A key focus of the PEAN99 campaign (Photochemical Experimentat Neumayer (70°S, 8°W) January/February 1999) was to studythe photochemistry of reactive nitrogen compounds and theirchemical and physical exchange processes with the firn layer.For the latter, two sets of controlled experiments werecarried out, designed to i) show conclusively that NOxproduction in the snowpack is photochemical, ii) determinewhether NO or NO2 is produced, iii) quantify the productionrate of NO2 and NO inside a snowblock, iv) derive fluxes ofNox from the snow into the overlying atmosphere. Throughouta diurnal cycle, measurements were made of ambient air andof air from inside a snowblock. Enhanced concentrations ofNO and NO2 (up to 15 pptv and 32 pptv, respectively) weremeasured inside the snowblock. The production rate insidethe block varied with intensity of incident radiation, andreached a maximum of 1.1*10(6) molec cm(-3) s(-1) for NO and2.1*10(6) molec cm(-3) s(-1) for NO2. A second experiment,in which the snowblock was alternately exposed to sunlightand then shaded, showed that the diurnal production wasdriven by photochemistry rather than some other diurnallyvarying factor. Gradient measurements and subsequent fluxcalculations showed that the snowpack can be a source of NOxto the overlying atmosphere, but suggest that the sourcestrength varies considerably from day to day.