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We describe a dynamic chamber system to determine reactive trace gasexchange fluxes between plants and the atmosphere under laboratory and, withsmall modifications, also under field conditions. The system allowsmeasurements of the flux density of the reactive NO-NO2-O3triad and additionally of the non-reactive trace gases CO2 andH2O. The chambers are made of transparent and chemically inert wallmaterial and do not disturb plant physiology. For NO2 detection we useda highly NO2 specific blue light converter coupled to chemiluminescencedetection of the photolysis product, NO. Exchange flux densities derivedfrom dynamic chamber measurements are based on very small concentrationdifferences of NO2 (NO, O3) between inlet and outlet of thechamber. High accuracy and precision measurements are therefore required,and high instrument sensitivity (limit of detection) and the statisticalsignificance of concentration differences are important for thedetermination of corresponding exchange flux densities, compensation pointconcentrations, and deposition velocities. The determination of NO2concentrations at sub-ppb levels (<1 ppb) requires a highly sensitiveNO/NO2 analyzer with a lower detection limit (3σ-definition) of0.3 ppb or better. Deposition velocities and compensation pointconcentrations were determined by bi-variate weighted linearleast-squares fitting regression analysis of the trace gas concentrations,measured at the inlet and outlet of the chamber. Performances of the dynamicchamber system and data analysis are demonstrated by studies of Picea abies L. (NorwaySpruce) under field and laboratory conditions. Our laboratory data show thatthe quality selection criterion based on the use of only significantNO2 concentration differences has a considerable impact on theresulting compensation point concentrations yielding values closer to zero.The results of field experiments demonstrate the need to considerphoto-chemical reactions of NO, NO2, and O3 inside thechamber for the correct determination of the exchange flux densities,deposition velocities, as well as compensation point concentrations. Underour field conditions NO2 deposition velocities would have beenoverestimated up to 80%, if NO2 photolysis has not been considered.We also quantified the photolysis component for some previous NO2 fluxmeasurements. Neglecting photo-chemical reactions may have changed reportedNO2 compensation point concentration by 10%. However, the effect onNO2 deposition velocity was much more intense, ranged between 50 andseveral hundreds percent. Our findings may have consequences for the resultsfrom previous studies and ongoing discussion of NO2 compensation pointconcentrations

atmospheric measurement techniques

The dynamic chamber method: trace gas exchange fluxes (NO, NO 2 , O 3 ) between plants and the atmosphere in the laboratory and in the field