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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. 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 below detection limit at night to 100 pptv by day on average. In contrast, the oxygenated volatile organic compounds (OVOCs) methanol and acetone exhibited no diel cycle and were approximately an order of magnitude higher in mixing ratio (range: 1–8 ppbv) than the locally emitted isoprene (up to 320 pptv), total monoterpenes (up to 250 pptv) and aromatic compounds such as benzene and toluene (up to 100 pptv, spikes up to 400 pptv). Acetic acid was present at mixing ratios between 0.05 and 4 ppbv and followed a pronounced diel cycle in one specific period, which was related to local production and loss and local meteorological effects. During the rest of the campaign the impact of the free troposphere and long distance transport from source regions dominated over local processes and diel cycles were not observed. The Lagrangian model FLEXPART was used to determine transport patterns and photochemical processing times of air masses originating from Eastern and Western Europe. Eastern and Western European air masses showed distinct trace gas concentrations, with ca. 20 % higher ozone and ca. 30–50 % higher values for most of the OVOCs observed from the East. Using the FLEXPART calculated transport time, the contribution of photochemical processing, sea surface contact and dilution was estimated. Methanol, acetone and acetic acid all decreased with residence time in the marine boundary layer (MBL) with loss rates of 0.1 ± 0.01 ppbv/h, 0.06 ± 0.01 ppbv/h, 0.05 ± 0.01 ppbv/h from Eastern Europe and 0.06 ± 0.01 ppbv/h, 0.02 ± 0.004 ppbv/h and 0.03 ± 0.004 ppbv/h from Western Europe, respectively. The most soluble species, acetic acid, showed the lowest loss rates, indicating that solubility limited deposition to the ocean was not the only factor and that turbulent transport, plume dilution, microbial consumption within the surface of the ocean and especially entrainment from the free troposphere may also be important. Correlations between acetone, methanol and acetic acid were rather weak in western air masses (r2 = 0.52–0.62), but were stronger in air masses measured after the shorter transport time from the East (r2 = 0.53–0.81).
