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Nitric Acid Dry Deposition at Conifer Forests: Niwot Ridge, Colorado Subalpine Spruce-Fir Study

H. Sievering
Long Term Ecol. Res. Program of INSTAAR, Univ. of Colorado-Boulder, CO
MS-Environ. Science Prog., Geog. & Environ. Sci. Dept., Univ. of Colorado-Denver
PhD in Environmental Studies Program, Univ. of Colorado-Boulder
INSTAAR, CB 206, 1560 30th St., Boulder, CO

C. Seibold
Long Term Ecol. Res. Program of INSTAAR, Univ. of Colorado-Boulder, CO

G. Rattray
MS-Environ. Science Prog., Geog. & Environ. Sci. Dept., Univ. of Colorado-Denver

T. Tomaszewski
PhD in Environmental Studies Program, Univ. of Colorado-Boulder
INSTAAR, CB 206, 1560 30th St., Boulder, CO

 

The dry deposition velocity of nitric acid, Vd(HNO3), over a 13-m (mean ht.) spruce-fir forest at the 3000m asl NADP and AmeriFlux monitoring site of the Niwot Ridge Long Term Ecological Research (LTER) Program was estimated using the flux-gradient approach. Turbulence intensity at this site is high (mean u* of 0.65 m s-1 with U of 2.9 m s-1) and contributed to a large observed Vd(HNO3). However, the overriding contributor was found to be the small aerodynamic needle width of the site’s conifer trees. Two flux-gradient measurement cases had inflated Vd(HNO3) due to heightdifferentiated HNO3 loss to soil-derived particle surfaces. Not considering these cases, the mean Vd(HNO3) was 7.6 cm s-1. The mean laminar boundary layer resistance (Rb) was found to be 7.8 cm s-1 (of slightly lesser magnitude than the aerodynamic resistance, 8.5 cm s-1). The datadetermined Rb is bracketed by two theoretical estimates of the mean Rb of 5.9 and 8.6 cm s-1. These theoretical estimates include slightly differing considerations of the small canopy length scale (aerodynamic needle width) of = 1 mm at this subalpine spruce-fir forest (also some pine). The correlation of data-determined Rb values with both sets of theoretical estimates indicates that measurement error needs to be reduced and/or somewhat improved formulations of theoretical Rb values are in order. The most significant aspect of this HNO3 dry deposition study at the Niwot Ridge LTER is that quite large Vd(HNO3) in the 3-10 cm s-1 range were observed at a typical spruce-fir forest.