The atmosphere contains VOCs which are both man-made (anthropogenic) and
those produced by natural sources (biogenics). The biological production of volatile organic compounds, especially in the marine environment, constitutes an important source of gases into the atmosphere. Marine organisms can produce different kinds of trace gases, such as halocarbons, dimethylsulphide (DMS), nonmethane hydrocarbons (NMHCs) and others that are exchanged across the ocean-atmosphere boundary. Their contribution, pa1ticularly for halocarbons, has a direct influence in the composition and reactivity of the atmosphere. For instance, some of these compounds have been shown to deplete stratospheric ozone, typically referred to as ozone depleting substances (ODSs). However, they are also potent greenhouse gases, exceeding the values of the greenhouse effect of some gases such as carbon dioxide, methane and nitrogen-oxide compounds. Recognizing the harmful effects of these compounds on the ozone layer, many governments signed the Montreal Protocol in 1987 on substances that deplete the ozone layer. This limits the production and the importation of a number of halogenated compounds. Recently, the natural production of those compounds has become an important topic of research, studying their production in a variety of environments and conditions.
This thesis focuses on how external factors such as meteorology, physico-chemistry and biology drive the production and release of VOCs in the marine environment, and how individual sources contribute to the total amount of VOCs measured. The relatively new technique of solid-phase microextraction (SPME) coupled with GC-MS analysis was used for VOCs identification and quantification. Two marine environments were studied including The Menai Strait and Liverpool Bay. Multivariate analysis including Principal Component Analysis (PCA) and Partial Least Squares (PLS) was used to find relationships between VOC production and release with external factors.
Three potential natural contributors of VOCs were studied in the Menai Strait such
as sediments, macroalgae and microalgae. Analysis of sediment-derived gases had 61 different compounds classified into groups as halogenated (<0.1 - 13,435 pg g¯¹ w/w), sulphur containing compounds (1.1 - 5,671 pg g¯¹ w/w), aldehydes (1.4 - 462 pg g¯¹ w/w), BTEXs and monoaromatic compounds (<0.1 - 886 pg g¯¹ w/w) and aliphatic hydrocarbons (<0.1 - 195 pg g¯¹ w/w). Results of PCA showed that the chemical composition of extracted gas is influenced primarily by sediment type. Muddy/anoxic sediments were dominated by halogenated and sulphur containing compounds and sandy sediments had more aldehydes and BTEXs.
A range of intertidal macroalgae produced 47 different compounds: the algae
studied were Ascophyllum nodosum, Fucus vesiculosus, Fucus serratus, Laminaria
digitata, Ulva lactuca, Enteromorpha sp, Palmaria palmata and Grifithsia flocculosa. The main compound quantified was bromoform followed by dibromomethane,
tetrachloroethene, chloroform, dichloromethane, diiodomethane, iodoethane,
bromodichloromethane, dimethyl sulphide (DMS) and dimethyldisulphide (DMS2).
Results showed that production of VOCs increased after long immersion experiments and this production was enhanced when algae were exposed to desiccation, especially those living at low intertidal levels such as kelps.
The microalgae investigation principally focused on one species (Skeletonema
costatum) and was used to obtain a characteristic VOC signature appropriate to the algal blooms in the Menai Strait duing April. Key compounds obtained from it were
iodomethane, 1-iodobutane, diiodomethane, 1,1,2-trichloroethene, 2-chloropropane, chloroiodomethane, dibromochloromethane, 1-bromopentane, 1-bromopropane, linear hydrocarbons C5 to C8, DMS, DMS2 , hexanal and 2,4-dimethylfurane.
A one year time series of VOC in seawater was conducted in the Menai Strait. 64
different VOCs were quantified including halogenated (<LoD to 906 ng L¯¹ ), non-methane hydrocarbons (NMHC) (<LoD to 1539 ng L¯¹), mono-aromatics (<LoD to 4,232 ng L¯¹), oxygenated (<LoD to 1,539 ng L¯¹) and sulphur containing compounds (<LoD to 160 ng L¯¹). Pigments such as chlorophyll a correlated with halogenated compounds (e.g. chloroform and dichloromethane) and DMS was maximal during the spring bloom. Multivariate statistical analyses demonstrated the seasonal changes in the VOC signature (more BTEX and alkanes in the winter and halocarbons in the summer). PCA analysis demonstrated that physico-chemical and meteorological factors such as wind speed, water temperature can influence the detection of VOCs in surface waters as well their productions. A PLS analysis was conducted with all the signatures from the three potential sources. Signatures analysis highlighted the importance of the microalgae signature in spring while macroalgae and sediments dominated at other times. Short term variability in
concentrations and fluxes was due to such factors such as tidal state, wind speed and seawater temperature.
The survey conducted in the Irish Sea and Liverpool Bay area showed a suite of
compounds that can be classified as halogenated (0.2 - 1,400 ng L¯¹), BTEXs and mono-aromatics (1.5 - 2,900 ng L¯¹), aliphatic hydrocarbons and others (0.6 - 15,800 ng L¯¹). Day and night sampling was performed at a single station and suggested that factors such as sunlight and tide effect the presence of many of these compounds. Sample variability was very high under different weather conditions at the same station. Poor correlations were found with marine pigments and some selected VOCs. PCA analysis showed that chlorinated compounds such as 1,2-dichloroethane, 1,1,1-trichloroethane, trichloroethene, tetrachloroethene and carbon tetrachloride were predominantly anthropogenic in origin and originated from the River Mersey. Brominated and iodinated compounds were likely to be
from biogenic sources with presence of novel marine compounds such as 2-chloropropane, 1-bromoethane and 1-chlorobutane.
In conclusion, VOCs concentrations are governed by external factors influencing
their production, release and degradation in complex ways: there are factors effecting long time scale variations and also factors having an effect on a daily base. The presence of VOCs in the marine environment is the result of a combined production of natural and anthropogenic sources.