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

Abstract

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 massesPublished by Copernicus Publications on behalf of the European Geosciences Union. = 0.68), but were stronger in air masses measured after the shorter transport time from the east (r 2 = 0.73).

Measurements of carbonyl compounds around the Arabian Peninsula indicate large missing sources of acetaldehyde

“…The Mediterranean Sea had somewhat higher levels of aliphatic carbonyls than the clean regions (the Arabian Sea and the Gulf of Aden) but with acetone (above 2ppb) still dominating the distribution. The mean values of acetaldehyde, acetone and MEK are comparable with the results from a rural site on the west coast of Cyprus (Derstroff et al, 2017). Larger aliphatic carbonyls (C6-C9) were below the detection limit most of the time.…”

Chemical ionization quadrupole mass spectrometer with an electrical discharge ion source for atmospheric trace gas measurement

“…ing the CDC parameters can be found in Derstroff et al (2017). An exemplary time series showing the different measurement modes of the CI-QMS (scrubber, ambient, NO addition) and the differences in detection of PAA and acetic acid when applying a de-clustering voltage is provided in Fig.…”

“…During CYPHEX (Cyprus Photochemistry Experiment, summer 2014), located at a coastal site on the eastern Mediterranean island of Cyprus, we measured chemically aged air masses with origins in continental Europe (Meusel et al, 2016;Derstroff et al, 2017). A time series of SO 2 and HCl for a 3-week period of the campaign is shown in Fig.…”

A new marine biogenic emission: methane sulfonamide (MSAM), DMS and DMSO<sub>2</sub> measured in air over the Arabian Sea

“…For mass scale calibrations, 1,3,5-trichlorobenzene was continuously fed into the sample stream. The PTR-TOF-MS was calibrated at the beginning, during and at the end of the campaign (in total five humidity dependent calibrations were conducted as described by Derstroff et al (2017)). Calibrations were performed by using a standard 10 gas mixture (Apel-Riemer Environmental inc., Broomfield, USA) of several VOCs with known, gravimetrically determined mixing ratios.…”