Influence of local air pollution on the deposition of peroxyacetyl nitrate to a nutrient-poor natural grassland ecosystem

Dry deposition of peroxyacetyl nitrate (PAN) is known to have a phytotoxic impact on plants under photo- chemical smog conditions, but it may also lead to higher pro- ductivity and threaten species richness of vulnerable ecosys- tems in remote regions. However, underlying mechanisms or controlling factors for PAN deposition are not well un- derstood and studies on dry deposition of PAN are limited. In this study, we investigate the impact of PAN deposition on a nutrient-poor natural grassland ecosystem situated at the edge of an urban and industrialized region in Germany. PAN mixing ratios were measured within a 3.5 months sum- mer to early autumn period. In addition, PAN fluxes were determined with the modified Bowen ratio technique for a selected period. The evaluation of both stomatal and non- stomatal deposition pathways was used to model PAN de- position over the entire summer-autumn period. We found that air masses at the site were influenced by two contrasting pollution regimes, which led to median diurnal PAN mixing ratios ranging between 50 and 300 ppt during unpolluted and between 200 and 600 ppt during polluted episodes. The mea- sured PAN fluxes showed a clear diurnal cycle with maxi- mal deposition fluxes of 0.1 nmol m 2 s 1 (correspond- ing to a deposition velocity of 0.3 cm s 1 / during daytime and a significant non-stomatal contribution was found. The ratio of PAN to ozone deposition velocities was found to be 0.1, which is much larger than assumed by current depo- sition models. The modelled PAN flux over the entire pe- riod revealed that PAN deposition over an entire day was 333 µg m 2 d 1 under unpolluted and 518 µg m 2 d 1 under polluted episodes. Additionally, thermochemical decompo- sition PAN deposition accounted for 32 % under unpolluted episodes and 22 % under polluted episodes of the total atmo- spheric PAN loss. However, the impact of PAN deposition as a nitrogen source to the nutrient-poor grassland was esti- mated to be only minor, under both unpolluted and polluted episodes.