Seasonal effects of additional HONO sources and the heterogeneous reactions of N2O5 on nitrate in the North China Plain

We coupled the heterogeneous hydrolysis of N₂O₅ into the newly updated Weather Research and Forecasting model with Chemistry (WRF-Chem) to reveal the relative importance of the hydrolysis of N₂O₅ and additional nitrous acid (HONO) sources for the formation of nitrate during high PM_2.5 events in the North China Plain (NCP) in four seasons. The results showed that additional HONO sources produced the largest nitrate concentrations in winter and negligible nitrates in summer, leading to a 10% enhancement of a PM_2.5 peak in southern Beijing and a 15% enhancement in southeastern Hebei in winter. In contrast, the hydrolysis of N₂O₅ produced high nitrate in summer and low nitrate in winter, with the largest contribution of nearly 20% for a PM_2.5 peak in southeastern Hebei in summer. During PM_2.5 explosive growth events, the additional HONO sources played a key role in nitrate increases in southern Beijing and southwestern Hebei in winter, whereas the hydrolysis of N₂O₅ contributed the most to a rapid increase in nitrate in other seasons. HONO photolysis produced more hydroxyl radicals, which were >1.5 μg m^-3 h^-1 in the early explosive stage and led to a rapid nitrate increase at the southwestern Hebei sites in winter, while the heterogeneous reaction of N₂O₅ contributed greatly to a significant increase in nitrate in summer. The above results suggest that the additional HONO sources and the heterogeneous hydrolysis of N₂O₅ contributed the most to nitrate formation in NCP in winter and summer, respectively.

Effect of hydrolysis of N2O5 on nitrate and ammonium formation in Beijing China: WRF-Chem model simulation

Beijing, the capital of China, is a mega city with a population of >20 million. In recent years, the city has experienced heavy air pollution, with particulate matter (PM) being one of its top pollutants. In the last decade, extensive efforts have been made to characterize the sources, properties, and processes of PM in Beijing. Despite progress made by previous studies, there are still some important questions to be answered and addressed. The focus of this research is to study the impact of the heterogeneous hydrolysis of N₂O₅ on the formation of nitrate (NO₃^-) and ammonium (NH₄⁺) in Beijing. The results show that during heavy pollution days (e.g., during 14-17 September 2015, with PM_2.5 concentration over 100μg/m³), the concentrations of NO₂ and O₃ were high, with maxima of 90 and 240μg/m³, respectively, providing high precursors for the formation of N₂O₅. In addition, the aerosol and sulfate concentrations were also high, with maxima of 201μg/m³ and 23μg/m³ respectively, providing reacting surface for the heterogeneous reaction. As a result, the hydrolysis of N₂O₅ led to 21.0% enhancement of nitrate (NO₃^-) and 7.5% enhancement of ammonium (NH₄⁺). It is worth to note that this important effect only occurred in high pollution days (PM_2.5 concentration over 100μg/m³). During low-pollution periods (PM_2.5 concentration <100μg/m³), the effect of hydrolysis of N₂O₅ on the formation of nitrate and ammonium was insignificant (variation rate <5%). This study suggests that during heavy pollution periods, the hydrolysis of N₂O₅ enhances the level of aerosol pollution in Beijing, and needs to be further studied in order to perform efficient air pollution control and mitigation strategies.

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.

The Inhibition of N2O5 Hydrolysis in Sulfuric Acid by 1-Butanol and 1-Hexanol Surfactant Coatings

Gas-liquid scattering experiments are used to measure the fraction of N2O5 molecules that are converted to HNO3 after colliding with 72 wt % H2SO4 containing 1-hexanol or 1-butanol at 216 K. These alcohols segregate to the surface of the acid, with saturation coverages estimated to be 60% of a close-packed monolayer for 1-hexanol and 44% of a close-packed monolayer for 1-butanol. We find that the alkyl films reduce the conversion of N2O5 to HNO3 from 0.15 on bare acid to 0.06 on the hexyl-coated acid and to 0.10 on the butyl-coated acid. The entry of HCl and HBr, however, is enhanced by the hexanol and butanol films. The hydrolysis of N2O5 may be inhibited because the alkyl chains restrict the transport of this large molecule and because the alcohol OH groups dilute the surface region, suppressing reaction between N2O5 and near-interfacial H3O+ or H2O. In contrast, the interfacial alcohol OH groups provide additional binding sites for HCl and HBr and help initiate ionization. These and previous scattering experiments indicate that short-chain alcohol surfactants impede or enhance sulfuric acid-mediated reactions in ways that depend on the chain length, liquid phase acidity, and nature of the gas molecule.

Chemical characterizations of soluble aerosols in southern China

Soluble aerosols are measured at Guangdong and Hainan Provinces of southern China. The measured chemical composition of aerosols includes F-, Cl-, NO3-, SO4=, Na+, NH4+, K+, Ca2+, and Mg2+. The locations of measurements include a mega city (Guangzhou), a medium city along the coastline (Haiko), a small city along the coastline (Shanya), and a remote island site in the South China Sea (Yongxing island). The results reveal that aerosols in this region are complex and heterogeneous. Sulfate aerosol (SO4=) has the highest concentrations in Guangzhou (approximately 41% of total soluble aerosol mass), suggesting that anthropogenic activities (e.g., coal burning) play important roles in controlling aerosol concentrations in Guangzhou. By contrast, the concentrations of chlorine (Cl-) and sodium (Na+) are higher in Yongxing than in Guangzhou, indicating that the sea salt is the dominant aerosol in this marine environment site. In the medium (Haiko) and small (Shanya) city sites, the effects of anthropogenic and marine activities on aerosols fall in between the values in the mega city and the remote island site. The measured ratio of Cl-/Na+ shows that the ratio is less than 1.16 in all observation sites. The ratio in the Guangzhou city, the Haiko city, the Shanya city, and the Yongxing island is 0.52, 0.91, 0.24, and 0.53, respectively, indicating that significantly heterogeneous chemical reactions occur on sea salt particles. Unlike those in Europe and North America, there are high concentrations of calcium (Ca+) in all observation sites. The percentage of calcium mass to the measured total soluble aerosols mass is 21, 32, 34, and 30 at Guangzhou, Haiko, Sanya, and Yongxing, respectively. The calculations show that calcium plays an important role in neutralizing aerosols. The calculated "cation/anion" (summation operator[ion+]/summation operator[ion-]) ratio is 2.5, 2.5, 3.2, and 2.1, at Guangzhou, Haiko, Shanya, and Yongxing, respectively. The high "cation/anion" ratios suggest that SO4=, NO3-, and Cl- are neutralized, and the aerosols as a whole (internally mixed), appear to be in an alkaline mode in this region. However, without taking into account for calcium, the calculated "cation/anion" ratio reduces to 1.2, 0.98, 1.3, and 0.8 at Guangzhou, Haiko, Sanya, and Yongxing, respectively. The property of aerosols switches from an alkaline mode to an acidity mode at the Haiko and Yongxing sites.

An ab Initio Molecular Orbital Study of the Mechanism for the Gas-Phase Water-Mediated Decomposition and the Formation of Hydrates of Peroxyacetyl Nitrate (PAN)

There is uncertainty in the mechanism for the hydrolysis of peroxyacetyl nitrate (PAN), and experimental attempts to detect products of the direct reaction have been unsuccessful. Ab initio calculations are used to examine the energetics of water-mediated decomposition of gas-phase PAN into acetic acid and peroxynitric acid. On the basis of ab initio calculations, an alternative reaction mechanism for the decomposition of PAN is proposed. The calculations indicate that the barrier for one water addition to PAN is large. However, including additional water molecules reveals a substantially lower energy route. The calculations suggest that the formation of PAN hydrate complexes are energetically favorable and stable. Additional waters are increasingly efficient at stabilizing hydrated PAN.