Technical Abstract: Cumulative emission or gas-phase concentration of ammonia (NH3) are commonly measured by trapping gaseous NH3 in an acidic solution that is later analyzed for total ammonia content. This traditional acid trap method is inexpensive, reliable, and accurate, but it is labor-intensive and inconvenient for high-frequency sampling. This paper describes a new acid trap method in which total ammonia concentration captured in a citric acid solution is calculated from measured pH by applying a speciation model. With this new method, ammonia concentration in a single acid trap can be determined repeatedly over time. Testing through titrations, laboratory emission measurements, and field measurement of gaseous NH3 showed that the method is accurate and reasonably precise. (For the most sensitive case standard deviation in titration results was 8 micromol/kg, and 10-20 micromol/kg in the field trials. The quantitation limit from the field trials was about 300 micromol/kg.) Speciation modeling was used to assess the importance of interferences. Results showed that error due to minor contamination of chemicals and evaporation of water can be kept at low levels. Acidic gases may be a significant interference. However, their presence is easy to recognize, and significant error can be avoided by selecting an appropriate acid concentration (at the expense of sensitivity in some cases).