Estimation of dry deposition velocity using inferential models and site-specific meteorology—uncertainty due to siting of meteorological towers

Application of inferential models is currently the only practical way of quantifying seasonal and annual dry deposition rates of acidifying chemical species. A number of inferential modelling approaches have been developed, but the most up-to-date approaches use easily obtained meteorological measurements and information on the surface characteristics to estimate pollutant-specific deposition velocities (vd). These are then multiplied by air concentration measurements with hourly to weekly resolution. There are a number of sources of uncertainty associated with vd estimates, one of which arises from spatial heterogeneity in surface and meteorological conditions. Due to this spatial variability it is difficult to determine how representative a point estimate of vd is of other nearby locations (i.e. with 1 km) and whether a point estimate represents the prevailing conditions in the area of interest. This paper examines the uncertainty in vd due to small-scale spatial variability in meteorological variables by comparing vd and related parameters among up to four measurement locations all within 500 m of each other. These locations were selected to provide an estimate of the upper limit on the spatial variability in vd around typical air pollutant/dry deposition monitoring sites. Under most conditions (i.e. 75% of the time) the variability or uncertainty in a point measurement of hourly vd was within ±40% for O3 and SO2 and ±90% for SO42− and HNO3. Hourly estimates of aerodynamic resistance and friction velocity, which are needed to compute vd, varied spatially by an average of ±115 and ±41%, respectively. When vd was averaged over an annual period, the spatial variability was less pronounced. For SO2 and O3, the annual vd did not vary in space by more than ±10% from the annual average value determined across all of the locations. For SO42− and HNO3, which are more dependent on meteorological and less on surface conditions, annual vd varied in space by about ±30 and ±40%, respectively.