High Gaseous Nitrous Acid (HONO) Emissions from Light-Duty Diesel Vehicles

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.

A review of indoor nitrous acid (HONO) pollution: Measurement techniques, pollution characteristics, sources, and sinks

Indoor air quality is of major concern for human health and well-being. Nitrous acid (HONO) is an emerging indoor pollutant, and its indoor mixing ratios are usually higher than outdoor levels, ranging from a few to tens of parts per billion (ppb). HONO exhibits adverse effects to human health due to its respiratory toxicity and mutagenicity. Additionally, HONO can easily undergo photodissociation by ultraviolet light to produce hydroxyl radicals (OH•), which in turn trigger a series of further photochemical oxidation reactions of primary or secondary pollutants. The accumulation of indoor HONO can be attributed to both direct emissions from combustion sources, such as cooking, and secondary formation resulting from enhanced heterogeneous reactions of NOx on indoor surfaces. During the day, the primary sink of indoor HONO is photolysis to OH• and NO. Moreover, adsorption and/or reaction on indoor surfaces, and diffusion to the outside atmosphere contribute to HONO loss both during the day and at night. The level of indoor HONO is also affected by human occupancy, which can influence household factors such as temperature, humidity, light irradiation, and indoor surfaces. This comprehensive review article summarized the research progress on indoor HONO pollution based on indoor air measurements, laboratory studies, and model simulations. The environmental and health effects were highlighted, measurement techniques were summarized, pollution levels, sources and sinks, and household influencing factors were discussed, and the prospects in the future were proposed.

Process-specified emission factors and characteristics of VOCs from the auto-repair painting industry

Daily use of passenger vehicles leads to considerable emission of volatile organic compounds (VOCs), which are key precursors to the ground-level ozone pollution. While evaporative and tailpipe emission of VOCs from the passenger vehicles can be eliminated largely, or even completely, by electrification, VOCs emission from the use of coatings in auto-repair is unavoidable and has long been ignored. Here, we present for the first time, to the best of our knowledge, a comprehensive investigation on the emission factors and process-specified characteristics of VOCs from auto-repair painting, based on field measurements over 15 representative auto-repair workshops in the Pearl-River-Delta area, China. Replacement of solvent-borne coatings with water-borne counterparts, which was only achieved partially in the Basecoat step but not in the Putty, Primer and Clearcoat steps, could reduce the per automobile VOCs emission from 756.5 to 489.6 g and the per automobile ozone formation potential (OFP) from 2776.5 to 1666.4 g. Implementation of exhaust after-treatment led to a further reduction of the per automobile VOCs emission to 340.9 g, which is still ca. 42% higher than that from the state-of-art painting processes for the manufacture of passenger vehicles. According to the analysis of VOCs compositions, the Putty process was dominated by the emission of styrene, while Primer, Basecoat (solvent-borne) and Clearcoat steps were all characterized by the emission of n-butyl acetate and xylenes. By contrast, water-borne Basecoat step showed a prominent emission of n-amyl alcohol. Notably, for the full painting process to repair an automobile, n-butyl acetate emerged as the most abundant species in the VOCs emission, whereas xylenes contributed most significantly to the OFP. Scenario analysis suggested that reducing VOCs contents in the coatings, as well as improving the after-treatment efficiency, were highly potential solutions for effective reduction of VOCs emission from auto-repair. Our study contributes to an update of industrial inventories of VOCs emission, and may provide valuable insights for reducing VOCs emission and OFPs from the auto-repair industry.

Additional HONO and OH Generation from Photoexcited Phenyl Organic Nitrates in the Photoreaction of Aromatics and NOx

HONO acts as a major OH source, playing a vital role in secondary pollutant formation to deteriorate regional air quality. Strong unknown sources of daytime HONO have been widely reported, which significantly limit our understanding of radical cycling and atmospheric oxidation capacity. Here, we identify a potential daytime HONO and OH source originating from photoexcited phenyl organic nitrates formed during the photoreaction of aromatics and NOx. Significant HONO (1.56-4.52 ppb) and OH production is observed during the photoreaction of different kinds of aromatics with NOx (18.1-242.3 ppb). We propose an additional mechanism involving photoexcited phenyl organic nitrates (RONO2) reacting with water vapor to account for the higher levels of measured HONO and OH than the model prediction. The proposed HONO formation mechanism was evidenced directly by photolysis experiments using typical RONO2 under UV irradiation conditions, during which HONO formation was enhanced by relative humidity. The 0-D box model incorporated in this mechanism accurately reproduced the evolution of HONO and aromatic. The proposed mechanism contributes 5.9-36.6% of HONO formation as the NOx concentration increased in the photoreaction of aromatics and NOx. Our study implies that photoexcited phenyl organic nitrates are an important source of atmospheric HONO and OH that contributes significantly to atmospheric oxidation capacity.

Revisiting the Ultraviolet Absorption Cross Section of Gaseous Nitrous Acid (HONO): New Insights for Atmospheric HONO Budget

Nitrous acid (HONO) is an important source of hydroxyl radicals (OH) in the atmosphere. Precise determination of the absolute ultraviolet (UV) absorption cross section of gaseous HONO lays the basis for the accurate measurement of its concentration by optical methods and the estimation of HONO loss rate through photolysis. In this study, we performed a series of laboratory and field intercomparison experiments for HONO measurement between striping coil-liquid waveguide capillary cell (SC-LWCC) photometry and incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS). Specified HONO concentrations prepared by an ultrapure standard HONO source were utilized for laboratory intercomparisons. Results show a consistent ∼22% negative bias in measurements of the IBBCEAS compared with a SC-LWCC photometer. It is confirmed that the discrepancies occurring between these techniques are associated with the overestimation of the absolute UV absorption cross sections through careful analysis of possible uncertainties. We quantified the absorption cross section of gaseous HONO (360-390 nm) utilizing a custom-built IBBCEAS instrument, and the results were found to be 22-34% lower than the previously published absorption cross sections widely used in HONO concentration retrieval and atmospheric chemical transport models (CTMs). This suggests that the HONO concentrations retrieved by optical methods based on absolute absorption cross sections may have been underestimated by over 20%. Plus, the daytime loss rate and unidentified sources of HONO may also have evidently been overestimated in pre-existing studies. In summary, our findings underscore the significance of revisiting the absolute absorption cross section of HONO and the re-evaluation of the previously reported HONO budgets.

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.

Developing a model-ready highly resolved HONO emission inventory in Guangdong using domestic measured emission factors

Nitrous acid (HONO) plays an important role in the budget of hydroxyl radical (OH) in the atmosphere. However, current chemical transport models (CTMs) typically underestimate ambient concentrations of HONO due to a dearth of high resolution primary HONO emission inventories. To address this issue, we have established a highly resolved bottom-up HONO emission inventory for CTMs in Guangdong province, utilizing the best available domestic measured emission factors and newly obtained activity data. Our results indicate that emissions from various sources in 2020, including soil, on-road traffic, non-road traffic, biomass burning, and stationary combustion, were estimated at 21.5, 10.0, 8.2, 2.5, and 0.7 kt, respectively. Notably, the HONO emissions structure differed between the Pearl River Delta (PRD) and the non-PRD regions. Specifically, traffic sources were the dominant contributors (62 %) to HONO emissions in the PRD, whereas soil sources accounted for the majority (65 %) of those in the non-PRD. Among on-road traffic sources, diesel vehicles played a significant role, contributing 99.7 %. Comparisons with previous methods suggest that HONO emissions from diesel vehicles are underestimated by approximately 2.5 times. Higher HONO emissions, dominated by soil emissions, were observed in summer months, particularly in August. Furthermore, diesel vehicle emissions were pronounced at night, likely contributing to the nighttime accumulation of HONO and the morning peak of OH. The emission inventories developed in this study can be directly applied to widely used CTMs, such as CMAQ, CAMx, WRF-Chem, and NAQPMS, to support the simulation of OH formation and secondary air pollution.

Direct identification of total and missing OH reactivities from light-duty gasoline vehicle exhaust in China based on LP-LIF measurement

Considerable efforts have been devoted to characterising the chemical components of vehicle exhaust. However, these components may not accurately reflect the contribution of vehicle exhaust to atmospheric reactivity because of the presence of species not accounted for ("missing species") given the limitations of analytical instruments. In this study, we improved the laser photolysis-laser-induced fluorescence (LP-LIF) technique and applied it to directly measure the total OH reactivity (TOR) in exhaust gas from light-duty gasoline vehicles in China. The TOR for China I to VI-a vehicles was 15.6, 16.3, 8.4, 2.6, 1.5, and 1.6 × 10⁴ sec^-1, respectively, reflecting a notable drop as emission standards were upgraded. The TOR was comparable between cold and warm starts. The missing OH reactivity (MOR) values for China I to IV vehicles were close to zero with a cold start but were much higher with a warm start. The variations in oxygenated volatile organic compounds (OVOCs) under different emission standards and for the two start conditions were similar to those of the MOR, indicating that OVOCs and the missing species may have similar production processes. Online measurement revealed that the duration of the stable driving stage was the primary factor leading to the production of OVOCs and missing species. Our findings underscore the importance of direct measurement of TOR from vehicle exhaust and highlight the necessity of adding OVOCs and other organic reactive gases in future upgrades of emission standards, such that the vehicular contribution to atmospheric reactivity can be more effectively controlled.

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.

Direct Observation of HONO Emissions from Real-World Residential Natural Gas Heating in China

Atmospheric nitrous acid (HONO) is an important precursor of atmospheric hydroxyl radicals. Vehicle emissions and heterogeneous reactions have been identified as major sources of urban HONO. Here, we report on HONO emissions from residential natural gas (RNG) for water and space heating in urban areas based on in situ measurements. The observed HONO emission factors (EFs) of RNG heating vary between 6.03 and 608 mg·m^-3 NG, which are highly dependent on the thermal load. The highest HONO EFs are observed at a high thermal load via the thermal NO homogeneous reaction. The average HONO EFs of RNG water heating in winter are 1.8 times higher than that in summer due to the increased thermal load caused by the lower inlet water temperatures in winter. The power-based HONO EFs of the traditional RNG heaters are 1085 times and 1.7 times higher than those of gasoline and diesel vehicles that meet the latest emission standards, respectively. It is estimated that the HONO emissions from RNG heaters in a typical Chinese city are gradually close to emissions from on-road vehicles when temperatures decline. These findings highlight that RNG heating is a non-negligible source of urban HONO emissions in China. With the continuous acceleration of coal-to-gas projects and the continuous tightening of NOx emission standards for vehicle exhaust, HONO emissions from RNG heaters will become more prominent in urban areas. Hence, it is urgently needed to upgrade traditional RNG heaters with efficient emission reduction technologies such as frequency-converted blowers, secondary condensers, and low-NOx combustors.

Characterizing Operating Condition-Based Formaldehyde Emissions of Light-Duty Diesel Trucks in China Using a PEMS-HCHO System

Formaldehyde (HCHO) plays a critical role in atmospheric photochemistry and public health. While existing studies have suggested that vehicular exhaust is an important source of HCHO, the operating condition-based diesel truck HCHO emission measurements remain severely limited due to the limited temporal resolution and accuracy of measurement techniques. In this study, we characterized the second-by-second HCHO emissions from 29 light-duty diesel trucks (LDDTs) in China over dynamometer and real-world driving tests using a portable online HCHO emission measurement system (PEMS-HCHO), considering various operating conditions. Our results suggested that the HCHO emissions from LDDTs might be underestimated by the widely used offline DNPH-HPLC method. The HCHO emissions at a 200 s cold start from China V LDDT can be up to 50 mg/start. Different driving conditions over dynamometer and real-world driving tests led to a 2-4 times difference in the HCHO emission factors (EFs). Under real-world hot-running conditions, the HCHO EFs of China III, IV, V, and VI LDDTs were 43.5 ± 35.7, 10.6 ± 14.2, 8.8 ± 5.1, and 3.2 ± 1.2 mg/km, respectively, which significantly exceeded the latest California low emission vehicle III HCHO emission standard (2.5 mg/km). These findings highlighted the significant impact of vehicle operating conditions on HCHO emissions and the urgency of regulating HCHO emissions from LDDTs in China.

Atmospheric chemistry of nitrous acid and its effects on hydroxyl radical and ozone at the urban area of Beijing in early spring 2021

The atmospheric chemistry of nitrous acid (HONO) has received extensive attention because of its significant contribution to hydroxyl (OH) radicals. Heterogeneous reaction of NO₂ is an important HONO source, and its reaction mechanism is affected by many factors, such as concentration of gaseous NO₂, surface adsorbed water, relative humidity and temperature. Although laboratory studies have confirmed the effect of temperature on heterogeneous reaction of NO₂, there are few field observations reporting about it. We have conducted a field observation in the early spring 2021 when the temperature ranges widely (-0.1-24.7 °C). Concentrations of HONO and related pollutants at the urban area of Beijing are obtained. The hourly averaged HONO concentration reaches 4.87 ppb with a mean value of 1.48 ± 1.09 ppb. Combined with box model and RACM2 mechanism, we found an optimal temperature (∼10 °C) existing for heterogeneous reaction of NO₂ during this measurement. When considering the promotion effect of optimal temperature, the contribution of heterogeneous reaction of NO₂ to HONO can increase by 10%. This result will provide essential information for developing an accurate model of HONO chemistry in the atmosphere especially for certain periods or regions with temperature changing largely. Moreover, heterogeneous reaction of NO₂ is the vital source of HONO, contributing 63-76% to simulated HONO during this measurement. Note that HONO photolysis is the most important formation pathway of OH radicals, and ambient HONO concentration is the obbligato constraint for evaluating atmospheric oxidation by model simulations.

Upgrading Emission Standards Inadvertently Increased OH Reactivity from Light-Duty Diesel Truck Exhaust in China: Evidence from Direct LP-LIF Measurement

Vehicular exhaust is an important source of reactive gases responsible for the formation of ozone and secondary organic aerosols (SOAs) in the atmosphere. Although significant efforts have been made to characterize the chemical compounds associated with vehicular exhaust, there is still a wealth of compounds that are unable to be detected, posing uncertainties in estimating their contribution to atmospheric reactivity. In this study, by improving laser-induced fluorescence techniques, we achieved the first-ever direct measurement of the total OH reactivity (TOR) from light-duty diesel truck (LDDT) exhaust with different emission standards. We found that the TOR from the LDDT exhaust was 80-130 times the TOR from the gasoline exhaust measured in Japan. Unexpectedly, we discovered increased TOR emissions along with upgrading emission standards, possibly as a collective result of high combustion temperature in the engine and the oxidation catalysts in the exhaust after-treatment that favor production of highly oxidized organics in the stricter emission standard. Most of these oxidized organics are unable to be speciated by routine measurements, resulting in the missing OH reactivity increasing rapidly from 1.91% for China III to 42.0% for China V LDDT. Upgrading the emission standard failed to reduce the TOR from LDDT exhaust, which may inadvertently promote the contribution of LDDT to the formation of ozone and SOA pollution in China.

Nitrous acid in the polluted coastal atmosphere of the South China Sea: Ship emissions, budgets, and impacts

Nitrous acid (HONO) can significantly contribute to hydroxyl radicals (OH) and thus regulate atmospheric oxidation chemistry; however, ambient HONO sources are not well quantified and vary in different environments. In this study, we conducted comprehensive field observations at a coastal site in the South China Sea and performed chemical box modelling to demonstrate contrasting budgets and impacts of diurnal atmospheric HONO derived from the sea, coastline and continent. The ship emission ratio of HONO/nitrogen oxides (NO_x) (1.21 ± 0.99%) was calculated from hundreds of night-time fresh plume measurements. Offshore marine air was frequently influenced by ship exhausts, and the sea acted as an HONO sink. Heterogeneous conversions of nitrogen dioxide (NO₂) on underlying surfaces and photolysis of adsorbed nitric acid (HNO_3(ads)) were the major HONO sources in coastal air, when heterogeneous NO₂ conversions on the ground surface and the homogeneous NO + OH reaction dominated HONO formation in continental air. HONO photolysis was a significant source of reactive radicals (RO_x = OH + HO₂ + RO₂) in these air masses. Atmospheric box model including only homogeneous HONO source of the NO + OH reactions significantly underpredicted the OH concentration and atmospheric oxidising capacity in coastal and continental air. This study provides new insights into the complex sources and significant impacts of HONO in the polluted coastal boundary layer.

Effect of Different Combustion Processes on Atmospheric Nitrous Acid Formation Mechanisms: A Winter Comparative Observation in Urban, Suburban and Rural Areas of the North China Plain

Atmospheric nitrous acid (HONO) is a dominant precursor of hydroxyl (OH) radicals, and its formation mechanisms are still controversial. Few studies have simultaneously explored effects of different combustion processes on HONO sources. Hereby, synchronous HONO measurement in urban (BJ), suburban (XH) and rural (DBT) areas with different combustion processes is performed in the North China Plain in winter. A box model is utilized to analyze HONO formation mechanisms. HONO concentration is the highest at the DBT site (2.51 ± 1.90 ppb), followed by the XH (2.18 ± 1.95 ppb) and BJ (1.17 ± 1.20 ppb) sites. Vehicle exhaust and coal combustion significantly contribute to nocturnal HONO at urban and rural sites, respectively. During a stagnant pollution period, the NO+OH reaction and combustion emissions are more crucial to HONO in urban and rural areas; meanwhile, the heterogeneous reaction of NO₂ is more significant in suburban areas. Moreover, the production rate of OH from HONO photolysis is about 2 orders of magnitude higher than that from ozone photolysis. Consequently, vehicle exhaust and coal combustion can effectively emit HONO, further causing environmental pollution and health risks. It is necessary to expand the implementation of the clean energy transition policy in China, especially in areas with substantial coal combustion.

A meteorologically adjusted ensemble Kalman filter approach for inversing daily emissions: A case study in the Pearl River Delta, China

The conventional Ensemble Kalman filter (EnKF), which is now widely used to calibrate emission inventories and to improve air quality simulations, is susceptible to simulation errors of meteorological inputs, making accurate updates of high temporal-resolution emission inventories challenging. In this study, we developed a novel meteorologically adjusted inversion method (MAEInv) based on the EnKF to improve daily emission estimations. The new method combines sensitivity analysis and bias correction to alleviate the inversion biases caused by errors of meteorological inputs. For demonstration, we used the MAEInv to inverse daily carbon monoxide (CO) emissions in the Pearl River Delta (PRD) region, China. In the case study, 60% of the total CO simulation biases were associated with sensitive meteorological inputs, which would lead to the overestimation of daily variations of posterior emissions. Using the new inversion method, daily variations of emissions shrank dramatically, with the percentage change decreased by 30%. Also, the total amount of posterior CO emissions estimated by the MAEInv decreased by 14%, indicating that posterior CO emissions might be overestimated using the conventional EnKF. Model evaluations using independent observations revealed that daily CO emissions estimated by MAEInv better reproduce the magnitude and temporal patterns of ambient CO concentration, with a higher correlation coefficient (R, +37.0%) and lower normalized mean bias (NMB, -17.9%). Since errors of meteorological inputs are major sources of simulation biases for both low-reactive and reactive pollutants, the MAEInv is also applicable to improve the daily emission inversions of reactive pollutants.

Nitrous acid (HONO) emission factors for diesel vehicles determined using a chassis dynamometer

Vehicle exhaust gases are important sources of nitrous acid (HONO). In this study, HONO in diesel vehicle exhaust was measured by incoherent broadband cavity-enhanced absorption spectroscopy using a chassis dynamometer system. The mean HONO concentrations in exhaust gases emitted by passenger cars and light-duty trucks were high when the after treatment devices were not fully working during the warming up period. The HONO/NOx ratio is a good index of HONO formation. The HONO/NOx ratios were 9.7 × 10^-3-18.1 × 10^-3, and were higher than what we found in a previous study. The estimated HONO emission factors were 7.71-64.70 mg (kg fuel)^-1, and were lower than were found in previous studies. The results indicated that the frequency particulate matter is removed from a diesel particle filter affects the HONO concentration in the emitted gases and the HONO emission factor.

Contribution of Vehicle Emission and NO2 Surface Conversion to Nitrous Acid (HONO) in Urban Environments: Implications from Tests in a Tunnel

Nitrous acid (HONO) is an important photochemical precursor to hydroxyl radicals particularly in an urban atmosphere, yet its primary emission and secondary production are often poorly constrained. Here, we measured HONO and nitrogen oxides (NO_x) at both the inlet and the outlet in a busy urban tunnel (>30 000 vehicles per day) in south China. Multiple linear regression revealed that 73.9% of the inlet-outlet incremental HONO concentration was explained by NO₂ surface conversion, while the rest was directly emitted from vehicles with an average HONO/NO_x ratio of 1.31 ± 0.87%, which was higher than that from previous tunnel studies. The uptake coefficient of NO₂, γ(NO₂), on the tunnel surfaces was calculated to be (7.01 ± 0.02) × 10^-5, much higher than that widely used in models. As tunnel surfaces are typical of urban surfaces in the wall and road materials, the dominance of HONO from surface reactions in the poorly lit urban tunnel demonstrated the importance of NO₂ conversion on urban surfaces, instead of NO₂ conversion on the aerosol surface, for both daytime and night-time HONO even in polluted ambient air. The higher γ(NO₂) on urban surfaces and the elevated HONO/NO_x ratio from this study can help explain the missing HONO sources in urban areas.

Mapping the daily nitrous acid (HONO) concentrations across China during 2006-2017 through ensemble machine-learning algorithm

Nitrous acid (HONO) is a major source of the hydroxyl radical (OH) and plays a key role in atmospheric photochemistry. The lack of spatially resolved HONO concentration information results in large knowledge gaps of HONO and its role in atmospheric chemistry and air pollution in China. In this work, an ensemble machine learning model comprising of random forest, gradient boosting, and back propagation neural network was proposed, for the first time, to estimate the long-term (2006-2017) daily HONO concentrations across China in 0.25° resolution. Further, the key factors controlling the space-time variablity of HONO concentrations were analyzed based on variable importance values. The ensemble model well characterized the spatiotemporal distribution of daily HONO concentrations with the sampled-based, site-based and by-year cross-validation (CV) R² (RMSE) of 0.7 (0.36 ppbv), 0.67 (0.36 ppbv), and 0.62 (0.40 ppbv), respectively. HONO hotspots were mainly distributed in the Beijing-Tianjin-Hebei (BTH), Pearl River Delta (PRD), Yangtze River Delta (YRD), and several sites of Sichuan Basin, in line with the distribution patterns of the tropospheric NO₂ columns and assimilated surface NO₃^- levels. The national HONO levels stagnated during 2006-2013, then declined after 2013 benefiting from the implementation of the Action Plan for Air Pollution Prevention and Control. The NO₃^- concentration, urban area, NO₂ column density ranked as important variables for HONO prediction, while agricultral land, forest and grassland played minor roles in affecting HONO concentrations, suggesting the significant role of heterogeneous HONO production from anthropogenic precursor emissions. Leveraging the ground-level HONO observations, this study fills the gap of statistically modelling nationwide HONO in China, which provides essential data for atmospheric chemistry research.