Seasonal variation and origins of volatile organic compounds
observed during two 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: July 2020

Abstract. An original time series of about 300 atmospheric measurements of a wide range of volatile organic compounds (VOCs) has been obtained at a remote Mediterranean station on the northern tip of Corsica Island (Ersa, France) over 25 months from June 2012 to June 2014. This study presents the seasonal variabilities of 25 selected VOCs, and their various associated sources. The VOC speciation was largely dominated by oxygenated VOCs (OVOCs) along with primary anthropogenic VOCs having a long lifetime in the atmosphere. VOC temporal variations are then examined. Primarily of local origin, biogenic VOCs exhibited notable seasonal and interannual variations, related to temperature and solar radiation ones. Anthropogenic compounds have shown an increasing concentration trend in winter (JFM months) followed by a decrease in spring/summer (AMJ/JAS months), and different concentration levels in winter periods of 2013 and 2014. OVOC concentrations were generally higher in summertime, mainly due to secondary and biogenic sources, whereas their concentrations during fall and winter were potentially more influenced by anthropogenic primary/secondary sources. Moreover, an apportionment factorial analysis was applied to a database comprising a selection of 14 primary individual or grouped VOCs by means of the positive matrix factorization (PMF) technique. A PMF solution composed of 5 factors was taken on. It includes a biogenic factor (which contributed 4 % to the total VOC mass), three 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 remaining one (39 %) connected to the regional background pollution. Variations in these main sources impacting VOC concentrations observed at the receptor site are also investigated at seasonal and interannual scales. In spring and summer, VOC concentrations observed at Ersa were the lowest in the 2-yr period, despite higher biogenic source contributions and since anthropogenic sources advected to Ersa were largely influenced by chemical transformations and vertical dispersion phenomena and were mainly of regional origins. During fall and winter, anthropogenic sources showed higher accumulated contributions when European air masses were advected to Ersa and could be associated to potential emission areas located in Italy and possibly more distant ones in central Europe. Higher VOC concentrations during winter 2013 compared to winter 2014 ones could be related to anthropogenic source contribution variations probably governed by emission strength of the main anthropogenic sources identified in this study together with external parameters, i.e. weaker dispersion phenomena and pollutant depletion. High frequency observations collected during several intensive field campaigns conducted at Ersa during the three summers 2012–2014 confirmed findings from bi-weekly samples in terms of summer concentration levels and source apportionment. However, they suggest that higher sampling frequency and temporal resolution, in particular to observe VOC concentrations variation during the daily cycle, are needed to confirm the deconvolution of the different anthropogenic sources identified by PMF approach. Finally, comparisons of the 25 months of observations at Ersa with VOC measurements conducted at 17 other European monitoring stations highlight the representativeness of the Ersa background station for monitoring seasonal variations in VOC regional pollution impacting continental Europe. Nevertheless, winter VOC 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. Interannual and spatial VOC concentration variations in winter were also 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 studied period.