In December 2024, EPA released a 3-year modernization plan for CASTNET. The plan, which is available at https://www.epa.gov/system/files/documents/2025-01/castnet_modernization_plan_dec2024_final.pdf, lists the steps the program will take to respond to input from an external scientific review of CASTNET and address the evolving data needs of the EPA and the scientific community. The scientific review made several key recommendations among them to maintain long-term multipollutant monitoring at as many locations as possible, add PM2.5 measurements using non-regulatory sensors, and continue to address spatial gaps in the Central United States. The review also recommended expanding Tribal partnerships to address gaps in data coverage and better understand environmental impacts to those communities.

The ultimate goal of the modernization effort is to establish a cost-effective, sustainable future for CASTNET. This involved difficult decisions about decommissioning select sites and ending certain measurements to enable the long-term viability of the network. The scientific review suggested criteria for evaluating existing sites, and these were used to perform a network assessment and determine which sites would be discontinued. Also, SO₂ measurements from the CASTNET filter pack were ended at most sites (EPA and NPS-sponsored), and continuous NO_y measurements were ended with the exception of several special study sites and the NCore site at Bondville, IL.

During 2025 and early 2026, WSP has worked to begin implementation of the modernization plan. WSP has recommended and procured replacement, regulatory ozone analyzers and transfer standards for all EPA-sponsored sites. This equipment will begin deployment during the second half of 2026. We have acquired and deployed lower-cost PM sensors at all EPA and Indian River Lagoon (IRL) Council-sponsored CASTNET sites. WSP decommissioned seven CASTNET sites and transitioned two sites to ozone-only. We also worked on repairing existing infrastructure including several major shelter refurbishments. Since the fourth quarter of 2024, CASTNET has seen the addition of three sites (two with Tribal partners and one in conjunction with IRL Council) and the restart of a site in West Virginia whose operations had been suspended.