Emission of biogenic halocarbons in temperate and tropical coastal zones

Biogenically-produced halocarbons play an important role in regional and global biogeochemical processes. These compounds are short-lived (lifetimes <6 months) and so have temporal and spatial variability in their atmospheric distributions. Marine regions, in particular coastlines, have been identified as important source regions for these compounds, and within these regions macroalgae (seaweeds) are an important source. Despite their short lifetime, it is believed that biogenic bromocarbons may contribute to stratospheric inorganic bromine (Bry). Measurement and model studies have identified a 6 (1-8) ppt excess of stratospheric Bry that cannot be accounted for via known sources of longer-lived halocarbons. Tropical regions are believed to play an important role in this process, as deep convection may act as a rapid transport mechanism allowing these compounds to reach the upper troposphere within their atmospheric lifetimes. Despite this potential importance, gaps still remain in our knowledge of halocarbon biogeochemistry in this region. This study provides the first dedicated measurements of tropical macroalgae via laboratory incubations of 15 species. Laboratory studies on temperate macroalgae were also performed, with a focus on the impact of exposure and desiccation on halocarbon emissions. Desiccation-related halocarbon emissions are of interest due to a growing seaweed aquaculture industry; seaweeds are often left to dry before processing. In situ atmospheric measurements of halocarbons around Malaysia as part of the SHIVA campaign are also reported here. A study of halocarbon concentrations in Malaysia allowed the identification of different regions characterised by different source and atmospheric transport processes. We identified that strong coastal sources do exist in this region, but that their distribution is patchy and model studies should not assume a constant, strong coastal source. Laboratory and field measurements were combined in a final discussion providing annual emission estimates for the Malaysian and south east Asian region. Of particular interest is the potential impact of aquaculture, which, if projected expansions in production are met, could account for a considerable proportion of future Malaysian annual bromoform emissions

The effect of desiccation on the emission of volatile bromocarbons from two common temperate macroalgae

Exposure of intertidal macroalgae during low tide has been linked to the emission of a variety of atmospherically-important trace gases into the coastal atmosphere. In recent years, several studies have investigated the role of inorganic iodine and organoiodides as antioxidants and their emission during exposure to combat oxidative stress, yet the role of organic bromine species during desiccation is less well understood. In this study the emission of dibromomethane (CH2Br2) and bromoform (CHBr3) during exposure and desiccation of two common temperate macroalgae, Fucus vesiculosus and Ulva intestinalis, is reported. Determination of the impact exposure may have on algal physiological processes is difficult as intertidal species are adapted to desiccation and may undergo varying degrees of desiccation before their physiology is affected. For this reason we include comparisons between photosynthetic capacity (Fv/Fm) and halocarbon emissions during a desiccation time series. In addition, the role of rewetting with freshwater to simulate exposure to rain was also investigated. Our results show that an immediate flux of bromocarbons occurs upon exposure, followed by a decline in bromocarbon emissions. We suggest that this immediate bromocarbon pulse may be linked to volatilisation or emissions of existing bromocarbon stores from the algal surface rather than the production of bromocarbons as an antioxidant response

Increasing concentrations of dichloromethane, CH2Cl2, inferred from CARIBIC air samples collected 1998–2012

Atmospheric concentrations of dichloromethane, CH2Cl2, a regulated toxic air pollutant and minor contributor to stratospheric ozone depletion, were reported to have peaked around 1990 and to be declining in the early part of the 21st century. Recent observations suggest this trend has reversed and that CH2Cl2 is once again increasing in the atmosphere. Despite the importance of ongoing monitoring and reporting of atmospheric CH2Cl2, no time series has been discussed in detail since 2006. The CARIBIC project (Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container) has analysed the halocarbon content of whole-air samples collected at altitudes of between ~10–12 km via a custom-built container installed on commercial passenger aircraft since 1998, providing a long-term record of CH2Cl2 observations. In this paper we present this unique CH2Cl2 time series, discussing key flight routes which have been used at various times over the past 15 years. Between 1998 and 2012 increases were seen in all northern hemispheric regions and at different altitudes, ranging from ~7–10 ppt in background air to ~13–15 ppt in regions with stronger emissions (equating to a 38–69% increase). Of particular interest is the rising importance of India as a source of atmospheric CH2Cl2: based on CARIBIC data we provide regional emission estimates for the Indian subcontinent and show that regional emissions have increased from 3–14 Gg yr^-1 (1998–2000) to 16–25 Gg yr^-1 (2008). Potential causes of the increasing atmospheric burden of CH2Cl2 are discussed. One possible source is the increased use of CH2Cl2 as a feedstock for the production of HFC-32, a chemical used predominantly as a replacement for ozone-depleting substances in a variety of applications including air conditioners and refrigeration

Trends and emissions of six perfluorocarbons in the Northern Hemisphere and Southern Hemisphere

Perfluorocarbons (PFCs) are potent greenhouse gases with global warming potentials up to several thousand times greater than CO2 on a 100-year time horizon. The lack of any significant sinks for PFCs means that they have long atmospheric lifetimes of the order of thousands of years. Anthropogenic production is thought to be the only source for most PFCs. Here we report an update on the global atmospheric abundances of the following PFCs, most of which have for the first time been analytically separated according to their isomers: c-octafluorobutane (c-C4F8), n-decafluorobutane (n-C4F10), n-dodecafluoropentane (n-C5F12), n-tetradecafluorohexane (n-C6F14), and n-hexadecafluoroheptane (n-C7F16). Additionally, we report the first data set on the atmospheric mixing ratios of perfluoro-2-methylpentane (i-C6F14). The existence and significance of PFC isomers have not been reported before, due to the analytical challenges of separating them. The time series spans a period from 1978 to the present. Several data sets are used to investigate temporal and spatial trends of these PFCs: time series of air samples collected at Cape Grim, Australia, from 1978 to the start of 2018; a time series of air samples collected between July 2015 and April 2017 at Tacolneston, UK; and intensive campaign-based sampling collections from Taiwan. Although the remote “background” Southern Hemispheric Cape Grim time series indicates that recent growth rates of most of these PFCs are lower than in the 1990s, we continue to see significantly increasing mixing ratios that are between 6 % and 27 % higher by the end of 2017 compared to abundances measured in 2010. Air samples from Tacolneston show a positive offset in PFC mixing ratios compared to the Southern Hemisphere baseline. The highest mixing ratios and variability are seen in air samples from Taiwan, which is therefore likely situated much closer to PFC sources, confirming predominantly Northern Hemispheric emissions for most PFCs. Even though these PFCs occur in the atmosphere at levels of parts per trillion molar or less, their total cumulative global emissions translate into 833 million metric tonnes of CO2 equivalent by the end of 2017, 23 % of which has been emitted since 2010. Almost two-thirds of the CO2 equivalent emissions within the last decade are attributable to c-C4F8, which currently also has the highest emission rates that continue to grow. Sources of all PFCs covered in this work remain poorly constrained and reported emissions in global databases do not account for the abundances found in the atmosphere

The emission of volatile halocarbons by seaweeds and their response towards environmental changes

Volatile halocarbons can deplete the protective stratospheric ozone layer contributing to global climate change and may even affect local climate through aerosol production. These compounds are produced through anthropogenic and biogenic processes. Biogenic halocarbons may be produced as defence compounds, anti-oxidants or by-products of metabolic processes. These compounds include very short-lived halocarbons (VSLH), e.g. bromoform (CHBr3), dibromomethane (CH2Br2), methyl iodide (CH3I), diiodomethane (CH2I2). Efforts to quantify the biogenic sources of these compounds, especially those of marine origin, e.g. seaweeds, phytoplankton and seagrass meadows, are often complicated by inherent biological variability as well as spatial and temporal changes in emissions. The contribution of the coastal region and the oceans to the stratospheric load of halocarbons has been widely debated. This highlights the need to understand the factors affecting the release of these compounds from marine sources for which data for modelling purposes are generally lacking. Seaweeds are important sources of biogenic halocarbons subjected to changing environmental conditions. Huge uncertainties in the prediction of current and future global halocarbon pool exist due to the lack of spatial and temporal data input from coastal and oceanic sources. Therefore, investigating the effect of changing environmental conditions on the emission of VSLH by the seaweeds could help towards better estimations of halocarbon emissions. This is especially important in light of global changes in both climate and the environment, the expansion of seaweed cultivation industry and the interactions between halocarbon emission and their environment. In this paper, we review current knowledge of seaweed halocarbon emissions, how environmental factors affect these emissions and identify gaps in understanding. Our aim is to direct much needed research to improve understanding of the contribution of marine biogenic sources of halocarbons and their impact on the environment

Mercury plumes in the global upper troposphere observed during flights with the CARIBIC observatory from May 2005 until June 2013

Tropospheric sections of flights with the CARIBIC (Civil Aircraft for Regular Investigation of the Atmosphere Based on an Instrumented Container) observatory from May 2005 until June 2013, are investigated for the occurrence of plumes with elevated Hg concentrations. Additional information on CO, CO, CH, NO, O, hydrocarbons, halocarbons, acetone and acetonitrile enable us to attribute the plumes to biomass burning, urban/industrial sources or a mixture of both. Altogether, 98 pollution plumes with elevated Hg concentrations and CO mixing ratios were encountered, and the Hg/CO emission ratios for 49 of them could be calculated. Most of the plumes were found overEast Asia, in the African equatorial region, over South America and over Pakistan and India. The plumes encountered over equatorial Africa and over South America originate predominantly from biomass burning, as evidenced by the low Hg/CO emission ratios andelevated mixing ratios of acetonitrile, CHCl and particle concentrations. The backward trajectories point to the regions around the Rift Valley and the Amazon Basin, with its outskirts, as the source areas. The plumes encountered over East Asia and over Pakistan and India are predominantly of urban/industrial origin, sometimes mixed with products of biomass/biofuel burning. Backward trajectories point mostly to source areas in China andnorthern India. The Hg/CO and Hg/CH emission ratios for several plumes are also presented and discussed

Effect of ocean acidification and elevated fCO2 on trace gas production from the Baltic Sea summer phytoplankton community.

The Baltic Sea is a unique environment as the largest body of brackish water in the world. Acidification of the surface oceans due to absorption of anthropogenic CO2 emissions is an additional stressor facing the pelagic community of the already challenging Baltic Sea. To investigate its impact on trace gas biogeochemistry, a large-scale mesocosm experiment was performed off Tvärminne Research Station, Finland, in summer 2012. During the second half of the experiment, dimethylsulfide (DMS) concentrations in the highest-fCO2 mesocosms (1075–1333 µatm) were 34 % lower than at ambient CO2 (350 µatm). However, the net production (as measured by concentration change) of seven halocarbons analysed was not significantly affected by even the highest CO2 levels after 5 weeks' exposure. Methyl iodide (CH3I) and diiodomethane (CH2I2) showed 15 and 57 % increases in mean mesocosm concentration (3.8 ± 0.6 increasing to 4.3 ± 0.4 pmol L−1 and 87.4 ± 14.9 increasing to 134.4 ± 24.1 pmol L−1 respectively) during Phase II of the experiment, which were unrelated to CO2 and corresponded to 30 % lower Chl a concentrations compared to Phase I. No other iodocarbons increased or showed a peak, with mean chloroiodomethane (CH2ClI) concentrations measured at 5.3 (±0.9) pmol L−1 and iodoethane (C2H5I) at 0.5 (±0.1) pmol L−1. Of the concentrations of bromoform (CHBr3; mean 88.1 ± 13.2 pmol L−1), dibromomethane (CH2Br2; mean 5.3 ± 0.8 pmol L−1), and dibromochloromethane (CHBr2Cl, mean 3.0 ± 0.5 pmol L−1), only CH2Br2 showed a decrease of 17 % between Phases I and II, with CHBr3 and CHBr2Cl showing similar mean concentrations in both phases. Outside the mesocosms, an upwelling event was responsible for bringing colder, high-CO2, low-pH water to the surface starting on day t16 of the experiment; this variable CO2 system with frequent upwelling events implies that the community of the Baltic Sea is acclimated to regular significant declines in pH caused by up to 800 µatm fCO2. After this upwelling, DMS concentrations declined, but halocarbon concentrations remained similar or increased compared to measurements prior to the change in conditions. Based on our findings, with future acidification of Baltic Sea waters, biogenic halocarbon emissions are likely to remain at similar values to today; however, emissions of biogenic sulfur could significantly decrease in this region

Evaluation of stratospheric age of air from CF4_{4}, C2_{2}F6_{6}, C3_{3}F8_{8}, CHF3_{3}, HFC-125, HFC-227ea and SF6_{6}; Implications for the calculations of halocarbon lifetimes, fractional release factors and ozone depletion potentials

In a changing climate, potential stratospheric circulation changes require long-term monitoring. Stratospheric trace gas measurements are often used as a proxy for stratospheric circulation changes via the “mean age of air” values derived from them. In this study, we investigated five potential age of air tracers – the perfluorocarbons CF4, C2F6 and C3F8 and the hydrofluorocarbons CHF3 (HFC-23) and HFC-125 – and compare them to the traditional tracer SF6 and a (relatively) shorter-lived species, HFC-227ea. A detailed uncertainty analysis was performed on mean ages derived from these “new” tracers to allow us to confidently compare their efficacy as age tracers to the existing tracer, SF6. Our results showed that uncertainties associated with the mean age derived from these new age tracers are similar to those derived from SF6, suggesting that these alternative compounds are suitable in this respect for use as age tracers. Independent verification of the suitability of these age tracers is provided by a comparison between samples analysed at the University of East Anglia and the Scripps Institution of Oceanography. All five tracers give younger mean ages than SF6, a discrepancy that increases with increasing mean age. Our findings qualitatively support recent work that suggests that the stratospheric lifetime of SF6 is significantly less than the previous estimate of 3200 years. The impact of these younger mean ages on three policy-relevant parameters – stratospheric lifetimes, fractional release factors (FRFs) and ozone depletion potentials – is investigated in combination with a recently improved methodology to calculate FRFs. Updates to previous estimations for these parameters are provided

Evaluation of stratospheric age of air from CF4, C2F6, C3F8, CHF3, HFC-125, HFC-227ea and SF6; implications for the calculations of halocarbon lifetimes, fractional release factors and ozone depletion potentials

In a changing climate, potential stratospheric circulation changes require long-term monitoring. Stratospheric trace gas measurements are often used as a proxy for stratospheric circulation changes via the <q>mean age of air</q> values derived from them. In this study, we investigated five potential age of air tracers – the perfluorocarbons CF₄, C₂F₆ and C₃F₈ and the hydrofluorocarbons CHF₃ (HFC-23) and HFC-125 – and compare them to the traditional tracer SF₆ and a (relatively) shorter-lived species, HFC-227ea. A detailed uncertainty analysis was performed on mean ages derived from these <q>new</q> tracers to allow us to confidently compare their efficacy as age tracers to the existing tracer, SF₆. Our results showed that uncertainties associated with the mean age derived from these new age tracers are similar to those derived from SF₆, suggesting that these alternative compounds are suitable in this respect for use as age tracers. Independent verification of the suitability of these age tracers is provided by a comparison between samples analysed at the University of East Anglia and the Scripps Institution of Oceanography. All five tracers give younger mean ages than SF₆, a discrepancy that increases with increasing mean age. Our findings qualitatively support recent work that suggests that the stratospheric lifetime of SF₆ is significantly less than the previous estimate of 3200 years. The impact of these younger mean ages on three policy-relevant parameters – stratospheric lifetimes, fractional release factors (FRFs) and ozone depletion potentials – is investigated in combination with a recently improved methodology to calculate FRFs. Updates to previous estimations for these parameters are provided