Excess nitrogen (N) is a serious water-quality problem in most of the estuaries in the United States, especially those downstream of developed coastal basins. Understanding sources of N is a key first step in managing and mitigating N pollution. While the major sources of this N, atmospheric deposition, wastewater, fertilizer, and other agricultural sources are well-known, their relative importance as N sources to particular estuaries is not. Much of this uncertainty is due to difficulties associated with estimating the amount of atmospheric N deposition. Here, we show that deposition is 47% higher in urban and 22% higher in suburban areas compared to nonurban areas and that this deposition is primarily due to dry deposition. Moreover, this deposition is not being measured by the current deposition monitoring networks that were designed to measure broader regional patterns causing an underestimation of N inputs in urban areas.