Wet deposition (wetfall) measurements are a key component of long-term biogeochemical research and monitoring at the Fraser Experimental Forest, central, Colorado.  Measurements have been ongoing since 1984 at the 2745 m, snow-dominated, lodge pole pine forest site.  During the majority of this time the Aerochem Metrics (ACM) model 301 collector has been used.  The ACM model has been used for decades nationwide, and has been the primary sampler used within the NADP program.  In 2011 a sampler of different design, the N-CON Systems Company model 00-120, was installed at the site, and the instruments operated in tandem and the number of opening/closing events were recorded. Design differences in precipitation detection between the two samplers resulted in N-CON sampler opening approximately 10 times more frequently than the Aerochem. Based on this difference in collection sensitivity, frequency and duration, we hypothesized differences in wetfall collection amounts and chemistry between the two sampler types.

We compared collection weights and chemistry between the two collocated samplers over the course of almost two years (January 2017-November 2018). Weekly samples (n=132 per collector), were analyzed for total collection weight, EC, ANC, pH, sodium, calcium, chloride, nitrate and sulfate.  Summer and shoulder season comparisons between the samplers show no weight differences between the samplers (summer p = 0.67; shoulder p = 0.37). However, winter collection weights were, on average, marginally higher in the N-CON (0.64 kg), than in the ACM (0.49 kg) (p = 0.1).  This is in line with our field observations noting the N-CON collector reacting to the onset of storms well before the ACM. Wetfall concentrations were strongly correlated (R² = 0.85) between collectors with slopes near 1 for EC, calcium, nitrate and sulfate.  Nitrate (p=0.1) and sulfate (p=0.05) concentrations were higher in the N-CON collector, but other constituents did not differ between the collectors.   

Though we document differences in constituent concentrations for a few constituents, the distinct collection duration of the two samplers had little overall effect on weekly chemical inputs.  The lack of disparity may be due to dilution effects of smaller event differences by the larger volume of bulk collection at a weekly timescale.   Although this study reports few differences in chemical input between the two collector types, further investigation may highlight event-based differences in precipitation volume and chemistry.