Concurrent dominant pathways of multifunctional products formed from nocturnal isoprene oxidation

Determination of dominant chemical pathways toward the formation of nocturnal secondary organic aerosols (SOA) remains ambiguous by which nitrogen oxides (NO_x) always affect oxidation of volatile alkenes. Here, comprehensive chamber simulations on dark isoprene ozonolysis were conducted under different nitrogen dioxides (NO₂) mixing ratios to exam multiple functionalized isoprene oxidation products. Aside from that the oxidation processes were concurrently driven by nitrogen radical (NO₃) and small hydroxyl radicals (OH), ozone (O₃) cycloaddition at isoprene was launched initially regardless of NO₂ to rapidly form first-generation oxidation products, i.e., carbonyls and Criegee intermediates (CI) referred to carbonyl oxides. They could further undergo complicated self- and cross-reactions to produce alkylperoxy radicals (RO₂). Corresponding to yields of the C₅H₁₀O₃ tracer, weak OH pathway at night was credited to ozonolysis of isoprene but suppressed by unique NO₃ chemistry. Following the ozonolysis of isoprene, NO₃ played a crucial supplementary role in nighttime SOA formation. The ensuing production of gas-phase nitrooxy carbonyls (the first-generation nitrates) became dominant in the production of a sizeable pool of organic nitrates (RO₂NO₂). By contrast, isoprene dihydroxy dinitrates (C₅H₁₀N₂O₈) were outstanding with the elevated NO₂, related to typical second-generation nitrates. As such, the yielding number concentrations of dark SOA were promoted to approximately 1.8 × 10⁴ cm^-3 but presented a nonlinear relation with excess high-NO₂ condition. This study provides valuable insights into importance of multifunctional organic compounds from alkene oxidation to constitute nighttime SOA.

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-NO_x 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 NO_x 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.