Table S2.

Summary of data from US national networks used in the study

Network	Deposition species	Data period	Source
AMoN*	Dry deposition: gaseous NH3	2011–2013	nadp.isws.illinois.edu/AMoN
CASTNET†	Dry deposition: gaseous HNO3, particulate NH4+, NO3−	2011–2013	https://www.epa.gov/castnet
NTN‡	Wet deposition: NH4+, NO3−	1990–2013	nadp.isws.illinois.edu/NTN
IMPROVE NHx§	Dry deposition: gaseous NH3, particulate NH4+, NO3−	2011–2012	(28)

^*The AMoN is operated by the NADP, which measures biweekly NH₃ concentrations using passive diffusion (Radiello) samplers.

^†The CASTNET is funded by the Environmental Protection Agency (EPA) and NPS, which measures weekly HNO₃ and particulate NH₄⁺ and NO₃⁻ concentrations using three-stage filter pack samplers.

^‡The NTN is operated by the NADP, which measures weekly NH₄⁺, NO₃⁻ concentrations in precipitation samples.

^§The IMPROVE NHx study was conducted from April 2011 to August 2012, which measured the sum of gaseous NH₃ and fine particle NH₄⁺ concentrations using a single, acid-coated filter with 1-in-3 d sampling period. Colocated measurements of NH₄⁺, NO₃⁻ and sulfate (SO₄²⁻) collected on nylon filters provide two methods to determine the split of measured NHx between gaseous NH₃ and fine particle NH₄⁺; here we assume that fine particle NO₃⁻ and SO₄²⁻ are fully neutralized by NH₄⁺ to estimate the NH₄⁺ concentration, which was then subtracted from the NHx concentration to obtain a lower bound estimate of the NH₃ concentration (28).