Near-highway air pollutant levels were measured to determine how seasonal changes in early morning atmospheric mixing conditions affect pollutant concentrations. Early morning was chosen because the highest diurnal pollutant levels generally occur after the start of morning rush hour before the breakup of the nocturnal surface boundary layer. Monitoring was conducted near Interstate 93 in Somerville (Massachusetts) from 05:30 to 10:00 on 14 days between 20 October 2009 and 15 April 2010 using a mobile laboratory equipped with rapid-response instruments to measure fine and ultrafine particle concentrations and size distribution (6-225 nm), CO, NO, NO_x, and polycyclic aromatic hydrocarbons (PAHs). Significant differences in near-highway pollutant levels before and after sunrise were observed, with changes highly dependent on meteorology and season. Pre-sunrise particle number concentrations were ∼20% greater than post-sunrise levels on all 14 monitoring days, while a mix of pre- to post-sunrise increases ranging from 0–40% were found for CO, NO, NO _x, and PAHs. Winter concentrations were 10–70% higher than in Fall-Spring. Particle size diameter changes showed varying trends based on wind speed, with the greatest pre- to post-sunrise particle diameter increases (∼25%) occurring with the lightest winds. Pollutant concentrations generally did not correlate with morning traffic (R² < 0.25), indicating the importance of meteorology on pollutant levels. The results indicate an increase in atmospheric mixing beginning around the time of sunrise, leading to attenuated pollutant concentrations from early to late morning. These findings can be used to help assess exposure for people who live near highways.