Global trends, seasonal cycles, and European emissions of dichloromethane, trichloroethene, and tetrachloroethene from the AGAGE observations at Mace Head, Ireland, and Cape Grim, Tasmania

Abstract

In situ observations (every 4 hours) of dichloromethane (CH2Cl2) from April 1995 to December 2004 and trichloroethene (C2HCl3) and tetrachloroethene (C2Cl4) from September 2000 to December 2004 are reported for the Advanced Global Atmospheric Gases Experiment (AGAGE) station at Mace Head, Ireland. At a second AGAGE station at Cape Grim, Tasmania, CH2Cl2 and C2Cl4 data collection commenced in 1998 and 2000, respectively. C2HCl3 is below the limit of detection at Cape Grim except during pollution episodes. At Mace Head CH2Cl2 shows a downward trend from 1995 to 2004 of 0.7 ± 0.2 ppt yr−1 (ppt: expressed as dry mole fractions in 1012), although from 1998 to 2004 the decrease has been only 0.3 ± 0.1ppt yr−1. Conversely, there has been a small but significant growth of 0.05 ± 0.01 ppt yr−1 in CH2Cl2 at Cape Grim. The time series for C2HCl3 and C2Cl4 are relatively short for accurate trend analyses; however, we observe a small but significant decline in C2Cl4 (0.18 ± 0.05 ppt yr−1) at Mace Head. European emissions inferred from AGAGE measurements are compared to recent estimates from industry data and show general agreement for C2HCl3. Emissions estimated from observations are lower than industry emission estimates for C2Cl4 and much lower in the case of CH2Cl2. A study of wildfires in Tasmania, uncontaminated by urban emissions, suggests that the biomass burning source of CH2Cl2 may have been previously overestimated. All three solvents have distinct annual cycles, with the phases and amplitudes reflecting their different chemical reactivity with OH as the primary sink.

Multi-year (2004–2008) record of nonmethane hydrocarbons and halocarbons in New England: seasonal variations and regional sources

“…A seasonal variation was apparent in the C 2 Cl 4 background (monthly 10th percentile) mixing ratios which were highest in winter (8-9 pptv) and lowest in late summer (3-5 pptv). The monthly mean halocarbon mixing ratios at TF are higher than observed at remote sites, such as Mace Head, Ireland (Simmonds et al, 2006), along the US west coast , NOAA CMDL sites (Thompson et al, 2004), Chebogue Point, Nova Scotia , and over the North Atlantic Ocean (Dimmer et al, 2001), reflecting the closer proximity to industrial sources and their continued use in North America, but are lower than observed in heavily urbanized areas, such as Pittsburgh, PA (Millet et al, 2005), Houston, TX (Gilman et al, 2009), and Mexico City, Mexico (Velasco et al, 2007). The C 2 Cl 4 mixing ratios at TF are similar to background levels in Massachusetts during -1999(Kleiman and Prinn, 2000Barnes et al, 2003) which suggests that C 2 Cl 4 emission rates in more populated areas in the US did not change considerably between the late 1990's and 2004.…”

A growing threat to the ozone layer from short-lived anthropogenic chlorocarbons

“…A growing use of CH 2 CL 2 is in the production of HFC-32 (CH 2 F 2 ), an ozone-friendly replacement for HCFC-22 (CHF 2 Cl) in refrigeration applications. Around 10 % of global CH 2 Cl 2 emissions comes from natural marine and biomass burning sources (Simmonds et al, 2006;Montzka et al, 2011). Whilst the strong enhancements of CH 2 Cl 2 are not entirely unexpected, the presence of high concentrations of CH 2 ClCH 2 Cl most certainly are.…”

Continental outflow from the US to the upper troposphere over the North Atlantic during the NASA INTEX-NA Airborne Campaign

“…First, we used trace gas data from the NOAA Global Monitoring Division (GMD) monitoring site on Bermuda (http: //www.esrl.noaa.gov/gmd/dv/ftpdata.html) and from Mace Head, Ireland (Simmonds et al, 2006) to determine representative background mixing ratios over the North Atlantic. The monthly average surface mixing ratio at Bermuda in July 2004 was 1800 ppbv for CH 4 , 86 ppbv for CO, and 377 ppmv for CO 2 .…”