Difference in ammonia air concentration at two sampling heights from ground surface: implications for deposition research

Monitoring atmospheric ammonia levels (NH 3 ) is essential for accurately assessing nitrogen deposition, especially in agricultural regions. Conventional dry deposition measurement techniques are costly, complex, and often sparsely deployed, especially in emission hotspots. This limits the empirical basis for evaluating spatial and temporal variability and uncertainty in dry deposition estimates. In this study, we evaluated a low-cost passive sampling approach to monitor vertical NH 3 concentration gradients between two heights (100 cm and 180 cm) across 70 locations in the Netherlands, covering rural, semi-urban, and natural areas. We assessed whether systematically observed NH 3 air concentration gradients can serve as a proxy for estimating near-surface deposition processes. Over 1600 monthly measurement sets were collected and analyzed. We observed a consistent and statistically significant vertical concentration gradient, with higher concentrations at 1.8m height than at 1.0m height. The difference (Δ) averaged 0.49 μg/m 3 , about 4% of the overall sampling area average ambient ammonia concentration. This vertical gradient was detectable despite measurement uncertainty and a low signal-to-noise ratio. Total variability in Δ was only partly explained by spatial and temporal factors, but remained largely unexplained (residual variation) and thus reflected analytical and sampling errors. Spatial and temporal variables which were related to (a higher) Δ were in particular proximity to livestock farms and wind speed. Rather than providing direct flux estimates, our findings indicate that vertical NH 3 gradients can function as a diagnostic, proxy for surface atmosphere exchange patterns at local scales. By demonstrating that statistically significant gradients can be detected using low-cost passive samplers when spatial coverage and replication are sufficient, this study extends previous NH 3 monitoring work that has largely focused on single-height concentration measurements. Our study offers a scalable approach to empirically explore spatial variability and uncertainty in dry deposition assessments, particularly in regions with high agricultural emission where conventional flux measurements remain impractical.