Methyl chloroform (MCF) is a man-made chlorinated solvent contributing to the destruction of stratospheric ozone and is controlled under the "Montreal Protocol on Substances that Deplete the Ozone Layer" and its amendments, which called for its phase-out in 1996 in developed countries and 2015 in developing countries. Long-term, high-frequency observations of MCF carried out at three European sites show a constant decline in the background mixing ratios of MCF. However, we observe persistent non-negligible mixing ratio enhancements of MCF in pollution episodes, suggesting unexpectedly high ongoing emissions in Europe. In order to identify the source regions and to give an estimate of the magnitude of such emissions, we have used a Bayesian inversion method and a point source analysis, based on high-frequency long-term observations at the three European sites.

The inversion identified southeastern France (SEF) as a region with enhanced MCF emissions. This estimate was confirmed by the point source analysis. We performed this analysis using an 11-year data set, from January 2002 to December 2012. Overall, emissions estimated for the European study domain decreased nearly exponentially from 1.1 Gg yrg'1 in 2002 to 0.32 Gg yrg'1 in 2012, of which the estimated emissions from the SEF region accounted for 0.49 Gg yrg'1 in 2002 and 0.20 Gg yrg'1 in 2012. The European estimates are a significant fraction of the total semi-hemisphere (30-90° N) emissions, contributing a minimum of 9.8% in 2004 and a maximum of 33.7% in 2011, of which on average 50% are from the SEF region. On the global scale, the SEF region is thus responsible for a minimum of 2.6% (in 2003) and a maximum of 10.3% (in 2009) of the global MCF emissions.

This study is a comparison of CFC-12 (CCl2F2) measurements in firn air and ice core samples from three Antarctic sites: South Pole, West Antarctic Ice Sheet Divide (79.46°S, 112.13°W), and Siple Dome (81.65°S, 148.81°W). CFC-12 is a synthetic chlorofluorocarbon manufactured during the mid-late 20th century for use as a refrigerant and an aerosol spray propellant. Its atmospheric history is well established with agreement among instrumental time series measurements and industry-reported production data [Walker et al., 2000], the distribution of dissolved CFC-12 in the oceans [e.g. Weiss et al., 1985], and firn air measurements [Butler et al., 1999]. The atmospheric history indicates that there was no measureable CFC-12 in the atmosphere prior to the 1940’s. The firn air CFC-12 profiles are consistent with the known atmospheric history of this gas. In contrast, the air in ice core samples collected near the close-off depth exhibit anomalously high CFC-12 levels. We propose that this is due to entrapment of modern air in open pores that close after drilling, resulting in elevated CFC-12 mixing ratios. These results demonstrate how the composition of air trapped in shallow ice cores can be altered during the post-drilling period through purely physical processes. Comparison of firn air and ice core bubble composition is one of the commonly used tools for studying the bubble close-off process. The post-drilling entrapment process detected in this study represents a potential complication for such investigations.

Climate models incorporate photosynthesis-climate feedbacks, yet we lack robust tools for large-scale assessments of these processes. Recent work suggests that carbonyl sulfide (COS), a trace gas consumed by plants, could provide a valuable constraint on photosynthesis. Here we analyze airborne observations of COS and carbon dioxide concentrations during the growing season over North America with a three-dimensional atmospheric transport model. We successfully modeled the persistent vertical drawdown of atmospheric COS using the quantitative relation between COS and photosynthesis that has been measured in plant chamber experiments. Furthermore, this drawdown is driven by plant uptake rather than other continental and oceanic fluxes in the model. These results provide quantitative evidence that COS gradients in the continental growing season may have broad use as a measurement-based photosynthesis tracer.

The East Asian Summer Monsoon driven by temperature and moisture gradients between the Asian continent and the Pacific Ocean, leads to approximately 50% of the annual rainfall in the region across 20-40°N. Due to its increasing scientific and social importance, there have been several previous studies on identification of moisture sources for summer monsoon rainfall over East Asia mainly using Lagrangian or Eulerian atmospheric water vapor models. The major source regions for EASM previously proposed include the North Indian Ocean, South China Sea and North western Pacific. Based on high-precision and high-frequency 6-year measurement records of hydrofluorocarbons (HFCs), here we report a direct evidence of rapid intrusion of warm and moist tropical air mass from the Southern Hemisphere (SH) reaching within a couple of days up to 33°N into East Asia. We further suggest that the combination of direct chemical tracer record and a back-trajectory model with physical meteorological variables helps pave the way to identify moisture sources for monsoon rainfall. A case study for Gosan station (33.25°N, 126.19°E) indicates that the meridional transport of precipitable water from the SH accompanying the southerly/southwesterly flow contributes most significantly to its summer rainfall.

The future resilience of coast redwoods (Sequoia sempervirens) is now of critical concern due to the detection of a 33% decline in California coastal fog over the 20th century. However, ecosystem-scale measurements of photosynthesis and stomatal conductance are challenging in coast redwood forests, making it difficult to anticipate the impacts of future changes in fog. To address this methodological problem, we explore coastal variations in atmospheric carbonyl sulfide (COS or OCS), which could potentially be used as a tracer of these ecosystem processes. We conducted atmospheric flask campaigns in coast redwood sites, sampling at surface heights and in the canopy (~70 m), at the University of California Landels-Hill Big Creek Reserve and Big Basin State Park. We simulated COS atmosphere-biosphere exchange with a high-resolution 3-D model to interpret these data. Flask measurements indicated a persistent daytime drawdown between the coast and the downwind forest (45 ± 6 ppt COS) that is consistent with the expected relationship between COS plant uptake, stomatal conductance, and gross primary production. Other sources and sinks of COS that could introduce noise to the COS tracer technique (soils, anthropogenic activity, nocturnal plant uptake, and surface hydrolysis on leaves) are likely to be small relative to daytime COS plant uptake. These results suggest that COS measurements may be useful for making ecosystem-scale estimates of carbon, water, and energy exchange in coast redwood forests.

The 1,1,1,2-tetrafluoroethane (HFC-134a), an important alternative to CFC-12 in accordance with the Montreal Protocol on Substances that Deplete the Ozone Layer, is a high global warming potential greenhouse gas. Here we evaluate variations in global and regional HFC-134a emissions and emission trends, from 1995 to 2010, at a relatively high spatial and temporal (3.75° in longitude × 2.5° in latitude and 8 day) resolution, using surface HFC-134a measurements. Our results show a progressive increase of global HFC-134a emissions from 19 ± 2 Gg/yr in 1995 to 167 ± 5 Gg/yr in 2010, with both a slowdown in developed countries and a 20%/yr increase in China since 2005. A seasonal cycle is also seen since 2002, which becomes enhanced over time, with larger values during the boreal summer.