Traffic-related air pollution near roadways: discerning local impacts from background
Authors: Hilker Nathan , Wang Jonathan M. , Jeong Cheol-Heon , Healy Robert M. , Sofowote Uwayemi , Debosz Jerzy , Su Yushan , Noble Michael , Munoz Anthony , Doerksen Geoff , White Luc , Audette Céline , Herod Dennis , Brook Jeffrey R. , Evans Greg J. 
Published: October 2019

Abstract. Adverse health outcomes related to exposure to air
pollution have gained much attention in recent years, with a particular
emphasis on traffic-related pollutants near roadways, where concentrations
tend to be most severe. As such, many projects around the world are being
initiated to routinely monitor pollution near major roads. Understanding the
extent to which local on-road traffic directly affects these measurements,
however, is a challenging problem, and a more thorough comprehension of it
is necessary to properly assess its impact on near-road air quality. In this
study, a set of commonly measured air pollutants (black carbon; carbon
dioxide; carbon monoxide; fine particulate matter, PM2.5; nitrogen
oxides; ozone; and ultrafine particle concentrations) were monitored
continuously between 1 June 2015 and 31 March 2017 at six
stations in Canada: two near-road and two urban background stations in
Toronto, Ontario, and one near-road and one urban background station in
Vancouver, British Columbia. Three methods of differentiating between local
and background concentrations at near-road locations were tested: (1) differences in average pollutant concentrations between near-road and urban
background station pairs, (2) differences in downwind and upwind pollutant
averages, and (3) interpolation of rolling minima to infer background
concentrations. The last two methods use near-road data only, and were
compared with method 1, where an explicit difference was measured, to assess
accuracy and robustness. It was found that method 2 produced average local
concentrations that were biased high by a factor of between 1.4 and 1.7 when
compared with method 1 and was not universally feasible, whereas method 3
produced concentrations that were in good agreement with method 1 for all
pollutants except ozone and PM2.5, which are generally secondary and
regional in nature. The results of this comparison are intended to aid
researchers in the analysis of data procured in future near-road monitoring
studies. Lastly, upon determining these local pollutant concentrations as a
function of time, their variability with respect to wind speed (WS) and wind
direction (WD) was assessed relative to the mean values measured at the
specific sites. This normalization allowed generalization across the
pollutants and made the values from different sites more comparable. With
the exception of ozone and PM2.5, local pollutant concentrations at
these near-road locations were enhanced by a factor of 2 relative to their
mean in the case of stagnant winds and were shown to be proportional to
WS−0.6. Downwind conditions enhanced local concentrations by a factor
of ∼2 relative to their mean, while upwind conditions
suppressed them by a factor of ∼4. Site-specific factors such
as distance from roadway and local meteorology should be taken into
consideration when generalizing these factors. The methods used to determine
these local concentrations, however, have been shown to be applicable across
pollutants and different near-road monitoring environments.