Relationships between roadside and setback air quality at two nearby suburbs bisected by major motorways

Abstract

Monitoring of air quality alongside heavily-trafficked motorways has shown air pollutant levels are typically much higher than levels at background monitoring sites. In many large cities, a significant portion of the population resides on land immediately adjacent to motorways. For long-term residents, this has been associated with adverse health outcomes. However, generalizing the impact of major roads on nearby air quality is complicated by local influences such as wind climate, topography and vehicle fleet emission rates. This study aimed to quantify and explain differences in pollutant levels at two roadside and several associated setback sites in Auckland, New Zealand. Levels of nitrogen oxides (NOx) and carbon monoxide (CO) were monitored over the 2010 and 2011 autumn-winter transition (April - July), around two urban motorways roughly 5 km apart. Limited monitoring of ultrafine particles (UFP) was also conducted at two sites. Annual Average Daily Traffic (AADT) was 33% greater at the primary study site (122,098) than at the secondary site (81,075). Unmonitored ultrafine particle (UFP) concentrations were modeled from NOx data using linear regression (R2>0.75). Missing CO data was predicted using nitric oxide (NO) concentrations (R2=0.93). Dispersion modeling was performed using The Air Pollution Model (TAPM) in order to assess the influence of meteorology on pollutant decay. Results show that, for both study sites, mean levels of NOx and UFPs were elevated by 41 - 52% at the roadside compared to setback sites as close as 134 m downwind. CO concentrations were elevated by 44 - 64%. Mean roadside UFP, NOx and CO at the busier study area was 25,672 pt/cm3, 128.6 µg/m3 and 1.1 ppm, respectively; higher than the lesser trafficked area by 43% (UFP, NOx) and 56% (CO). Overall results suggest that outdoor exposure along the roadside corridor is approximately 50% greater than for those living at setback distances. As New Zealand homes are particularly prone to infiltration of outdoor air, these results may have significant long-term exposure implications.