Seasonal variation and origins of volatile organic compounds observed during 2 years at a western Mediterranean remote background site (Ersa, Cape Corsica)
Authors: Debevec Cécile , Sauvage Stéphane , Gros Valérie , Salameh Thérèse , Sciare Jean , Dulac François , Locoge Nadine 
Published: February 2021

Abstract. An original time series of about 300 atmospheric measurements of a wide range of volatile organic compounds (VOCs) was obtained at a
remote Mediterranean station on the northern tip of Corsica (Ersa, France) over 25 months from June 2012 to June 2014. This study presents the
seasonal variabilities of 35 selected VOCs and their various associated sources. The VOC abundance was largely dominated by oxygenated VOCs (OVOCs)
along with primary anthropogenic VOCs with a long lifetime in the atmosphere. VOC temporal variations were then examined. Primarily of local
origin, biogenic VOCs exhibited notable seasonal and interannual variations,
related to temperature and solar radiation. Anthropogenic compounds showed
increased concentrations in winter (JFM months) followed by a decrease in
spring/summer (AMJ/JAS months) and higher winter concentration levels in 2013 than in 2014 by up to 0.3 µg m−3 in the cases of propane,
acetylene and benzene. OVOC concentrations were generally high in
summertime, mainly due to secondary anthropogenic/biogenic and primary
biogenic sources, whereas their lower concentrations during autumn and winter were potentially more influenced by primary/secondary anthropogenic sources.
Moreover, an apportionment factorial analysis was applied to a database
comprising a selection of 14 individual or grouped VOCs by means of the
positive matrix factorization (PMF) technique. A PMF five-factor solution was taken on. It includes an anthropogenic factor (which contributed 39 % to
the total concentration of the VOCs selected in the PMF analysis) connected
to the regional background pollution, three other anthropogenic factors
(namely short-lived anthropogenic sources, evaporative sources, and
long-lived combustion sources, which together accounted for 57 %) originating from either nearby or more distant emission areas (such as Italy
and south of France), and a local biogenic source (4 %). Variations in
these main sources impacting VOC concentrations observed at the Ersa station
were also investigated at seasonal and interannual scales. In spring and
summer, VOC concentrations observed at Ersa were the lowest in the 2-year
period, despite higher biogenic source contributions. During these seasons,
anthropogenic sources advected to Ersa were largely influenced by chemical
transformations and vertical dispersion phenomena and were mainly of
regional origins. During autumn and winter, anthropogenic sources showed higher contributions when European air masses were advected to Ersa and
could be associated with potential emission areas located in Italy and
possibly more distant ones in central Europe. Higher VOC winter
concentrations in 2013 than in 2014 could be related to contribution
variations in anthropogenic sources probably governed by their emission
strength with external parameters, i.e. weaker dispersion phenomena and the pollutant depletion. High-frequency observations collected during several intensive field campaigns conducted at Ersa during the three summers
2012–2014 confirmed findings drawn from bi-weekly samples of the 2-year period
in terms of summer concentration levels and source apportionment. However,
they also suggested that higher sampling frequency and temporal resolution,
in particular to observe VOC concentration variations during the daily
cycle, would have been necessary to confirm the deconvolution of the
different anthropogenic sources identified following the PMF approach.
Finally, comparisons of the 25 months of Ersa observations with VOC
measurements conducted at 17 other European monitoring stations highlighted
the representativeness of the Ersa station for monitoring seasonal
variations in VOC regional pollution impacting continental Europe.
Nevertheless, VOC winter concentration levels can significantly vary between
sites, pointing out spatial variations in anthropogenic source
contributions. As a result, Ersa concentration variations in winter were
more representative of VOC regional pollution impacting central Europe.
Moreover, interannual and spatial variations in VOC winter concentration
levels were significantly impacted by synoptic phenomena influencing
meteorological conditions observed in continental Europe, suggesting that
short observation periods may reflect the variability of the identified
parameters under the specific meteorological conditions of the study period.