Monoethanolamine decay mediated by photolysis of nitrate in atmospheric particles: a brown carbon and organic phase formation pathway

The massive industrial release of monoethanolamine (MEA) into the atmosphere highlights MEA as a potential environmental risk. Nitrate (NO₃⁻) is one of the most abundant inorganic compounds and has been found to co-exist with amines in ambient particles. The photolysis of NO₃⁻ can produce oxidants (OH radicals, NO₂, O(³P), and N(iii)), which lead to particulate MEA decay. Furthermore, MEA degradation products are likely to yield brown carbon (BrC) due to the formation of carbonyl species. Here, we investigated the aging of MEA-containing particles mediated by NO₃⁻ photolysis. Particles under different relative humidity (RH) and initial pH conditions were irradiated with 300 nm UV light. After reactions, the more acidic particles (MEA : H₂SO₄ : NaNO₃ : HNO₃ molar ratio = 4 : 1:1 : 3 and 4 : 0.75 : 1:3) show an increase in pH, while the 4 : 0.5 : 1:3 particles show a decrease in pH. We attributed these contrary pH changes to the combined results of HONO evaporation which increases the pH against MEA reactions which decreases the pH. NO₃⁻ and MEA decay rates are more sensitive to the initial pH than RH. Unlike the monotonically slow decay trends at all RH for the 4 : 0.5 : 1:3 particles, NO₃⁻ and MEA in more acidic 4 : 1:1 : 3 and 4 : 0.75 : 1:3 particles decay rapidly in the first few hours but followed by a slower decay. MEA reaction mechanisms in the presence of oxidants produced from NO₃⁻ photolysis were proposed by combining quantum chemistry computations and speciation of the products. Furthermore, water-soluble BrC and an organic phase were formed as potential secondary organic aerosols (SOAs). This study reveals the particulate sink of MEA and its potential in BrC and SOA formation mediated by NO₃⁻ photolysis in the atmosphere, which may give a new insight into the aging of amines in atmospheric aerosols.