Atmospheric aerosols degrade visibility, alter the radiative balance of the planet, are the leading environmental cause of premature mortality, and are a vector for the deposition of both nutrients and toxic species to the environment. Inorganic aerosol (sulfate, nitrate, and ammonium) make up a substantial fraction of fine aerosol particles in surface air. These particles largely reflect human activities, where emissions from mobile sources (nitrogen oxides, NOx), power generation (sulfur oxides, SOx) and agriculture (ammonia, NH3) are the major precursors. National networks in the United States have been making measurements of inorganic aerosol deposition both in precipitation (National Atmospheric Deposition Program, NADP) and from surface uptake (Clean Air Status and Trends Network, CASTNET) for decades. The goal of this project is to explore the trend in these observations of inorganic aerosol deposition and how these measurements can constrain aerosol precursor emissions and removal. Specifically, we propose to quantify the interannual variability and trend in inorganic aerosol deposition from 1995-2015 in the continental United States using quantile regression. We will then investigate how well measurements from the current networks of deposition monitoring reflect trends in bottom-up emissions over the same time interval. Finally, we will use these deposition measurements to test and identify biases in the representation of aerosol loss in a global model (GEOS-Chem). The proposed work directly addresses the AC4 program research priorities and solicitation by exploring long-term records of deposition collected by the NADP (supported by NOAA) and CASTNET monitoring networks. In addition, the proposed work will focus not only on trends, but also on extremes in the multi-decadal observational record as highlighted in the solicitation. Finally, we will make use of complementary datasets and use these to help comment on the intrinsic value of the United States deposition monitoring networks.