Seasonal variations of volatile organic compounds in the coastal Baltic Sea

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Abstract
Environmental context. Volatile organic compounds (VOCs) play a significant role in the global climate and are engaged in several atmospheric reactions. Relatively large amounts of VOCs are emitted from coastal waters, which is why these zones are expected to have significant impact on the atmospheric chemistry. The abundance of a single compound depends on its source and removal processes as well as on environmental parameters. Thus, seasonal changes can greatly affect the occurrence and behaviour of these trace gases. In order to investigate temporal changes in combination with the influence of different environmental parameters on the concentration and the composition of volatile organic compounds (VOCs), seawater samples from the coastal Baltic Sea were weekly measured from January to November 2008. In most cases, concentrations of VOCs varied seasonally and were influenced by changes in temperature and light conditions or biological species composition. A nearly two-fold increase in the mean concentration was noticed for isoprene, iodomethane and bromoform in the season with higher water temperature. The strongest flux of dimethylsulfide to the atmosphere appeared in May and July. Its high production was related to the presence of Prymnesiophyceae. The highest concentrations of diiodomethane and chloroiodomethane were observed with the spring and autumn phytoplankton bloom; their distribution was strongly controlled by light intensity. Flux calculations showed that coastal regions can affect local atmosphere, especially during biologically active periods. The strongest emission of bromoform and iodomethane was in July and August. The data presented here highlights the need to include seasonal cycles when calculating the global budgets and modelling sea–air fluxes of trace gases.

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