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Field Methods

A special collector was designed for MDN which is a modification of the wet-only deposition collector used by NADP for other observations. The modifications include replacing the collection bucket with a smaller insulated sampling chimney. The lower portion of the sampler is enclosed and insulated. A small space heater is used to maintain the winter temperature between 10 and 15 °C to prevent samples from freezing. Heat from the interior rises up the chimney to warm the collection funnel which melts ice and snow on contact with the funnel. A ventilation fan pulls filtered air through the sampler during warm weather to moderate the inside temperatures.

The mercury sampling train consists of an approximately 128-mm diameter, borosilicate glass funnel, an acid-cleaned, wide-bore (3-mm) capillary tube, and a 2-liter, borosilicate glass bottle. The capillary tube is connected to the funnel by a ground glass fitting. The opposite end of the capillary tube is expanded to form a sphere with a diameter slightly larger than the mouth of the sample bottle. A small hole in the sphere allows water to drain into the sample bottle. Sample bottles are pre-charged with 20.0 mL of dilute (0.12 M) hydrochloric acid as a preservative to prevent microbial action and provides sufficient chloride ion to complex mercury in solution and prevent conversion to and loss of Hgo.

The field operators receive a pre-cleaned sampling train each week. Every Tuesday, the exposed sampling train is removed using clean techniques and returned to the MDN laboratory, Frontier Global sciences, Inc. in Seattle, WA (Hg Analytical Laboratory, "HAL"), along with the sample bottle containing any collected precipitation. All operators wear plastic gloves when handling the sampling train and follow special procedures to avoid contaminating the sample. Any overflow from the bottle is collected and measured to determine the total precipitation amount, but it is not included with the sample sent to the lab.

Each site is also equipped with a weighing-bucket rain gauge that provides a continuous record of rainfall amounts. Rainfall is recorded to the nearest 0.01 in. The rain gauge also monitors the precipitation sampler, recording whether the sampler was properly open during wet periods and closed during dry periods.