Characterizing the influence of highways on springtime NO2 and NH3 concentrations in regional forest monitoring plots

• Springtime concentrations of NO2 and NH3 in Ontario forest plots vary greatly.
• Concentrations of NO2 and NH3 can be predicted by surrounding road density.
• Plant and lichen N concentrations are positively related to predicted NO2 and NH3.
• Epiphytic lichen richness in negatively related to NO2 and NH3.
• Vascular plant richness is positively related to NO2 and NH3.

Highways are major sources of nitrogen dioxide (NO2) and ammonia (NH3). In this study, springtime NO2 and NH3 concentrations were measured at 17 Ontario Forest Biomonitoring Network (OFBN) plots using passive samplers. Average springtime NO2 concentrations were between 1.3 μg m−3 and 27 μg m−3, and NH3 concentrations were between 0.2 μg m−3 and 1.7 μg m−3, although concentrations measured in May (before leaf out) were typically twice as high as values recorded in June. Average NO2 concentrations, and to a lesser extent NH3, could be predicted by road density at all radii (around the plot) tested (500 m, 1000 m, 1500 m). Springtime NO2 concentrations were predicted for a further 50 OFBN sites. Normalized plant/lichen N concentrations were positively correlated with estimated springtime NO2 and NH3 concentrations. Epiphytic foliose lichen richness decreased with increasing NO2 and NH3, but vascular plant richness was positively related to estimated springtime NO2 and NH3.