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.

Strong impacts of biomass burning, nitrogen fertilization, and fine particles on gas-phase hydrogen peroxide (H2O2).

Gas-phase hydrogen peroxide (H2O2) plays an important role in atmospheric chemistry as an indicator of the atmospheric oxidizing capacity. It is also a vital oxidant of sulfur dioxide (SO2) in the aqueous phase, resulting in the formation of acid precipitation and sulfate aerosol. However, sources of H2O2 are not fully understood especially in polluted areas affected by human activities. In this study, we reported some high H2O2 cases observed during one summer and two winter campaigns conducted at a polluted rural site in the North China Plain. Our results showed that agricultural fires led to high H2O2 concentrations up to 9 ppb, indicating biomass burning events contributed substantially to primary H2O2 emission. In addition, elevated H2O2 and O3 concentrations were measured after fertilization as a consequence of the enhanced atmospheric oxidizing capacity by soil HONO emission. Furthermore, H2O2 exhibited unexpectedly high concentration under high NOx conditions in winter, which are closely related to multiphase reactions in particles involving organic chromophores. Our findings suggest that these special factors (biomass burning, fertilization, and ambient particles), which are not well considered in current models, are significant contributors to H2O2 production, thereby affecting the regional atmospheric oxidizing capacity and the global sulfate aerosol formation.

Particle-Phase Photoreactions of HULIS and TMIs Establish a Strong Source of H2O2 and Particulate Sulfate in the Winter North China Plain.

During haze periods in the North China Plain, extremely high NO concentrations have been observed, commonly exceeding 1 ppbv, preventing the classical gas-phase H2O2 formation through HO2 recombination. Surprisingly, H2O2 mixing ratios of about 1 ppbv were observed repeatedly in winter 2017. Combined field observations and chamber experiments reveal a photochemical in-particle formation of H2O2, driven by transition metal ions (TMIs) and humic-like substances (HULIS). In chamber experiments, steady-state H2O2 mixing ratios of 116 ± 83 pptv were observed upon the irradiation of TMI- and HULIS-containing particles. Correspondingly, H2O2 formation rates of about 0.2 ppbv h-1 during the initial irradiation periods are consistent with the H2O2 rates observed in the field. A novel chemical mechanism was developed explaining the in-particle H2O2 formation through a sequence of elementary photochemical reactions involving HULIS and TMIs. Dedicated box model studies of measurement periods with relative humidity >50% and PM2.5 ≥ 75 µg m-3 agree with the observed H2O2 concentrations and time courses. The modeling results suggest about 90% of the particulate sulfate to be produced from the SO2 reaction with OH and HSO3- oxidation by H2O2. Overall, under high pollution, the H2O2-caused sulfate formation rate is above 250 ng m-3 h-1, contributing to the sulfate formation by more than 70%.

Development and application of a twin open-top chambers method to measure soil HONO emission in the North China Plain.

HONO (nitrous acid) is a crucial precursor for tropospheric OH radicals, and its sources are not well understood. In the past decade, soil was proven to be a potential source for HONO. However, more field measurements of soil HONO emission flux are needed to explore the mechanism and its impact on regional air quality. Here, we developed a system based on twin open-top chambers (OTCs) and wet chemical methods to measure HONO emission flux from agricultural soil in the North China Plain (NCP). The performance of the OTC system was tested under laboratory and field measurement conditions. The results showed that the system could reflect the strength (>90%) and variation of gas emission with an average residence time of 4-5 min. The greenhouse effect and chemical reaction interference in the chamber was proven to have no significant influence on the HONO flux measurement. Field measurement revealed that agricultural soil before fertilization was an important source of HONO. The emission flux showed radiation-dependent or temperature-dependent variation, with a peak of 3.21 ng m-2 s-1 at noontime that could account for approximately 67 pptv h-1 of the missing HONO source under an assumed mixing layer height of 300 m. Fertilization substantially accelerated HONO emission, which was rationally attributed to biological processes including nitrification. Considering the high fertilization rate in the NCP and other similar regions in China, HONO emission from agricultural soil likely has enormous impact on regional photochemistry and air quality, suggesting that more research should be conducted on this aspect.

The levels, sources and reactivity of volatile organic compounds in a typical urban area of Northeast China.

Air concentrations of volatile organic compounds (VOCs) were continually measured at a monitoring site in Shenyang from 20 August to 16 September 2017. The average concentrations of alkanes, alkenes, aromatics and carbonyls were 28.54, 6.30, 5.59 and 9.78 ppbv, respectively. Seven sources were identified by the Positive Matrix Factorization model based on the measurement data of VOCs and CO. Vehicle exhaust contributed the most (36.15%) to the total propene-equivalent concentration of the measured VOCs, followed by combustion emission (16.92%), vegetation emission and secondary formation (14.33%), solvent usage (10.59%), petrochemical industry emission (9.89%), petrol evaporation (6.28%), and liquefied petroleum gas (LPG) usage (5.84%). Vehicle exhaust, solvent usage and combustion emission were found to be the top three VOC sources for O3 formation potential, accounting for 34.52%, 16.55% and 11.94%, respectively. The diurnal variation of the total VOCs from each source could be well explained by their emission characteristics, e.g., the two peaks of VOC concentrations from LPG usage were in line with the cooking times for breakfast and lunch. Wind rose plots of the VOCs from each source could reveal the possible distribution of the sources around the monitoring site. The O3 pollution episodes during the measurement period were found to be coincident with the elevation of VOCs, which was mainly due to the air parcel from the southeast direction where petrochemical industry emission was found to be dominant, suggesting that the petrochemical industry emission from the southeast was probably a significant cause of O3 pollution in Shenyang.

The influence of OH concentration on SOA formation from isoprene photooxidation.

The formation of secondary organic aerosol (SOA) from isoprene photooxidation was investigated to reveal the influence of OH concentration on SOA formation through varying the concentrations of isoprene and H2O2 in a smog chamber. The results indicated that the higher the OH concentration was, the less the isoprene consumed for the detectable SOA mass concentration, for example, the lowest isoprene consumption for the detectable SOA was about 14.4 ppb under the OH concentration of about 1.65 × 107 molecules cm-3, whereas tens ppb of isoprene consumption were needed under the OH concentrations <1.0 × 107 molecules cm-3, and even no detectable SOA was observed with isoprene consumption of 75.1 ppb under OH concentration of 7.2 × 105 molecules cm-3. SOA yield was also found to increase with increasing OH concentration for a given aerosol loading (M0) at atmospherically relevant conditions, confirming that OH concentration played important role in SOA formation from isoprene photooxidation. The maximal SOA yields (5.8-42.8%) obtained by this study were a factor of 1.5-3.1 greater than those reported by previous study for the almost the same initial reactant concentrations of isoprene and H2O2, and the difference was mainly ascribed to the higher OH concentrations in the reaction systems of this study than those of previous study. The OH concentrations adopted in this study closed to those in the real atmosphere around noontime, and hence the SOA yield obtained from the isoprene photooxidation might be representative.

Development of stripping coil-ion chromatograph method and intercomparison with CEAS and LOPAP to measure atmospheric HONO.

Nitrous acid (HONO) is the major precursor of OH radicals in polluted areas. Accurate measurement of HONO provides vital evidence for exploring the formation of secondary pollution. Stripping coil (SC) equipped with ion chromatograph (IC) or spectrograph as one of wet chemical methods has been already used to measure HONO. The reliability of the method mainly depends on the collection efficiency and the interference from other species. In this study, a SC-IC method was set up to measure HONO. The performance of the method was assessed in the chamber using two kinds of absorption solutions i.e. ultrapure water and 25 µM Na2CO3 solution under different concentrations of SO2. Results indicated that HONO concentrations absorbed by ultrapure water and Na2CO3 solution were almost identical in the absence of SO2 in the chamber and both the collection efficiencies were >99%. However, the collection efficiency of ultrapure water decreased with the increase of SO2, indicating that the presence of SO2 resulted in the penetration of HONO. The collection efficiency kept >90% when the concentration of SO2 was no >23 ppbv. Comparing with the situation without SO2, HONO performed a remarkable increase with the presence of SO2 when using Na2CO3 absorption solution, indicating that the extra generation of HONO from the reaction between SO2 and NO2 in alkaline solution. Consequently, ultrapure water as the absorption solution could provide a high collection efficiency and avoid the interferences from SO2 when the concentration of SO2 was below 23 ppbv. High correlations (slope = 0.94-1.06, r2 > 0.90) were found during the intercomparisons between SC-IC and other three techniques, suggesting the SC-IC method developed in this study was able to measure atmospheric HONO in the field campaigns.