Long-term Trends in Atmospheric Sulfur and Nitrogen Species across the United States:  30 years of Clean Air Status Network (CASTNET) operations

David Schmeltz1, Melissa Puchalski2, Gary Lear3, Greg Beachley4, Kemp Howell5, Taylor Macy6, Chris Rogers7, Tim Sharac8 and Marcus Stewart9

The Clean Air Status and Trends Network (CASTNET) and the National Atmospheric Deposition Program (NADP) offer a robust capability to investigate long-term trends in atmospheric concentrations and deposition of sulfur and nitrogen species in rural areas of the U.S.  The U.S. EPA, National Park Service, and the Bureau of Land Management (Wyoming State Office) manage the CASTNET program and network operations at 90+ sites located throughout the contiguous US, Alaska, and Canada.  For nearly three decades, CASTNET has produced data of proven quality following consistent methods required for long-term trends analysis.  Observational data from CASTNET (and NADP) are used commonly to assess the effectiveness of emission control policies, to evaluate and develop air quality models, and are important inputs to critical load assessments and ecosystem studies.  CASTNET is recognized as a dynamic and resilient program that has evolved to respond to changing agency objectives and emerging issues while still maintaining a consistent set of measurements (SO2, HNO3, SO4, NO3, NH4, base cations, CL, and O3) over time in the face of static or declining federal budgets.  CASTNET complements measurements from co-located or nearby air quality monitoring network sites (i.e. NCore, Chemical Speciation Network, Interagency Monitoring of Protected Visual Environments). Further, the CASTNET platform has facilitated new deposition and effects research and methods development geared toward reducing uncertainties in the nitrogen budget.   Here we present results from CASTNET, featuring regional long-term trends (1990-2016) in atmospheric concentrations and estimates of dry deposition of key sulfur and nitrogen species.  We also look at how the network has adapted to advance deposition research, reduce spatial gaps in remote areas, and build or expand upon existing partnerships to meet the evolving needs of the policy and scientific communities. Finally, we discuss the challenges the network will face in the next 5-years and plans for minimizing the impacts of those challenges.


1EPA, schmeltz.david@epa.gov
2EPA, puchalski.melissa@epa.gov
3EPA, lear.gary@epa.gov
4EPA, beachley.gregory@epa.gov
5Amec Foster Wheeler, Inc., kemp.howell@amecfw.com
6EPA, macy.taylor@epa.gov
7Amec Foster Wheeler, Inc., christopher.rogers@amecfw.com
8EPA, sharac.timothy@epa.gov
9Amec Foster Wheeler, Inc., marcus.stewart@amecfc.com