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

Abstract

AbstractMany 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.