Spatial and temporal variability of high resolution reduced nitrogen concentrations in the Fraser Valley

Abstract

We present high resolution measurements of ammonia (NH3) and particle-ammonium (NH4+) collected during the Pacific 2001 experiment at an urban, sub-urban and high elevation site. These data are used to conduct a detailed instrument inter-comparison and to characterize the sites. The data are interpreted with respect to; the degree to which the sites are representative, the degree of sensitivity to inorganic particle burdens to NH3 availability and controls on reduced nitrogen (NHx) variability and partitioning. Ammonia measurements from wet effluent diffusion denuders (WEDD), annular diffusion denuders and a tunable diode laser spectrometer generally lie within a factor of 1.5 across a range of observed concentrations of 0–0.25 μmol m−3, although individual observations differed by more than a factor of two. The high resolution (2 min average) data from the WEDD systems are used to calculate a coefficient of variability which is used to assess the degree to which the measurement sites are spatially representative. The results indicate the sub-urban site is the most spatially representative. Ammonia concentrations at this site exhibit variability on time scales of less than 4 h that is comparable to that of ozone. Data collected in the urban setting indicate persistent measurable NH3 concentrations and that NH4+ dominates NHx. Conversely, the high elevation site shows a much greater degree of variability due in part to the influence of meteorological coupling and decoupling from the underlying valley, domination of NHx by NH3 and a stronger coupling of NH3 and NH4+.