Overview of IADN

Over the past few decades, the Great Lakes have been the recipient of many different persistent organic pollutants (POPs) from a variety of sources. In an attempt to better understand atmospheric deposition as one of these sources of contamination, the Integrated Atmospheric Deposition Network (IADN) was formed in 1990 through mandates of the Clean Air Act and Great Lakes Water Quality Agreement. The project is a joint venture between Environment Canada, the Ontario Ministry of the Environment, and the United States EPA’s Great Lakes National Program Office which has provided funding to the Hites’ Lab to carry out analysis. We operate master sampling stations in rural areas near each of the five Great Lakes and operate a series of satellite stations throughout the Great Lakes Basin (see Figure 1). At all master and some satellite sites, vapor and particle phase air samples are taken concurrently every 12 days for 24 hours. Precipitation samples are also taken and represent a composite over 28 days at US sites and 14 days at Canadian sites. We measures 56 polychlorinated biphenyl (PCB) congeners or congener groups, 20 organochlorine pesticides (both banned and in use), and 16 polycyclic aromatic hydrocarbons (PAHs) in each of the phases.

With ten or more years worth of data collected, we have been able to make several observations pertaining to the trends of POPs in the Great Lakes atmosphere. The most pronounced spatial trend is seen in the concentration gradient of PCBs and PAHs between urban and rural sites. PCBs are industrial chemicals that were banned in the late seventies while PAHs are a continually released byproduct of fossil fuel combustion. Because of their use and production history, both compound classes are strongly associated with urban areas. In fact, concentrations for these compounds measured at Chicago are an order magnitude higher than at any other sampling site. Moreover, pollutants from urban centers can significantly impact concentrations measured at nearby rural sites. Our research has indicated that slightly elevated PAH and PCB concentrations at some rural sites are closely associated with winds coming from urban areas upwards of 40 km away.

One of the more interesting aspects of IADN is the analysis of temporal trends in vapor phase concentrations of banned POPs. Though specific trends may vary between compounds, they all indicate an overall decline in atmospheric concentrations. The pesticide a-hexachlorocyclohexane (a-HCH) is a good example of this decline (see Figure 2). a-HCH was the dominant isomer in a technical mixture that was used as a broad spectrum insecticide and was banned in the United States and Canada in 1978. As depicted by the plot, concentrations of a-HCH have dramatically declined at all sites over the past 10 years. Based on the rate of decline and current analytical limitations, we will no longer be able to detect this pesticide in the vapor phase by the year 2040. On average, based on the similar declining patterns of PCBs and other banned pesticides, we will no longer be able to detect these compounds by the year 2034. Thus, based on the long term monitoring data we have gathered, it is clear that the bans on these POPs have worked.


Figure 2. Annual average concentrations (in pg/m3) for a-HCH at the IADN master stations.

Despite such encouraging results, we are continuing to respond to growing concerns in the region by incorporating additional compounds into the list of analytes and doubling the number of urban sampling sites. Long-term monitoring networks like IADN are essential in understanding the behavior of pollutants in the atmosphere and making the link between policy and science. For more information, visit the official IADN website. Click to view the IADN Quality Assurance Project Plan (QAPP)

Source Apportionment

An important goal of IADN is determining the sources that contribute to the continuing contamination of POPs to the Great Lakes. One way of doing this is through the use of backwards air trajectories, which serve as an approximation of the path an air parcel took in arriving at the sampling site. A series of back trajectories calculated for each sample were used in a probabilistic model called the potential source contribution function (PSCF). The model works on the assumption that samples with high (or low) concentrations should have trajectories coming from similar directions. The source regions for PCBs, various pesticides and PAH were plotted on a 0.5° x 0.5° latitude/longitude grid centered over the Great Lakes basin.

PCBs primarily have sources in urban areas, and atmospheric transport is strongly affected by air/water exchange with the Great Lakes. Like PCBs, PAH show a strong urban signature, but these compounds also seem to come from rural sites. The source regions of PAH become less distinct as the molecular weight of the compound increases. Since reactivity increases with PAH size, this diminishing trend may be an indication that atmospheric degradation plays a large role in PAH transport. The pesticides have the strongest source regions and are typically transported the farthest, often from areas distant from the Great Lakes basin. The PSCF plot of chlordane for all 5 US IADN sites is shown in Figure 3. From 1983 to 1988, chlordane was exclusively used as a termiticide in the foundations of homes. The source plot for chlordane closely matches the known range of termite infestations, suggesting that chlordanes presence in the Great Lakes air is due to volatilizion from past termite extermination efforts. For a more detailed account, see Potential Sources of Pesticides, PCBs, and PAHs to the Atmosphere .

Figure 3. The potential source region for chlordane to the 5 US IADN sites. The scale ranges from red to blue. Dark red depicts most likely sources, white are nuetral sources, and dark blue are least likely sources. The source region for chlordane to the Great Lakes has a strongly southern trend.

Precipitation Trends

IADN measures the concentrations of twenty pesticides and related analytes in 28-day integrated precipitation samples at each site. Decreases in concentration over time were observed only for p,p’-DDE and p,p’-DDD, while increases in b-HCH were observed at all sites. The most prominent behavior was significant annual variations observed for most analytes at each site, with higher concentrations in the summer for current-use pesticides (endosulfan and g-HCH), and peaks in the winter for most others. The increased concentrations in the winter are likely the result of the increased scavenging efficiency of snow compared to rain and, for some analytes, higher concentrations in the particulate phase during winter. These seasonal differences appear to account for a large portion of the observed variability in pesticide concentrations in precipitation samples. Figure 4 shows the concentration of the pesticide dieldrin in precipitation at the site Sleeping Bear Dunes. Blue points are samples from the winter and spring, while yellow points are sample from the summer and fall. The green line is a model fit to the data.

Figure 4. Seasonality of dieldrin in precipitation measured at Sleeping Bear Dunes.