Chemical Structure Regulates the Formation of Secondary Organic Aerosol and Brown Carbon in Nitrate Radical Oxidation of Pyrroles and Methylpyrroles.

Nitrogen-containing heterocyclic volatile organic compounds (VOCs) are important components of wildfire emissions that are readily reactive toward nitrate radicals (NO) during nighttime, but the oxidation mechanism and the potential formation of secondary organic aerosol (SOA) and brown carbon (BrC) are unclear. Here, NO oxidation of three nitrogen-containing 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-initiated hydrogen abstraction from the 1-position in pyrrole and 2-MP followed by radical shift and NO addition leads to light-absorbing nitroaromatic products. In the absence of a 1-position hydrogen, NO 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.