Long term trends of halogenated trace gases, hydrocarbons, alkyl nitrates and of the oxidative capacity of the atmosphere

Atmospheric trends of a series of ozone depleting substances (halons) and greenhouse gases (PFCs, SF5CF3, HFC-227ea) are reported using measurements from Cape Grim, Tasmania. A two-dimensional global atmospheric chemistry transport model is used to derive global annual emission histories of these long lived gases. Emissions of the halons are found to have decreased since peaking in the 1990s. There is still some considerable uncertainty in the remaining banks of the halons caused by uncertainties in their atmospheric lifetimes and possibly in the reported production data. The contribution of the halons to total tropospheric bromine is predicted to fall from current mixing ratios of about 8.1 ppt to about 3 ppt by 2060 assuming reported lifetimes to be correct. Emissions of the higher perfluorocarbons (PFCs) (C4F10, C5F12, C6F14 and C7F16) have decreased since peaking in the 1990s, with the exception of C7F16, emissions of which have remained stable since the 1990s. Emissions of C3F8 have also decreased since the 1990s; those of C2F6 appear to be increasing though this disagrees with recent work of other groups. However the growth rate of the radiative forcing of the atmosphere from the PFCs reported herein has been relatively constant for the past decade largely because of increasing emissions of c-C4F8. Emissions of SF5CF3 appear to have ceased in the mid-2000s seeming to confirm the source of the gas as being from production of certain fluorochemicals by the company 3M. Emissions of the hydrofluorocarbon (HFC) HFC-227ea have increased rapidly since the mid-1990s. Long term trends of six alkanes (ethane, propane, n-butane, iso-butane, n-pentane and iso- pentane) and six alkyl nitrates (methyl, ethyl, 2-propyl, 2-butyl, 2+3-pentyl and 3-methyl-2- butyl) have been derived from measurements from firn air from the two Greenland sites, NEEM and North GRIP. These suggest that alkane mixing ratios peaked in the Arctic around 1980 and that alkyl nitrate mixing ratios in the Arctic peaked in the early 1990s. A detailed chemical mechanism has been developed in a 2-D atmospheric model to investigate the tropospheric chemistry relevant to the production of alkyl nitrates. Using this model, long term emission histories of the alkanes have been derived. These suggest that alkane emissions from Europe, North America and Russia roughly halved between 1980 and the early 2000s while those from Asia roughly doubled. A historic trend of the changing photochemical age of air masses reaching the Arctic has been derived and used as a proxy for changing OH concentrations in the mid-high latitude northern hemisphere. This suggests that OH concentrations decreased from 1950 to about 1975, increased from 1975 to 1995 and have subsequently decreased again. This trend appears to correlate with the trend in the growth rate of methane in the atmosphere over the past sixty years. Using a box model the chemistry of the alkyl nitrate system both in background air and polluted air masses has been investigated, focusing on the response of the system to varying levels of atmospheric NO, OH and HO2. It appears that the observed historic trends of the alkyl nitrates are a result of both changing OH and NO concentrations.