On tracer correlations in the troposphere: The case of ethane and propane

To investigate the reasons for and the utilities of tropospheric tracer correlations, we examine the interrelationship between ethane and propane on the basis of the observations and global 3-D chemical transport model (GEOS-CHEM) simulations. Ethane and propane have reasonably well defined sources and sinks. We chose to examine the correlation between propane and ethane/propane ratio because it is more sensitive to mixing and is less dependent on temperature than that between ethane and propane. We find that the linear correlation in the log space of propane and ethane/propane ratio is better characterized away from the source regions. Observations at northern mid and high latitudes (TOPSE) and over the tropical Pacific (PEM-Tropics B) are therefore investigated. We show that the correlation generally follows the one determined by chemical (loss) kinetics and that the deviation from the kinetics slope reflects the difference between ethane and propane in the effect of mixing relative to chemical loss. The comparisons of observed and simulated correlation slopes (and probability distributions) can therefore be used to evaluate the coupling of chemistry and transport simulated in the model. At northern mid and high latitudes, the model is generally in agreement with the observations in February and March but simulates a wrong seasonal change of the correlation from March to May. The model appears to overestimate the transport from lower to mid latitudes and the horizontal transport and mixing at high latitudes in May. Over the tropical Pacific, the model reproduces well the observed two-branch structure of the correlation. The minor branch results from different transport and chemistry coupling characteristics in the southern from northern hemisphere. The discrepancy between observed and simulated correlation slope values