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

The dry deposition velocity of nitric acid, V d(HNO 3), 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 V d(HNO 3). The overriding contributor is identified to be the small aerodynamic needle width of the conifer trees. Two cases had inflated V d(HNO 3) due to height-differentiated nitric acid loss to soil-derived particle surfaces. Not considering these cases, the mean V d(HNO 3) was 7.6 cm s −1. The mean laminar boundary layer resistance ( R b) 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 R b is bracketed by two theoretical estimates of the mean R b, 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 R b values with both sets of theoretical estimates indicates that measurement error needs to be reduced and/or improved formulations of theoretical R b values are in order. The large observed V d(HNO 3) at this conifer forest site is attributed to high turbulence intensity, and, especially, to small aerodynamic needle width.