Roadside increments in PM10, NOx and NO2 concentrations observed over 2 months at a major highway in New Zealand

Continuous and simultaneous observational particulate matter (measured as PM₁₀), nitrogen dioxide (NO₂) and oxides of nitrogen (NO_x) data were captured at a kerbside site alongside a major highway in Auckland, New Zealand, and at a pair of setback sites within 250 m of the highway, day and night over 8 weeks. The three measurement sites were intended to allow emissions from the highway to be largely isolated from other sources. By filtering the data and subtracting upwind concentrations, the average roadside increment was calculated to be 1.8, 7.2 and 101.4 μg m^-3 for PM₁₀, NO₂ and NO_x, respectively, relative to a predominantly upwind setback site, and -0.1, 9.4 and 98.5 μg m^-3 for PM₁₀, NO₂ and NO_x, respectively, relative to a downwind setback site. The negative value for PM₁₀ was attributed to local evening heating sources impacting the setback site. On days when peak 24 h PM₁₀ concentrations were observed, the absolute kerbside increment was 2.1 μg m^-3. The absolute roadside 24 h average PM₁₀ increment varied diurnally, peaking (on average) at 2.4 μg m^-3 during peak traffic hours. The largest observed 24-h average PM₁₀ roadside increment was 6.9 μg m^-3 and exceeded 5 μg m^-3 on nine occasions. On each of these occasions, the daily mean wind speed was less than 2 m s^-1. The diurnally averaged difference in NO_x concentrations between the kerbside site and the setback sites clearly resembled the diurnal cycle in traffic volume, and peaked during the morning traffic peak at around 180 μg m^-3. Background NO_x concentrations were slightly higher in our study compared to a similar study in Las Vegas but absolute roadside concentrations were higher. This may be consistent with higher NO_x emission factors in Auckland, but differences in the precise distance of the monitor from the road lanes and differences in meteorology need to be considered.