Nitrogen-containing heterocyclic volatile organic compounds (VOCs) are important components of wildfire emissions that are readily reactive toward nitrate radicals (NO 3 ) during nighttime, but the oxidation mechanism and the potential formation of secondary organic aerosol (SOA) and brown carbon (BrC) are unclear. Here, NO 3 oxidation of three nitrogencontaining heterocyclic VOCs, pyrrole, 1-methylyrrole (1-MP), and 2-methylpyrrole (2-MP), was investigated in chamber experiments to determine the effect of precursor structures on SOA and BrC formation. The SOA chemical compositions and the optical properties were analyzed using a suite of online and offline instrumentation. Dinitro-and trinitro-products were found to be the dominant SOA constituents from pyrrole and 2-MP, but not observed from 1-MP. Furthermore, the SOA from 2-MP and pyrrole showed strong light absorption, while that from 1-MP were mostly scattering. From these results, we propose that NO 3initiated hydrogen abstraction from the 1-position in pyrrole and 2-MP followed by radical shift and NO 2 addition leads to lightabsorbing nitroaromatic products. In the absence of a 1-position hydrogen, NO 3 addition likely dominates the 1-MP chemistry. We also estimate that the total SOA mass and light absorption from pyrrole and 2-MP are comparable to those from phenolic VOCs and toluene in biomass burning, underscoring the importance of considering nighttime oxidation of pyrrole and methylpyrroles in air quality and climate models.
Atmospheric organic aerosols (OA)
are complex mixtures of organic
molecules that are usually highly functionalized through various oxidative
processes. Understanding the volatilities and chemical compositions of OA is key to elucidating their
environmental impacts. Thermal desorption coupling to mass spectrometry
has been used as the main approach to examine both aspects of OA.
In this work, we investigated the thermal desorption-induced chemical
compositional change of OA from heterogeneous oxidation of glutaric
acid and α-pinene ozonolysis. Using an ion mobility spectrometry
mass spectrometer, coupled with total peroxide analysis and a mass
transfer evaporation model, we determined diverse reactions in the
particle phase during rapid heating under moderate desorption temperatures
(less than 100 °C). These reactions include irreversible oligomer
(e.g., esters and organic peroxides) decomposition into monomers and
new oligomer formation from decarboxylation, CO elimination, decarbonylation,
and dehydration. These chemical processes may effectively modify the
volatility and chemical characteristics of the residual OA particles.
Further, the monomeric products from thermal desorption could interfere
with quantification of the original constituents without isomer separation.
These findings could help reconcile the previously observed inconsistency
of OA evaporation kinetics versus volatility distribution. Further,
the results from this study could help interpret and constrain thermal
desorption-based measurements of OA volatility and compositions.
Organic aerosols (OA) in the atmosphere are composed of molecules with highly diverse chemical structures and functionalities. The OH-initiated heterogeneous aging of these organic molecules could occur in multiple generations, exponentially increasing the complexity of the oxidation products. Furthermore, the detailed reaction mechanisms are likely different and site-specific as the OA's chemical structures vary. To systematically study these mechanisms, in this work, heterogeneous OH oxidation of five surrogate diacid OA with different positions and numbers of branched methyl groups was examined in a flow tube reactor. The oxidation products were characterized by an ion mobility mass spectrometer, which could resolve isomers and unambiguously identify individual products. Through detailed analysis of the molecular compositions of the oxidized OA, we suggest that the site-specific oxidation mechanisms largely modulate the oxidation products from the functionalization, fragmentation, and oligomerization pathways. Depending on the molecular structures and adjacent functional group(s), oxidation on primary carbons could play a key role, while that on secondary and tertiary carbons could be less important if hindered by the mesomeric effect. Our results also suggest that fragmentation products are most likely formed from later-generation reactions, while functionalization occurs only at certain carbon sites in later generations. Finally, oligomers are also largely affected by the molecular structures of the parent OA, evidenced by the distinct carbon oxidation state patterns and distributions across carbon numbers. This work provides new insights into the detailed reaction mechanisms from heterogeneous OH oxidation for diverse organic molecules.
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