The reaction between HgBr and O₃: kinetic study and atmospheric implications

Area of Science:

• Atmospheric Chemistry
• Mercury Speciation
• Chemical Kinetics

Background:

• Atmospheric mercury chemistry is crucial for understanding mercury's environmental fate.
• Key reaction rate constants, particularly for mercury bromide (HgBr), are often unmeasured.
• Accurate kinetic data is essential for atmospheric mercury modeling.

Purpose of the Study:

• To experimentally determine the rate constant for the gas-phase reaction between HgBr and ozone.
• To measure the rate constants for HgBr reduction side reactions.
• To elucidate the primary atmospheric reaction pathway for HgBr.

Main Methods:

• Utilized laboratory experiments to measure gas-phase reaction rate constants.
• Employed techniques to determine the rate of HgBr + O3 reaction.
• Concurrently measured rate constants for HgBr + O and HgBrO + O reactions.

Main Results:

• The rate constant for HgBr + O3 was determined to be (7.5 ± 0.6) × 10^-11 cm^3 molecule^-1 s^-1 at room temperature.
• Rate constants for reduction side reactions were measured: k(HgBr + O) = (5.3 ± 0.4) × 10^-11 cm^3 molecule^-1 s^-1 and k(HgBrO + O) = (9.1 ± 0.6) × 10^-11 cm^3 molecule^-1 s^-1.
• The HgBr + O3 rate constant is near the collision limit, suggesting no significant energy barrier.

Conclusions:

• The measured rate constant for HgBr + O3 supports its role as the main atmospheric sink for HgBr.
• The reaction of HgBr with ozone forms BrHgO, a key step in mercury's atmospheric transformation.
• This research provides critical kinetic data for refining atmospheric mercury models.