Collection of Nitrogen Dioxide for Nitrogen and Oxygen Isotope Determination─Laboratory and Environmental Chamber Evaluation.

The family of atmospheric oxides of nitrogen, NOy (e.g., nitrogen oxides (NOx) + nitric acid (HNO3) + nitrous acid (HONO) + peroxyacetyl nitrate (PAN) + particulate nitrate (pNO3-) + other), have an influential role in atmospheric chemistry, climate, and the environment. The nitrogen (δ15N) and oxygen (δ18O and Δ17O) stable isotopes of NOy are novel tools for potentially tracking emission sources and quantifying oxidation chemistry. However, there is a lack of well-established methods, particularly for speciated gas-phase components of NOy, to accurately quantify δ15N, δ18O, and Δ17O. This work presents controlled laboratory experiments and complex chamber α-pinene/NOx oxidation experiments of a sampling apparatus constructed for the simultaneous capture of multiple NOy species for isotope analysis using a series of coated denuders, with a focus on nitrogen dioxide (NO2•). The laboratory tests indicate complete NO2• capture for the targeted concentration of 15 ppbv for at least 24 h collections at 10 liters per minute, with δ15N and δ18O precisions of ±1.3‰ and 1.0‰, respectively, and minimal (2.2% ± 0.1%) NO2• collection on upstream denuders utilized for the capture of HNO3 and other acidic gases. The multispecies NOy collection system showed excellent concentration correlations with online instrumentation for both HNO3 and NO2• and isotope reproducibility of ±1.7‰, ±1.8‰, and ±0.7‰ for δ15N, δ18O, and Δ17O, respectively, for replicate experiments and highly time-resolved collections. This work demonstrates a new method that can enable the simultaneous collection of HNO3 and NO2• for accurate quantification of concentration and isotopic composition.

Photolysis of Gas-Phase Atmospherically Relevant Monoterpene-Derived Organic Nitrates.

Organic nitrates (ONs) can impact spatial distribution of reactive nitrogen species and ozone formation in the atmosphere. While photolysis of ONs is known to result in the release of NO2 back to the atmosphere, the photolysis rate constants and mechanisms of monoterpene-derived ONs (MT-ONs) have not been well constrained. We investigated the gas-phase photolysis of three synthetic ONs derived from α-pinene, ß-pinene, and d-limonene through chamber experiments. The measured photolysis rate constants ranged from (0.55 ± 0.10) × 10-5 to (2.3 ± 0.80) × 10-5 s-1 under chamber black lights. When extrapolated to solar spectral photon flux at a solar zenith angle of 28.14° in summer, the photolysis rate constants were in the range of (4.1 ± 1.4) × 10-5 to (14 ± 6.7) × 10-5 s-1 (corresponding to lifetimes of 2.0 ± 0.96 to 6.8 ± 2.4 h) and (1.7 ± 0.60) × 10-5 to (8.3 ± 4.0) ×10-5 s-1 (3.3 ± 1.6 to 17 ± 6.0 h lifetimes) by using wavelength-dependent and average quantum yields, respectively. Photolysis mechanisms were proposed based on major products detected during photolysis. A zero-dimensional box model was further employed to simulate the photolysis of α-pinene-derived ON under ambient conditions. We found that more than 99% of α-pinene-derived ON can be converted to inorganic nitrogen within 12 h of irradiation and ozone was formed correspondingly. Together, these findings show that photolysis is an important atmospheric sink for MT-ONs and highlight their role in NOx recycling and ozone chemistry.

Non-linear effects of secondary organic aerosol formation and properties in multi-precursor systems.

Secondary organic aerosol (SOA) contributes significantly to ambient fine particulate matter that affects climate and human health. Monoterpenes represent an important class of biogenic volatile organic compounds (VOCs) and their oxidation by nitrate radicals poses a substantial source of SOA globally. Here, we investigate the formation and properties of SOA from nitrate radical oxidation of two common monoterpenes, α-pinene and limonene. When two monoterpenes are oxidized simultaneously, we observe a ~50% enhancement in the formation of SOA from α-pinene and a ~20% reduction in limonene SOA formation. The change in SOA yields is accompanied by pronounced changes in aerosol chemical composition and volatility. These non-linear effects are not observed in a sequential oxidation experiment. Our results highlight that unlike currently assumed in atmospheric models, the interaction of products formed from individual VOCs should be accounted for to accurately describe SOA formation and its climate and health impacts.