Dual isotopic evidence of δ15N and δ18O for priority control of vehicle emissions in a megacity of East China: Insight from measurements in summer and winter

The source apportionment and main formation pathway of nitrate aerosols in China are not yet fully understood. In this study, PM2.5 samples were collected in Shanghai in the summer and winter of 2019. Water-soluble inorganic ions and isotopic signatures of stable nitrogen (δ15N-NO3-) and stable oxygen (δ18O-NO3-) in PM2.5 were determined. The results showed that NO3- was less important in summer (NO3-/SO42- = 0.4 ± 0.8), while it became the dominant species in winter (52.1 %). The average values of δ15N-NO3- and δ18O-NO3- in summer were + 2.0 ± 6.1 ‰ and 63.3 ± 9.4 ‰ respectively, which were significantly lower than those in winter (+7.2 ± 3.4 ‰ and 88.3 ± 12.1 ‰), indicating discrepancies between NOx sources and nitrate formation pathways. Both δ15N-NO3- and δ18O-NO3- were elevated at night, demonstrating that N2O5 hydrolysis contributed to the nocturnal nitrate increase even in summer. The contribution of the OH oxidation pathway to nitrate aerosols averaged at 70.5 ± 17.0 % in summer and N2O5 hydrolysis dominated the nitrate production in winter (approximately 80 %). On average, vehicle exhaust, coal combustion, natural gas burning, and soil emission contributed 50.7 %, 21.5 %, 15.9 %, and 11.9 %, respectively, to nitrate aerosols in summer, and contributed 56.8 %, 23.9 %, 13.6 %, and 5.7 %, respectively, to nitrate production in winter. Notably, natural gas burning is a non-negligible source of nitrate aerosols in Shanghai. In contrast to an inverse correlation between δ15N-NO3- and PM2.5, the value of δ18O-NO3- was positively correlated with nitrate concentration and aerosol liquid water content (ALWC) in winter, suggesting that explosive growth of nitrate was driven by continuous accumulation of N-depleted NOx and rapid N2O5 hydrolysis under calm and humid conditions. To continuously improve air quality, priority control should be given to vehicle emissions as the dominant source of NOx and volatile organic compounds (VOCs) in Shanghai.

Sensitivity of fine particulate matter concentrations in South Korea to regional ammonia emissions in Northeast Asia

Ammonia (NH3) is an important precursor for forming PM2.5. In this study, we estimated the impact of upwind transboundary and local downwind NH3 emissions on PM2.5 and its inorganic components via photochemical grid model simulations. Nine sensitivity scenarios with ±50% perturbations of upwind (China) and/or downwind (South Korea) NH3 emissions were simulated for the year 2016 over Northeast Asia. The annual mean PM2.5 concentrations in the downwind area were predicted to change from -3.3 (-18%) to 2.4 μg/m3(13%) when the NH3 emissions in the upwind and downwind areas were perturbed by -50% to +50%. The change in PM2.5 concentrations in the downwind area depending on the change in NH3 emissions in the upwind area was the highest in spring, followed by winter. This was mainly attributed to the change in nitrate (NO3-), a secondary inorganic aerosol (SIA) that is a predominant constituent of PM2.5. Since NH3 is mainly emitted near the surface and vertical mixing is limited during the night, it was modeled that the aloft nitric acid (HNO3)-to-NO3- conversion in the morning hours was increased when the NH3 accumulated near the surface during nighttime begins to mix up within the Planetary Boundary Layer (PBL) as it develops after sunrise. This implies that the control of upwind and/or downwind NH3 emissions is effective at reducing PM2.5 concentrations in the downwind area even under NH3 rich conditions in Northeast Asia.

Changes in nitrate accumulation mechanisms as PM2.5 levels increase on the North China Plain: A perspective from the dual isotopic compositions of nitrate

Nitrate (NO₃^-) has become recognized as the most important water-soluble ion in fine particulate (PM_2.5), and has been proposed as a driving factor for regional haze formation. However, nitrate formation mechanisms are still poorly understood. In this study, PM_2.5 samples were collected from September 2017 to August 2018 in Shijiazhuang, a city located on the North China Plain, and NO₃^-concentration, δ¹⁸O-NO₃^- and δ¹⁵N-NO₃^- values in PM_2.5 were analyzed. NO₃^- concentrations increased as PM_2.5 levels increased during both polluted and non-polluted days over the entire year. δ¹⁸O-NO₃^- values during cold months (63.5-103‰) were higher than those during warm months (50.3-85.4‰), these results suggested that the nitrate formation pathways shifted from the NO₂ + OH (P_OH) in warm months to the N₂O₅ + H₂O (P_N2O5) and NO₃ + VOCs (P_NO3) pathways in cold months. Especially during cold months, δ¹⁸O-NO₃^- values increased from 65.2-79.9‰ to 80.7-96.2‰ when PM_2.5 increased from ∼25 to >100 μg/m³, but when PM_2.5 > 100 μg/m³, there were relatively small variations in δ¹⁸O-NO₃^-. These results suggested that nitrate formation pathways changed from P_OH to P_N2O5 and P_NO3 pathways when PM_2.5 < 100 μg/m³, but that P_N2O5 and P_NO3 dominated nitrate production when PM_2.5 > 100 μg/m³. Higher δ¹⁵N-NO₃^- values in warm months (-11.8-13.8‰) than in cold months (-0.7-22.6‰) may be attributed to differences in NO_x emission sources and nitrogen isotopic fractionation among NO_x and NO₃^-. These results provide information on the dual isotopic compositions of nitrate to understand nitrate formation pathways under different PM_2.5 levels.

Comparative study on water-soluble inorganic ions in PM2.5 from two distinct climate regions and air quality

Recently, air quality has significantly improved in developed country, but that issue is of concern in emerging megacity in developing country. In this study, aerosols and their precursor gas were collected by NILU filter pack at two distinct urban sites during the winter and summer in Osaka, Japan and dry and rainy seasons in Ho Chi Minh City (HCMC), Vietnam. The aims are to investigate the contribution of water-soluble inorganic ions (WSIIs) to PM_2.5, thermodynamic characterization and possible formation pathway of secondary inorganic aerosol (SIA). The PM_2.5 concentration in Osaka (15.8 μg/m³) is lower than that in HCMC (23.0 μg/m³), but the concentration of WSIIs in Osaka (9.0 μg/m³) is two times higher than that in HCMC (4.1 μg/m³). Moreover, SIA including NH₄⁺, NO₃^- and SO₄^2- are major components in WSIIs accounting for 90% and 76% (in molar) in Osaka and HCMC, respectively. Thermodynamic models were used to understand the thermodynamic characterization of urban aerosols. Overall, statistical analysis results indicate that very good agreement (R² > 0.8) was found for all species, except for nitrate aerosol in HCMC. We found that when the crustal species present at high amount, those compositions should be included in model calculation (i.e. in the HCMC situation). Finally, we analyzed the characteristics of NH₄⁺- NO₃^-- SO₄^2- system. A possible pathway to produce fine nitrate aerosol in Osaka is via the homogeneous reaction between NH₃ and HNO₃, while non-volatile nitrate aerosols can be formed by the heterogeneous reactions in HCMC.

Wintertime nitrate formation during haze days in the Guanzhong basin, China: A case study

In this study, the formation of nitrate aerosol from 16 to 24 December 2015 in the Guanzhong basin, China is simulated using the WRF-Chem model. The predicted near-surface O₃, NO₂, and fine particulate matters (PM_2.5) in the basin and inorganic aerosols and nitrous acid (HONO) in Xi'an are generally in good agreement with the observations. Sensitivity studies show that the heterogeneous HONO sources play an appreciable role in the nitrate formation in the basin, contributing 9.2% of nitrate mass concentrations during heavy haze days. Nitrate formation is also affected by sulfate due to their competition for ammonia, particularly in urban areas. A 50% decrease in SO₂ emissions enhances the nitrate concentration by 6.2% during heavy haze days on average in the basin, and a 50% increase in SO₂ emission reduces the nitrate concentration by 9.7%. The roles of HONO and sulfate competition in nitrate formation are strongly modulated by ammonia. Agricultural emissions predominate the nitrate level in the basin (93.5%), but the non-agricultural sources cannot substantially influence nitrate formation (3.7%-14.6%). Reducing agricultural emission is an effective control strategy to mitigate nitrate pollution in the basin.

Humidity plays an important role in the PM₂.₅ pollution in Beijing

Heavily-polluted PM₂.₅ (fine particulate matter) episodes frequently impacting Beijing, especially during winter, have become a substantial concern. We found that during winter, the daily variation of PM2.5 in Beijing tracked the pattern of relative humidity (RH). With the increase of PM₂.₅ (or RH), water-soluble components (especially inorganic ions) became more abundant, and the water-soluble organic carbon to organic carbon ratios increased. The nitrate to sulfate ratios also exhibited dependence on RH, and were higher than those measured about a decade ago, consistent with the increasing trend of nitrogen oxides emissions. Surprisingly, the ratios of water-insoluble organic carbon to elemental carbon showed significant increase at high RH levels, presumably indicating the formation of secondary organic aerosol that is not soluble in water. In addition, humid winters were occasionally identified during 1996-2013 which are expected to be favorable for the formation of air pollution episodes with high PM₂.₅ concentrations.

Particulate nitrate formation in a highly polluted urban area: a case study by single-particle mass spectrometry in Shanghai

An aerosol time-of-flight mass spectrometer was deployed in August 2007 to characterize the 0.1-2.0 microm diameter particles in Shanghai to examine nitrate-containing particles. About 39% of the mass spectra of single particles contained nitrate ion peaks. The relative intensity of nitrate signals showed a pronounced diurnal profile, peaking in the late night or early morning during highly polluted days, and is closely correlated with the ambient relative humidity (RH). However, during the sampling days with good air quality, the diurnal pattern of nitrate changed by showing much lower signal intensity of nitrate with irregular variation. Poor correlation between the signals of ammonium and nitrate inthe mass spectra excluded the possibility of NH4NO3 as a major form of particulate nitrate, whose formation is favored by high RH and low temperature. The peak intensities of nitrate during the nighttime and high concentrations of O3 and NO2 strongly suggest that the heterogeneous reactions of N2O5 and NO3 onthe aerosol surface dominated the particulate nitrate formation on polluted days.

Molecular Dynamics Simulations of the Solution−Air Interface of Aqueous Sodium Nitrate

Molecular dynamics simulations have been used to investigate the behavior of aqueous sodium nitrate in interfacial environments. Polarizable potentials for the water molecules and the nitrate ion in solution were employed. Calculated surface tension data at several concentrations are in good agreement with measured surface tension data. The surface potential of NaNO3 solutions at two concentrations also compare favorably with experimental measurements. Density profiles suggest that NO3- resides primarily below the surface of the solutions over a wide range of concentrations. When the nitrate anions approach the surface of the solution, they are significantly undercoordinated compared to in the bulk, and this may be important for reactions where solvent cage effects play a role such as photochemical processes. Surface water orientation is perturbed by the presence of nitrate ions, and this has implications for experimental studies that probe interfacial water orientation. Nitrate ions near the surface also have a preferred orientation that places the oxygen atoms in the plane of the interface.