Atmospheric variability and emissions of halogenated trace gases near New York City

Elevated mixing ratios of chlorofluorocarbons (CFC-11 and CFC-12), and sulfur hexafluoride (SF6) have been observed at Lamont-Doherty Earth Observatory (LDEO), located approximately 25 km north of New York City (NYC). Emissions and transport of these gases are of interest because of their global warming potential, the role of CFCs in depletion of stratospheric ozone and information they provide on the transport of atmospheric pollutants. Comparison of trace gas time series with meteorological data indicates that both NYC and the region to the southwest (New Jersey and the Philadelphia –Washington DC area) are significant sources of CFCs, and confirms that NYC is an unusually large source of SF6. From 1996 to 2005 the elevation of CFC-12 mixing ratio above that of the remote (well mixed) atmosphere has decreased on average by 5.2 ± 0.6 ppt y−1, whereas that of CFC-11 has not changed significantly (0.0 ± 2.0 ppt y−1). From 1998 to 2006, the elevation of SF6 mixing ratios above that of the remote atmosphere declined by 0.4 ± 0.1 ppt y−1. Time series of the same gases measured at Harvard Forest, 205 km northeast of LDEO, demonstrate transport of air masses with elevated levels of these gases from their source region to central Massachusetts. Emissions in the local area around LDEO were quantified through analysis of diurnal cycles. Local CFC-12 emissions decreased ca. 95% between 1996 and 2005 while CFC-11 emission decreased ca. 51% during the same period. Local SF6 emissions decreased by 47% between 1998 and 2005.