Isoprene photochemistry over the Amazon rainforest

Area of Science:

• Atmospheric Chemistry
• Biogeochemical Cycles
• Environmental Science

Background:

• Isoprene photooxidation significantly influences atmospheric chemistry, particularly in forested areas.
• Key reactions involve hydroxyl radicals (OH), leading to isoprene peroxy radicals (ISOPOO).
• Uncertainty exists regarding the dominant reaction pathways of ISOPOO and the impact of anthropogenic pollution.

Purpose of the Study:

• To measure concentrations of hydroxyhydroperoxides (ISOPOOH) and methyl vinyl ketone (MVK) + methacrolein (MACR) in the Amazon basin.
• To investigate the impact of anthropogenic pollution on isoprene photooxidation pathways.
• To compare field measurements with global chemical transport model simulations.

Main Methods:

• Field measurements of ISOPOOH, MVK, and MACR concentrations were conducted during the wet season in the Amazon.
• Air masses sampled included both background and polluted conditions, influenced by an urban region.
• Data analysis focused on the ratio of ISOPOOH to MVK + MACR under varying pollution levels.

Main Results:

• Under background conditions, the ratio of ISOPOOH to MVK + MACR was 0.4-0.6, implying a near-unity ratio for ISOPOO reactions with hydroperoxyl radicals (HO2) and nitric oxide (NO).
• This observed ratio differs significantly from current global chemical transport model simulations for low-NOx forested regions.
• Under polluted conditions (>1 ppb reactive nitrogen), ISOPOOH concentrations fell below detection limits (<60 ppt).

Conclusions:

• The reaction rate ratio of ISOPOO with HO2 to NO is approximately unity under background Amazonian conditions.
• Anthropogenic pollution causes an abrupt shift in isoprene photooxidation, drastically reducing ISOPOOH formation.
• These findings highlight potential future changes in Amazon rainforest photochemistry due to human activities.