ABSTRACT

Air pollutant concentrations are often higher near major roadways than in the surrounding environments owing to emissions from on-road mobile sources. In this study, we quantified the gradient in black carbon (BC) and air toxics concentrations from the I-70 freeway in the Elyria-Swansea environmental justice neighborhood in Denver, Colorado, during three measurement campaigns in 2017–2018. The average hourly upwind-downwind gradient of BC concentrations from the roadway was 500–800 ng/m³, equal to an increment of approximately 30-80% above local background levels within 180 m of the freeway. When integrated over all wind directions, the gradients were smaller, approximately 150–300 ng/m³ (~11-18%) over the course of nearly four months of measurements. No statistically significant gradient in air toxics (e.g., benzene, formaldehyde, etc.) was found, likely because the uncertainties in the mean concentrations were larger than the magnitude of the gradient (<25%). This finding is in contrast to some earlier studies in which small gradients of benzene and other VOCs were found. We estimate that sample sizes of at least 100 individual measurements would have been required to estimate mean concentrations with sufficient certainty to quantify gradients on the order of ±10% uncertainty. These gradient estimates are smaller than those found in previous studies over the past two decades; more stringent emissions standards, the local fleet age distribution, and/or the steady turnover of the vehicle fleet may be reducing the overall impact of roadway emissions on near-road communities.

Implications: Gradients of near-road pollution may be declining in the near-road environment as tailpipe emissions from the vehicle fleet continue to decrease. Near-road concentration gradients of mobile source air toxics, including benzene, 1,3-butadiene, and ethylbenzene, will require higher sample sizes to quantify as emissions continue to decline.

摘要

由于道路移动源的排放，主要道路附近的空气污染物浓度通常高于周围环境。在这项研究中，我们在 2017 年至 2018 年的三项测量活动中量化了科罗拉多州丹佛市 Elyria-Swansea 环境正义社区的 I-70 高速公路的黑碳 (BC) 和空气有毒物质浓度的梯度。来自道路的 BC 浓度的平均每小时上风-下风梯度为 500-800 ng/m ³，相当于在高速公路 180 m 内比当地背景水平高出约 30-80%。当在所有风向上积分时，梯度更小，大约 150–300 ng/m ³(~11-18%) 在近四个月的测量过程中。没有发现空气毒物（例如苯、甲醛等）的统计显着梯度，可能是因为平均浓度的不确定性大于梯度的幅度（<25%）。这一发现与一些早期的研究形成对比，在这些研究中发现了小梯度的苯和其他 VOC。我们估计至少需要 100 次单独测量的样本量才能以足够的确定性估计平均浓度，以量化 ±10% 不确定性的梯度。这些梯度估计值比过去二十年以前的研究中发现的要小；更严格的排放标准，当地车队的车龄分布，

影响：随着车队尾气排放的持续减少，近路环境中近路污染的梯度可能正在下降。随着排放量持续下降，移动源空气有毒物质（包括苯、1,3-丁二烯和乙苯）的近路浓度梯度将需要更大的样本量来量化。