Progress in Monitoring and Modelling Estimates of Nitrogen Deposition at Local, Regional and Global Scales

Abstract

This chapter discusses the status and progress of activities around the world to measure and model dry and wet deposition of reactive nitrogen (N_r), i.e. the final removal processes from the atmosphere, at the local, regional and global scales. It gives an overview of present status and developments in networks and techniques for measuring deposition of N_r. We describe recent developments in the modelling of emissions and deposition, and finish by giving research and policy recommendations regarding N-deposition measurement and modelling, including the need for;

• an increase in the number of regional-scale, long-term monitoring sites for the routine measurement/estimation of wet and particularly dry deposition worldwide;

• an increase in the number of N species routinely measured, especially dissolved organic N (DON), ammonia (NH₃) and nitrogen dioxide (NO₂);

• global and regional models to improve their estimation of dry deposition and their resolution, especially in situations with complex topography.

Appendix: Overview of Networks

1.1 DEBITS/IDAF (http://medias.obs-mip.fr)

Under the auspices of the international DEBITS program, with about 25 stations in the tropical belt , the objective of the African IDAF project is to measure wet- and dry-deposition fluxes and identify the relative contribution of natural and anthropogenic source of a number of components. The IDAF project implemented 8 monitoring sites covering the major African ecosystems over West and Central Africa: dry savanna (Niger, Mali, South Africa), wet savanna (Côte d’Ivoire and Benin) and equatorial forest (Cameroon, Congo) . Wet deposition is measured by precipitation collectors specially designed for the IDAF network, and all the analysis are quality controlled by the quality assurance and quality control (known as ‘QA/QC’) intercomparison WMO programme . Typical values for wet deposition of N components range from 0.9 kg N-NO₃ ⁻ ha⁻¹ year⁻¹ and 1.5 kg N-NH₄ ⁺ ha⁻¹ year⁻¹ in dry savannas, 1.3 kg N-NO₃ ⁻ ha⁻¹ year⁻¹ and 2.9 kg N-NH₄ ⁺ ha⁻¹ year⁻¹ in wet savannas and 2 kg N-NO₃ ⁻ ha⁻¹ year⁻¹ and 3 kg N-NH₄ ⁺ ha⁻¹ year⁻¹ in forests. Dry deposition is estimated using measurements of gaseous and particulate species based on continuous measurements of gaseous concentrations through passive gas sampling (NO₂, NH₃, HNO₃) , and on bulk air sampling (ammonium and nitrate particulate content). These concentrations are multiplied by site and species specific dry deposition velocities. A detailed overview of the IDAF network can be made available by C. Galy-Lacaux .

1.2 EANET (www.eanet.cc)

The Acid Deposition Monitoring Network in East Asia (EANET) was established in 1998 and the regular phase activities were started in 2001, with the UNEP Regional Resource Centre of Asia and the Pacific (UNEP RRC.AP) in Thailand as the secretariat, and the Acid Deposition and Oxidant Research Center (ADORC) in Japan as the Network Center . The objectives of the EANET are to create a common understanding on the state of acid deposition problems in East Asia, provide useful inputs for decision making at various levels with the aim of preventing or reducing the adverse impacts on the environment, and promote cooperation among countries (http://www.eanet.cc/eanet/outline.html). EANET monitors four environmental items—wet deposition, dry deposition, soil and vegetation, and inland aquatic environment. Currently there are 56/47 wet/dry deposition monitoring sites and 19 ecological monitoring sites from fourteen countries in East Asia. Using common methods, wet acid deposition is estimated by analyzing the rainwater’s concentrations of sulphate (SO₄ ²⁻) and nitrate (NO₃ ⁻), and dry acid deposition by calculating measured concentrations of sulfur dioxide (SO₂), nitrogen dioxide (NO₂), ozone (O₃) and particulate components (for more details see http://www.eanet.cc/index.html) .

1.3 South Asian Network: Composition of Asian Deposition

Several initiatives like the CAD (Composition of Asian Deposition) associated with DEBITS, and the Malé Declaration provides a framework for the measurement of rain chemistry and gaseous concentrations . Nevertheless, in India, for example, there is no national deposition network, although there are several individual measurement activities. Recently, Kuhlshrestha et al. (2005) compiled a data set of about 100 stations, of which 50 stations which can be characterized as “rural” and “sub-urban.” Only a few stations operated wet-only measurements.

In the South Asian region, specific studies to monitor N deposition have not been carried out. As part of individual efforts or through limited networks, wet and dry deposition of N has been reported, unfortunately most results are not reliable due to the delay in chemical analysis and improper sample storage . Estimation of nitrate or ammonium needs to be done immediate after sample collection. Moreover, very few publications reveal their quality assurance (QA) procedures, and clearly future network activities will have to address these QA/QC issues. It is worth mentioning here that through CAD, a few groups working on wet and dry deposition studies have been trained for good quality analysis. Good results in a recent EANET inter- comparison exercise are encouraging. More encouragement for QA/QC in measurements in the South Asian region will strengthen the data reliability. More information can be obtained from Umesh C. Kulshrestha .

1.4 European EMEP network

The framework for EMEP (European Monitoring and Evaluation Programme) is provided by the Convention on Long-range Transboundary Air Pollution), ratified by 50 countries. The network (tarantula.nilu.no/projects/ccc/network/index.html) consists of some 100 operating stations, which cover measurements of acidifying/eutrophying components, ozone, heavy metals, POPs, VOCs and PM . The stations are not equally distributed over Europe, with very few stations in Southern Europe. Likewise, the quality and completeness of the measurements may differ from country to county. Given the large heterogeneity of land-use and emissions in Europe, it is sometimes difficult to classify stations as truly background, rural, or urban. More information can be obtained from Wenche Aas.

Acknowledgments

The authors gratefully acknowledge travel support to present this paper through INI from the Packard Foundation and from the European Science foundation under the COST729 and NinE programmes.