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 be an developed, but the most up-to-date approaches use easily obtained m eteorological measurements and information on the surface characterist ics to estimate pollutant-specific deposition velocities (nu(d)). Thes e are then multiplied by air concentration measurements with hourly to weekly resolution. There are a number of sources of uncertainty assoc iated with nu(d) estimates, one of which arises from spatial heterogen eity in surface and meteorological conditions. Due to this spatial var iability it is difficult to determine how representative a point estim ate of a, is of other nearby locations (i.e. with 1 km) and whether a point estimate represents the prevailing conditions in the area of int erest. This paper examines the uncertainty in nu(d) due to small-scale spatial variability in meteorological Variables by comparing nu(d) an d related parameters among up to four measurement locations all within 500 m of each other. These locations were selected to provide an esti mate of the upper limit on the spatial variability in nu(d) around typ ical air pollutant/dry deposition monitoring sites. Under most conditi ons (i.e. 75% of the time) the variability or uncertainty in a point m easurement of hourly nu(d) was within +/-40% for O-3 and SO2 and +/-90 % for SO42- and HNO3. Hourly estimates of aerodynamic resistance and f riction velocity, which are needed to compute nu(d), varied spatially by an average of +/-115 and +/-41%, respectively. When nu(d), was aver aged over an annual period, the spatial variability was less pronounce d. For SO2 and O-3, the annual nu(d) did not vary in space by more tha n +/- 10% from the annual average value determined across all of the l ocations. For SO42- and HNO3, which are more dependent on meteorologic al and less on surface conditions, annual nu(d), varied in space by ab out +/-30 and +/-40%, respectively. (C) 1997 Elsevier Science Ltd.