UV-induced NOy emissions from Scots pine: Could they originate from photolysis of deposited HNO3?

Production of NOy on Scots pine branch surfaces by ultraviolet radiation is observed in Hyytiälä, southern Finland. Similar irradiance-dependent NOy emissions from snow and different chamber surfaces have been observed to originate from HNO3 or nitrate photolysis. In this study, we investigate whether also the NOy emitted from pine shoots could originate from photolysis of HNO3 attached to the needle surface. We used field data of several years from Hyytiälä to test this hypothesis. The HNO3 deposition, estimated for the Hyytiälä site, had been high enough to account for the NOy emission rates observed from the chambers. We could not find the particular characteristics of the daily pattern of CO2 exchange or stomatal control reflected in the NOy flux. When a pine branch was rinsed, which reduced the amount of water-soluble nitrogen compounds (e.g., HNO3, nitrates and HONO) from the needle surface, NOy emissions from that branch decreased compared to another non-rinsed branch. Therefore, we conclude that the results support the hypothesis and that HNO3 photolysis on plant surfaces needs to be taken into account both from air chemistry and plant sciences point of view.