Measurements and Interpretation of Surface Mixing Ratios of CH4 and CO and δ 13C and δ D of CH4 in Air from Pacific Ocean Transects Between Auckland, New Zealand and Los Angeles, California

We report on measurements of atmospheric CH₄ and CO mixing ratios and δ ¹³C of CH₄ from air samples collected every 2.5 to 5° latitude along a transect over the Pacific Ocean using container ships of P&O Nedlloyd (formerly Blue Star) shipping line. Data presented here begins in June 1996 and extends to January 2002. Scientists from the National Institute of Water and Atmospheric Research in New Zealand and from University of California, Irvine alternate sampling trips so that a transect between Auckland, New Zealand (35° S) and Los Angeles, California (35° N) can be sampled over a period of ∼15 days approximately every four months. Data sets from the two laboratories are intercalibrated through a sample exchange program. The data provide detail on the spatial and seasonal variation of CH₄ and CO mixing ratios and stable isotope ratios of CH₄ over the Pacific equatorial region, including the Intertropical Convergence Zone (ITCZ) and both northern and southern temperate zones to about 30° latitude, including the South Pacific Convergence Zone (SPCZ). Data from 18 transect samplings so far clearly show that δ ¹³C in the mid latitudes of both hemispheres are ∼6 months out of phase. In June, a minimum in δ ¹³C CH₄ in the southern hemisphere (SH) coincides approximately with the maximum in the northern hemisphere (NH) seasonal cycle. Because the NH is less enriched in ¹³C than the SH this situation results in a remarkably flat gradient between 30° N and 30° S. In November the opposite situation occurs with the SH mid latitude maximum coinciding with the minimum in the NH cycle, leading to a relatively large gradient of ∼0.5‰ between the hemispheres. We discuss how CH₄ and CO mixing ratios are related to the changing positions and strengths of the ITCZ and SPCZ and how this data can be used in multi-dimensional models of atmospheric chemistry and transport to better define CH₄ sources and sinks both temporally and spatially.