Size-resolved aerosol composition at an urban and a rural site in the Po Valley in summertime: implications for secondary aerosol formation
The aerosol size-segregated chemical composition was analyzed at an urban (Bologna) and a rural (San Pietro Capofiume) site in the Po Valley, Italy, during June and July 2012, by ion-chromatography (major water-soluble ions and organic acids) and evolved gas analysis (total and water-soluble carb...
| Main Authors: | , , , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-09-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/16/10879/2016/acp-16-10879-2016.pdf |
| Summary: | The aerosol size-segregated chemical composition was analyzed at an urban
(Bologna) and a rural (San Pietro Capofiume) site in the Po Valley, Italy,
during June and July 2012, by ion-chromatography (major water-soluble ions
and organic acids) and evolved gas analysis (total and water-soluble carbon),
to investigate sources and mechanisms of secondary aerosol formation during
the summer. A significant enhancement of secondary organic and inorganic
aerosol mass was observed under anticyclonic conditions with recirculation of
planetary boundary layer air but with substantial differences between the
urban and the rural site. The data analysis, including a principal component
analysis (PCA) on the size-resolved dataset of chemical concentrations,
indicated that the photochemical oxidation of inorganic and organic gaseous
precursors was an important mechanism of secondary aerosol formation at both
sites. In addition, at the rural site a second formation process, explaining
the largest fraction (22 %) of the total variance, was active at
nighttime, especially under stagnant conditions. Nocturnal chemistry in the
rural Po Valley was associated with the formation of ammonium nitrate in
large accumulation-mode (0.42–1.2 µm) aerosols favored by local
thermodynamic conditions (higher relative humidity and lower temperature
compared to the urban site). Nocturnal concentrations of fine nitrate were,
in fact, on average 5 times higher at the rural site than in Bologna. The
water uptake by this highly hygroscopic compound under high RH conditions
provided the medium for increased nocturnal aerosol uptake of water-soluble
organic gases and possibly also for aqueous chemistry, as revealed by the
shifting of peak concentrations of secondary compounds (water-soluble organic
carbon (WSOC) and sulfate) toward the large accumulation mode
(0.42–1.2 µm). Contrarily, the diurnal production of WSOC (proxy
for secondary organic aerosol) by photochemistry was similar at the two sites
but mostly affected the small accumulation mode of particles
(0.14–0.42 µm) in Bologna, while a shift to larger accumulation
mode was observed at the rural site. A significant increment in carbonaceous
aerosol concentration (for both WSOC and water-insoluble carbon) at the urban
site was recorded mainly in the quasi-ultrafine fraction (size range
0.05–0.14 µm), indicating a direct influence of traffic emissions
on the mass concentrations of this range of particles. |
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| ISSN: | 1680-7316 1680-7324 |