Use an ammonia sensitive electrode. Lowest LOD will be reached with a gas electrode in which an ammonium-ion selective (NH4+)-membrane electrode is used instead of a pH-glass electrode used in commercially available ammonia electrodes. We used a stretched Teflon gas permeable membrane and an inner puffer solution of around pH4 to convert the ammonia gas into ammonium ions. This resulted in LOD’s in the lower ppb range. Those sensors were used by the US army to detect hidden soldiers in the Vietnam War.

For longer terms, something cheap and easy is to use passive samplers such as the http://ogawausa.com/passive-sampler/

For shorter term something somehow cheaper but also complicated is to use denuder coated with some acid substrate. You will need to find a lab to help you with that.

You might also check: Li, Y., Schwandner, F. M., Sewell, H. J., Zivkovich, A., Tigges, M., Raja, S., Holcomb, S., Molenar, J. V., Sherman, L., Archuleta, C., Lee, T., and Collett, Jr., J. L. (2014) Observations of ammonia, nitric acid, and fine particles in a rural gas production region, Atmos. Env. 83, 80-89.

One very good tool for online measurement of NH3 at ambient levels of few ppb is provided by PIcarro. It provides continuous online measurements. of ammonia (plus CO2, CH4, water vapor and N2O). You can check the product at the link with this reply