Downwind evolution of the volatility and mixing state of near-road aerosols near a US interstate highway
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 roa...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-02-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/18/2139/2018/acp-18-2139-2018.pdf |
| Summary: | 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, <i>C</i><sup>∗</sup> ≥ 1µm<sup>−3</sup>) 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
<q>background</q> 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. |
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| ISSN: | 1680-7316 1680-7324 |