Secondary organic aerosol formation from photooxidation of C3H6 under the presence of NH3: Effects of seed particles.

Frequently-occurred secondary organic aerosols (SOAs) under low-NOx conditions contribute to the winter haze episodes and remain unclear in the abundant presence of NH3. Here, the effects of CaCl2 seed particles on the photooxidation of low-molecular-weight C3H6 with co-existing NO2 and NH3 were highlighted and investigated through a chamber-simulation study equipped with high-resolution proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS). The influences of NH3 are often overestimated to exclusively enhance SOA yields under a low-[NO2]0 condition. Instead, the seeds played a central role in the heterogeneous formation of SOAs in this reaction with two orders of magnitudes higher than that in the absence of seeds at relative humidity (RH) of 82%. Interestedly, the O3 production was unchanged whether the seeds existed or not, small changes in the production of O3 were observed whether the seeds existed or not, indicating that the gas-phase conversions of C3H6 and NOx into C1-C3 oxygenated volatile organic compounds (OVOCs) and nitrogen-containing compounds (NOCs) were not affected by seed particles. Given that the ensuing formation of these low-volatile compounds was condensed into nucleation on the seeds, the explosive growth of C3H6 SOAs was then stimulated in the addition of NH3. Besides NO2 photolysis, the producing O3 was related to the formation of secondary carbonyls such as formaldehyde and then was consumed in the ·OH generation of approximately 3.40 × 10-12 molecules cm-3. This study provides a new insight to better understand the new gas-to-particle formation mechanisms when the haze pollution outbreaks in the complex air mixture.