Abstract
Abstract. We present spatial measurements of particle volatility and mixing state at a site near a North Carolina interstate highway (I-40) applying several heating (thermodenuder; TD) experimental approaches. Measurements were conducted in summer 2015 and winter 2016 in a roadside trailer (10 m from road edge) and during downwind transects at different distances from the highway under favorable wind conditions using a mobile platform. Results show that the relative abundance of semi-volatile species (SVOCs) in ultrafine particles decreases with downwind distance, which is consistent with the dilution and mixing of traffic-sourced particles with background air and evaporation of semi-volatile species during downwind transport. An evaporation kinetics model was used to derive particle volatility distributions by fitting TD data. While the TD-derived distribution apportions about 20–30 % of particle mass as semi-volatile (SVOCs; effective saturation concentration, C∗ ≥ 1µm−3) at 10 m from the road edge, approximately 10 % of particle mass is attributed to SVOCs at 220 m, showing that the particle-phase semi-volatile fraction decreases with downwind distance. The relative abundance of semi-volatile material in the particle phase increased during winter. Downwind spatial gradients of the less volatile particle fraction (that remaining after heating at 180 °C) were strongly correlated with black carbon (BC). BC size distribution and mixing state measured using a single-particle soot photometer (SP2) at the roadside trailer showed that a large fraction (70–80 %) of BC particles were externally mixed. Heating experiments with a volatility tandem differential mobility analyzer (V-TDMA) also showed that the nonvolatile fraction in roadside aerosols is mostly externally mixed. V-TDMA measurements at different distances downwind from the highway indicate that the mixing state of roadside aerosols does not change significantly (e.g., BC mostly remains externally mixed) within a few hundred meters from the highway. Our analysis indicates that a superposition of volatility distributions measured in laboratory vehicle tests and of background aerosol can be used to represent the observed partitioning of near-road particles. The results from this study show that exposures and impacts of BC and semi-volatile organics-containing particles in a roadside microenvironment may differ across seasons and under changing ambient conditions.
Highlights
Motor vehicles are a large source of ambient fine particulate matter (PM; Dallmann and Harley, 2010; Fraser et al, 1999; Kumar et al, 2011; Zhang et al, 2015)
Vehicle-emitted PM largely consists of primary organic aerosol (POA) and black carbon (BC; Dallmann et al, 2014; Maricq, 2007)
Results shows that the evaporation observed in a TD at 60 ◦C decreases with downwind distance during transects in both seasons, which suggests a reduction in the relative abundance of the semi-volatile fraction in particles with distance
Summary
Motor vehicles are a large source of ambient fine particulate matter (PM; Dallmann and Harley, 2010; Fraser et al, 1999; Kumar et al, 2011; Zhang et al, 2015). Vehicle-emitted PM largely consists of primary organic aerosol (POA) and black carbon (BC; Dallmann et al, 2014; Maricq, 2007). Vehicle exhaust undergoes rapid cooling and dilution with ambient air on the road. Emissions undergo further evolution from road to background-like conditions within a few hundred meters downwind from the roadway (Robinson et al, 2010; Zhang et al, 2004), which involves complex physicochemical processes.
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