Composition and origin of PM2.5 aerosol particles in the upper Rhine valley in summer
Authors: Shen Xiaoli , Vogel Heike , Vogel Bernhard , Huang Wei , Mohr Claudia , Ramisetty Ramakrishna , Leisner Thomas , Prévôt André S. H. , Saathoff Harald 
Published: October 2019

Abstract. We conducted a 6-week measurement campaign in summer 2016 at a
rural site about 11 km north of the city of Karlsruhe in southwest Germany
in order to study the chemical composition and origin of aerosols in the
upper Rhine valley. In particular, we deployed a single-particle mass
spectrometer (LAAPTOF) and an aerosol mass spectrometer (AMS) to provide
complementary chemical information on aerosol particles smaller than 2.5 µm. For the entire measurement period, the total aerosol particle
mass was dominated by sodium salts, contributing on average (36±27) % to the total single particles measured by the LAAPTOF. The total
particulate organic compounds, sulfate, nitrate, and ammonium contributed on
average (58±12) %, (22±7) %, (10±1) %, and
(9±3) % to the total non-refractory particle mass measured by the
AMS. Positive matrix factorization (PMF) analysis for the AMS data suggests
that the total organic aerosol (OA) consisted of five components, including
(9±7) % hydrocarbon-like OA (HOA), (16±11) %
semi-volatile oxygenated OA (SV-OOA), and (75±15) %
low-volatility oxygenated OA (LV-OOA). The regional transport model
COSMO-ART was applied for source apportionment and to achieve a better
understanding of the impact of complex transport patterns on the field
observations. Combining field observations and model simulations, we
attributed high particle numbers and SO2 concentrations observed at
this rural site to industrial emissions from power plants and a refinery in
Karlsruhe. In addition, two characteristic episodes with aerosol particle
mass dominated by sodium salts particles comprising (70±24) %
of the total single particles and organic compounds accounting
for (77±6) % of total non-refractory species, respectively, were
investigated in detail. For the first episode, we identified relatively
fresh and aged sea salt particles originating from the Atlantic Ocean more
than 800 km away. These particles showed markers like m∕z 129
C5H7NO3+, indicating the influence of anthropogenic
emissions modifying their composition, e.g. from chloride to nitrate salts
during the long-range transport. For a 3 d episode including high organic
mass concentrations, model simulations show that on average (74±7) % of the particulate organics at this site were of biogenic origin.
Detailed model analysis allowed us to find out that three subsequent peaks
of high organic mass concentrations originated from different sources,
including local emissions from the city and industrial area of Karlsruhe,
regional transport from the city of Stuttgart (∼64 km away),
and potential local night-time formation and growth. Biogenic (forest) and
anthropogenic (urban) emissions were mixed during transport and contributed
to the formation of organic particles. In addition, topography, temperature
inversion, and stagnant meteorological conditions also played a role in the
build-up of higher organic particle mass concentrations. Furthermore, the
model was evaluated using field observations and corresponding
sensitivity tests. The model results show good agreement with trends and
concentrations observed for several trace gases (e.g. O3, NO2,
and SO2) and aerosol particle compounds (e.g. ammonium and nitrate).
However, the model underestimates the number of particles by an order of
magnitude and underestimates the mass of organic particles by a factor of
2.3. The discrepancy was expected for particle number since the model does
not include all nucleation processes. The missing organic mass indicates
either an underestimated regional background or missing sources and/or
mechanisms in the model, like night-time chemistry. This study demonstrates
the potential of combining comprehensive field observations with dedicated
transport modelling to understand the chemical composition and complex
origin of aerosols.