Relative Humidity Modulates the Physicochemical Processing of Secondary Brown Carbon Formation from Nighttime Oxidation of Furan and Pyrrole.

Light-absorbing secondary organic aerosols (SOAs), also known as secondary brown carbon (BrC), are major components of wildfire smoke that can have a significant impact on the climate system; however, how environmental factors such as relative humidity (RH) influence their formation is not fully understood, especially for heterocyclic precursors. We conducted chamber experiments to investigate secondary BrC formation from the nighttime oxidation of furan and pyrrole, two primary heterocyclic precursors in wildfires, in the presence of pre-existing particles at RH < 20% and ∼ 50%. Our findings revealed that increasing RH significantly affected the size distribution dynamics of both SOAs, with pyrrole SOA showing a stronger potential to generate ultrafine particles via intensive nucleation processes. Higher RH led to increased mass fractions of oxygenated compounds in both SOAs, suggesting enhanced gas-phase and/or multiphase oxidation under humid conditions. Moreover, higher RH reduced the mass absorption coefficients of both BrC, contrasting with those from homocyclic precursors, due to the formation of non-absorbing high-molecular-weight oxygenated compounds and the decreasing mass fractions of molecular chromophores. Overall, our findings demonstrate the unique RH dependence of secondary BrC formation from heterocyclic precursors, which may critically modulate the radiative effects of wildfire smoke on climate change.

Contribution of Carbonyl Chromophores in Secondary Brown Carbon from Nighttime Oxidation of Unsaturated Heterocyclic Volatile Organic Compounds.

The light absorption properties of brown carbon (BrC), which are linked to molecular chromophores, may play a significant role in the Earth's energy budget. While nitroaromatic compounds have been identified as strong chromophores in wildfire-driven BrC, other types of chromophores remain to be investigated. Given the electron-withdrawing nature of carbonyls ubiquitous in the atmosphere, we characterized carbonyl chromophores in BrC samples from the nighttime oxidation of furan and pyrrole derivatives, which are important but understudied precursors of secondary organic aerosols primarily found in wildfire emissions. Various carbonyl chromophores were characterized and quantified in BrC samples, and their ultraviolet-visible spectra were simulated by using time-dependent density functional theory. Our findings suggest that chromophores with carbonyls bonded to nitrogen (i.e., imides and amides) derived from N-containing heterocyclic precursors substantially contribute to BrC light absorption. The quantified N-containing carbonyl chromophores contributed to over 40% of the total light absorption at wavelengths below 350 nm and above 430 nm in pyrrole BrC. The contributions of chromophores to total light absorption differed significantly by wavelength, highlighting their divergent importance in different wavelength ranges. Overall, our findings highlight the significance of carbonyl chromophores in secondary BrC and underscore the need for further investigation.

Remediation of Thirdhand Tobacco Smoke with Ozone: Probing Deep Reservoirs in Carpets.

We assessed the efficacy of ozonation as an indoor remediation strategy by evaluating how a carpet serves as a sink and long-term source of thirdhand tobacco smoke (THS) while protecting contaminants absorbed in deep reservoirs by scavenging ozone. Specimens from unused carpet that was exposed to smoke in the lab ("fresh THS") and contaminated carpets retrieved from smokers' homes ("aged THS") were treated with 1000 ppb ozone in bench-scale tests. Nicotine was partially removed from fresh THS specimens by volatilization and oxidation, but it was not significantly eliminated from aged THS samples. By contrast, most of the 24 polycyclic aromatic hydrocarbons detected in both samples were partially removed by ozone. One of the home-aged carpets was installed in an 18 m3 room-sized chamber, where its nicotine emission rate was 950 ng day-1 m-2. In a typical home, such daily emissions could amount to a non-negligible fraction of the nicotine released by smoking one cigarette. The operation of a commercial ozone generator for a total duration of 156 min, reaching concentrations up to 10,000 ppb, did not significantly reduce the carpet nicotine loading (26-122 mg m-2). Ozone reacted primarily with carpet fibers, rather than with THS, leading to short-term emissions of aldehydes and aerosol particles. Hence, by being absorbed deeply into carpet fibers, THS constituents can be partially shielded from ozonation.

Peroxy Radical Autoxidation and Sequential Oxidation in Organic Nitrate Formation during Limonene Nighttime Oxidation.

Limonene is an abundant monoterpene released into the atmosphere via biogenic emissions and biomass burning. However, the atmospheric oxidation and secondary organic aerosol (SOA) formation mechanisms of limonene, especially during nighttime, remain largely understudied. In this work, limonene was oxidized synergistically by ozone (O3) and nitrate radicals (NO3) in a flow tube reactor and a continuous flow stirred tank reactor. Upon oxidation, many highly oxidized organic nitrates and nitrooxy peroxy radicals (RO2) were observed in the gas phase within 1 min. Combining quantum chemical calculations with kinetic simulations, we found that the primary nitrooxy RO2 (C10H16NO5) through NO3 addition at the more substituted endocyclic double bond and at the exocyclic double bond (previously considered as minor pathways) can undergo autoxidation with rate constants of around 0.02 and 20 s-1 at 298 K, respectively. These pathways could explain a major portion of the observed highly oxidized organic nitrates. In the SOA, highly oxidized mono- and dinitrates (e.g., C10H17NO7-8 and C10H16,18N2O8-10) make up a significant contribution, highlighting nitrooxy RO2 autoxidation and sequential NO3 oxidation of limonene. The same organic nitrates are also observed in ambient aerosol during biomass burning and nighttime in the southeastern United States. Therefore, the present work provides new insights into the nighttime oxidation of limonene and SOA formation in the atmosphere.

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 (NO3) during nighttime, but the oxidation mechanism and the potential formation of secondary organic aerosol (SOA) and brown carbon (BrC) are unclear. Here, NO3 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 NO3-initiated hydrogen abstraction from the 1-position in pyrrole and 2-MP followed by radical shift and NO2 addition leads to light-absorbing nitroaromatic products. In the absence of a 1-position hydrogen, NO3 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.