Nitrous acid (HONO) is a major precursor of tropospheric hydroxyl radical (OH) that accelerates the formation of secondary pollutants. The HONO sources, however, are not well understood, especially in polluted areas. Based on a comprehensive winter field campaign conducted at a rural site of the North China Plain, a box model (MCM v3.3.1) was used to simulate the daytime HONO budget and nitrate formation. We found that HONO photolysis acted as the dominant source for primary OH with a contribution of more than 92%. The observed daytime HONO could be well explained by the known sources in the model. The heterogeneous conversion of NO2 on ground surfaces and the homogeneous reaction of NO with OH were the dominant HONO sources with contributions of more than 36% and 34% to daytime HONO, respectively. The contribution from the photolysis of particle nitrate and the reactions of NO2 on aerosol surfaces were found to be negligible in clean periods (2%) and slightly higher during polluted periods (8%). The relatively high OH levels due to fast HONO photolysis at the rural site remarkably accelerated gas-phase reactions, resulting in the fast formation of nitrate as well as other secondary pollutants in the daytime.

The pulsed laser photolysis-laser induced fluorescence (PLP-LIF) technique has been used to measure the rate constant for the gas phase reaction of OH radicals with pyruvic acid (CH 3 C(O)C(O)OH) in the temperature range 273-371 K. The data obtained were found to be well represented by the conventional Arrhenius expression: k PA = (4.9 ± 1.9) × 10 −14 exp[(276 ± 123)/T ] cm 3 molecule −1 s −1 , and at 298 K, k PA = (1.2 ± 0.4) × 10 −13 cm 3 molecule −1 s −1 . In addition, the UV-Vis absorption cross-sections of pyruvic acid were measured in the wavelength range 290-380 nm at 298 K; the obtained values are compared with literature ones. Using the PLP-LIF system, the photolysis of pyruvic acid was found to yield OH radicals with a quantum yield of 5 ± 3% at 355 nm. Evidence was also found for the OH production from the photolysis of pyruvic acid using a set of black lamps as the light source. The data obtained confirm that the photolysis is the major gas phase atmospheric loss process of pyruvic acid.