Measurements of HCHO, CH3CHO, and CH3C(O)CH3 were made at the Narwhal ice floe camp in the Lincoln Sea at 84 ¿N latitude from April 10 to 24, 1994. During the period April 13 to 18, O3 was below the detection limit of the measurement (i.e.,<1 ppb), and the average HCHO, CH3CHO, and CH3C(O)CH3 concentrations were 193, 93, and 1730 ppt, respectively. A box model of the chemistry involved in the surface O3 depletion shows that the majority of BrOx termination reactions occur via Br atom reaction with the aldehydes. The reaction of Br atoms with CH3CHO is shown to be very effective in removing NOx from the Arctic marine boundary layer (MBL), via formation of peroxyacetyl nitrate (PAN). This denitrification of the surface layer has a significant impact on the radical chemistry. In particular, the model indicates that the observed levels of HCHO and CH3CHO cannot be reproduced if, as discussed in recent reports of Arctic ozone chemistry at sunrise, both Br atom and Cl atom chemistry occur simulataneously (at estimated concentrations of 1¿104 and 1¿107 atoms/cm3, respectively). However, if only chlorine atoms are present (at 1¿104 atoms/cm3), reasonable steady state CH3CHO levels (~80 ppt), but rather low HCHO levels (~50 ppt) are produced. The model HCHO levels for chlorine-atom-only chemistry are as much as a factor of 10 lower than those observed (by these authors and others) in the Arctic MBL at sunrise. Model simulations show that the ratio CH3C(O)CH3/C2H5CHO could be a useful indicator of the relative importance of Br atom and Cl atom chemistry. ¿ American Geophysical Union 1996