Air-surface exchange of ammonia at an agricultural site in the northern San Joaquin Valley during CalNex

In agricultural areas, elevated ammonia levels can be linked to widespread fertilizer use and livestock farms. Recent estimates attribute ~80% of the U.S. ammonia (NH3) emission budget to agriculture. However, large uncertainties in emissions inventories persist for ammonia, a key aerosol precursor. In conjunction with the 2010 CalNex study, measurements of ammonia air-surface exchange were conducted for approximately three weeks at an agricultural site north of Modesto, CA in the San Joaquin Valley. The site was adjacent to fertilized farmland and surrounded by several dairies. A Picarro ammonia analyzer continuously sampled ambient air while switching between two heights, 0.5 m and 2.5 m at 30-minute intervals. Ammonia concentrations from the Picarro were utilized to estimate fluxes via the flux-gradient theory. A relaxed eddy accumulation system with annular denuders was collocated at the site to measure ammonia in up, mid, and down drafts.