The Impact of Humidity, Temperature and Ultraviolet Light on the Near-Field Environmental Fate of Pinacolyl Alcohol, Methyl Iodide, Methylphosphonic Dichloride (DCMP) and Thionyl Chloride Using an Environmental Wind Tunnel

Understanding the near-field fate of parent chemicals and their decay products in the atmosphere provides essential information for the development of remote chemical sensors. To elucidate the near-field fate of candidate chemical signatures, selected gas phase compounds were introduced into atmospheres of varying humidity, temperature and incident light flux. These atmospheres were maintained in an environmental wind tunnel for periods typical of near-field transport scenarios. The range of humidity and temperature into which the compounds were emitted encompassed arid, temperate, and tropical values. Simulated sunlight exposure was used to evaluate the impact of time of release on signature composition. The rates of compound decay and evolution of transformation products under the various environmental conditions were monitored in real time. A Fourier transform infrared spectrometer and a gas chromatograph/mass spectrometer were used to determine chemical concentration, evaluate detectability, and identify potential interferences to the detection capability. Specifically, this report describes the initial system function tests with pinacolyl alcohol and methyl iodide and subsequent atmospheric fate experiments with methylphosphonic dichloride and thionyl chloride. Test system function was evaluated using pinacolyl alcohol because as a relatively non-reactive compound, it served as a negative control for the system. Methyl iodide is a compound knownmore » to photodissociate in the atmosphere and therefore was used to evaluate the effectiveness of the test system to detect a known positive effect under specific conditions. Results from the function tests showed that sufficient vapor generation into the large volume of the wind tunnel could be accomplished within a reasonable time period and that the operating conditions of the wind tunnel did not appear to affect the decay rate of the two initial test chemicals. As expected, no near-field decay of pinacolyl alcohol was observed under a wide range of temperature and humidity conditions. Further, both analytical techniques could detect pinacolyl alcohol at and below field-relevant concentrations. No significant difference in the disappearance of pinacolyl alcohol from the wind tunnel atmosphere was observed when the contaminated atmosphere was illuminated with simulated sunlight. We also observed that plants exposed to pinacolyl alcohol absorbed it and then continued to outgas the absorbed compound for a period of hours after being removed from the contaminated atmosphere. Methyl iodide similarly proved unreactive as a function of humidity and temperature. It is known to photodissociate when exposed to ultraviolet light. Relative humidity was found to play an important role in the environmental fate of both compounds. DCMP exhibited an exponential decay rate whose lifetime was a function of the relative humidity. Compound dissociation was so rapid at high relative humidity that sufficient concentrations of the parent chemical could not be attained to permit reliable determination of the decay rate. These indicate an atmospheric lifetime of less than 10 minutes for typical conditions. This study looked at environmental conditions typical of midlatitude environmental conditions, particularly humidity. The results indicate that the near field fate of parent compounds must be considered when choosing chemical signatures for detection. Failure to do so may cause one to incorrectly identify pollution sources or inaccurately estimate their production. « less