Litterfall Mercury Monitoring Initiative


The National Atmospheric Deposition Program (NADP) has developed a Litterfall Mercury Monitoring Initiative to complement both the Mercury Deposition Network (MDN) that monitors mercury in precipitation and the Atmospheric Mercury Network (AMNet) that monitor mercury in air.

The Litterfall Mercury Monitoring Initiative offers a means by which an NADP site sponsor can obtain measurements that provide an estimate of an important source of mercury dry deposition to a forested landscape. A site sponsor can subscribe to the Litterfall Mercury Monitoring Initiative for a fee and receives:

  • supplies for collecting annual samples of litterfall from a forested study plot near their site,
  • analysis of the samples for mercury,
  • data on mercury concentrations and litterfall mercury deposition.

The litterfall mercury monitoring data will be hosted on a public website. In combination with data from the MDN and AMNet networks, the litterfall data can be used to examine ranges of mercury dry deposition, to estimate combined wet and dry mercury deposition, and to evaluate mercury models.

The importance of litterfall mercury data for quantifying atmospheric mercury deposition to forests was demonstrated with studies at NADP sites in the eastern USA from 2007-2009 and 2007 to 2014. Maps on this web page show the NADP study sites and mean annual atmospheric mercury deposition from litterfall and precipitation at the sites. Litterfall mercury deposition was compared with estimated mercury dry deposition based on NADP data in another study

Siting Requirements/Start Up

There are two requirements for an NADP site to participate in the litterfall mercury monitoring initiative as part of the transition program:

  1. forest near the NADP site that is suitable for establishing a representative study plot for collecting annual litterfall samples; and
  2. joining the NADP as a cooperator.

The Litterfall Mercury Monitoring Initiative began sample collection prior to the onset of autumn leaf fall in 2012, a date that varies with latitude, altitude, and forest type (approximately August through September). Sites can join the Litterfall Mercury Monitoring Initiative during any year, but will need to deploy sample collectors before leaf fall begins. NADP has litterfall sampling kits available and ready to ship to site operators.

The protocols for the litterfall mercury monitoring initiative explain the forest study plot and sample collection. Passive collectors are systematically deployed and emptied to obtain an annual sample of ambient litterfall during the entire autumn leaf drop season in deciduous and mixed deciduous-coniferous forests. Contact NADP to start litterfall monitoring in other forest types..


The NADP litterfall mercury monitoring fee is $2,000 per site. This fee pays for sampling supplies, sample analysis, shipping to the site, and site support. Like other NADP networks, the site pays to ship the monthly litterfall samples to NADP (2 to 6 shipments, depending on location and forest type). Also, the site operator will spend approximately 1 hour each month, first to deploy 8 sample collectors and then to retrieve and ship the 8 monthly litterfall samples. The samples can be collected at the same time as the MDN sample is collected (about 1 hour per month). No extra equipment or supplies are needed.

For More Information

Additional details about the new initiative for litterfall mercury monitoring are in the 12-Point Plan, including protocols for site selection, sample collection, and data management. For more information about starting litterfall sample collection, contact Mark Olson (608-263-9041,

The 12-Point Plan (the official NADP proposal) is available here, as is a printable announcement of the initiative.

National Atmospheric Deposition Program forest study sites 2007-2014, with forest-cover types (Click to enlarge)

National Atmospheric Deposition Program forest study sites with quartile distribution of mean annual litterfall mercury and precipitation mercury deposition, 2007-2014 (High is top two quartiles, Medium is middle two quartiles, Low is bottom two quartiles, two sites excluded) (Click to enlarge)

Passive collector for annual samples of ambient litterfall at forest study sites.
(Click to enlarge)

National Atmospheric Deposition Program Litterfall Mercury Monitoring Initiative Sites, 2012-2016.
(Click to enlarge)