Field investigations of nitrogen dioxide (NO 2) exchange between plants and the atmosphere
The nitrogen dioxide (NO 2) exchange between the atmosphere and needles of Picea abies L. (Norway Spruce) was studied under uncontrolled field conditions using a dynamic chamber system. This system allows measurements of the flux density of the reactive NO-NO 2-O 3 triad and additionally of the non-reactive trace gases CO 2 and H 2O. For the NO 2 detection a highly NO 2 specific blue light converter was used, which was coupled to chemiluminescence detection of the photolysis product NO. This NO 2 converter excludes known interferences with other nitrogen compounds, which occur by using more unspecific NO 2 converters. Photo-chemical reactions of NO, NO 2, and O 3 inside the dynamic chamber were considered for the determination of NO 2 flux densities, NO 2 deposition velocities, as well as NO 2 compensation point concentrations. The calculations are based on a bi-variate weighted linear regression analysis (y- and x-errors considered). The NO 2 deposition velocities for spruce, based on projected needle area, ranged between 0.07 and 0.42 mm s −1. The calculated NO 2 compensation point concentrations ranged from 2.4 ± 9.63 to 29.0 ± 16.30 nmol m −3 (0.05–0.65 ppb) but the compensation point concentrations were all not significant in terms of compensation point concentration is unequal to zero. These data challenge the existence of a NO 2 compensation point concentration for spruce. Our study resulted in lower values of NO 2 gas exchange flux densities, NO 2 deposition velocities and NO 2 compensation point concentrations in comparison to most previous studies. It is essential to use a more specific NO 2 analyzer than used in previous studies and to consider photo-chemical reactions between NO, NO 2, and O 3 inside the chamber.