Sampling Sites and MethodsThe 2. The seasonal change in the occurrence and strength of the surface inversion may contribute to the variability of all short-lived species in the boundary layer except locally produced sea-salt aerosol. As summarized by K6nig-Langlo et al.[this issue], the period from November to February shows a relatively weak or absent surface inversion at NM and DDU but generally stronger temperature gradients at NM (due to radiative cooling). Cyclonic activity, however, may regularly interrupt the isolation from higher atmospheric levels •' at both sites throughout all seasons.3. The surface'wind system is dominated by zonal flow at NM but by a meridional one at DDU and Mawson [Parish, 1988]. The latter two sites are therefore more directly influenced by air masses flowing down from the Antarctic plateau and by upper tropospheric air from midlatitudes eventually entrained in the downslope winds.In summary, the climatologies at DDU and Mawson are similar, whereas NM is characterized by its ice shelf position (leading to zonal winds) and by its relatively more frequent location inside the polar vortex. Palmer station stands out by its position on the Antarctic Peninsula and reflects mainly the subpolar maritime regime.
Sampling and AnalysesDetails on the sampling procedure and analytical methods used for aerosol chemical analyses are summarized in Table 1 2.2.1. Assessing local contamination at DDU. The very small size of the island (Ile des P6trels) on which DDU is located makes it impossible to define a "clean air sector" there. In order to estimate the effects both of human activities (which confined to the summer period and to individual filters with very low sea-salt loads. On the other hand, the HNO 3 collection efficiency for the backup aerosol filter was found to be between 60 and 90%, allowing resampling of most of the evaporated nitrate. Again, inspection of the summer nitrate variability at NM did not reveal a systematic relationship to the sea-salt or acidity loads of the filters. In particular, the general nitrate trend during the summer season is not substantially influenced by the dramatic change in the aerosol acidity during that season. We cannot exclude, however, the possibility that some filters collected during the acidity peak in January under low sea-salt loads may underestimate the total atmospheric nitrate concentration. We expect that the same holds true for the total nitrate data reported by Savoie et al. [1992] from Mawson station since sea salt is lower there by a factor of-4 compared to NM and since nonhygroscopic polycarbonate fibre filters are used.In conclusion, total atmospheric nitrate records reported here are affected to a minor extent by year-round contamination events at DDU and by HNO 3 remobilization from summer filters at NM, respectively. None of these shortcomings occurring on individual filter samples could, however, substantially perturb the observed mean nitrate levels and their seasonal patterns.
ResultsBecause of the large difference in the sampl...