Wendell W. Walters

The outbreak of COVID-19 and subsequent lockdowns caused widespread restrictions on human activities. This presented a unique opportunity to track atmospheric chemistry changes under rapid emission reductions. Using stable nitrogen (δ15N) and triple oxygen isotopes (δ18O and Δ17O) of particulate nitrate (pNO3) collected from 2019 to 2020 in the northeastern US, combined with 0-D box-model simulations, we investigate the response of atmospheric nitrate chemistry during the COVID-19 lockdown period (March 2020) in the northeastern US. Triple oxygen isotope measurements indicate that emission reductions altered the relative contributions of NOx oxidation pathways, with a shift toward the NO2 + OH pathway and a reduced contribution from N2O5 hydrolysis. This shift was consistent with the modeled increase in OH concentrations and the observed and modeled decrease in O3 during the COVID-19 lockdown, which limited N2O5 formation and subsequent heterogeneous nitrate formation. Moreover, nitrogen isotope-constrained sensitivity analysis suggests that a reduction of approximately 40(±10)% in transportation NOx emissions relative to the annual emission inventory estimates may be required to reproduce the observed decrease in δ15N(pNO3) during the COVID-19 period. These findings highlight the utility of isotopes to detail the complex interactions between emission reductions, atmospheric oxidation chemistry, and secondary aerosol formation. Understanding these interactions is critical for effectively determining air pollution mitigation strategies and testing air-quality models, particularly under rapidly evolving emission conditions associated with energy transitions and policy interventions.

Ammonia (NH3) is the main alkaline gas in the atmosphere and a key regulator of aerosol formation, nitrogen deposition, and air quality. Yet the relationship between NH3 emissions and deposited ammonium (NH4+) in remote archives, remains poorly constrained. Here, we present the first ice-core record of δ15N(NH4+), covering 1000 years of European atmosphere. Ice-core δ15N(NH4+) is systematically enriched relative to inventory-based δ15N(NH3), consistent with atmospheric isotopic fractionation between emitted NH3 and deposited NH4+. The 10-year seasonally resolved record shows lower δ15N(NH4+) in winter than in summer, which is consistent with NH3–NH4+ partitioning integrated during transport, although no unique pathway can be identified as other mechanisms remain insufficiently constrained. Between 1900 and 1970, lower δ15N(NH4+) broadly coincides with higher acidity and lower neutralisation factor, consistent with increasing proportion of NH4+ through NH3–NH4+ partitioning, although cloud processing and other atmospheric processes cannot be separated. After 1980, this relationship breaks down, with a divergence between ice-core NH4+ and NH3 emission inventories, the isotope–concentration relationship disappears, and a decrease in apparent isotopic fractionation. Boundary layer observations suggest that a large underestimation of NH3 emissions is unlikely. This shift therefore indicates a change in the source-to-deposition atmospheric processes, although the responsible pathway remains unresolved. Ice-core δ15N(NH4+) therefore provides a long-term view of the source-to-deposition regime shifts, a perspective increasingly needed as Europe moves toward a less acidic, more NH3-rich atmosphere.

Reactive nitrogen oxides (NO_x = NO + NO₂) are critical drivers of atmospheric chemistry, with far-reaching consequences for air quality, climate, and ecosystem health. Stable nitrogen isotopes provide a useful constraint for NO_x source attribution, but source-derived isotopic signals can be substantially modified by atmospheric processes after emission. In this study, we used a previously developed ¹⁵N-enabled NO_x emission dataset based on the 2008 Multi-resolution Emission Inventory for China (MEIC) as input to the Community Multiscale Air Quality modeling system (CMAQ) to simulate the spatiotemporal variation in δ¹⁵N(NO_x) over South China, with a focus on the Pearl River Delta region. In the simulations, ¹⁴NO_x and ¹⁵NO_x were implemented as nonreactive tracers under three scenarios to isolate the effects of atmospheric physical processes, without explicitly simulating NO_x oxidation chemistry, isotope fractionation, or nitrate formation. Under the “emission + transport + default deposition”, simulated δ¹⁵N(NO_x) values generally ranged from approximately −2‰ to +6‰, with most areas showing values between +2‰ and +4‰. Compared with the corresponding “emission-only” baseline, atmospheric transport, mixing, and deposition tended to increase δ¹⁵N(NO_x), especially in rural and low-emission regions, by redistributing anthropogenic NO_x signals. Default deposition had only a minor effect on δ¹⁵N(NO_x), whereas enhanced deposition, used to approximate effective near-source removal, produced spatially heterogeneous but mostly modest changes. Simulated δ¹⁵N(NO_x) values were both systematically lower than measured δ¹⁵N(NO₃⁻) at the rural Dinghushan and urban Guangzhou sites under all scenarios. This discrepancy indicates that atmospheric physical processes alone cannot fully explain observed nitrate isotope variability, and that isotope fractionation during NO_x oxidation, source-endmember uncertainty, and emission inventory biases likely need to be considered. This study serves as a proof of concept and a necessary step toward fully ¹⁵N-enabled CMAQ simulations for evaluating atmospheric reactive nitrogen sources and improving NO_x emission inventories.

The successful implementation of the Clean Air Act has reduced anthropogenic nitrogen oxide (NO_x) emissions, significantly reducing acid rain and improving air quality in the U.S. However, particulate nitrate (pNO₃) concentrations have remained elevated, posing persistent challenges to link NO_x emission sources with pNO₃ formation, which limits the ability to evaluate the effectiveness of emission controls for particulate end points. Measured nitrogen isotopes (δ¹⁵N) in pNO₃ from a 2005–2015 aerosol record in the northeastern U.S. provide direct empirical confirmation of the effectiveness of emission regulations in reducing coal-derived NO_x, concurrent with an increasing influence from oil and natural gas combustion. These findings have broad implications for improving our understanding of how changes in NO_x emission sources influence pNO₃ formation and evaluating the success of past clean air policies.

Nitrogen oxides (NO_x), comprising nitric oxide (NO) and nitrogen dioxide (NO₂), are key precursors of atmospheric nitrate, a major component of fine particulate matter (PM_2.5) that critically affects air quality, human health, and ecosystems. Emission inventories provide detailed spatial and temporal information on NO_x sources, while stable isotope techniques offer an additional constraint for source apportionment. Here, we incorporated stable nitrogen isotopes (¹⁴N, ¹⁵N) into the widely used Multi-resolution Emission Inventory for China (MEIC) over South China, with a focus on the Pearl River Delta (PRD) region, one of the most highly urbanized and industrialized regions in China, using an isotopic mass–balance model. The 2008 MEIC inventory indicated that NO_x emissions across South China were spatially heterogeneous, dominated by transportation sources, and concentrated mainly in the PRD and other urban clusters. We then compared the simulated isotopic composition of emitted NO_x with atmospheric measurements to assess the role of emission sources in controlling atmospheric nitrate (NO₃⁻). The simulated δ¹⁵N(NO_x) values were found to generally underestimate the observed δ¹⁵N(NO₃⁻) values. This discrepancy highlights the need for future ¹⁵N-enabled air quality modeling to better represent both source contributions and atmospheric processing, thereby improving source apportionment, emission inventory evaluation, and our understanding of reactive nitrogen cycling.

Transformation in the vegetation structure influences atmospheric chemistry by modulating the flux and composition of reactive boundary layer gases, yet quantifying its direct impact on oxidation processes remains challenging due to pervasive anthropogenic interference. Here, we present an unprecedented data set of year-round isotopic observations of atmospheric nitrate (δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-, and Δ¹⁷O-NO₃^-) from 12 minimally disturbed sites across the remote Tibetan Plateau (TP), allowing us to isolate vegetation-driven effects on atmospheric oxidant cycling. We observed consistently lower δ¹⁵N-NO₃^-, Δ¹⁷O-NO₃^-, and δ¹⁸O-NO₃^- values in forested regions, with progressively higher values across shrubland and grassland. Isotopic constraints reveal enhanced soil nitrogen oxide (NO_x) emissions in forests, while oxygen isotope signals point to a shift toward biogenic volatile organic compound (VOC)-driven NO_x oxidation via peroxy radicals (RO₂/HO₂). This highlights vegetation-specific modulation of atmospheric oxidant regimes. With projected warming and wetting expected to drive forest expansion across the TP, our findings suggest a consequential shift in atmospheric oxidant levels and NO_x cycling. We provide direct isotopic evidence that vegetation distribution─not merely biomass extent─systematically structures the chemical pathways of NO_x oxidation in remote regions. This vegetation-oxidant coupling highlights a key, underappreciated biogeochemical feedback within high-altitude ecosystems undergoing rapid climate change.

Human activities have increased atmospheric reactive nitrogen (N) deposition, with important consequences for ecosystem biogeochemical cycles. In forest ecosystems, tree canopies act as active filters that intercept, transform and redistribute atmospheric N before it reaches to the soil. These canopy-level processes determine the chemical forms of N that become available for biological uptake or are transferred to the soil. Despite their importance, the temporal variability of these processes remains poorly understood, particularly in Mediterranean ecosystems with pronounced seasonal contrasts.In this study, we investigated the seasonal dynamics of atmospheric N transformations within the canopy of a Mediterranean Holm oak forest (Quercus ilex / Q. rotundifolia) in Spain. We focused on canopy-scale nitrification processes and the abundance of N-fixing and nitrifying microorganisms associated with the phyllosphere and precipitation. Canopy N fluxes were quantified, nitrate sources were identified using Δ¹⁷O isotopic signatures, and microbial abundances were quantified by qPCR, to explore seasonal dynamics and their environmental drivers.Our results show that the canopy acted as a net sink for atmospheric N throughout the year, indicating that N inputs did not exceed ecosystem demand and suggesting potential N limitation. The nitrate measured in throughfall samples indicated a predominantly atmospheric origin during most of the year (76–92%). In contrast, during summer up to 76% of the nitrate was derived from in situ biological processes at canopy level. These enhanced biological transformations were correlated with weather conditions, particularly the higher temperatures and dry conditions typical of summer, which may favour nitrification and promote the accumulation of N compounds on leaf surfaces. Reduced plant activity and lower N uptake during summer further prolong N residence time within the canopy, increasing the likelihood of microbial transformations. Both archaeal and bacterial nitrifiers, as well as N-fixing microorganisms, were detected year-round in the phyllosphere and precipitation. Archaeal nitrifiers consistently outnumbered bacterial ones, and showed a marked increase during summer, driven by higher radiation, temperature and lower humidity. This pattern suggests that archaea may play a significant important role in nitrification, coinciding with the highest nitrification rates observed in summer. These findings highlight the crucial function of canopy processes in regulating N fluxes in Mediterranean forests, particularly during summer. The seasonal dynamics of biological transformations and microbial communities emphasise the influence of environmental conditions on N cycling.

The successful implementation of the US Clean Air Act has reduced nitrogen oxides (NO_x) emissions, with important improvements in air quality and acid deposition. However, predicting the impact of emission reductions on tropospheric chemistry remains challenging, as NO_x influences oxidant concentrations and aerosol formation. Stable oxygen isotopes (Δ¹⁷O and δ¹⁸O) of atmospheric nitrate (pNO₃) serve as powerful observational constraints for the understanding of oxidation processes. Here, we quantified Δ¹⁷O and δ¹⁸O in pNO₃ from a unique 10-year aerosol record in the northeastern US to constrain chemistry changes over a time of large reductions in NO_x (2005-2015). We observed a significant decreasing trend in δ¹⁸O(pNO₃) (-0.32‰yr^-1), which reflects a shift in oxidation chemistry driven by emission reductions. Air quality modeling indicated that the decrease in δ¹⁸O(pNO₃) between 2005 and 2015 was primarily driven by enhanced nighttime NO oxidation by ozone, with a relative reduction in N₂O₅ hydrolysis and increased NO₂ + OH, leading to more efficient nitrate formation in the recent past. This explains the continued elevated wintertime nitrate that plagues air quality in the eastern US despite significant NO_x decreases. The isotopic constraint on oxidizing chemistry greatly enhances our ability to test and predict the effectiveness of emissions reductions on oxidation chemistry.