Ongoing US-Canada collaborations on nitrogen and sulfur deposition

History of US-Canada collaborations on atmospheric deposition

Atmospheric deposition, both wet and dry, can be an important contributor to nitrogen and sulfur loading to ecosystems. Excessive deposition can cause acidification and eutrophication which can lead to harmful effects such as algal blooms, decreases in forest growth, loss of species richness, and shifts in species distribution. In more moderate amounts, atmospheric deposition can be a source of nutrients reducing the need for fertilization of agricultural areas. Quantifying the amount of atmospheric deposition is therefore a critical activity for both the US and Canada.

The United States and Canada have a long history of collaborating to improve the understanding of the amount of deposition and the effects of deposition in both countries. Early efforts began in the late 1970s with the establishment of networks to monitor wet deposition of acidifying species (sulfate and nitrate) and other relevant components, with the National Atmospheric Deposition Program National Trends Network (NADP/NTN) in the United States in 1978, and the Canadian Network for Sampling Precipitation (CANSAP) in 1976, followed by the Canadian Air and Precipitation Monitoring Network (CAPMoN) in 1983.

Formal collaboration on the issue of acid rain between the two countries began with the 1986 Joint Report of the Special Envoys on Acid Rain and the 1991 U.S.–Canada Air Quality Agreement. Data from both networks have been used to generate interpolated maps of wet deposition across both countries and jointly report on reductions in nitrate and sulfate wet deposition under the 1991 Air Quality Agreement (e.g., progress reports 1996–2014, https://www.epa.gov/airmarkets/us-canada-air-quality-agreement-progress-reports; and progress report 2016, https://www.canada.ca/en/environment-climate-change/services/air-pollution/publications/canada-united-states-air-quality-report-2016.html). Figure 1 shows an example of sulfate wet deposition changes similar to those in the joint reports. In addition, co-located NADP/ NTN and CAPMoN sites in Pennsylvania, USA, and Québec, Canada, have been maintained since 1986 to quantify the long-term comparability of measurement and modeling approaches across the networks.^1,2

Figure 1.

Annual non-sea-salt sulphate wet deposition interpolated using site measurements (dots) from US and Canadian networks.

In addition to wet deposition monitored by precipitation networks, dry deposition is an important component of total deposition. Long-standing networks, CAPMoN in Canada and the Clean Air Status and Trends Network (CASTNET) in the United States, measure gas and particle species that contribute to nitrogen and sulfur dry deposition. Since neither network measures dry deposition directly, fluxes at the network sites are determined by multiplying the measured concentrations by modeled deposition velocities.

Scientists from the United States and Canada have collaborated on several studies using these data. For example, an extensive comparison of the inferential models used to determine the dry deposition velocities for the networks revealed important differences between the approaches.³ In a follow-on study, concentration and meteorological data from the Borden forest site in Canada were used to conduct an evaluation of the inferential models and other deposition approaches used by the U.S. Environmental Protection Agency (EPA) and Environment and Climate Change Canada (ECCC) in their regional air quality models (see Figure 2).⁴ These collaborative efforts have served to further model development in both countries as well as provide estimates of dry deposition uncertainty.

Figure 2.

Comparison of performance of models used in the U.S. and Canada for predicting deposition velocity of O3 during the summer at Borden Forest (from Wu et al., 2018⁴).

Measurement-Model Fusion (MMF) Approaches

Scientists from the United States and Canada are active participants in the National Atmospheric Deposition Program Total Deposition Science Committee (TDep; http://nadp.slh.wisc.edu/committees/tdep). The mission of this committee is to improve measurement and model estimates of dry, wet, and total deposition of sulfur and nitrogen. One of the efforts undertaken by this committee was the development of maps of total deposition to support critical loads assessments.

A new methodology was developed to produce the values of total deposition for the contiguous United States (CONUS) that uses both measured and modeled values.⁵ Measurement networks have a limited number of sites and dry deposition cannot be easily interpolated between sites due to its dependence on the underlying land use type. Additionally, not all chemical species that comprise important parts of the sulfur and nitrogen budgets are measured. Wet deposition values are typically representative of only inorganic species. At CASTNET sites, species such as NO, NO2, N2O5, and organic N compounds are not measured. Chemical transport models (CTMs) provide complete spatial coverage across the modeling domain and a more robust estimation of the depositing species. However, concentration and deposition values predicted by CTMs are subject to uncertainties in emissions inputs, modeled meteorology, and atmospheric processes.

To adjust for these uncertainties, the measured concentrations of gas and particle species from CASTNET are used to adjust modeled values from EPA’s Community Multiscale Air Quality (CMAQ) modeling system using inverse distance weighting to calculated dry deposition values of measured species while CMAQ values for species not measured at network sites are used directly. Wet deposition values are developed from a fusion of measurements of precipitation and chemistry from NADP/NTN and precipitation from the Parameter-elevation Regressions on Independent Slopes Model (PRISM; http://www.prism.oregonstate.edu/). The dry deposition and wet deposition values are combined to produce grids and maps of total deposition, which are available for download at https://nadp.slh.wisc.edu/committees/tdep/tdepmaps/.

A similar effort was undertaken by ECCC under the project Atmospheric Deposition Analysis Generated from optimal Interpolation from Observations (ADAGIO). Measured concentrations of nitrogen and sulfur species as gases, in particulate matter, and in precipitation from Canadian and U.S. networks are used to adjust predicted concentrations from ECCC’s GEM–MACH (Global Environmental Multiscale–Modelling Air quality and Chemistry) model using optimal interpolation techniques. Optimal interpolation is a statistical method for minimizing the differences between the model and measurements. These techniques have already been developed and used successfully to combine measured and modelled air concentrations of ozone, sulfur dioxide and nitrogen dioxide, and particulate matter in Canada.⁶ This process generates maps of nitrogen and sulfur species concentrations in air and precipitation at a 10-km horizontal resolution. A similar method is used to generate precipitation amounts. These are then combined with modelled dry deposition velocities to yield total deposition.

The approaches and models used for the TDEP maps and the ECCC ADAGIO maps are slightly different but produce comparable results for deposition, as seen in Figures 3 and 4. A primary goal under the U.S.–Canada Air Quality Agreement is to combine the results from the two approaches to obtain one set of maps for North America. A detailed comparison of the two methods, species by species, is expected to be completed by 2020 and result in combined maps that are accepted by both countries. The measurement-model fusion approaches used by the United States and Canada are now leading the way for the use of measurement model fusion on a global scale as part of an effort sponsored by the World Meteorological Organization (WMO; https://library.wmo.int/index.php?lvl=notice_display&id=19885#.XIvp7jN7mHs). Two workshops have been held to work toward developing an implementation plan that would result in measurement-model fusion total deposition values globally.

Figure 3.

Total (wet and dry) sulfur deposition for 2010 produced by the ADAGIO (Canada) and TDep (US) methods.

Figure 4.

Total (wet and dry) nitrogen deposition for 2010 produced by the ADAGIO (Canada) and TDep (US) methods.

Future Plans to Improve Deposition Estimates

Building on this long history of joint deposition activities, new collaborative projects using satellite data and model intercomparisons are also planned. For example, ECCC provided satellite-derived ammonia (NH3) estimates for EPA scientists to evaluate bidirectional (emission and deposition) NH3 exchange in the CMAQ model. In turn, EPA will provide ECCC with dry deposition measurements that can be used to help evaluate satellitederived dry deposition of NH3 and NO2 over North America.⁷

Both Canada and the United States are participating in the Phase 4 of the Air Quality Modelling Evaluation International Initiative (http://aqmeii.jrc.ec.europa.eu/phase4.html), which is focused on atmospheric deposition. Goals of this study include quantifying the performance and variability of deposition fields predicted by regional air quality models, documenting deposition schemes, and analyzing model outputs to understand differences in model predictions. With continued collaboration between the measurement, modeling, satellite, and measurement-model fusion communities in both countries, our ability to provide nitrogen and sulfur deposition estimates will continue to improve.