Nitrogen loss through ammonia volatilization is an environmental and economic concern. When acid traps are used with wind tunnels to measure ammonia volatilization, loss of solution volume is observed. As the loss mechanism affects volatilization estimates, a field study was conducted to determine if solution loss from acid traps was due to either selective loss of water through evaporation, loss of bulk solution, or a combination. Two methods for calculating air flow volume through the acid traps were also examined. Solution losses from acid traps averaged 40 mL d^-1 (±9.2 mL) from an initial 100 mL, and ammonium concentration increased in close accordance with the dilution–concentration relationship for aqueous solutions. Hence, solution loss was due to evaporation, with virtually no ammonium loss, confirming that the flux calculations using corrected acid trap volumes are required. Failure to correct for the reduced volumes resulted in 9%–224% overestimation of ammonium concentrations. Air flow volumes through acid traps were underestimated by 18.5% when initial and final air flow rates were used compared with continuous cumulative flow measurements. Using cumulative flows and accounting for evaporation loss from acid traps help ensure that treatment differences are not masked by the inherent variability in field-based measurements.