In the late 1940s the Navy's BuAer was responsible for the development of airborne countermeasures systems and the idea of spraying chemicals into the air to produce large radar echoes as a "liquid chaff" attracted significant effort. Iron pentacarbonyl, a straw-colored liquid used in the cores of electrical transformers and in the magnetic coils of certain radio and TV coils, was the subject to much experimentation in 1948. Upon contact with the air, iron pentacarbonyl undergoes a chemical reaction which results in a cloud of iron oxide particles which the Navy surmised might block radar beams. Initial experiments involved spraying the chemical from a boat, but results were inconclusive.
On 10 November 1948 a Curtiss SB2C Helldiver was used to spray 60 gallons of iron pentacarbonyl over Chesapeake Bay while flying at 130 knots and 500 feet altitude. Along the shoreline, the Navy set up various radar systems at six different locations operating at different wavelengths from 200 to 9100 MHz to track the Helldiver as it sprayed the chemical.
There was a brief signal at 700 MHz, but for the most part the radars saw nothing. Visually, however, it was spectacular according to eyewitnesses to the tests. As the clear chemical came in contact with the air, it turned into a black vapor which several feet behind the aircraft then burst into a brilliant flame that varied between dark red and light orange that extended past the Helldiver for approximately 10 plane lengths and persisted for several seconds. As the flame darkened to a red color, the cloud turned black again and then a rust color before dispersing.
The pilot during the tests refused to fly further missions to test the iron pentacarbonyl and the Navy ended its tests with no indication that it blocked radar beams. However, for days after that November tests, reports came from different communities along Chesapeake Bay of some sort of "burning rain" that damaged paint on cars, discolored houses, and allegedly damaged clothing on clotheslines. With the Pentagon and the Navy quiet, the state of Maryland conducted an investigation and concluded that the culprit were rotting skunk cabbages on the bay shore that exuded sulphur dioxide that reacted with water vapor to produce sulphuric acid vapor (acid rain).
Naturally, the Navy didn't feel the need to correct the conclusions of the state investigation and no further tests of liquid radar countermeasures were attempted again.
Source: The History of U.S. Electronic Warfare, Volume II- The Renaissance Years, 1946-1964 by Alfred Price. The Association of Old Crows/Port City Press, 1989, p24-25.