Recent trends in global emissions of hydrochlorofluorocarbons and hydrofluorocarbons: reflecting on the 2007 adjustments to the Montreal Protocol.

Global-scale atmospheric measurements are used to investigate the effectiveness of recent adjustments to production and consumption controls on hydrochlorofluorocarbons (HCFCs) under the Montreal Protocol on Substances that Deplete the Ozone Layer (Montreal Protocol) and to assess recent projections of large increases in hydrofluorocarbon (HFC) production and emission. The results show that aggregate global HCFC emissions did not increase appreciably during 2007-2012 and suggest that the 2007 Adjustments to the Montreal Protocol played a role in limiting HCFC emissions well in advance of the 2013 cap on global production. HCFC emissions varied between 27 and 29 kt CFC-11-equivalent (eq)/y or 0.76 and 0.79 GtCO2-eq/y during this period. Despite slower than projected increases in aggregate HCFC emissions since 2007, total emissions of HFCs used as substitutes for HCFCs and chlorofluorocarbons (CFCs) have not increased more rapidly than rates projected [Velders, G. J. M.; Fahey, D. W.; Daniel, J. S.; McFarland, M.; Andersen, S. O. The Large Contribution of Projected HFC Emissions to Future Climate Forcing. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 10949-10954] for 2007-2012. HFC global emission magnitudes related to this substitution totaled 0.51 (-0.03, +0.04) GtCO2-eq/y in 2012, a magnitude about two times larger than emissions reported to the United Nations Framework Convention on Climate Change (UNFCCC) for these HFCs. Assuming accurate reporting to the UNFCCC, the results imply that developing countries (non-Annex I Parties) not reporting to the UNFCCC now account for nearly 50% of global HFC emissions used as substitutes for ozone-depleting substances (ODSs). Global HFC emissions (as CO2-eq) from ODS substitution can be attributed approximately equally to mobile air conditioning, commercial refrigeration, and the sum of all other applications.

Observational evidence for interhemispheric hydroxyl-radical parity.

The hydroxyl radical (OH) is a key oxidant involved in the removal of air pollutants and greenhouse gases from the atmosphere. The ratio of Northern Hemispheric to Southern Hemispheric (NH/SH) OH concentration is important for our understanding of emission estimates of atmospheric species such as nitrogen oxides and methane. It remains poorly constrained, however, with a range of estimates from 0.85 to 1.4 (refs 4, 7-10). Here we determine the NH/SH ratio of OH with the help of methyl chloroform data (a proxy for OH concentrations) and an atmospheric transport model that accurately describes interhemispheric transport and modelled emissions. We find that for the years 2004-2011 the model predicts an annual mean NH-SH gradient of methyl chloroform that is a tight linear function of the modelled NH/SH ratio in annual mean OH. We estimate a NH/SH OH ratio of 0.97 ± 0.12 during this time period by optimizing global total emissions and mean OH abundance to fit methyl chloroform data from two surface-measurement networks and aircraft campaigns. Our findings suggest that top-down emission estimates of reactive species such as nitrogen oxides in key emitting countries in the NH that are based on a NH/SH OH ratio larger than 1 may be overestimated.

Background monitoring and long-range transport of atmospheric CFC-11 and CFC-12 at Kosan, Korea.

The background concentrations of atmospheric CFC-11 and CFC-12 were monitored to assess their impact on stratospheric ozone depletion and global warming from September 1995 to March 1999 at Kosan, Korea, located at eastern margin of the Asian Continent. The concentrations of atmospheric CFC-11 at Kosan have decreased slightly, at a rate of -2.5 pptv yr(-1), over the period in response to the Montreal Protocol. The CFC-12 mixing ratio at Kosan continues to increase in the atmosphere at a rate of 5.7 pptv yr(-1) despite international regulations, because of its extreme atmosphere persistence. Recent trends of these two chlorofluorocarbons at Kosan, Korea were concordant with those of the northern hemispheric background monitored unit at Mauna Loa, Hawaii. The maximum seasonal mean mixing ratios of CFC-11 and CFC-12 at Kosan, Korea, were 270 +/- 4 pptv in the spring and 538 +/- 9 pptv in the winter, and the corresponding seasonal minima were 267 +/- 7 and 529 +/- 12 pptv. This occurred in the summer and was due to southeasterly winds from the northwestern Pacific Ocean. By performing a three-day isentropic backward trajectory analysis, it was shown that air masses at Kosan, and with the exception of summer, mainly originated from central and northern China. In particular, the mixing ratios of these two contaminant species are closely related with their air mass trajectories.

The detection of large HNO3-containing particles in the winter Arctic stratosphere.

Large particles containing nitric acid (HNO3) were observed in the 1999/2000 Arctic winter stratosphere. These in situ observations were made over a large altitude range (16 to 21 kilometers) and horizontal extent (1800 kilometers) on several airborne sampling flights during a period of several weeks. With diameters of 10 to 20 micrometers, these sedimenting particles have significant potential to denitrify the lower stratosphere. A microphysical model of nitric acid trihydrate particles is able to simulate the growth and sedimentation of these large sizes in the lower stratosphere, but the nucleation process is not yet known. Accurate modeling of the formation of these large particles is essential for understanding Arctic denitrification and predicting future Arctic ozone abundances.

A Net Sink for Atmospheric CH3Br in the East Pacific Ocean.

Surface waters along a cruise track in the East Pacific Ocean were undersaturated in methyl bromide (CH(3)Br) in most areas except for coastal and upwelling regions, with saturation anomalies ranging from + 100 percent in coastal waters to -50 percent in open ocean areas, representing a regionally weighted mean of -16 (-13 to -20) percent. The partial lifetime of atmospheric CH(3)Br with respect to calculated oceanic degradation along this cruise track is 3.0 (2.9 to 3.6) years. The global, mean dry mole fraction of CH3Br in the atmosphere was 9.8 +/- 0.6 parts per trillion, with an interhemispheric ratio of 1.31 +/- 0.08. These data indicate that approximately 8 percent (0.2 parts per trillion) of the observed interhemispheric difference in atmospheric CH3Br could be attributed to an uneven global distribution of oceanic sources and sinks.

Chlorine chemistry on polar stratospheric cloud particles in the arctic winter.

Simultaneous in situ measurements of hydrochloric acid (HCl) and chlorine monoxide (ClO) in the Arctic winter vortex showed large HCl losses, of up to 1 part per billion by volume (ppbv), which were correlated with high ClO levels of up to 1.4 ppbv. Air parcel trajectory analysis identified that this conversion of inorganic chlorine occurred at air temperatures of less than 196 +/- 4 kelvin. High ClO was always accompanied by loss of HCI mixing ratios equal to (1/2)(ClO + 2Cl(2)O(2)). These data indicate that the heterogeneous reaction HCl + ClONO(2) --> Cl(2) + HNO(3) on particles of polar stratospheric clouds establishes the chlorine partitioning, which, contrary to earlier notions, begins with an excess of ClONO(2), not HCl.

Chemical loss of ozone in the arctic polar vortex in the winter of 1991-1992.

In situ measurements of chlorine monoxide, bromine monoxide, and ozone are extrapolated globally, with the use of meteorological tracers, to infer the loss rates for ozone in the Arctic lower stratosphere during the Airborne Arctic Stratospheric Expedition II (AASE II) in the winter of 1991-1992. The analysis indicates removal of 15 to 20 percent of ambient ozone because of elevated concentrations of chlorine monoxide and bromine monoxide. Observations during AASE II define rates of removal of chlorine monoxide attributable to reaction with nitrogen dioxide (produced by photolysis of nitric acid) and to production of hydrochloric acid. Ozone loss ceased in March as concentrations of chlorine monoxide declined. Ozone losses could approach 50 percent if regeneration of nitrogen dioxide were inhibited by irreversible removal of nitrogen oxides (denitrification), as presently observed in the Antarctic, or without denitrification if inorganic chlorine concentrations were to double.

Ozone loss inside the northern polar vortex during the 1991-1992 winter.

Measurements made in the outer ring of the northern polar vortex from October 1991 through March 1992 reveal an altitude-dependent change in ozone, with a decrease at the bottom of the vortex and a substantial increase at the highest altitudes accessible to measurement. The increase is the result of ozone-rich air entering the vortex, and the decrease reflects ozone loss accumulated after the descent of the air through high concentrations of reactive chlorine. The depleted air that is released out of the bottom of the vortex is sufficient to significantly reduce column ozone at mid-latitudes.

High Nitrite Levels off Northern Peru: A Signal of Instability in the Marine Denitrification Rate.

During February and March 1985, nitrite levels along the northern (approximately 7 degrees to 10 degrees S) Peruvian coast were unusually high. These accumulations occurred in oxygen-deficient waters, suggesting intensified denitrification. In a shallow offshore nitrite maximum, concentrations were as high as 23 micromoles per liter (a record high). Causes for the unusual conditions may include a cold anomaly that followed the 1982-83 El Niño. The removal of combined nitrogen (approximately 3 to 10 trillion grams of nitrogen per year) within zones of new or enhanced denitrification observed between 7 degrees to 16 degrees S suggests a significant increase in oceanic denitrification.

Transformations of nitrogen in a polluted estuary: nonlinearities in the demand for oxygen at low flow.

Oxidation of sewage ammonium in the Potomac River is described in terms of a simple kinetic model, with growth of nitrifying bacteria limited by the supply of ammonium ion. The oxidation rate varies inversely with freshwater inflow, and the associated demand for oxygen varies as the inverse square of the freshwater inflow rate. Similar behavior is observed for the Delaware River. The model accounts for the observed concentrations of ammonium and nitrous oxide.