Atmosphere × Canopy Interactions of Nitric Acid Vapor in Loblolly Pine Grown in Open-Top Chambers

Many studies that address the impact of tropospheric O₃ on agricultural and forested ecosystems utilize the open-top chamber. During the production of O₃ using electrical discharge generators fed with dry air, there is an inadvertent addition of HNO₃ vapor, a highly reactive trace gas. While several studies have proposed that HNO₃ vapor introduces artifacts, none has measured concentrations of the odd-N₂ trace gas in the chamber or investigated the fate of the N in the context of whole-plant physiology and growth. These questions were investigated using open-top chambers containing seedlings of loblolly pine (Pinus taeda L.) during the 1988 growing season in Oak Ridge, TN. The O₃ treatments consisted of charcoal-filtered or subambient (0.96 µmol m⁻³, 24-h mean), ambient (1.62 µmol m⁻³, 24-h mean), and elevated (2.36 µmol m⁻³, 24-h mean) concentrations, the last being accomplished by proportional O₃ addition over the diurnal period. Measurements of the HNO₃ vapor concentration during dry periods only (no rainfall or ground-level fog) averaged 28.6 nmol m⁻³ (subambient), 55.4 nmol m⁻³ (ambient air), and 240.0 nmol m⁻³ (elevated O₃), an 8.4-fold range. For every 100 mol of O₃ added to the chamber, 28 mol of HNO₃ vapor were inadvertently added; this ratio is several times higher than that previously reported. This result, taken with published estimates of leaf conductance to HNO₃ vapor, indicates a maximum N deposition in the form of HNO₃ vapor ranging from 19.5 pmol N cm⁻² leaf area h⁻¹ (subambient O₃) to 171.9 pmol N cm⁻² h⁻¹ (elevated O₃). Given the nutrient content of the seedlings and knowledge of the fate of HNO₃ vapor on the leaf surface and leaf interior, the degree to which N deposition via HNO₃ vapor met the N requirements of the loblolly pine seedlings was estimated. Seedlings in the elevated O₃ treatment had an upper-limit estimate of 3.5% for the needles and 1.8% for the whole plant of N derived from HNO₃ vapor. The concentration of HNO₃ vapor in the chambers, site of HNO₃ vapor deposition, N requirements of the loblolly pine seedlings, and estimated threshold for phytotoxic effects of HNO₃ vapor indicate that the inadvertent production of this odd-N₂ trace gas is important in understanding the atmospheric chemistry within the chambers, but that the level of N loading in this study is unlikely to affect physiology or growth. However, we recommend that studies that employ higher O₃-exposure scenarios recognize the potential for inadvertent N deposition, particularly in trees grown in N-deficient substrate.