Trace Methylamine Drives Efficient Photochemical HONO Formation on Aged 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.