Long-path Measurements of Pollutants and Micrometeorology over Highway 401 in Toronto

Highway traffic emissions are a significant source of air pollution, especially in urban areas. Traffic-related pollution has been linked to a number of adverse health effects, such as respiratory problems, cancer, heart failure, and a general increase in mortality. Determining the pollutants involved and quantifying their emission rates is paramount in developing high resolution air quality models to predict future developments with some confidence.

Highway 401, as it passes through Toronto, is the world's busiest highway with a typical traffic volume of over 450,000 vehicles per day. In July 2015, three weeks of measurements were conducted over the 401 with an Open-path Fourier Transform Infrared (OP-FTIR) spectrometer in parallel with a scintillometer, integrating a variety of gas concentrations as well as turbulence parameters over a 310 m path length. Concurrent in-situ measurements were made from a trailer adjacent to the highway with standard NAPS (National Air Pollution Surveillance network) instrumentation for comparison.

Relationships between traffic volume, micrometeorological conditions and the build-up and venting of pollutants on the highway are analyzed and quantified, demonstrating the role of stratification and turbulence in the accumulation and dispersal of traffic emissions. Concentrations of some gaseous pollutants are found to have diurnal cycles correlated with traffic volume, turbulence and local micrometeorology in various ways. High ammonia levels (up to 15 ppb) were observed, most likely coming from catalytic converters. Hydrogen cyanide (HCN) was found between 0 to 4 ppb, generally consistent with previous measurement and model results which have shown that HCN can be formed during catalytic reduction of nitric oxides (NOx) as well as directly in fossil fuel combustion processes. Methanol was typically 0 to 20ppb, and formaldehyde 0-5 ppb. Significant discrepancies were found between NO and NO2 derived from FTIR spectra and in-situ NAPS measurements, illustrating the difficulty of correcting for the ubiquitous water vapour interferences under high humidity conditions.

The feasibility of deriving emission rates by combining the micrometeorological data from the scintillometer with the gas concentrations from the FTIR using an inverse dispersion approach is investigated. The findings will be compared with output from a high-resolution air quality model (GEM-MACH) to aid future improvements in the sub-grid parameterization of pollutant dispersion.