Large Contribution of Nitrous Acid to Soil-Emitted Reactive Oxidized Nitrogen and Its Effect on Air Quality

The potentials of uncertainty analysis and Bayesian optimization in HONO source modeling diagnosis and improvement

Nitrous acid (HONO) plays a critical role in atmospheric chemistry, significantly influencing hydroxyl radical (OH) production and the formation of secondary pollutants. However, current atmospheric chemical transport models (CTMs) still underestimate HONO formation, due to uncertainties in source parameterizations. This study proposed a new framework that combines uncertainty analysis with Bayesian optimization (RFM-BMC) to diagnose and reduce uncertainties in HONO source parameterizations, using the North China Plain (NCP) as a case study. The results show that uncertainties in source parameterizations cause HONO simulation concentrations varying by 8-20 times the baseline values. The primary contributors to uncertainties in HONO simulations include heterogeneous reactions on aerosol (33-59 %) and ground surfaces (18-30 %), vehicle emissions (12-33 %), and nitrate photolysis (26-30 %). By optimizing these parameters using observational data, the accuracy of HONO simulations significantly improves, reducing the normalized mean bias by 59 %. Additionally, this study identifies soil emissions, light-induced NO2 heterogeneous reactions and underestimated nitrate as important underrepresented HONO sources in CTMs. These sources contribute to the systematic underestimation of HONO concentrations during midday (08:00-14:00). This work provides valuable insights for refining HONO source parameterizations and improving air quality simulations. Furthermore, the RFM-BMC framework can be applied to optimize parameterizations of other atmospheric chemical processes, such as sulfate and secondary organic aerosol formation.

Evaluating the contribution of residential coal combustion to atmospheric nitrous acid (HONO) in winter

Nitrous acid (HONO) is an important precursor of hydroxy radicals (OH) that determine atmospheric oxidation capacity and thus influence air quality and climate. Like vehicle exhausts and biomass burning, coal combustion is also recognized as a potential direct source of HONO. However, the quantification of HONO emissions from coal combustion is currently limited. Here, by comparing HONO levels and sources at a rural site in the North China Plain during the winters of 2017 and 2023, we found the large reduction in HONO concentrations during wintertime of 2023 compared to 2017, which was mainly attributed to the implementation of the "coal-to-gas" policy in this area. Based on the nocturnal HONO budget calculations between the two years, the HONO/NOx ratio from coal combustion was reasonably deduced to be about 18.8 %, which was 1-3 orders of magnitude higher than that from vehicle emissions. When considering HONO direct emission from coal combustion, the box model could well reproduce the observed HONO levels in the two years, with the normalized mean bias (NMB) of -11.4 % and 4.8 %, respectively. Although coal combustion only played an important role in nocturnal HONO production in 2017, the emitted HONO at nighttime could be kept to contribute to daytime OH as soon as the sun rises, in turn leading to a considerable contribution of the homogeneous reaction of NO + OH to atmospheric HONO at daytime. Our findings highlight the efficacy of the energy structural adjustments on reducing atmospheric HONO, and call for more attention to coal combustion as a potential source of HONO.

Enhanced Soil Emissions of Reactive Nitrogen Gases by Fertilization and Their Impacts on Secondary Air Pollution in Eastern China

Nitrogen fertilizer application is accompanied by intense release of multiple reactive nitrogen (Nr) gases such as nitrous acid (HONO), ammonia (NH3), and nitric oxide (NO) from the soil, influencing atmospheric chemistry and air pollution. In current emission inventories, postfertilization soil emissions are poorly characterized due to inaccurate identification of fertilization timing and location. Moreover, pre-existing studies predominantly focus on individual Nr gases, and a comprehensive understanding of simultaneously emitted Nr gases from fertilization and their impacts on air quality is still limited. Here, we developed a novel method to identify the dryland fertilization activity based on satellite and reanalysis data sets. Then, we updated a dynamic soil Nr emissions model (WRF-SoilN-Chem) with lab-derived parametrization and applied it to analyze the time- and space-varying Nr emissions and their effects on air quality. It is estimated that the Nr emissions from a typical fertilization event in the Yangtze River Delta (YRD) region increased ozone (O3) and nitrate concentrations by 2.5 and 18.2%, respectively. HONO and NH3 emissions jointly enhanced nitrate production via gas-particle partitioning. An accurate representation of fertilization and meteorology-emission-chemistry coupled modeling would greatly improve the understanding of the soil Nr emissions and their impacts on regional air pollution.

Increasing soil nitrous acid emissions driven by climate and fertilization change aggravate global ozone pollution

Soil microbial nitrous acid (HONO) production is an important source of atmospheric reactive nitrogen that affects air quality and climate. However, long-term global soil HONO emissions driven by climate change and fertilizer use have not been quantified. Here, we derive the global soil HONO emissions over the past four decades and evaluate their impacts on ozone (O3) and vegetation. Results show that climate change and the increased fertilizer use enhanced soil HONO emissions from 9.4 Tg N in 1980 to 11.5 Tg N in 2016. Chemistry-climate model simulations show that soil HONO emissions increased global surface O3 mixing ratios by 2.5% (up to 29%) and vegetation risk to O3, with increasing impact during 1980s-2016 in low-anthropogenic-emission regions. With future decreasing anthropogenic emissions, the soil HONO impact on air quality and vegetation is expected to increase. We thus recommend consideration of soil HONO emissions in strategies for mitigating global air pollution.

Reactive nitrogen emissions from cropland and their dominant driving factors in China

The environmental impacts of reactive nitrogen (Nr) emitted from fertilized cropland present significant challenges for balancing food security, air pollution and climate change mitigation. As a leading agricultural producer, China requires high-resolution Nr emissions modeling within a comprehensive processed-based framework to address these issues effectively. In this study, we applied a process-based agroecological model (FEST-C*) to estimate daily Nr emissions at 0.25° in China during 2020 and analyzed the driving factors by using Structural Equation Modeling, Random Forest, and Dominance Analysis. The hotspots of annual Nr emissions were in North China, Southeast China, and Southwest China, collectively responsible for over 80 % of the total emissions. Approximately 81 % of the total Nr emissions were from wheat, maize, and rice fields. Timing and amount of basal and topdressing fertilization under different crop rotation systems determined the monthly and seasonal variations of Nr emissions. The impacts of various factors on Nr emissions varied with NH3 being mainly driven by fertilizer consumption and other Nr species (N2O, NO, and HONO) also affected by soil temperature and water content. The spatial distributions of monthly Nr emissions calculated by FEST-C* were more realistic than currently available emission inventories compared to satellite or field observations. These findings will enable policymakers to develop effective control measures that alleviate cropland Nr emissions while sustaining crop production in China.

BVOCs' role in dynamic shifts of summer ozone formation regimes across China and policy implications

Biogenic volatile organic compounds (BVOCs) are crucial players in atmospheric chemistry, significantly impacting the formation of tropospheric ozone (O₃). While China has made substantial strides in reducing anthropogenic VOC (AVOCs) emissions, O₃ levels persist, highlighting the complex interplay between biogenic and anthropogenic sources. A critical knowledge gap exists in understanding how BVOC emissions influence ozone formation regimes (OFRs) and how this knowledge can inform effective air quality policies. This study employs the Model of Emissions of Gases and Aerosols from Nature (MEGAN) version 3.2 and the Community Multiscale Air Quality Modeling System (CMAQ) version 5.3.3 models, combined with process analysis (PA) and the Integrated Source Apportionment Method (ISAM), to evaluate the impact of BVOC emissions on OFRs in China. The models simulate BVOC emissions and their effects on OFRs across various regions during July 2019. The findings highlight that BVOCs play a pivotal role in shifting OFRs, with significant implications for ozone mitigation strategies in China. The study suggests that effective ozone control measures must consider the dual impact of BVOCs and AVOCs, with tailored strategies for different regions and times of day. The study also proposes potential challenges in mitigating BVOC emissions and outlines future research directions for interdisciplinary collaboration to address the complexities of ozone pollution management. This research advances the understanding of BVOCs' roles in ozone formation dynamics and provides a foundation for developing more effective air quality management policies in China, especially as global greening and climate change continue to influence BVOC emissions.

Refined source apportionment of nitrate aerosols based on isotopes and emission inventories in coastal city of northern China

The increasing mass percentage of nitrate (NO3-) in PM2.5 in North China Plain (NCP) from 2013 (20.5 %) to 2019 (28.7 %) indicates that NO3- became the most prominent composition of atmospheric aerosols. However, accurately quantifying the sources of NO3- in aerosols remained questionable. In this study, we coupled dual isotopic composition of NO3- with multiple emission inventories during winter 2018 and summer 2019 to accurately identify the sources of NO3-. Source apportionment revealed that mobile sources (including road traffic and shipping) contributed 36.7 % to NO3-, followed by coal combustion (18.6 %), lightning (10.1 %), biomass burning (9.8 %), industry oil (8.8 %), natural gas (8.6 %), and soil (7.4 %) during summer. In winter, the contributions to NO3- shifted to mobile sources (39.6 %), coal combustion (32.3 %), biomass burning (12.0 %), natural gas (8.1 %), and industry oil (8.0 %). The contribution of major sources was consistent with regional emissions inventories, supporting us in further analyzing the contribution of regional emission. Marine air-mass contributed 33.7 ± 19.6 % of NO3- during summer. In winter, in addition to local emissions, regional transport from the Shandong area (outside Qingdao) and Beijing-Tianjin-Hebei (BTH) regions was particularly significant (62.2 ± 12.5 %). This study for the first time established a refined methodology for quantifying the contribution of emission sources and regional transport, providing basis for precise and effective control of the sustained increase of proportion of atmospheric NO3-.

Soil Emissions of Reactive Oxidized Nitrogen Reduce the Effectiveness of Anthropogenic Source Control in China

Nitrogen dioxide (NO2) has decreased by ∼33% across over 1200 monitoring sites in China during 2015-2023, following a series of clean air policies. However, most of these sites are located in or near cities, leading to uncertainties in NO2 trends beyond urban regions due to limited observations. Here, we used satellite measurements to examine the differences in NO2 trends between urban and rural China. In urban areas, NO2 columns decreased by 4.0% per annum (a-1) during summer 2011-2023, consistent with bottom-up anthropogenic emission inventory and in situ measurements. In contrast, rural NO2 columns showed a slower than expected reduction (-2.6 to -0.0% a-1) during the same period. Model simulations with updates in the soil reactive oxidized nitrogen (Nr) scheme indicated that increasing soil Nr emissions can be an important factor contributing to the observed slow NO2 decrease in rural areas. This unregulated source increased summertime pollutant levels, partially offsetting the national efforts to mitigate NO2, ozone (O3), and particulate nitrate (NO3-) levels by 20.9%, 15.4%, and 4.7%, respectively, from 2011 to 2020. In the agriculture-intensive North China Plain, the increase in soil Nr emissions offset 46.6% of the NO2 reductions achieved by clean air policies. Our results highlight the increasing significance of soil emissions and the need to control them in future air-quality policies.

The observation of atmospheric HONO by wet-rotating-denuder ion chromatograph in a coastal city: Performance and influencing factors

Due to the significance of atmospheric HONO as a reservoir for radicals and the presence of substantial unknown sources of HONO, there is a pressing need for accurate and consistent measurement of its concentration. In this study, we compared the measurements obtained from the monitor for aerosols and gases in ambient air (MARGA) based on wet chemical method with those from the incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) based on optical method to assess the suitability of the MARGA instrument for accurate HONO detection. The diurnal patterns obtained by the two instruments are similar, with peaks at 8 a.m. and lows at 5 p.m. Over the course of the observation period, it was often observed that HONO concentrations recorded by the MARGA instrument consistently exceeded those obtained through the IBBCEAS technique, accounting for approximately 91.33% of the total observation time. Throughout the entire observation period, the R2 value between the two instruments was 0.49, indicating relatively good correlation. However, with a slope of only 0.27, it suggests poor agreement between the two instruments. Furthermore, the R2 and slopes between the two instruments vary with the seasons and day-night. The larger the quartile values of NO2, NH3, and BC, the greater the slopes of both MARGA and IBBCEAS instruments, and the higher the concentrations of NO2, NH3, and BC (indicator of semivolatile oxidizable hydrocarbons), the greater the differences between the two instruments, all indicating that NH3 may promote the reaction of NO2 with semivolatile oxidizable hydrocarbons to produce HONO. The O3 with its strong oxidizing properties may cause underestimation in the MARGA instrument by oxidizing NO2- to NO3- in the absorbing solution. It is challenging to derive a universal correction formula due to the interference of various chemical substances. Hence, MARGA should not be used for HONO research in the future.

Impact of soil-atmosphere HONO exchange on concentrations of HONO and O3 in the North China Plain

Nitrous acid (HONO) is an important precursor of the hydroxyl radical (OH) and plays a vital role in atmospheric photochemistry and nitrogen cycling. Soil emissions have been considered as a potential source of HONO. Lately, the HONO emission via soil-atmosphere exchange (ESA-exchange) from soil nitrite has been validated and quantified through chamber experiments, but has not been assessed in the real atmosphere. We coupled ESA-exchange and the other seven potential sources of HONO (i.e., traffic, indoor and soil bacterial emissions, heterogeneous reactions on ground and aerosol surfaces, nitrate photolysis, and acid displacement) into the Weather Research and Forecasting model with Chemistry (WRF-Chem), and found that diurnal variations of the soil emission flux at the Wangdu site were well simulated. During the non-fertilization period, ESA-exchange contributed ∼28 % and ∼35 % of nighttime and daytime HONO, respectively, and enhanced the net ozone (O3) production rate by ∼8 % across the North China Plain (NCP). During the preintensive/intensive fertilization period, the maximum ESA-Exchange contributions attained ∼70 %/83 % of simulated HONO in the afternoon across the NCP, definitely asserting its dominance in HONO production. ESA-Exchange enhanced the OH production rate via HONO photolysis by ∼3.5/7.0 times, and exhibited an increase rate of ∼13 %/20 % in the net O3 production rate across the NCP. The total enhanced O3 due to the eight potential HONO sources ranged from ∼2 to 20 ppb, and ESA-exchange produced O3 enhancements of ∼1 to 6 ppb over the three periods. Remarkably, the average contribution of ESA-exchange to the total O3 enhancements remained ∼30 %. This study suggests that ESA-exchange should be included in three-dimensional chemical transport models and more field measurements of soil HONO emission fluxes and soil nitrite levels are urgently required.

Reducing Soil-Emitted Nitrous Acid as a Feasible Strategy for Tackling Ozone Pollution

Severe ozone (O3) pollution has been a major air quality issue and affects environmental sustainability in China. Conventional mitigation strategies focusing on reducing volatile organic compounds and nitrogen oxides (NOx) remain complex and challenging. Here, through field flux measurements and laboratory simulations, we observe substantial nitrous acid (HONO) emissions (FHONO) enhanced by nitrogen fertilizer application at an agricultural site. The observed FHONO significantly improves model performance in predicting atmospheric HONO and leads to regional O3 increases by 37%. We also demonstrate the significant potential of nitrification inhibitors in reducing emissions of reactive nitrogen, including HONO and NOx, by as much as 90%, as well as greenhouse gases like nitrous oxide by up to 60%. Our findings introduce a feasible concept for mitigating O3 pollution: reducing soil HONO emissions. Hence, this study has important implications for policy decisions related to the control of O3 pollution and climate change.

Atmospheric HONO emissions in China: Unraveling the spatiotemporal patterns and their key influencing factors

Nitrous acid (HONO) can be photolyzed to produce hydroxyl radicals (OH) in the atmosphere. OH plays a critical role in the formation of secondary pollutants like ozone (O3) and secondary organic aerosols (SOA) via various oxidation reactions. Despite the abundance of recent HONO studies, research on national HONO emissions in China remains relatively limited. Therefore, this study employed a "wetting-drying" model and bottom-up approach to develop a high-resolution gridded inventory of HONO emissions for mainland China using multiple data. We used the Monte Carlo method to estimate the uncertainty in HONO emissions. In addition, the primary sources of HONO emissions were identified and their spatiotemporal distribution and main influencing factors were studied. The results indicated that the total HONO emissions in mainland China in 2016 were 0.77 Tg N (R50: 0.28-1.42 Tg N), with soil (0.42 Tg N) and fertilization (0.26 Tg N) as the primary sources, jointly contributing to over 87% of the total. Notably, the North China Plain (NCP) had the highest HONO emission density (3.51 kg N/ha/yr). Seasonal HONO emissions followed the order: summer (0.38 kg N/ha) > spring (0.19 kg N/ha) > autumn (0.17 kg N/ha) > winter (0.06 kg N/ha). Moreover, HONO emissions were strongly correlated with fertilization, cropland, temperature, and precipitation. This study provides vital scientific groundwork for the atmospheric nitrogen cycle and the formation of secondary pollutants.

Quantifying the Nitrogen Sources and Secondary Formation of Ambient HONO with a Stable Isotopic Method

Nitrous acid (HONO) is a reactive gas that plays an important role in atmospheric chemistry. However, accurately quantifying its direct emissions and secondary formation in the atmosphere as well as attributing it to specific nitrogen sources remains a significant challenge. In this study, we developed a novel method using stable nitrogen and oxygen isotopes (δ15N; δ18O) for apportioning ambient HONO in an urban area in North China. The results show that secondary formation was the dominant HONO formation processes during both day and night, with the NO2 heterogeneous reaction contributing 59.0 ± 14.6% in daytime and 64.4 ± 10.8% at nighttime. A Bayesian simulation demonstrated that the average contributions of coal combustion, biomass burning, vehicle exhaust, and soil emissions to HONO were 22.2 ± 13.1, 26.0 ± 5.7, 28.6 ± 6.7, and 23.2 ± 8.1%, respectively. We propose that the isotopic method presents a promising approach for identifying nitrogen sources and the secondary formation of HONO, which could contribute to mitigating HONO and its adverse effects on air quality.

Soil Emissions of Reactive Nitrogen Accelerate Summertime Surface Ozone Increases in the North China Plain

Summertime surface ozone in China has been increasing since 2013 despite the policy-driven reduction in fuel combustion emissions of nitrogen oxides (NOx). Here we examine the role of soil reactive nitrogen (Nr, including NOx and nitrous acid (HONO)) emissions in the 2013-2019 ozone increase over the North China Plain (NCP), using GEOS-Chem chemical transport model simulations. We update soil NOx emissions and add soil HONO emissions in GEOS-Chem based on observation-constrained parametrization schemes. The model estimates significant daily maximum 8 h average (MDA8) ozone enhancement from soil Nr emissions of 8.0 ppbv over the NCP and 5.5 ppbv over China in June-July 2019. We identify a strong competing effect between combustion and soil Nr sources on ozone production in the NCP region. We find that soil Nr emissions accelerate the 2013-2019 June-July ozone increase over the NCP by 3.0 ppbv. The increase in soil Nr ozone contribution, however, is not primarily driven by weather-induced increases in soil Nr emissions, but by the concurrent decreases in fuel combustion NOx emissions, which enhance ozone production efficiency from soil by pushing ozone production toward a more NOx-sensitive regime. Our results reveal an important indirect effect from fuel combustion NOx emission reduction on ozone trends by increasing ozone production from soil Nr emissions, highlighting the necessity to consider the interaction between anthropogenic and biogenic sources in ozone mitigation in the North China Plain.

Ambient ozone at a rural Central European site and its vertical concentration gradient close to the ground

The representativeness of ambient air quality of an in situ measurement is key in the use and correct interpretation of the measured concentration values. Though the horizontal representativeness aspect is generally not neglected in air pollution studies, a detailed, high-resolution vertical distribution of ambient air pollutant concentrations is rarely addressed. The aim of this study is twofold: (i) to explore the vertical distribution of ground-level ozone (O3) concentrations measured at four heights above the ground-namely at 2, 8, 50, and 230 m-and (ii) to examine in detail the vertical O3 concentration gradient in air columns between 2 and 8, 8 and 50, and 50 and 230 m above the ground. We use the daily mean O3 concentrations measured continuously at the Košetice station, representing the rural Central European background ambient air quality observed during 2015-2021. We use the semiparametric GAM (generalised additive model) approach (with complexity or roughness-penalised splines implementation) to analyse the data with sufficient flexibility. Our models for both O3 concentrations and O3 gradients use (additive) decomposition into annual trend and seasonality (plus an overall intercept). The seasonal and year-to-year patterns of the modelled O3 concentrations look very similar at first glance. Nevertheless, a more detailed look through O3 gradients shows that they differ substantially with respect to their seasonal and long-term dynamics. The vertical O3 concentration gradient in 2-230 m is not uniform but changes substantially with increasing height and shows by far the highest dynamics near the ground between 2 and 8 m, differing in both the seasonal and annual aspects for all the air columns inspected. We speculate that non-linear changes of both seasonal and annual components of vertical O3 gradients are due to atmospheric-terrestrial interactions and to meteorological factors, which we will explore in a future study.