The Mercury Deposition Network (MDN) is the only network providing a long-term record of total mercury (Hg) concentration and deposition in precipitation in the United States and Canada. In an effort to decrease expense thereby increasing the accessibility of joining the MDN program, the Program Office (PO) proposed the possibility of extending sampling periods from one-week to two-weeks. This change would allow the shipping cost of MDN coolers to be cut in half for a given year.

Starting in Summer 2025, three sites (TN11, WA04, and WI06) were able to participate in a collocated study between one- and two-week sampling to analyze if there exists any statistically significant difference in the precipitation volumes collected, and the reported Hg concentration measured in each sample. Both types of precipitation collectors provided supported by NADP, the Aerochem (ACM) collector, and the NCON collector, were tested during this study. The same collector types were used for both samples at each site; TN11 tested using ACM collectors, while WA04 and WI06 tested using NCON collectors. All factors other than sampling period, including during deployment and sample processing, were kept consistent between all samples.

Here we will present the results of this co-located intercomparison study by comparing the sample volume and Hg deposition of the two-week sample to that of the two one-week samples that correspond with that deployment time. Results are consistent in both sample volume and Hg deposition, showing that there exists no significant loss of sample volume due to evaporation over the two-week sampling period. Additionally, comparison of the volume corrected Hg deposition between the two sample periods do not show significant difference in analytical results.