There is growing evidence that ultrafine particles (UFP; particles smaller than 100 nm) are likely more toxic than larger particles. However, the health effects of UFP remain uncertain due in part to the lack of large-scale population-based exposure assessment. We develop a national-scale empirical model of particle number concentration (PNC; a measure of UFP) using data from mobile monitoring and fixed sites across the United States and a land-use regression (LUR) modeling framework. Traffic, commercial land use, and urbanicity-related variables explain much of the spatial variability of PNC (base model R² = 0.77, RMSE = 2400 cm^–3). Model predictions are robust across a diverse set of evaluations [random 10-fold holdout cross-validation (HCV): R² = 0.72, RMSE = 2700 cm^–3; spatially defined HCV: R² = 0.66, RMSE = 3000 cm^–3; evaluation against an independent data set: R² = 0.54, RMSE = 2600 cm^–3]. We apply our model to predict PNC at ∼6 million residential census blocks in the contiguous United States. Our estimates are annual average concentrations for 2016–2017. The predicted national census-block-level mean PNC ranges between 1800 and 26 600 cm^–3 (population-weighted average: 6500 cm^–3), with hotspots in cities and near highways. Our national PNC model predicts large urban–rural, intra-, and inter-city contrasts. PNC and PM_2.5 are moderately correlated at the city scale, but uncorrelated at the regional/national scale. Our high-spatial-resolution national PNC estimates are useful for analyzing population exposure (socioeconomic disparity, epidemiological health impact) and environmental policy and regulation.

中文翻译：

---

美国大陆超细粒子浓度的高分辨率估计

越来越多的证据表明，超细颗粒（UFP；小于 100 nm 的颗粒）可能比较大的颗粒更具毒性。然而，UFP 的健康影响仍然不确定，部分原因是缺乏大规模的基于人群的暴露评估。我们使用来自美国各地的移动监测和固定站点的数据以及土地利用回归 (LUR) 建模框架开发了一个全国范围的粒子数浓度经验模型 (PNC；UFP 的度量)。交通、商业用地和城市相关变量解释了 PNC 的大部分空间变异性（基本模型R ² = 0.77，RMSE = 2400 cm ^–3）。模型预测在各种评估中都是稳健的 [随机 10 倍保持交叉验证 (HCV)：R² = 0.72，RMSE = 2700 cm ^–3；空间定义的 HCV：R ² = 0.66，RMSE = 3000 cm ^–3；对独立数据集的评估：R ² = 0.54，RMSE = 2600 cm ^–3 ]。我们应用我们的模型来预测美国本土约 600 万个住宅人口普查街区的 PNC。我们的估计是 2016-2017 年的年平均浓度。预测的全国人口普查块级平均 PNC 范围在 1800 到 26 600 cm ^{–3 之间}（人口加权平均值：6500 cm ^–3），热点位于城市和高速公路附近。我们的国家 PNC 模型预测了城市与农村、城市内和城市间的大对比。PNC 和 PM _2.5在城市尺度上适度相关，但在区域/国家尺度上不相关。我们的高空间分辨率国家 PNC 估计可用于分析人口暴露（社会经济差异、流行病学健康影响）和环境政策和法规。