The R/V Meteor cruise M91 (Callao–Callao) took place off Peru from 1 December to 26 December 2012. The overall goal of M91 was to conduct an integrated biogeochemical study on the upwelling region off Peru and its adjacent oxygen minimum zone in order to assess its importance for the emissions of various climate-relevant atmospheric trace gases and tropospheric chemistry. The various work packages of M91 included measurements of (1) atmospheric and dissolved trace gases, (2) aerosols, (3) nitrogen processes and isotopes in the water column, (4) dissolved organic matter in the surface microlayer, (5) upwelling velocity, and (6) exchange fluxes across the ocean–atmosphere interface. M91 was funded by the German BMBF project SOPRAN (Surface Ocean Processes in the Anthropocene; http://www.sopran.pangaea.de ), which is a contribution to the International SOLAS (Surface Ocean – Lower Atmosphere Study; http://www.solas-int.org).