Abstract
Abstract. We present here vertical fluxes of oxygenated volatile organic compounds (OVOCs) measured with eddy covariance (EC) during the period of March to July 2012 near the southwest coast of the United Kingdom. The performance of the proton-transfer-reaction mass spectrometer (PTR-MS) for flux measurement is characterized, with additional considerations given to the homogeneity and stationarity assumptions required by EC. Observed mixing ratios and fluxes of OVOCs (specifically methanol, acetaldehyde, and acetone) vary significantly with time of day and wind direction. Higher mixing ratios and fluxes of acetaldehyde and acetone are found in the daytime and from the direction of a forested park, most likely due to light-driven emissions from terrestrial plants. Methanol mixing ratio and flux do not demonstrate consistent diel variability, suggesting sources in addition to plants. We estimate air–sea exchange and photochemical rates of these compounds, which are compared to measured vertical fluxes. For acetaldehyde, the mean (1σ) mixing ratio of 0.13 (0.02) ppb at night may be maintained by oceanic emission, while photochemical destruction outpaces production during the day. Air–sea exchange and photochemistry are probably net sinks of methanol and acetone in this region. Their nighttime mixing ratios of 0.46 (0.20) and 0.39 (0.08) ppb appear to be affected more by terrestrial emissions and long-distance transport, respectively.
Highlights
(PTR-MS) for flux measurement is characterized, with additional considerations given to the homogeneity and stationarity assumptions required by eddy covariance (EC)
We report here Oxygenated volatile organic compounds (OVOCs) fluxes measured during selected periods between March and July 2012
Three time periods of flux measurements are shown here to demonstrate the application of quality control filters and the variability in OVOC fluxes
Summary
The city of Plymouth (population of ∼ 250 000) is situated on the SW coast of the United Kingdom. Measurements of OVOC mixing ratios and winds were made from the rooftop of the PML building, which is 200– 300 m from the water’s edge (depending on direction). Meteorological parameters were measured every 5 min from a station secured on a ∼ 2 m mast near the SW corner of the PML rooftop, including horizontal wind speed and direction (Mierji Meteo solid-state wind sensor, MMW-005), temperature and humidity (Hygroclip S3 sensor), precipitation (Omni Instruments 15 cm tipping bucket rain gauge, RG200), solar irradiance (Li-Cor pyranometer, LI-200SZ), and photosynthetically active radiation (Chelsea Instruments half-hemisphere PAR sensor). Just outside of Plymouth Sound, the autonomous L4 buoy (50◦15.0 N, 4◦13.0 W) is equipped with sensors that measure surface ocean parameters hourly, including chlorophyll a from fluorescence (WetLabs WQM), nitrate (Satlantic ISUS), sea surface temperature, salinity, etc.
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