Articles | Volume 17, issue 15
Research article
09 Aug 2017
Research article |  | 09 Aug 2017

Volatile organic compounds (VOCs) in photochemically aged air from the eastern and western Mediterranean

Bettina Derstroff, Imke Hüser, Efstratios Bourtsoukidis, John N. Crowley, Horst Fischer, Sergey Gromov, Hartwig Harder, Ruud H. H. Janssen, Jürgen Kesselmeier, Jos Lelieveld, Chinmay Mallik, Monica Martinez, Anna Novelli, Uwe Parchatka, Gavin J. Phillips, Rolf Sander, Carina Sauvage, Jan Schuladen, Christof Stönner, Laura Tomsche, and Jonathan Williams

Abstract. During the summertime CYPHEX campaign (CYprus PHotochemical EXperiment 2014) in the eastern Mediterranean, multiple volatile organic compounds (VOCs) were measured from a 650 m hilltop site in western Cyprus (34° 57′ N/32° 23′ E). Periodic shifts in the northerly Etesian winds resulted in the site being alternately impacted by photochemically processed emissions from western (Spain, France, Italy) and eastern (Turkey, Greece) Europe. Furthermore, the site was situated within the residual layer/free troposphere during some nights which were characterized by high ozone and low relative humidity levels. In this study we examine the temporal variation of VOCs at the site. The sparse Mediterranean scrub vegetation generated diel cycles in the reactive biogenic hydrocarbon isoprene, from very low values at night to a diurnal median level of 80–100 pptv. In contrast, the oxygenated volatile organic compounds (OVOCs) methanol and acetone exhibited weak diel cycles and were approximately an order of magnitude higher in mixing ratio (ca. 2.5–3 ppbv median level by day, range: ca. 1–8 ppbv) than the locally emitted isoprene and aromatic compounds such as benzene and toluene. Acetic acid was present at mixing ratios between 0.05 and 4 ppbv with a median level of ca. 1.2 ppbv during the daytime. When data points directly affected by the residual layer/free troposphere were excluded, the acid followed a pronounced diel cycle, which was influenced by various local effects including photochemical production and loss, direct emission, dry deposition and scavenging from advecting air in fog banks. The Lagrangian model FLEXPART was used to determine transport patterns and photochemical processing times (between 12 h and several days) of air masses originating from eastern and western Europe. Ozone and many OVOC levels were  ∼  20 and  ∼  30–60 % higher, respectively, in air arriving from the east. Using the FLEXPART calculated transport time, the contribution of photochemical processing, sea surface contact and dilution was estimated. Methanol and acetone decreased with residence time in the marine boundary layer (MBL) with loss rate constants of 0.74 and 0.53 day−1 from eastern Europe and 0.70 and 0.34 day−1 from western Europe, respectively. Simulations using the EMAC model underestimate these loss rates. The missing sink in the calculation is most probably an oceanic uptake enhanced by microbial consumption of methanol and acetone, although the temporal and spatial variability in the source strength on the continents might play a role as well. Correlations between acetone and methanol were weaker in western air masses (r2  =  0.68), but were stronger in air masses measured after the shorter transport time from the east (r2  =  0.73).

Short summary
The aim of the study was to examine aged air masses being transported from the European continent towards Cyprus. Longer-lived oxygenated volatile organic compounds (OVOCs) such as methanol were mainly impacted by long-distance transport and showed higher values in air masses from eastern Europe than in a flow regime from the west. The impact of the transport through the marine boundary layer as well as the influence of the residual layer/free troposphere on OVOCs were studied.
Final-revised paper