Measuring shoot-level NOx flux in field conditions: the role of blank chambers

We monitored NOx fluxes in an empty chamber and in chambers containing Scots pine shoots. Our aims were to observe how the chamber blank behaves in non-controlled field conditions and at low ambient NOx concentrations, to find ways of reducing deleterious wall effects, and to evolve a method for a blank correction. We found that solar ultraviolet radiation induced NOx emission from the chamber walls. Comparisons between blanks of several chambers showed that the chambers were not identical. We therefore recommend that a separate blank chamber will be monitored together with the branch chambers in long-term field measurements. At regular time intervals, all of the chambers should be measured simultaneously while empty and the regression between the UV irradiance and NOx flux determined for each chamber. In case the chambers differ from each other, the blank could be corrected with the UV-regression coefficients obtained.