Nitric acid dry deposition to conifer forests: : Niwot Ridge spruce–fir–pine study

The dry deposition velocity of nitric acid, Vd(HNO3), over a 12-m (mean height) spruce–fir forest at Niwot Ridge, Colorado was estimated during 13 daytime periods 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 the large observed Vd(HNO3). The overriding contributor is identified to be the small aerodynamic needle width of the conifer trees. Two cases had inflated Vd(HNO3) due to height-differentiated nitric acid 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 s m−1 (of similar magnitude to that of the aerodynamic resistance, 8.5 s m−1). The data-determined Rb is bracketed by two theoretical estimates of the mean Rb, 5.9 and 8.6 s m−1, that include consideration of the small canopy length scale (aerodynamic needle width), 1 mm or less, at this conifer forest. However, the poor correlation of data-determined Rb values with both sets of theoretical estimates indicates that measurement error needs to be reduced and/or improved formulations of theoretical Rb values are in order. The large observed Vd(HNO3) at this conifer forest site is attributed to high turbulence intensity, and, especially, to small aerodynamic needle width.