter-comparison studies conducted by the GAW Quality Assurance/Science Activity Centre for the Americas. These laboratory inter-comparison studies, initiated in 1978, are designed to assess and track the performance of laboratories based on data quality objectives for major ions typically found in precipitation (see GAW Report No. 160). As part of the study, laboratories are sent simulated rain samples of known ion concentrations for analysis. The QA-SAC Americas performs a statistical analysis of the results of each chemical measurement reported by each participating laboratory and generates products for laboratory managers and data users to assess and improve performance. Study results can be found for 19xx to the present via the QASAC-Americas web site.
Past Relevant Meetings
Acid Rain 2015, Rochester, New York, USA, 19-23 October 2015.
GAW Workshop on Measurement-Model Fusion for Global Total Atmospheric Deposition, Geneva, Switzerland, 28 Feb to 2 March, 2017.
Related Reports and Publications
Vet, R., R.S. Artz, Silvina Carou, Mike Shaw, Chul-Un Ro, Wenche Aas, Alex Baker, Van C. Bowersox, Frank Dentener, Corinne Galy-Lacaux, Amy Hou, Jacobus J. Pienaar, Robert Gillett, M. Cristina Forti, Sergey Gromov, Hiroshi Hara, Tamara Khodzher, Natalie M. Mahowald, Slobodan Nickovic, P.S.P. Rao, Neville W. Reid, 2014: Special Issue: A global assessment of precipitation chemistry and deposition of sulphur, nitrogen, sea salt, base cations, organic acids, acidity and pH, and phosphorous, Atmospheric Environment, 93: 3-100.
WMO, 2017: Global Atmosphere Watch (GAW) Implementation Plan: 2016-2023, GAW Report No. 228.
WMO, 2017: Global Atmosphere Watch Workshop on Measurment-Model Fusion for Global Total Atmospheric Deposition (MMF-GTAD), Geneva, Switzerland, 28 February-2 March, 2017, 45 pp., May 2017.GAW Report No. 234.
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