posted on 2018-07-25, 00:00authored bySarah
A. Styler, Alyson M. Baergen, D. J. Donaldson, Hartmut Herrmann
In polluted urban environments, windows
and building surfaces are
coated with a complex film of chemicals. Despite its high surface-to-volume
ratio and direct exposure to sunlight, few studies have directly investigated
the role that this “urban film” may play in promoting
the chemistry and photochemistry of semivolatile organic species contained
within it. Here, we report results from a field investigation of the
organic composition of urban film and particulate matter (PM10) samples collected at an urban site in Leipzig, Germany, in which
we provide clear evidence for the influence of anthropogenic processes
on film composition. In this study, we find that the ratio of water-soluble
organic carbon (WSOC) to the total ionic content of film samples decreases
with atmospheric exposure time, which suggests that urban film growth
proceeds first via the condensation of semivolatile species, and that
the coating thus formed enhances the dry deposition of particles.
Further, we find that the polycyclic aromatic hydrocarbon (PAH) abundance
profiles in light-exposed films are different from those in films
collected under light-shielded conditions, which represents the first
direct evidence that urban films serve as a photochemical sink for
semivolatile organic pollutants. Finally, we find that the PAH and n-alkane profiles of urban film samples differ substantially
from colocated PM10 samples, which we suggest reflects
both the contribution of settled coarse particulate matter to the
overall film composition and the influence of in-film oxidative processes.
Together, these results highlight the unique reactive environment
afforded by urban film and underscore the need for further studies
of urban surface chemistry.