Polycyclic aromatic hydrocarbons (PAHs) are widespread pollutants that are costly to measure with air quality instruments. We used a spatially balanced sample of an epiphytic moss, Orthotrichum lyellii, to develop fine-scale maps of atmospheric PAHs across residential areas of Portland, Oregon. The unusual abundance of this stress-tolerant species enabled us to systematically collect a large sample (n = 350) on a 1 × 1km grid across the city. This provided the unique opportunity to investigate potential effects of more than 20 factors on PAH concentrations in moss, including emissions sources, tree canopy cover, relative elevation, short-term weather, and nuance of sample collection. We measured 20 PAHs in moss samples and assigned them to 4 groups for analysis based on molecular weight (MW): the Naphthalene, Low, Medium, and High MW groups. Spatial regression models explained a modest amount of variation in PAH concentrations in moss, with spatial structure increasing as MW increased. Motor vehicle emissions were the only PAH source we detected and was highly significant in all models. Deciduous tree cover correlated with lower PAH concentrations in moss for all except the high MW group. Short-term weather and nuance of sample collection, like height on the sampled tree, tree taxonomic family, and whether sample was primarily collected from a branch or bole, had surprisingly large effects on all groups. Overall, results aligned with expectations based on each group's dispersal potential due to MW. All PAH groups were highest over downtown Portland, along major highways, and lowest in parks and outer neighborhoods, although the details varied. Our results supported two common themes in studies directly measuring PAHs: the importance of roads as an emissions source in residential areas, and the potential for trees to reduce atmospheric concentrations. Moreover, the large effects of weather and sampling nuance have critical implications for the design of future studies.

多环芳烃（PAH）是广泛的污染物，使用空气质量仪器进行测量很昂贵。我们使用了附生苔藓Orthotrichum lyellii的空间平衡样本，以绘制俄勒冈州波特兰市居民区的大气多环芳烃的精细比例图。这种耐压物种的异常丰富使得我们能够在整个城市的1×1km网格上系统地收集大量样本（n = 350）。这提供了独特的机会来调查20多种因素对苔藓中PAH浓度的潜在影响，包括排放源，树冠覆盖，相对海拔，短期天气和样品采集的细微差别。我们测量了苔藓样品中的20种PAH，并根据分子量（MW）将其分为4组进行分析：萘，低，中和高MW组。空间回归模型解释了苔藓中PAH浓度的适度变化，其空间结构随MW的增加而增加。机动车排放是我们检测到的唯一PAH来源，并且在所有模型中都非常重要。除高分子量组外，所有树木的落叶乔木与较低的苔藓中PAH浓度相关。短期天气和样本采集的细微差别，例如被采样树的高度，树木分类学家族，以及样品主要是从树枝还是胆中采集的，对所有组的影响都出乎意料的大。总体而言，结果与基于每个组的分子量分散潜力所得出的预期相符。尽管细节各异，但所有PAH组在波特兰市区，主要公路沿线最高，而在公园和外围地区则最低。我们的结果支持了直接测量PAHs的研究中的两个共同主题：道路作为住宅区排放源的重要性，以及树木降低大气浓度的潜力。此外，天气和采样细微差别的巨大影响对未来研究的设计具有关键意义。