Stefan Reimann

The anthropogenic contribution to isoprene concentrations in a rural atmosphere

Abstract Atmospheric hydrocarbons are continuously monitored at the rural site of Taenikon, Switzerland. As expected for a rural area, highest isoprene concentrations are found in summer. However, elevated concentrations are also measured on some occasions in winter, in particular during events with long-lasting surface inversions, temperatures constantly below 0°C and snow covering the vegetation. During such events, concentrations of isoprene are strongly correlated with those of 1,3-butadiene, a substance that is mainly due to human activities. For these periods, a molar ratio between the concentrations of isoprene and those of 1,3-butadiene of 0.42 is observed. This value, together with the concentrations of 1,3-butadiene, is used to estimate the anthropogenic fraction of the atmospheric isoprene for the whole of 1997. It is found that the fraction is close to 100% in January–February and again in November–December. On the other hand, as early as March, a considerable amount of the observed isoprene appears to be of biogenic origin, although isoprene emissions by trees are negligible. The relative anthropogenic contribution is minimal in midsummer, when biogenic emissions are highest. For this time of the year, the anthropogenic contribution is largest during the early morning hours, in agreement with the traffic peak on nearby country roads.

Preserving Montreal Protocol Climate Benefits by Limiting HFCs

With no impending global controls on HFCs, the Montreal Protocol offers a near-term path to preserve its climate benefits. The Montreal Protocol is perhaps the most successful international environmental treaty, responsible for global phaseout of the consumption and production of ozone-depleting substances (ODSs), e.g., chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). Hydrofluorocarbons (HFCs), which do not destroy stratospheric ozone, were considered long-term substitutes for ODSs and are not controlled by the Montreal Protocol. Because most HFCs are potent greenhouse gases (GHGs), they are included in the Kyoto Protocol. But climate benefits provided by this protocol are limited as they apply only to developed countries and over a short time (2008–2012). As we describe below, with no impending global controls on HFCs, inclusion of HFCs under the Montreal Protocol offers a path, starting in the short term, to preserve the climate benefits already achieved by this protocol.

Low European methyl chloroform emissions inferred from long-term atmospheric measurements

Methyl chloroform (CH3CCl3, 1,1,1,-trichloroethane) was used widely as a solvent before it was recognized to be an ozone-depleting substance and its phase-out was introduced under the Montreal Protocol. Subsequently, its atmospheric concentration has declined steadily and recent European methyl chloroform consumption and emissions were estimated to be less than 0.1 gigagrams per year. However, data from a short-term tropospheric measurement campaign (EXPORT) indicated that European methyl chloroform emissions could have been over 20 gigagrams in 2000 (ref. 6), almost doubling previously estimated global emissions. Such enhanced emissions would significantly affect results from the CH3CC13 method of deriving global abundances of hydroxyl radicals (OH) (refs 7–12)—the dominant reactive atmospheric chemical for removing trace gases related to air pollution, ozone depletion and the greenhouse effect. Here we use long-term, high-frequency data from Mace Head, Ireland and Jungfraujoch, Switzerland, to infer European methyl chloroform emissions. We find that European emission estimates declined from about 60 gigagrams per year in the mid-1990s to 0.3–1.4 and 1.9–3.4 gigagrams per year in 2000–03, based on Mace Head and Jungfraujoch data, respectively. Our European methyl chloroform emission estimates are therefore higher than calculated from consumption data, but are considerably lower than those derived from the EXPORT campaign in 2000 (ref. 6).

Future Emissions and Atmospheric Fate of HFC-1234yf from Mobile Air Conditioners in Europe

HFC-1234yf (2,3,3,3-tetrafluoropropene) is under discussion for replacing HFC-134a (1,1,1,2-tetrafluoroethane) as a cooling agent in mobile air conditioners (MACs) in the European vehicle fleet. Some HFC-1234yf will be released into the atmosphere, where it is almost completely transformed to the persistent trifluoroacetic acid (TFA). Future emissions of HFC-1234yf after a complete conversion of the European vehicle fleet were assessed. Taking current day leakage rates and predicted vehicle numbers for the year 2020 into account, European total HFC-1234yf emissions from MACs were predicted to range between 11.0 and 19.2 Gg yr(-1). Resulting TFA deposition rates and rainwater concentrations over Europe were assessed with two Lagrangian chemistry transport models. Mean European summer-time TFA mixing ratios of about 0.15 ppt (high emission scenario) will surpass previously measured levels in background air in Germany and Switzerland by more than a factor of 10. Mean deposition rates (wet + dry) of TFA were estimated to be 0.65-0.76 kg km(-2) yr(-1), with a maxium of ∼2.0 kg km(-2) yr(-1) occurring in Northern Italy. About 30-40% of the European HFC-1234yf emissions were deposited as TFA within Europe, while the remaining fraction was exported toward the Atlantic Ocean, Central Asia, Northern, and Tropical Africa. Largest annual mean TFA concentrations in rainwater were simulated over the Mediterranean and Northern Africa, reaching up to 2500 ng L(-1), while maxima over the continent of about 2000 ng L(-1) occurred in the Czech Republic and Southern Germany. These highest annual mean concentrations are at least 60 times lower than previously determined to be a safe level for the most sensitive aquatic life-forms. Rainwater concentrations during individual rain events would still be 1 order of magnitude lower than the no effect level. To verify these results future occasional sampling of TFA in the atmospheric environment should be considered. If future HFC-1234yf emissions surpass amounts used here studies of TFA accumulation in endorheic basins and other sensitive areas should be aspired.

European emissions of halogenated greenhouse gases inferred from atmospheric measurements.

European emissions of nine representative halocarbons (CFC-11, CFC-12, Halon 1211, HCFC-141b, HCFC-142b, HCFC-22, HFC-125, HFC-134a, HFC-152a) are derived for the year 2009 by combining long-term observations in Switzerland, Italy, and Ireland with campaign measurements from Hungary. For the first time, halocarbon emissions over Eastern Europe are assessed by top-down methods, and these results are compared to Western European emissions. The employed inversion method builds on least-squares optimization linking atmospheric observations with calculations from the Lagrangian particle dispersion model FLEXPART. The aggregated halocarbon emissions over the study area are estimated at 125 (106-150) Tg of CO(2) equiv/y, of which the hydrofluorocarbons (HFCs) make up the most important fraction with 41% (31-52%). We find that chlorofluorocarbon (CFC) emissions from banks are still significant and account for 35% (27-43%) of total halocarbon emissions in Europe. The regional differences in per capita emissions are only small for the HFCs, while emissions of CFCs and hydrochlorofluorocarbons (HCFCs) tend to be higher in Western Europe compared to Eastern Europe. In total, the inferred per capita emissions are similar to estimates for China, but 3.5 (2.3-4.5) times lower than for the United States. Our study demonstrates the large benefits of adding a strategically well placed measurement site to the existing European observation network of halocarbons, as it extends the coverage of the inversion domain toward Eastern Europe and helps to better constrain the emissions over Central Europe.

Volatile Organic Compounds in the Global Atmosphere

Volatile organic compounds (VOCs) include saturated, unsaturated, and other substituted hydrocarbons. VOCs play an important role in the chemistry of the atmosphere by influencing ozone and hydroxyl radical (OH) concentrations, and the conversion rates of nitrogen oxides (NOx). Elevated levels of VOCs and NOx have led to an approximate doubling of ozone in the lower troposphere over the past couple of centuries, making tropospheric ozone the third most important anthropogenic greenhouse gas after carbon dioxide (CO2) and methane. Because of ozones strong oxidizing properties, increases in tropospheric ozone are a concern for living systems on Earth. Ozone stresses and damages vegetation, resulting in a reduction of terrestrial CO2 sequestration. VOCs also serve as a source of atmospheric secondary organic aerosol (SOA), which influences the solar radiation budget and cloud droplet nucleation. Through these complex interactions, VOCs play an important role in air quality and climate.

Reconciling reported and unreported HFC emissions with atmospheric observations

Significance Hydrofluorocarbons (HFCs) are among the atmosphere’s fastest growing, and most potent, greenhouse gases. Proposals have been made to phase down their use over the coming decades. Such initiatives may largely be informed by existing emissions inventories, which, we show, are the subject of significant uncertainty. In this work, we use atmospheric models and measurements to examine the accuracy of these inventories for five major HFCs. We show that, when aggregated together, reported emissions of these HFCs from developed countries are consistent with the atmospheric measurements, and almost half of global emissions now originate from nonreporting countries. However, the agreement between our results and the inventory breaks down for individual HFC emissions, suggesting inaccuracies in the reporting methods for individual compounds. We infer global and regional emissions of five of the most abundant hydrofluorocarbons (HFCs) using atmospheric measurements from the Advanced Global Atmospheric Gases Experiment and the National Institute for Environmental Studies, Japan, networks. We find that the total CO2-equivalent emissions of the five HFCs from countries that are required to provide detailed, annual reports to the United Nations Framework Convention on Climate Change (UNFCCC) increased from 198 (175–221) Tg-CO2-eq⋅y–1 in 2007 to 275 (246–304) Tg-CO2-eq⋅y–1 in 2012. These global warming potential-weighted aggregated emissions agree well with those reported to the UNFCCC throughout this period and indicate that the gap between reported emissions and global HFC emissions derived from atmospheric trends is almost entirely due to emissions from nonreporting countries. However, our measurement-based estimates of individual HFC species suggest that emissions, from reporting countries, of the most abundant HFC, HFC-134a, were only 79% (63–95%) of the UNFCCC inventory total, while other HFC emissions were significantly greater than the reported values. These results suggest that there are inaccuracies in the reporting methods for individual HFCs, which appear to cancel when aggregated together.

First Observations of the Fourth Generation Synthetic Halocarbons HFC-1234yf, HFC-1234ze(E), and HCFC-1233zd(E) in the Atmosphere

Halogenated alkenes are a class of anthropogenic substances, which replace ozone-depleting substances and long-lived greenhouse gases in the foam-blowing, refrigeration, and solvent sectors. We report the first multiyear atmospheric measurements of the hydrofluorocarbons HFC-1234yf (2,3,3,3-tetrafluoroprop-1-ene, CF3CF═CH2), and HFC-1234ze(E) (E-1,3,3,3-tetrafluoroprop-1-ene trans-CF3CH═CHF), and the hydrochlorofluorocarbon HCFC-1233zd(E) (E-1-chloro-3,3,3-trifluoroprop-1-ene trans-CF3CH═CHCl) from the high altitude observatory at Jungfraujoch and from urban Dubendorf (Switzerland). When observations started in 2011 HFC-1234yf was undetectable at Jungfraujoch (mole fractions <0.003 ppt, parts-per-trillion, 10(-12)) but since then the percentage of measurements with detectable mole fractions has steadily increased to 4.5% in 2014. By contrast, in 2014 HFC-1234ze(E) was detectable in half of our samples at Jungfraujoch and in all samples at Dubendorf demonstrating the wide use of this compound within the air mass footprints of the stations. Our back trajectory analysis for the Jungfraujoch observations suggests high emission strength of HFC-1234ze(E) in the Belgium/Netherlands region. HCFC-1233zd(E) is present at very low mole fractions (typically <0.03 ppt) at both stations, and features pronounced seasonality and a general absence of pollution events during our 2013-2014 measurements. This is indicative of the presence of significant emissions from source locations outside the footprints of the two stations. Based on a simple one-box model calculation we estimate globally increasing HCFC-1233zd(E) emissions from 0.2 Gg yr(-1) in 2013 to 0.5 Gg yr(-1) for 2014.

Modern inhalation anesthetics: Potent greenhouse gases in the global atmosphere

Modern halogenated inhalation anesthetics undergo little metabolization during clinical application and evaporate almost completely to the atmosphere. Based on their first measurements in a range of environments, from urban areas to the pristine Antarctic environment, we detect a rapid accumulation and ubiquitous presence of isoflurane, desflurane, and sevoflurane in the global atmosphere. Over the past decade, their abundances in the atmosphere have increased to global mean mole fractions in 2014 of 0.097ppt, 0.30ppt, and 0.13ppt (parts per trillion, 10−12, in dry air), respectively. Emissions of these long-lived greenhouse gases inferred from the observations suggest a global combined release to the atmosphere of 3.1 ± 0.6 million t CO2 equivalent in 2014 of which ≈80% stems from desflurane. We also report on halothane, a previously widely used anesthetic. Its global mean mole fraction has declined to 9.2ppq (parts per quadrillion, 10−15) by 2014. However, the inferred present usage is still 280 ±120t yr−1.

Constraining the carbon tetrachloride (CCl4) budget using its global trend and inter‐hemispheric gradient

Carbon tetrachloride (CCl4) is a major anthropogenic ozone-depleting substance and greenhouse gas and has been regulated under the Montreal Protocol. However, the near-zero 2007–2012 emissions estimate based on the UNEP reported production and feedstock usage cannot be reconciled with the observed slow decline of atmospheric concentrations and the inter-hemispheric gradient (IHG) for CCl4. Our 3-D model simulations suggest that the observed IHG (1.5 ± 0.2 ppt for 2000–2012) is primarily caused by ongoing current emissions, while ocean and soil losses and stratosphere-troposphere exchange together contribute a small negative gradient (~0 – −0.3 ppt). Using the observed CCl4 global trend and IHG, we deduce that the mean global emissions for the 2000–2012 period are 393445 Gg/yr (~30% of the peak 1980s emissions) and a corresponding total lifetime of 353732 years.