Vertical Gradients of Near-Ground Air Pollutants Reveal Substantial Biases in Inhalation Exposure Assessment

Differences in height-dependent near-ground air pollutant concentrations have not received sufficient attention in air quality monitoring and exposure measurement, of which the ignore could introduce significant bias in exposure assessment for people with different heights. Here, a modified passive air sampler was developed and used to characterize vertical profiles of typical hazardous polycyclic aromatic compounds (PACs) in near-ground microenvironments. Field measurements demonstrated significant vertical changes for most target air pollutants within the 2–150 cm height range, with the relative difference ranging from 1.1 to 40.0. The concentration of total PACs generally showed an increasing trend with height; however, the intensity of this vertical variation was closely linked to season, pollution levels, and photochemical activity, and it also varies among individual compounds. Children’s exposure to total BaP eq would be significantly overestimated, particularly during summer, with the overestimation ranging from 13% to 79%. This vertical heterogeneity implies that exposure assessment should not overlook the height-dependent variability of near-ground air pollution levels, and the sampling strategy employed in this study can be used to evaluate the vertical variation of air pollutants across different microenvironments.