Measurements of selected C2–C5 hydrocarbons in the troposphere: Latitudinal, vertical, and temporal variations

Results are presented on the tropospheric distribution of C2–C5 hydrocarbons based on (1) shipboard measurements over the eastern Pacific Ocean from latitude 47°N to 47°S, (2) ground-based measurements at a remote coastal site in northern California (39°N), and (3) airborne studies of the troposphere over Colorado and over the eastern Pacific Ocean (38°N) during the summer and winter seasons. A total of 1077 hydrocarbon samples were processed during these comprehensive field programs. In agreement with previous studies, the shipboard measurements show large north-to-south gradients for virtually all nonmethane hydrocarbons (NMHCs) and clearly indicate their northern hemispheric sources. Ethane, the most abundant NMHC, is present at a concentration of 2 ppb in the northern hemisphere (NH) mid-latitudes, with southern hemisphere (SH) concentrations of about 0.3 ppb. All other NMHC concentrations are in the sub-ppb range. The measurements at the remote coastal site of Point Arena, California, show that NMHC concentrations can decrease by a factor of 2 or more during the passage of cold fronts in winter and spring. During these events the behavior of NMHCs is strongly correlated with primary anthropogenic species (e.g., CH2Cl2, CH2ClCH2Cl, and CH3CCl3) as well as secondary oxygenates (e.g., peroxyacetyl nitrate (PAN) and ozone). Aircraft measurements of NMHC concentrations made over Colorado and near the coastal zone of northern California were higher over land than over the ocean in the lower troposphere (<5 km). Upper tropospheric concentrations were found to be lower and, in general, much less spatially variable than those present in the lower troposphere. Because of the differences in NMHC concentrations between ocean and land and between lower and upper troposphere evident in the measurements, it is possible that weather systems associated with significant horizontal and vertical transport can cause large temporal and spatial variations in NMHC concentrations on local and regional scales.