11.5: Carbon monoxide

CO Fate in the Atmosphere

CO is one of the most common air pollutants, and its concentration in contaminated continental air masses reaches up to several ppmv (Badr and Probert, 1994). CO is emitted to the atmosphere through anthropogenic processes, such as biomass combustion, fossil fuels, waste incineration, industrial processes, and transport. Additional contributors are natural sources (oceans, soils, plants, forest fires), atmospheric oxidation of CH_4, and other non-CH₄ hydrocarbons (NMHC) (Badr and Probert, 1995; Tarr et al., 1995; Schade and Crutzen, 1999; Bruhn et al., 2013). CH₄ oxidation has the largest share of these sources, producing approximately 700 Tg-CO·yr⁻¹ (Bergamaschi et al., 2000; Monson and Holland, 2001); fossil fuel combustion together with biofuel use and other industrial emissions are responsible for 500–900 Tg-CO·yr⁻¹, while biomass burning–for 400–800 Tg (Duncan et al., 2007). Photochemical CO production due to the oxidation of naturally emitted and anthropogenic NMHC equals 450 and 110 Tg-CO·yr⁻¹, respectively, (Rozante et al., 2017). In general, global CO levels rose from the Industrial Revolution until 1980; then a gradual decrease in its concentration, especially in the Northern Hemisphere, was observed in measurements from the global surface network from the National Oceanic and Atmospheric Administration (NOAA), caused by both the use of catalytic converters in cars and technological advances in combustion since 2000 (Bakwin et al., 1994; Voiland, 2015; Gaubert et al., 2017). More recently, the downward trends in CO observed for both the Northern and Southern Hemispheres have shown good consistency with long-term trends in bottom-up emissions in Europe, the United States, and China; where an improvement in combustion efficiency and a reduction in emissions from anthropogenic sources was observed (Gaubert et al., 2017). It is worth noting, however, that despite its main global sources, atmospheric CO levels show spatial as well as seasonal variability. While the oxidation of CH₄, a gas that is evenly distributed around the world, provides a similarly constant CO background of around 25 ppb, the remaining emission groups depend on space-time aspects. Thus, inter alia, CO from fossil fuels shows higher levels in the northern mid-latitudes, mainly in winter, while biomass combustion in tropical continents contributes to higher CO concentrations in the summer, during the dry season, along with the rainforests (Andreae et al., 2012). Additionally, the spatial variation in CO concentration is also characteristic on a smaller scale, e.g., in urban areas, where it depends not only on meteorological conditions or thermal inversion but also directly on atmospheric turbulence and traffic intensity (Oliveira et al., 2003).

CO is mainly utilized by the tropospheric reaction with the OH hydroxyl radical (Logan et al., 1981; Khalil and Rasmussen, 1984; Badr and Probert, 1995). CO is oxidized to CO₂ in the stratosphere, where it migrates via convection, turbulence, and mixing (Seiler and Warneck, 1972; Seiler, 1974). The reaction is fast and independent of temperature; thanks to it, the residence time of CO in the atmosphere is relatively short, from 2 weeks to 3 months (Rozante et al., 2017; Rakitin et al., 2021). Soils and oceans are also involved in CO capture (Ingersoll et al., 1974; Conrad et al., 1982) and higher plants and algae (Krall and Tolbert, 1957; Chappelle, 1962).

Taking into account the characteristics of greenhouse gases, CO is not considered one of them as it is not capable of absorbing infrared radiation (Rozante et al., 2017). However, because of the primary mechanism for removing atmospheric CO by reaction with the OH radical, CO is recognized as an essential trace gas that controls the oxidative ability of the atmosphere (Bruhn et al., 2013). An increase in CO concentration in the troposphere causes changes in the distribution and amount of OH; reactions of the radical with CO and CH₄ constitute about 97% of its destruction (Logan et al., 1981; Levine et al., 1985; Badr and Probert, 1995). The change in the atmospheric OH affects the concentration of other gases, including CH₄ and O₃ (Hameed et al., 1980). Thus, CO indirectly affects the energy budget of the atmosphere (Evans and Puckrin, 1995), increasing the concentration of GHGs and the time of their utilization in the troposphere, as well as controlling the transfer to the stratosphere, which in turn has an impact on stratospheric O₃ (Ramanathan et al., 1985; Thompson, 1992; Bruhn et al., 2013). The radiative forcing from CO is estimated more than from N₂O and halogenated hydrocarbons (Rakitin et al., 2021).

Further detailed information at Sobieraj, K., Stegenta-Dąbrowska, S., Luo, G., Koziel, J. A., & Białowiec, A. (2022). Carbon monoxide fate in the environment as an inspiration for biorefinery industry: A review. Frontiers in Environmental Science, 10, 822463. https://www.frontiersin.org/articles/10.3389/fenvs.2022.822463/full

CO Fate in Soil

CO is constantly supplied to the atmosphere, which results from both natural and anthropogenic sources. Despite the significant amount of gas emitted, its concentration in the atmosphere does not seem to increase (Bartholomew and Alexander, 1979). It’s because natural processes are responsible for utilizing CO shortly after its release (Inman et al., 1971).

Soil is considered one of the main sinks of atmospheric CO, responsible for 40% of total consumption (Seiler, 1978). However, soils can also be a CO source in the global CO cycle, as noted, especially in the savannas and deserts (Conrad and Seiler, 1985a; Kuhlbusch et al., 1998). For this reason, CO uptake by soils is a net flux consisting simultaneously of consumption and production (Figure 1) (Seiler, 1978; King and Crosby, 2002; Bruhn et al., 2013; van Asperen et al., 2015; Pihlatie et al., 2016). Simultaneous chemical, physical and microbiological processes (Kuhlbusch et al., 1998) depend on many climatic, biological, and physical soil factors, making the equilibrium CO vary between a few ppbv up to hundreds of ppbv (parts per billion by volume) (Conrad and Seiler, 1979; Conrad and Seiler, 1980b; Conrad and Seiler, 1982a). The most important soil parameters include water content, temperature, organic matter content, pH, soil type, the depth of CO consumption horizon, and CO concentration in the gas phase (Potter et al., 1996). Even small changes in this balancing between CO production and soil uptake can severely impact tropospheric chemistry (Moxley and Smith, 1998b).

Figure \(\PageIndex{1}\): Carbon monoxide cycle in soil. CC-BY, Frontiers, Sobieraj et al.

Further detailed information at Sobieraj, K., Stegenta-Dąbrowska, S., Luo, G., Koziel, J. A., & Białowiec, A. (2022). Carbon monoxide fate in the environment as an inspiration for biorefinery industry: A review. Frontiers in Environmental Science, 10, 822463. https://www.frontiersin.org/articles/10.3389/fenvs.2022.822463/full

CO Fate in Water

The ocean has been recognized as a source of CO released into the atmosphere since the early 1970s (Swinnerton et al., 1970; Lamontagne et al., 1971). Despite the low ocean share (0.4–9%) among all sources of CO in the atmosphere (Bates et al., 1995), it can constitute up to 50% of the load in the marine boundary layer (Erickson and Taylor, 1992; Stubbins et al., 2006a). The southern hemisphere is particularly important here, in which CO production accounts for almost 60% of the total CO flux from the ocean surface (Erickson, 1989). Surface ocean waters are saturated with CO compared to atmospheric equilibrium, which causes a net flux of this gas at the ocean-atmosphere interface (Linnenbom et al., 1973; Logan et al., 1981; Zuo et al., 1998). The CO emissions to air are controlled mainly by the concentration of this gas in water (Bates et al., 1995). It depends on several factors such as photochemical production, consumption by microorganisms, exchange between air and water, and physical mixing (Figure 2) (Wilson et al., 1970; Conrad et al., 1982; Butler et al., 1987; Jones, 1991). Due to the impact of these factors, CO concentration in waters shows diurnal, seasonal, and regional diversification (Bates et al., 1995).

Figure \(\PageIndex{2}\): Carbon monoxide cycle in water. CC-BY, Frontiers, Sobieraj et al.

CO was identified as the second most important product of dissolved organic matter (DOM) photolysis in water bodies (Mopper and Kieber, 2000; Stubbins et al., 2006b). The rate of CO formation is one order of magnitude higher compared to other low molecular weight carbon photoproducts produced under aqueous conditions (Mopper et al., 1991; Zuo and Jones, 1995). It is the photodegradation of the DOM by part of the UV solar radiation that is indicated as the main source of CO from both ocean and sea waters, as well as from the surface of lakes, rivers, wetlands, and coastal waters (Zuo and Jones, 1997; Pos et al., 1998; Zuo et al., 1998; Stubbins et al., 2006a; Blomquist et al., 2012).

Further detailed information at Sobieraj, K., Stegenta-Dąbrowska, S., Luo, G., Koziel, J. A., & Białowiec, A. (2022). Carbon monoxide fate in the environment as an inspiration for biorefinery industry: A review. Frontiers in Environmental Science, 10, 822463. https://www.frontiersin.org/articles/10.3389/fenvs.2022.822463/full

CO Impact on Plants

Biosynthesis and photoproduction of CO in plants were observed in the second half of the 20th century (Wilks, 1959; Schade et al., 1999). This compound is formed during oxidative heme catabolism due to the activity of the enzyme heme oxygenase (HO). The result is three products: CO, biliverdin, and free iron Fe²⁺ (Figure 3). The second one is immediately transformed into bilirubin, while the iron is involved in ferritin induction (Bilban et al., 2008). Among the three isoforms of HO discovered so far, HO-1, HO-2 and HO-3 have been distinguished, the last two of which are characterized by low activity (Maines, 1997).

Figure \(\PageIndex{3}\): Impact of CO on plants development. CC-BY, Frontiers, Sobieraj et al.

Further detailed information at Sobieraj, K., Stegenta-Dąbrowska, S., Luo, G., Koziel, J. A., & Białowiec, A. (2022). Carbon monoxide fate in the environment as an inspiration for biorefinery industry: A review. Frontiers in Environmental Science, 10, 822463. https://www.frontiersin.org/articles/10.3389/fenvs.2022.822463/full

Source

Sobieraj, K., Stegenta-Dąbrowska, S., Luo, G., Koziel, J. A., & Białowiec, A. (2022). Carbon monoxide fate in the environment as an inspiration for biorefinery industry: A review. Frontiers in Environmental Science, 10, 822463. CC-BY accessed December 2023 https://www.frontiersin.org/articles/10.3389/fenvs.2022.822463/full