Elucidating the effect of HONO on O pollution by a case study in southwest China.

Photolysis of nitrous acid (HONO) is one of the major sources for atmospheric hydroxyl radicals (OH), playing significant role in initiating tropospheric photochemical reactions for ozone (O) production. However, scarce field investigations were conducted to elucidate this effect. In this study, a field campaign was conducted at a suburban site in southwest China. The whole observation was classified into three periods based on O levels and data coverage: the serious O pollution period (Aug 13-18 as P1), the O pollution period (Aug 22-28 as P2) and the clean period (Sep 3-12 as P3), with average O peak values of 96 ppb, 82 ppb and 44 ppb, respectively. There was no significant difference of the levels of O precursors (VOCs and NOx) between P1 and P2, and thus the evident elevation of OH peak values in P1 was suspected to be the most possible explanation for the higher O peak values. Considering the larger contribution of HONO photolysis to HO primary production than photolysis of HCHO, O and ozonolysis of Alkenes, sensitivity tests of HONO reduction on O production rate in P1 are conducted by a 0-dimension model. Reduced HONO concentration effectively slows the O production in the morning, and such effect correlates with the calculated production rate of OH radicals from HONO photolysis. Higher HONO level supplying for OH radical initiation in the early morning might be the main reason for the higher O peak values in P1, which explained the correlation (R = 0.51) between average O value during daytime (10:00-19:00 LT) and average HONO value during early morning (00:00-05:00 LT). For nighttime accumulation, a suitable range of relative humidity that favored NO conversion within P1 was assumed to be the reason for the higher HONO concentration in the following early morning which promoted O peak values.