Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland: 1. Measurements and mechanisms

Abstract

We present 5 years of NOy and O3 eddy flux and concentration measurements and NOx concentration measurements at Harvard Forest (1990–1994), a mixed deciduous forest in central Massachusetts, and 2 months of data for a spruce woodland near Schefferville, Quebec, during the NASA ABLE3B/Northern Wetlands Study (1990). Mean midday values of net dry NOy flux from atmosphere to canopy were 3.4 and 3.2 μmole m−2 hr−1 at Harvard Forest in summer and winter, respectively, and 0.5 μmole m−2hr−1 at Schefferville during summer. Nighttime values were 1.3, 2.0, and 0.15 μmole m−2 hr−1, respectively. For 1990–1994, the net annual dry deposition of nitrogen oxides was 17.9 mmole m−2 yr−1 (2.49 kgN ha−1 y−1). Oxidized species such as HNO3 dominated N deposition, with minor contributions from direct deposition of NO2. Emissions of NO from the forest soil were negligible compared to deposition. Comparison of NOy deposition at Harvard Forest and Schefferville and analysis of the dependence on meteorological parameters show that anthropogenic sources dominate the nitrogen oxide inputs over much of North America. Heterogeneous reactions account for >90% of the conversion of NO2 to HNO3 in winter, leading to rates for dry deposition of NOy similar to fluxes in summer despite 10‐fold decrease in OH concentrations. In summer, formation of HNO3 by heterogeneous reactions (mainly at night) could provide 25–45% of the NO2 oxidation.

Dry nitrogen deposition estimates over a forest experiencing free air CO2 enrichment

“…On average, about 0.75 mg N m À2 was deposited daily. This is in reasonable agreement with the daily NO y flux estimates of 0.35-0.70 mg N m À2 day À1 reported by Munger et al (1996) over a deciduous forest in the northeast United States. Denuder/filterpack measurements above and below the canopy are summarized in Fig.…”

“…Scenario (2) builds on the observations presented by Munger et al (1996). Their long-term measurements over Harvard Forest (a deciduous forest located in Massachusetts, USA) suggested similar fluxes of NO y (but not deposition velocity) regardless of season.…”

“…Flux measurements were made at a height of 26 m. Three-dimensional wind velocities (u, v, w) were measured at 10 Hz by a sonic anemometer (ATI-K, Advanced Technologies Inc., Boulder, CO, USA). NO y fluxes were measured similar to the method described by Munger et al (1996). Sample air at 2000 sccm was drawn into a heated catalytic converter containing a 60 cm piece of 1 4 00 gold tube at 295 1C.…”

“…Micrometeorological techniques, including eddy covariance, resistance analogies, and Bowen ratio methods, have been used to estimate total dry deposition (e.g. Meyers et al, 1996;Munger et al, 1996;de Miguel & Bilbao, 1999;Nemitz et al, 2001). However, these studies often focus only on part of the total dry deposition based on a molecule of interest and do not quantify the entire flux and they are rarely conducted at research sites examining global change.…”

Increasing background ozone in surface air over the United States

“…The AIRS sites do not have data for NOy or CO with the sensitivity needed for such an analysis. One rural site where year-round, high-quality measurements for NOy and CO concentrations are available starting in 1990 is Harvard Forest in central Massachusetts [Munger et al, 1996]. Figure 1 (solid lines) shows the monthly probability distributions of daily maximum 8-hour average O3 concentrations at that site.…”