Photolysis of 1-nitropyrene: HONO and NO formations, optical property changes and mechanism insights

The emissions of HONO and NOx (NOx = NO + NO2) from 1-nitropyrene (1NPy) photolysis were investigated using a cylindrical reactor under different environmental conditions, such as total irradiance, relative humidity (RH), temperature, and O2 content. The emission flux densities of HONO (FHONO) and NO (FNO) had a positive correlation with the total irradiance (0–7.32 × 1016 photons cm−2 s−1). FHONO exhibited its maximum value at RH of 55% and temperature of 298 K. FNO decreased with increasing RH (7%–80%), whereas it linearly increased with the temperature (278–318 K). O2 participated in the production of HONO, whereas NO formation was independent of the O2 content. During the photolysis, changes in the composition of 1NPy occurred, including the consumption of nitro groups and the formation of aromatic C=O groups and phenolic hydroxyls. The 1NPy photolysis led to the modification of optical properties, as shown by a decrease at 300–320 and 355–440 nm and an increase at 320–355 and 440–500 nm for the mass absorption efficiency (MAEλ). The 1NPy photolysis frequency for HONO (J1NPy→HONO) and NO (J1NPy→NO) was calculated to be (1.6 ± 0.3) × 10−7 and (2.2 ± 0.1) × 10−6 s−1, respectively. Based on the experimental results, the HONO source strength from the nitroaromatic compounds (NACs) photolysis was estimated to be lower than 0.11 ppt h−1, suggesting that the photolysis of NACs in aerosols may not well explain daytime HONO sources.