Changes to the chemical state of the northern hemisphere atmosphere during the second half of the twentieth century
Atmospheric Chemistry and Physics Discussions
The NO<sub>X</sub> (NO and NO<sub>2</sub>) and HO<sub>X</sub> (OH and HO<sub>2</sub>) budgets of the atmosphere exert a major influence on atmospheric composition, controlling removal of primary pollutants and formation of a wide range of secondary products, including ozone, that can influence human health and climate. However, there remain large uncertainties in the changes to these budgets over recent decades. Due to their short atmospheric lifetimes, NO<sub>X</sub> and HO<sub>X</sub> are
... sub>X</sub> are highly variable in space and time, and so the measurements of these species are of very limited value for examining long term, large scale changes to their budgets. Here, we take an alternative approach by examining long-term atmospheric trends of alkyl nitrates, the formation of which is dependent on the atmospheric NO&thinsp;/&thinsp;HO<sub>2</sub> ratio. We derive long term trends in the alkyl nitrates from measurements in firn air from the NEEM site, Greenland. Their mixing ratios increased by a factor of 4&ndash;5 between the 1970s and 1990s. This was followed by a steep decline to the sampling date of 2008. Moreover, we examine how the trends in the alkyl nitrates compare to similarly derived trends in their parent alkanes (i.e. the alkanes which, when oxidised in the presence of NO<sub>X</sub>, lead to the formation of the alkyl nitrates). The ratios of the alkyl nitrates to their parent alkanes increase from around 1970 to the late 1990's consistent with large changes to the [NO]&thinsp;/&thinsp;[HO<sub>2</sub>] ratio in the northern hemisphere atmosphere during this period. These could represent historic changes to NO<sub>X</sub> sources and sinks. Alternatively, they could represent changes to concentrations of the hydroxyl radical, OH, or to the transport time of the air masses from source regions to the Arctic.