Biomass burning is a major global source of atmospheric ammonia (NH3), significantly influencing air quality, aerosol formation, and nitrogen cycling. Nitrogen isotope composition (delta 15N) of NH3 has been proposed as a powerful tool for source apportionment, yet values for several emission sources remain poorly constrained. This study presents the first field-based delta 15N of total reduced inorganic nitrogen (NH x = NH3 + pNH4) measurements from fresh and aged biomass-burning plumes, collected during the Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) campaign in the western United States during summer 2019. The NH x concentrations were strongly correlated with carbon monoxide (CO) and fine particulate matter (PM2.5), reflecting elevated emissions during smoldering conditions. The delta 15N(NH x ) ranged from -9.1 parts per thousand to 2.1 parts per thousand (x +/- sigma: -3.3 +/- 2.9 parts per thousand; n = 16). Using a Keeling plot approach, we derived a representative biomass-burning delta 15N(NH3) value of -4.7 +/- 1.3 parts per thousand, that integrates measurements across the sampled biomass burning events, while also accounting for background NH x influences. This field-based isotopic signature is clearly distinct from agricultural and vehicular sources and substantially lower than the +12 parts per thousand value commonly assumed for biomass burning in delta 15N-based source apportionment studies. Overall, this work improves our ability to track NH3 emissions using novel isotopic constraints. Field-based nitrogen isotope measurements of ammonia emissions from biomass burning reveal distinct isotopic signatures, enabling improved source apportionment and nitrogen cycling insights.

NO2) initiates complex photochemical processes that produce organic nitrates (RONO2) and influence atmospheric oxidation capacity, air quality, and the fate of reactive nitrogen. However, tracking the chemical fate of RONO2 remains challenging as it includes pathways such as renoxification, aerosol partitioning, deposition, and/or hydrolysis to nitric acid (HNO3). Stable oxygen (Delta O-17, delta O-18) and nitrogen (delta N-15) isotope measurements can provide a unique tool to probe these processes, as NOy species can exhibit distinct isotopic signatures due to characteristic oxygen-transfer dynamics and isotope fractionation. Here, we present chamber experiments of alpha-pinene oxidation in the presence of NOx under a range of oxidant and photochemical conditions, reporting the Delta O-17, delta O-18, and delta N-15 values of simultaneously collected NO2, HNO3, and particulate nitrate (pNO(3)), the latter of which derived predominantly from RONO2 in the conducted experiments. A strong linear relationship between delta O-18 and Delta O-17 across all NOy species (r=0.992; p<0.01) supports a two-endmember mixing model, in which oxygen atoms are transferred from isotopically distinct sources that include ozone (O-3) with high delta O-18 and Delta O-17 as well as peroxy and hydroxyl radicals (RO2, HO2, OH) with lower values. Nitrogen isotope fractionation, quantified as the difference in delta N-15 values (Delta delta N-15), revealed consistently positive Delta delta N-15(HNO3-NO2) values (+28.9 +/- 13.4 parts per thousand in daytime experiments; +22.2 +/- 1.4 parts per thousand at night) and negative Delta delta N-15(pNO(3)-NO2) values (-13.6 +/- 5.8 parts per thousand in daytime experiments). This reflected distinct formation pathways and isotope effects including NOx photochemical cycling, thermal dinitrogen pentoxide (N2O5)-nitrate radical (NO3)-NO2 equilibrium, and HNO3 production mechanisms. Box-model simulations based on Delta O-17 values as a constraint were conducted using a newly developed gas-phase mechanism, which reproduced Delta O-17(NO2) and Delta O-17(pNO(3)) (compared to simulated Delta O-17(RONO2)) accurately, with an average model bias of 0.9 +/- 2.4 parts per thousand (R-2=0.98) and -1.4 +/- 2.4 parts per thousand (R-2= 0.55 and R-2= 0.97 when excluding one outlier), respectively. We further empirically derived important isotopic parameters such as the Delta O-17 value transferred from O-3 through comparison of model-simulated oxygen atom source contributions with observed Delta O-17 values for NO2 and pNO(3) across experiments. This yielded best-fit slopes of 39.4 +/- 0.6 parts per thousand for NOx photochemical cycling and 41.7 +/- 1.2 parts per thousand for RONO2 formation, consistent with near-surface observations of Delta O-17 in the terminal oxygen atom of O-3. Despite the agreement with NO2 and RONO2, accurately simulating Delta O-17(HNO3) proved challenging. Sensitivity tests revealed that model biases likely stemmed from a combination of factors including background HNO3 chamber blanks affecting low-NOx experiments, missing N2O5 heterogeneous hydrolysis under nighttime conditions, and an overestimation in the Delta O-17(HNO3) mass balance resulting from the NO2+ OH reaction, which was improved by adjusting the contribution from (2/3)Delta O-17(NO2) to (1/2)Delta O-17(NO2). These adjustments reduced the average model bias in Delta O-17(HNO3) from 6.7 +/- 3.3 parts per thousand (R-2= 0.39) in the base mechanism to 1.6 +/- 1.3 parts per thousand (R-2= 0.48) in the modified mechanism. These findings demonstrate the utility of Delta O-17 and delta N-15 for disentangling nitrate formation mechanisms, while also highlighting critical gaps in our understanding of the isotope dynamics involving HNO3 formation. Future experimental work targeting isolated HNO3 pathways is essential to refine isotopic mass balance assumptions and nitrogen isotope fractionation.

Abstract. The chemical interaction between nitrogen oxides (NOx = NO + NO2) and α-pinene plays a critical role in air quality and climate. However, uncertainties remain regarding their coupling in NOx loss, renoxification, and oxidation chemistry. To address these gaps, we conducted controlled chamber experiments, analyzing nitric acid (HNO3), NO2, and particulate nitrate (pNO3) for their oxygen and nitrogen stable isotope variations (Δ17O, δ18O, and δ15N). A strong linear relationship between δ18O and Δ17O across experiments revealed contributions of oxygen from ozone (O3) and atmospheric oxygen (O2) in forming reactive radicals. The δ15N values followed the order δ15N(pNO3) < NO2 < HNO3, reflecting isotope fractionation during NOx oxidation. A new chemical mechanism accurately predicted aerosol precursor decay and simulated Δ17O and δ15N values. Simulations showed NOx photochemical cycling and pNO3 formation, primarily from organic nitrate, with Δ17O(NO2) simulations achieving a root mean square error (RMSE) of 1.7 ‰. Improved δ15N(NO2) and pNO3 simulations used a nitrogen isotope fractionation factor (15α) of 0.997 for NO2 + OH reactions. However, modeling Δ17O and δ15N of HNO3 proved challenging, likely due to sampling artifacts. This study provides insights into Δ17O transfer dynamics, nitrogen isotope fractionation, and the role of NOx-BVOC chemistry in air quality, highlighting the potential of Δ17O and δ15N as tools for evaluating complex atmospheric processes.

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.

Atmospheric ammonia (NH3) is a critical component of our atmosphere that contributes to air quality degradation and reactive nitrogen deposition; however, our knowledge of NH3 in urban environments remains limited. Year-long ambient NH3 and related species were measured for concentrations and the nitrogen isotopic compositions (δ15N) of NH3 and particulate ammonium (pNH4+) were measured to understand the temporal sources and chemistry of NH3 in a northeastern US urban environment. We found that urban NH3 and pNH4+ concentrations were elevated compared to regional rural background monitoring stations, with seasonally significant variations. Local and transported sources of NHx (NH3+ pNH4+) were identified using polar bivariate and statistical back trajectory analysis, which suggested the importance of vehicles, volatilization, industry, and stationary fuel combustion emissions. Utilizing a uniquely positive δ15N(NH3) emission source signature from vehicles, a Bayesian stable isotope mixing model (SIMMR) indicates that vehicles contribute 46.8±3.5 % (mean ±1σ) to the annual background level of urban NHx, with a strong seasonal pattern with higher relative contribution during winter (56.4±7.6 %) compared to summer (34.1±5.5 %). The decrease in the relative importance of vehicle emissions during the summer was suggested to be driven by temperature-dependent NH3 emissions from volatilization sources, seasonal fuel-combustion emissions related to energy generation, and change in seasonal transport patterns based on wind direction, back trajectory, and NH3 emission inventory analysis. This work highlights that reducing vehicle NH3 emissions should be considered to improve wintertime air quality in this region.

Abstract. Nitrous acid (HONO) is an important precursor to hydroxyl radical(OH) that determines atmospheric oxidative capacity and thus impacts climateand air quality. Wildfire is not only a major direct source of HONO, it alsoresults in highly polluted conditions that favor the heterogeneous formation ofHONO from nitrogen oxides (NOx= NO + NO2) and nitrate on bothground and particle surfaces. However, these processes remain poorlyconstrained. To quantitatively constrain the HONO budget under variousfire and/or smoke conditions, we combine a unique dataset of field concentrationsand isotopic ratios (15N / 14N and 18O / 16O) of NOxand HONO with an isotopic box model. Here we report the first isotopicevidence of secondary HONO production in near-ground wildfire plumes (over asample integration time of hours) and the subsequent quantification of therelative importance of each pathway to total HONO production. Mostimportantly, our results reveal that nitrate photolysis plays a minor role(<5 %) in HONO formation in daytime aged smoke, whileNO2-to-HONO heterogeneous conversion contributes 85 %–95 % to totalHONO production, followed by OH + NO (5 %–15 %). At nighttime, heterogeneousreduction of NO2 catalyzed by redox active species (e.g., iron oxideand/or quinone) is essential (≥ 75 %) for HONO production in additionto surface NO2 hydrolysis. Additionally, the 18O / 16O of HONOis used for the first time to constrain the NO-to-NO2 oxidationbranching ratio between ozone and peroxy radicals. Our approach provides anew and critical way to mechanistically constrain atmospheric chemistry and/or airquality models on a diurnal timescale.

Abstract. Nitrous acid (HONO) is an important precursor to hydroxyl radical (OH) that determines atmospheric oxidative capacity and thus impacts climate and air quality. Wildfire is not only a major direct source of HONO, but it also results in highly polluted conditions that favour heterogeneous formation of HONO from nitrogen oxides (NOx = NO + NO2) and nitrate on both ground and particle surfaces. However, these processes remain poorly constrained. To quantitatively constrain the HONO budget under various fire/smoke conditions, we combine a unique dataset of field concentrations and isotopic ratios (15N/14N and 18O/16O) of NOx and HONO, with an isotopic box model. Here we report the first isotopic evidence of secondary HONO production in near-ground wildfire plumes, and the subsequent quantification of relative importance of each pathway to total HONO production. Most importantly, our results reveal that nitrate photolysis plays a minor role (< 5 %) in HONO formation in daytime aged smoke, while photo-enhanced NO2-to-HONO heterogeneous conversion contributes 85–95 % to total HONO production, followed by OH+NO (5–15 %). In nighttime, heterogeneous reduction of NO2 catalysed by redox active species (e.g., iron oxide and/or quinone) is essential (≥ 75 %) for HONO production in addition to surface NO2 hydrolysis. Additionally, the 18O/16O of HONO is used for the first time to constrain the NO-to-NO2 oxidation branching ratio between ozone and peroxy radicals. Our approach provides a new and critical way to mechanistically constrain atmospheric chemistry/air quality models.

Stable isotopic composition of atmospheric nitrate (nitric acid (HNO3) + particulate nitrate (pNO3-)) provides a higher-order dimensional analysis of critical atmospheric components, enabling a process-level understanding of precursor emissions, oxidation chemistry, aerosol acidity, and depositional patterns. Current methods have not been evaluated for their ability to accurately speciate and determine nitrogen (δ15N) and oxygen (δ18O and Δ17O) isotope compositions for gaseous and particle phases. Suitability of a denuder-filter sampling system for the collection of speciated HNO3(g) and pNO3- for off-line concentration and isotopic determination was tested using both laboratory and field collections. Honeycomb denuders coated with either NaCl or Na2CO3 solutions were used to collect HNO3(g). Laboratory experiments found that both coating solutions quantitatively collected HNO3(g), with the Na2CO3 solution demonstrating a higher operative capacity (>1470 μg of HNO3; n = 25) compared to the NaCl solution (∼750 μg of HNO3; n = 25). The precision values for laboratory-tested HNO3(g) collections are ±0.6‰ and ±1.2‰ for δ15N and δ18O for the NaCl solution and ± 0.8‰ and ±1.2‰ for the Na2CO3 solution. Replicate (urban) samples indicate that the Na2CO3 solution is significantly less selective for HNO3(g) collection than the NaCl solution. Nylon filters were found to collect efficiently and retain laboratory-generated NaNO3 and NH4NO3 particles, with maximum standard deviations for δ15N and δ18O of ±0.3‰ and ±0.3‰, respectively. Field replicates, while predictably more variable, also show consistency for δ15N and δ18O of ±0.6‰ and ±1.3‰ for particulate species, respectively. Recommended methods for field collections of speciated HNO3(g) and pNO3- for isotopic measurements would best utilize the NaCl solution and Nylon filters.

Nitrogen stable isotope analysis (δ15N) of particulate ammonium (NH4+) may provide additional constraints on this critical component of fine particulate matter; however, no previous collection method has been verified for its ability to accurately and precisely characterize δ15N(NH4+). This is a critical point due to the difficulty of quantitative NH4+ collection and possible sampling artifacts. Here, we report on δ15N(NH4+) precision using an established denuder-filter pack combination with two filter configurations including (1) a nylon filter plus an acid-impregnated cellulose filter and (2) an acid-impregnated glass fiber filter for NH4+ collection in both laboratory-controlled environments and ambient air samples. Laboratory NH4+ were generated from the nebulization of ammonium salt solutions and collected using a filter pack sampling train for off-line concentration and isotopic measurement. Quantitative collection of NH4+ was achieved using both filter configurations in both laboratory and field collections. Laboratory experiments indicate a δ15N(NH4+) precision of ±0.9‰ (1σ; n = 24) and ±0.6‰ ( n = 9) for the nylon plus citric acid impregnated cellulose filter and for the citric acid impregnated glass fiber filter, respectively. Field sample reproducibility was assessed from 24 h collected side-by-side samples and indicated δ15N(NH4+) to be reproducible within 1.1‰, consistent with the laboratory findings. This work represents the first established method for speciated NH4+ collection for isotopic analysis with important implications for furthering our understanding of its atmospheric dynamics.