Ammonia flux and dry deposition velocity from near-surface concentration gradient measurements over a grass surface in North Carolina

Atmospheric ammonia concentrations were measured during fall 2001, winter, spring, and summer 2002 over natural surfaces in North Carolina where animal farms and waste storage and treatment lagoons (which are known to emit ammonia) at two heights (2 and 6 m) above the surface employing an arrangement of two Thermo Environmental Instruments, Inc. Model 17C chemiluminescent nitrogen oxides (NOX)–ammonia (NH3) analyzers along with a solenoid for each analyzer to alternate measurements between the two elevations. Simultaneously, mean winds and temperatures are also measured at the same two heights. The micrometeorological gradient method is used in conjunction with the Monin–Obukhov similarity theory, to estimate the vertical flux and dry deposition velocity of ammonia under different meteorological conditions. Diurnal and seasonal variations of ammonia flux and dry deposition velocity were investigated under a wide range of wind and atmospheric stability conditions yielding hourly variation of NH3 fluxes and deposition during each seasonal campaign. Fall average NH3 concentrations were generally the highest with daytime concentrations of 7.60±6.54 μg m−3 at 2 m, while nighttime NH3 concentrations were 7.25±6.14 μg m−3 at 2 m. The winter season had the lowest overall concentrations collected during each seasonal campaign with averages of 1.73±2.00 μg m−3 (2 m) during daytime and 1.37±1.50 μg m−3 (2 m) during nighttime. Deposition and emission occur, both on daily and seasonal scales. Summer measurements yielded the largest average daytime deposition velocity of 3.94±2.79 cm s−1, whereas winter measurements gave the lowest daytime velocities with an average of 2.41±1.92 cm s−1. The largest nighttime deposition velocities were estimated during the summer season with an average of 0.76±1.69 cm s−1, whereas fall nighttime deposition velocities were considerably lower at 0.07±0.17 cm s−1. Regression relationships between the ammonia deposition velocity and the friction velocity are obtained for different stability conditions.