Trace Methylamine Drives Efficient Photochemical HONO Formation on Aged Urban Grime

The aging process reduces the photochemical reactivity of urban grime. Here, we show that two-month accumulated urban grime from Guangzhou sustains highly efficient photochemical conversion of NO2 to nitrous acid (HONO) in the presence of trace methylamine (MA). Under UV-A irradiation and 80% relative humidity, MA addition increased HONO yields up to (91.25 ± 2.35)% at 40 ppb MA, while 50 ppb MA generated a maximum measured surface flux of 2.5 × 1010 molecules cm–2 s–1, of the same order of magnitude as values reported for NH3-promoted urban grime under different experimental conditions. Using density functional theory, we reveal that MA acts as a hydrogen carrier, lowering the activation barrier for HONO formation from 1.12 to 1.01 eV under photoexcitation and, critically, reduces the barrier for regenerating photoactive pyrene-like sites from 2.97 eV without MA to only 0.17 eV in the presence of MA. While MA substantially promotes HONO formation under laboratory conditions, its specific ambient contribution remains unquantified. Aged urban grime stays photochemically active with MA, marking it a persistent daytime HONO source. This MA-promoted pathway should be included in urban air quality models to quantitatively assess its contributions to urban HONO and OH radical budgets.