Methanol – climatic relevance, sources and sinks

Another bVOC is methanol that is after methane the second most abundant organic gas in the atmosphere, but its chemical reactivity is greater [Jacob et al., 2005; Seco et al., 2011;

Wohlfahrt et al., 2015]. In the troposphere a methanol loop occurs. Methanol can be formed through the reaction of methylperoxy radicals (CH₃O₂) with itself or higher peroxy radicals (RO2), and the oxidation of methanol or other VOCs leads in turn to the formation of peroxy radicals (RO₂) [Jacob et al., 2005]. In the atmosphere methanol reacts with hydroxyl radicals (OH) leading to the formation of formaldehyde, hydrogen radicals (H) and ozone (O₃) [Heikes et al., 2002] (Figure 1).

The average atmospheric concentration of methanol is estimated to range from 0.1 ppb up to 10 ppb [Jacob et al., 2005; Seco et al., 2011; Wohlfahrt et al., 2015]. However, on a regional scale the methanol concentrations can differ by a multiple, rendering the assessment of the global methanol budget more difficult [Wohlfahrt et al., 2015]. For example, the lowest mixing ratios were measured in the troposphere ranging from 0.2 - 1 ppb (average 0.6 ppb), followed by 0.9 ppb over the open ocean, 2 ppb for continental background (i.e., no impact of humans), 6 ppb for grasslands and 10 ppb for forests, and highest concentrations with more than 20 ppb for urban areas [Heikes et al., 2002]. Moreover, the atmospheric methanol concentrations vary depending on the latitude, hemisphere, seasonality, temperature, or even humidity which is in accordance with the reported sources and sinks [Millet et al., 2008].

The main sources of atmospheric methanol are of natural origin (biogenic), comprising marine systems (ocean) and terrestrial systems (plants) (Figure 2). Minor sources are anthropogenic activities such as solvent use, vehicle exhaust, industrial processes, and biomass burning, where wood pyrolysis (meaning the decomposition of wood at elevated temperatures in the absence of oxygen) of plant fibres (e.g. cellulose, hemicelluloses, lignin) is the reason of methanol formation and emission [Howard et al., 1990; Medeiros et al., 2008; Maleknia et al., 2009; Goeppert et al., 2014; Woolf et al., 2014]. In terrestrial environments methanol is mainly released by plant material as already mentioned. It is assumed that 40 - 80 % (large range due to the different models of methanol budget) of the annually arising methanol is originated from plant growth [Singh et al, 2000; Galbally &

Kristine, 2002; Heikes et al., 2002; Tie et al., 2003; Jacob et al., 2005; Millet et al., 2008], but also dead and decaying plant material provides remarkable amounts of methanol [Warneke

et al, 1999; Schade & Cluster, 2004; Karl et al, 2005a]. In growing plant biomass methanol is formed during demethylation processes of compounds rich in methoxy groups such as pectin and lignin [Schink & Zeikus, 1980; Fall & Benson, 1996; Warneke et al, 1999; Millet et al., 2008]. It was assumed that especially during plant or leaf growth methanol productions are high (up to 75 % of the emitted methanol is assumed as leaf-derived) [Fall & Benson, 1996;

Nemecek-Marshall et al., 1995; Hüve et al., 2007], but also during the abscission of plant leaves methanol is released [Willats et al., 2001]. In addition, methanol emissions depend on diurnal variations. For example, amounts of leaf-emitted methanol vary between night and day (especially in the morning) up to 18fold, which is correlated with stomata conductance, and a higher transpiration of plants also increases the amount of emitted methanol [Nemecek-Marshall et al., 1995; Hüve et al., 2007; Dorokhov et al., 2015]. In addition, methanol flux studies revealed seasonality for methanol concentrations according to plant growth behaviour with highest measured amounts in spring and fall [Tie et al., 2003]. Further, damaged plants release substantial amounts of methanol caused by cutting, mowing, or animal feeding (herbivore insects up to herbivore animals) [Karl et al., 2001; Peñuelas et al., 2005; Von Dahl et al., 2006; Brilli et al., 2011]. These emissions can be high for several days as it was reported for a lucerne-covered meadow [Warneke et al., 2002]. Also plant stress causes enhanced methanol emissions [Seco et al., 2007]. In dead or decaying plant material methanol can be released from residual in-leaf methanol, or it can be formed biotically by the activity of microorganisms degrading the plant fibers, or abiotically by physico-chemical degradation processes [Warneke et al., 1999; Galbally & Kirstine, 2002; Schade & Custer, 2004; Millet et al., 2008]. Thus, the role of plants in terms of methanol emission is crucial, and plants are the main providers of this C1 compound for methanol-utilising organisms.

The two major sinks for methanol are atmospheric reactions with hydroxyl radicals (OH) in the troposphere and clouds, as well as oceanic uptake, in which the role of the ocean is a net sink including gross methanol emission and uptake in the ocean mixed layer (OML) (Figure 2) [Millet et al., 2008]. Phytoplankton (i.e., unicellular algae, diatoms, dinoflagellates and bacteria) is a major source of methanol emission in the ocean, which is depending on several other factors (such as temperature, oxygen, nutrients, and light) [Heikes et al., 2002]. The major oceanic sink is likely caused by photochemical destruction and the high water solubility of methanol, as well as by the activity of marine methylotrophic microorganisms [Heikes et al., 2002; Millet et al., 2008]. Methanol-utilising microorganisms are also the main sink of methanol in terrestrial environments and are included in the term ‘dry deposition’ (Figure 2) [King, 1992; Oremland & Culbertson, 1992; Jacob et al., 2005; Dunfield, 2007; Trotsenko &

Murrell, 2008; Conrad, 2009; Kolb, 2009a; Vorholt, 2012; Knief, 2015]. Methylotrophic organisms in the phyllosphere are well known and well studied [Omer et al., 2004; Anda et al., 2011; Wellner et al., 2011; Madhaiyan et al., 2012; Meena et al., 2012]. They can even comprise up to 16 % of the total leaf microbiome [Vorholt, 2012]. However, the important role of methanol-utilising microorganisms in the soil has hardly been investigated in the last decades, and the main knowledge is based on pure cultures and artificial laboratory

experiments [Radajewski et al., 2000; Radajewski et al., 2002; Morris et al., 2002; Kolb, 2009a; Stacheter et al., 2013]. Thus, it is easily conceivable that for methanol-utilising organisms the same scenario like for methanotrophic organisms in terrestrial environments is likely – they might consume the highest proportion of produced methanol before it can reach the atmosphere acting as an important regulating agent in terms of methanol emissions.

Figure 2 Global sources and sinks for atmospheric methane, methanol and chloromethane.

The contribution of sources (left side) and sinks (right side) of atmospheric methane, methanol and chloromethane are already known, but the local amounts of formed gases or the local contribution of microbial degradation are not assessed and might be substantially higher. Sources are divided into their natural (light grey) and anthropogenic (dark grey) origin and sinks are divided based on their localisation in the atmosphere (dark grey) and on the Earth surface (light grey) in which ‘Earth surface sinks’ include deposition, ocean uptake, and microbial degradation. For detailed information please refer to the text in 1.1.1, 1.1.2, and 1.1.3. All charts are based on values mentioned in Nazaries et al., 2013 for methane, Millet et al., 2008 for methanol, and Keppler et al., 2005 for chloromethane.