Understanding Nitrous Acid (HONO) in the Urban Boundary Layer Using Continuous HONO Measurements at a 450 m Tall Tower in Guangzhou, China.

Nitrous acid (HONO) is a key precursor of hydroxyl radicals (OH) in the urban atmospheric boundary layer. However, most HONO observations so far are on the ground level, while HONO chemistry at higher altitude remains largely unknown. Through one-month observations at a 450 m platform of Canton Tower in Guangzhou, China, we have identified two distinct regimes of nocturnal HONO chemistry. One is dominated by heterogeneous reactions on the ground surface, likely corresponding to the period when the platform was within the stable nocturnal boundary layer. Another regime, occurring in the residual layer, is dominated by in situ formation from oxidation of nitric oxide (NO) by OH. During the daytime, HONO from emissions and heterogeneous sources at the ground undergoes ∼60% loss through photolysis before reaching 450 m. A detailed HONO budget analysis considering chemistry and vertical transport suggests that on average 32% of the observed HONO at 450 m is from OH oxidation of NO, while there remains 51% unidentified. These findings emphasize the increased contribution of NO + OH to the overall HONO budget throughout the urban boundary layer, in contrast to the diminished role of ground-related processes, and warrant future continuous measurements at high altitudes to supplement data at the ground to develop a complete understanding of HONO chemistry in the urban boundary layer.