Surface–atmosphere exchange of water-soluble gases and aerosols above agricultural grassland pre- and post-fertilisation

Abstract. The increasing use of intensive agricultural practices can lead to damaging consequences for the atmosphere through enhanced emissions of air pollutants. However, there are few direct measurements of the surface–atmosphere exchange of trace gases and water-soluble aerosols over agricultural grassland, particularly of reactive nitrogen compounds. In this study, we present measurements of the concentrations, fluxes and deposition velocities of the trace gases HCl, HONO, HNO 3 , SO 2 and NH 3 , and their associated water-soluble aerosol counterparts Cl − , NO 2 − , NO 3 − , SO 4 2− , NH 4 + as determined hourly for one month in May–June 2016 over agricultural grassland pre- and post-fertilisation. Measurements were made using the Gradient of Aerosols and Gases Online Registration (GRAEGOR) wet–chemical two–point gradient instrument. Emissions of NH 3 peaked at 1460 ng m −2 s −1 three hours after fertilisation, with an emission of HONO peaking at 4.92 ng m −2 s −1 occurring five hours after fertilisation. Apparent emissions of NO 3 − aerosol were observed after fertilisation which, coupled with a divergence of HNO 3 deposition velocity ( V d ) from its theoretical maximum value, suggested the reaction of emitted NH 3 with atmospheric HNO 3 to form ammonium nitrate aerosol. The use of the conservative exchange fluxes of tot-NH 4 + and tot-NO 3 − indicated net emission of tot-NO 3 − , implying a ground source of HNO 3 after fertilisation. Daytime concentrations of HONO remained above the detection limit (30 ng m −3 ) throughout the campaign, suggesting a daytime source for HONO at the site. Whilst the mean V d of NH 4 + was with 0.93 mm/s in the range expected for the accumulation mode, the larger average V d for Cl − (3.65 mm/s), NO 3 − (1.97 mm/s), SO 4 2− (1.89 mm/s) reflected the contribution of a super-micron fraction and decreased with increasing PM 2.5 / PM 10 ratio (a proxy measurement for aerosol size), providing direct evidence of a size-dependence of aerosol deposition velocity for aerosol chemical compounds.