Trace gas mixing ratio variability versus lifetime in the troposphere and stratosphere: Observations

Several archived data sets have been reviewed to examine the relationship between mixing ratio variability and lifetime for hydrocarbon and halocarbon species in the troposphere and stratosphere. The dependence on lifetime was described by the power law relationship s ln X = A τ − b where s ln X is the standard deviation of the ln of the mixing ratios, A is a proportionality coefficient, and b is an exponent that relates to the dominance of sink terms in the regional variability budget. At the Harvard forest ground site, winter and summer data displayed the same lifetime dependence, τ −0.18 , which was significantly weaker than the τ −0.5 dependence of remote tropospheric data, indicating that source terms dominated regional variability at Harvard. In addition, the ratio of summer to winter s ln X values was found to be similar for all species except ethane, averaging 1.54±0.04. This ratio is consistent with a factor of 11 seasonal change in the species lifetimes, given a τ −0.18 lifetime dependence. Stratospheric data displayed a stronger lifetime dependence than tropospheric trends, indicating a more dominant role for sink terms in describing spatial variability in this region of the atmosphere. We show that a unique power law relationship between s ln X ratios for two species X i and X j and the kinetic slope of ln( X i ) versus ln lpar; X j ) correlation plots is found to hold in both observations and theory. Thus knowledge of the coefficient b allows for a clearer understanding of the relationship between observed slopes of ln ( X i ) versus ln ( X j ) correlation plots and the ratio of the species lifetimes.