Mixing ratios of acetic (CH3COOH), formic (HCOOH), pyruvic (CH3COCOOH), and oxalic ((COOH)2) acids were measured both in gas and particulate phase in the marine boundary layer, over the Atlantic Ocean from 63 ¿N to 39 ¿S on board of the R/V Polarstern in October/November 1996. The average mixing ratios for gas phase CH3COOH, HCOOH, CH3COCOOH, and (COOH)2 were 291.2¿151.9, 448.7¿182.1, 1.1¿1.0, and 6.1¿5.4 parts per trillion by volume (pptv), respectively. The partitioning of these acids in the aerosol phase and the size distribution of their particulate form was also investigated. In the fine mode the mean mixing ratios for acetate, formate, pyruvate, and oxalate were 69.7¿47.5, 32.5¿39.4, 1.0¿1.0, and 5.1¿3.7 pptv, respectively. Elevated levels of all organic acids were encountered in the southern hemisphere (springtime) compared to the northern hemisphere (autumn), indicating a possible seasonal variation in their source strength and/or enhanced photochemical production. The observed distributions of formic and acetic acids have been compared to the results of a global chemistry/transport three-dimensional model. The model results show that acetic acid has mainly secondary photochemical sources (about 120 TgCH3COOH/yr). On the opposite, the known chemical sources of HCOOH are quite weak (20 Tg HCOOH/yr) and insufficient to simulate the HCOOH levels observed in the marine atmosphere. A local marine source of HCOOH of about 50 Tg/yr on a global scale is required to reasonably simulate the observations in the marine atmosphere. ¿ 2000 American Geophysical Union