Downwind evolution of the volatility and mixing state of near-road aerosols near a US interstate highway

摘要

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, 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) at 10 m from 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) was 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 non-volatile fraction in roadside aerosols are mostly externally mixed. V-TDMA measurements at different distances downwind from the highway indicate that mixing state of roadside aerosols does not change significantly (e.g., BC mostly remains externally mixed) within a few hundred meters from the highway. A preliminary analysis indicates that a super-position 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 highlight that exposures and impacts of BC and semi-volatile organics containing particles in a near-road microenvironment may differ across seasons and under changing ambient conditions.