Balloon-borne tropospheric CO 2 observations over the equatorial eastern and western Pacific

Abstract Vertical profiles of carbon dioxide (CO 2 ) mixing ratio in the equatorial eastern and western Pacific were measured by newly developed balloon-borne CO 2 sondes in February 2012 (two soundings) and February–March 2015 (four soundings), respectively. The 1–10 km vertically averaged CO 2 mixing ratios lie between the background surface values in the Northern Hemisphere (NH) and those in the Southern Hemisphere (SH) monitored at ground-based sites during these periods. A backward trajectory analysis, taking account of convective mixing processes using geostationary satellite cloud-image data, is applied to the measured CO 2 profiles to estimate the origin of the observed air masses. Air masses originating in the SH show low CO 2 mixing ratios that are similar to the background values in the SH. This relationship is confirmed by a positive correlation (∼0.6) between the CO 2 mixing ratio and the latitude of air mass origin which is found from trajectory calculations. This result suggests that the CO 2 distribution in the troposphere over the equatorial Pacific is controlled by monthly time-scale, large-scale CO 2 distribution and weekly time-scale atmospheric transport processes. Furthermore, this study shows that the combination of CO 2 sonde measurements and trajectory analysis, taking account of convective mixing, is a useful tool in investigating CO 2 transport processes.