After reviewing all the responses to this topic, I felt reasonably sure that the PM25 should be able to withstand severe rains we get during our Arizona Monsoons, and decided to sacrifice this unit, in the interest of... well, my curiosity... and put it to a test.
1) Disable sensor in WS View.
2) Remove batteries.
3) Replace battery door.
4) Place PM25 in kitchen sink.
5) Turn on cold water for a brisk flow of water.
6) Run water over top of PM25 for 1 and a half minutes.
7) Use hand to move water around sensor, simulating wind.
8 Turn off water after end of time frame.
9) Wait a couple of seconds for water beads to drip off.
10) Gently move PM25 out of sink and onto a flat surface.
At this point, I gently removed the battery compartment. The receptacle area for the batteries was dry. The flat part of the battery door, which lies directly under the batteries, was also dry. There was some water in the grooved area around this flat part, but I suspect it got in there when I remove the door -- trying to squeeze the clips, hold down the door so it doesn't move, and lift the sensor gave it a bit of a shake..
The PM25 is now on sitting on the the picnic table outside, with the battery door next to it, to dry out in the sun.
If after several hours of drying it decides to still work after putting the batteries back in and re-enabling the sensor in WS View, I'll go with plan A -- screw it into the top of a 4x4 fence post and let the Arizona Monsoons try their best to drown it.
If it doesn't work after several hours, I'll get another one and go with plan B -- screw it vertically on the south side of my shed, just below the protective overhang so the solar cell gets direct sunlight and hopefully not a lot of rain unless we get severe winds.
Many thanks to every one who provided input! I'll post the status of the sensor later on today.