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A Low Cost System for Detecting Fog Events and Triggering an Active Fog Water Collector

Category: Technical Note

Article In Press
DOI: 10.4209/aaqr.2016.11.0508
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Peter Weiss-Penzias 1, Daniel Fernandez2, Robert Moranville1, Chad Saltikov1

  • 1 University of California, Santa Cruz, CA 95064, USA
  • 2 California State University, Monterey Bay, Marina, CA 93933, USA


A moisture detection system for activating a fog water collector is described.
29 fog water samples were collected during the summer of 2016 in Santa Cruz, CA.
On average the system was triggered 4.5 ± 3.3 hours after visibility reduction.
In previous years a relative humidity-only trigger method was employed.
Monomethylmercury concentrations in fog were independent of sampler trigger method.


A simple method of activating the Caltech Active Strand Cloud-water Collector (CASCC) is described. This system detected the onset of wet deposition events associated with the advection of marine stratus clouds using an optical rain sensor (ORS) and a standard passive fog collector (SFC) in combination with a relative humidity threshold. The system was deployed on a rooftop between May 10 and September 20, 2016 (134 days) at the University of California, Santa Cruz, six km from Pacific Ocean, at 240 m elevation. Twenty-nine fog water samples (daily mean volume = 174 ± 71 mL) were collected for the purposes of quantifying the concentration of monomethylmercury (MMHg) and its possible marine origins. For 20 days during the study, a visibility sensor (VS) was collocated with the ORS and both sensors detected 7 fog events. The ORS detected 2 additional marine stratus drizzle events missed by the VS. The start time of the events detected by the ORS was delayed relative to the onset of visibility reduction in 6 of 7 events by 4.5 ± 3.3 hours. Low wind speeds at night at this location limited the wet deposition to the SFC. Average CASCC sampling time during these events was 6.2 ± 2.8 hours and 4 liquid samples were obtained (80 to > 275 mL). As a comparison, fog water collections at UCSC during the fog seasons of 2014 and 2015 yielded 35 and 12 samples, respectively using a trigger based on relative humidity (RH) and sampling times of > 12 h per day. The main benefit of triggering with the ORS in 2016 was to cut in half the sampling time without loss of sample collection volume. Mean MMHg concentrations between the 3 years were not significantly different suggesting that the SFC/ORS triggering system is appropriate for use at multiple fog collection sites simultaneously.


Fog water collector Activation mechanism Visibility Moisture detection Deposition Mercury

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