Heterogeneous Hydrolysis Of N2O5 Research Articles

Chemical characteristics and formation mechanisms of PM2.5 during wintertime in two cities with different industrial structures in the Sichuan Basin, China

PM2.5 remains one of the critical pollutants involving regional and complex air pollution in many big cities of China, and its improvement has become sluggish in recent years. In this study, we collected PM2.5 samples from two cities, Chengdu and Ya'an, which have different industrial structures in the Sichuan basin during wintertime and investigated the reasons behind their severe PM2.5 pollution. Throughout the entire sampling period, the average PM2.5 concentrations in Chengdu (71.3 ± 24.8 μg/m3) and Ya'an (72.6 ± 27.1 μg/m3) were similar. However, the chemical compositions of PM2.5 showed significant differences. Chengdu exhibited higher concentrations of water-soluble inorganic ions, whereas Ya'an had higher levels of carbonaceous compounds. Both cities experienced an ammonium-rich environment, which promoted the homogeneous generation of secondary pollutants. Moreover, as PM2.5 pollution worsened, the influence of heterogeneous reactions involving SO2 and NOx, as well as the heterogeneous hydrolysis of N2O5, gradually became more pronounced in particle formation. Additionally, adverse meteorological conditions facilitated pollutant accumulation in Ya'an. Using the Positive Matrix Factorization model, we identified 5 sources of PM2.5. The primary source of PM2.5 in both cities was secondary formation (35.4% in Chengdu and 32.5% in Ya'an), while their second-largest contributors varied (26.2% from vehicle emission in Chengdu, and 26.6% from combustion source in Ya'an). These discrepancies highlight the necessity for tailored government interventions, particularly during the winter season. By analyzing the light absorption of the carbonaceous at 370 nm, we discovered that brown carbon was the primary absorber of near-ultraviolet light, with vehicle emissions accounting for the largest portion in Chengdu (37.2%) and combustion emissions being the predominant factor in Ya'an (51.0%). These results could potentially help for having a long-term impact on climate change by simultaneously reducing PM2.5 pollution.

Measurement report: Atmospheric nitrate radical chemistry in the South China Sea influenced by the urban outflow of the Pearl River Delta

Abstract. The nitrate radical (NO3) is a critical nocturnal atmospheric oxidant in the troposphere, which widely affects the fate of air pollutants and regulates air quality. Many previous works have reported the chemistry of NO3 in inland regions of China, while fewer studies target marine regions. Here, we present a field measurement of the NO3 reservoir, dinitrogen pentoxide (N2O5), and related species at a typical marine site (Da Wan Shan Island) located in the South China Sea in the winter of 2021. Two patterns of air masses were captured during the campaign, including the dominant airmass from inland China (IAM) with a percentage of ∼ 84 %, and the airmass from eastern coastal areas (CAM) with ∼ 16 %. During the IAM period, the NO3 production rate reached 1.6 ± 0.9 ppbv h−1 due to the transportation of the polluted urban plume with high NOx and O3. The average nocturnal N2O5 and the calculated NO3 mixing ratios were 119.5 ± 128.6 and 9.9 ± 12.5 pptv, respectively, and the steady-state lifetime of NO3 was 0.5 ± 0.7 min on average, indicating intensive nighttime chemistry and rapid NO3 loss at this site. By examining the reaction of NO3 with volatile organic compounds (VOCs) and N2O5 heterogeneous hydrolysis, we revealed that these two reaction pathways were not responsible for the NO3 loss (< 20 %) since the NO3 reactivity (k(NO3)) towards VOCs was small (5.2×10-3 s−1) and the aerosol loading was low. Instead, NO was proposed to significantly contribute to nocturnal NO3 loss at this site, despite the nocturnal NO concentration always below the parts per billion by volume level and near the instrument detection limit. This might be from the local soil emission or something else. We infer that the nocturnal chemical NO3 reactions would be largely enhanced once without NO emission in the open ocean after the air mass passes through this site, thus highlighting the strong influences of the urban outflow to the downwind marine areas in terms of nighttime chemistry. During the CAM period, nocturnal ozone was higher, while NOx was much lower. The NO3 production was still very fast, with a rate of 1.2 ppbv h−1. With the absence of N2O5 measurement in this period, the NO3 reactivity towards VOCs and N2O5 uptake were calculated to assess NO3 loss processes. We showed that the average k(NO3) from VOCs (56.5 %, 2.6 ± 0.9 × 10−3 s−1) was higher than that from N2O5 uptake (43.5 %, 2.0 ± 1.5 × 10−3 s−1) during the CAM period, indicating a longer NO3 / N2O5 lifetime than that during IAM period. This study improves the understanding of the nocturnal NO3 budget and environmental impacts with the interaction of anthropogenic and natural activities in marine regions.

The Aggravation of Summertime Nocturnal Ozone Pollution in China and Its Potential Impact on the Trend of Nitrate Aerosols

AbstractTropospheric ozone (O3) concentration continued to increase over past years, thereby becoming a growing environmental concern in China. Most studies have focused on the analysis of daily maximum 8‐hr O3 concentration, while there is still a dearth of investigations of nocturnal O3. Here, by analyzing the data of 1,313 sites from the China National Environmental Monitoring Center, we show a remarkable increase in the nocturnal O3 concentration during summertime of 2015–2019 in most regions of China, revealing an aggravation of nocturnal O3 pollution. Combining with a GeoDetector model and statistical analysis, we clarify that the aggravation of nocturnal O3 pollution is mainly caused by reduction in both ambient nitrogen dioxide (NO2) concentration and wet scavenging in recent years. We further reveal that the increasing O3 may have enhanced the nocturnal particulate nitrate () formation through N2O5 heterogeneous hydrolysis, and thereby driving the variation and long‐term trend of nocturnal .

Chemical Characteristics of Water-Soluble Inorganic Ions in Different Types of Asian Dust in Wajima, a Background Site in Japan

Two Asian dust (AD) events were observed in March 2021 (AD1: 16 March 2021 00:00 UTC~17 March 2021 12:00 UTC and AD2: 28 March 2021 00:00 UTC~31 March 2021 12:00 UTC). To determine the chemical characteristics of water-soluble inorganic ions (WSIIs) in different types of Asian dust, the total suspended particulates (TSP) were collected at Kanazawa University Wajima Air Monitoring Station (KUWAMS), a background site in Japan from 27 February to 4 March, 2021. Based on the lidar observations and the backwards trajectory analysis results, AD events were divided into two types: ADN (aerosols were mainly mineral dust) and ADP (aerosols were mixtures of spherical particles). During ADs, the concentrations of the TSP and WSII increased, with the highest TSP concentration in ADN (38.6 μg/m3) and the highest WSII concentration in ADP (5.82 μg/m3). The increase in (cations)/(anions) during AD indicates that the input of AD aerosol buffered the aerosol acidity. Additionally, a significant increase in Cl depletion, along with ADN events, was found (Cl depletion = 73.8%). To comprehensively analyse the different types of ADs on WSIIs, we refer to the previous data from 2010 to 2015 at KUWAMS. As a result, the increased Cl depletion was caused by the heterogeneous reaction of HNO3 with sea salt when the air mass passed over the Japanese Sea. Additionally, the chemical form of SO42− was highly dependent on the source and pathway, while SO42− mainly came from natural soil dust in ADN and from anthropogenic emissions in ADP. The enhancement of secondary NO3− was observed in AD via the heterogeneous hydrolysis of N2O5.

Diesel vehicle emission accounts for the dominate NOx source to atmospheric particulate nitrate in a coastal city: Insights from nitrate dual isotopes of PM2.5

Nitrate has long been focused due to its dominant proportion in PM2.5. In this study, PM2.5 samples were non-consecutively collected in a coastal city located in Western Pacific from March 6, 2019 to January 16, 2020. On the basis of the determination of water soluble inorganic ions and nitrogen and oxygen isotopic signatures of nitrate, its formation pathways were estimated by isotope fractionation calculation and NOx source contributions were quantified with stable isotope analysis in R (SIAR) model. Results showed that δ15N-NO3− ranged from −3.7‰ to 32.1‰, and δ18O-NO3− varied from 38.1‰ to 101.4‰. Nitrate formation was dominated by oxidation of NO2 with hydroxyl radical (OH) in spring, summer, and fall, while it was mainly controlled by N2O5 heterogeneous hydrolysis in winter. Quantitative analysis of NOx sources contributing to nitrate by using SIAR model showed that estimated annual average contributions from diesel vehicle emission, gasoline vehicle emission, biogenic soil emission, biomass burning, and coal combustion were 25.6%, 21.4%, 21.1%, 17.4%, and 14.5%, respectively. The control of diesel vehicle emission is vital to nitrate reduction and air quality improvement in spring, summer, and fall, while coal combustion is the primary NOx source to nitrate in winter. Posterior distribution of source contribution results from SIAR model presented more stable results in spring.

HONO Production from Gypsum Surfaces Following Exposure to NO2 and HNO3: Roles of Relative Humidity and Light Source.

Nitrous acid (HONO) is a toxic household pollutant and a major source of indoor OH radicals. The high surface-to-volume ratio and diverse lighting conditions make the indoor photochemistry of HONO complex. This study demonstrates surface uptake of NO2 and gaseous HNO3 followed by gas-phase HONO generation on gypsum surfaces, model system for drywall, under reaction conditions appropriate for an indoor air environment. Tens of parts per billion of steady-state HONO are detected under these experimental conditions. Mechanistic insight into this heterogeneous photochemistry is obtained by exploring the roles of material compositions, relative humidities, and light sources. NO2 and HNO3 are adsorbed onto drywall surfaces, which can generate HONO under illumination and under dark conditions. Photoenhanced HONO generation is observed for illumination with a solar simulator as well as with the common indoor light sources such as compact fluorescence light and incandescent light bulbs. Incandescent light sources release more HONO and NO2 near the light source compared to the solar radiation. Overall, HONO production on the gypsum surface increases with the increase of RH up to 70% relative humidity; above that, the gaseous HONO level decreases due to surface loss. Heterogeneous hydrolysis of NO2 is predicted to be the dominant HONO generation channel, where NO2 is produced through the photolysis of surface-adsorbed nitrates. This hydrolysis reaction predominantly occurs in the first layer of surface-adsorbed water.

Air humidity affects secondary aerosol formation in different pathways

Haze pollution characteristics and PM2.5 chemical composition were distinctive in different air humidity-dependent haze episodes in winter of North China Plain (NCP). The impact of air humidity on particulate chemical composition was investigated based on the in situ observation in winter of 2017–2018 in Tianjin. Relative humidity (RH) and absolute humidity affect the secondary aerosol generation in different ways. Particularly, nitrate changes more obviously with absolute humidity, while sulfate changes more obviously with RH. In the daytime, at certain conditions, high water vapor content, O3 concentration and stronger solar radiation may promote the gas-phase oxidation of NOx by the addition of OH formed though O3 photolysis, especially during the transition periods between winter and autumn or spring. Whereas in the nighttime, temperature drop generated the high RH, which was favorable for the gas-particle portioning of HNO3 and the occurrence of the N2O5 heterogeneous hydrolysis reaction. At lower temperature and higher RH (T < 0 °C, RH > 80%) condition, SO42− mass fraction was relatively higher. Lower temperature can result in more SO2 dissolved in equilibrium and the relatively higher initial aerosol pH, which both generate faster aqueous oxidation rate. Given the currently low SO2 concentration in the regional scale, the meteorological condition in which the occurrence of sulfate formation through aqueous reaction may be more stringent.

Cross-regional transport of PM2.5 nitrate in the Pearl River Delta, China: Contributions and mechanisms

Cross-regional transport potentially contributes to PM2.5 nitrate (pNO3), and this can occur as indirect transport, through which pNO3 precursors are transported to targeted regions, wherein they subsequently react with locally emitted ones to produce pNO3. However, the process has been rarely studied, which limits its comprehensive understanding. We applied the CMAQ model to study the contributions and mechanisms of pNO3 transport during autumn in the Pearl River Delta (PRD), a metropolitan region under the growing influence of cross-regional transport on PM2.5 pollution. Results showed that cross-regional transport contributed to 58% pNO3 monthly in the PRD, and this mostly occurred as indirect transport contributions (accounting for 43% among all contributions). For the first time, we identified the mechanism of indirect pNO3 transport in the PRD, which mainly involves transported O3 and locally emitted NOx reacting to produce pNO3 through N2O5 heterogeneous hydrolysis. pNO3 contributions in different periods and regions indicated differences in the indirect transport contributions to N2O5 heterogeneous hydrolysis under varying O3 availability conditions, which are determined by wind fields and the intensity of NOx emissions. On the regional scale, the pNO3 level is controlled by both transported O3 and local NOx emissions, but pNO3 sensitivity to these two precursors varies among cities. This study demonstrates the notable effect and complex process of cross-regional pNO3 transport in the PRD. Considering the important role of transported O3 for pNO3, O3 reduction within a larger scale is required to achieve PM2.5 pollution control target.

Wintertime nitrate formation pathways in the north China plain: Importance of N2O5 heterogeneous hydrolysis

Strict emission control measures have been implemented in the North China Plain (NCP) to improve air quality since 2013. However, heavy particulate matter (PM) pollution still frequently occurs in the region especially during wintertime, and the nitrate contribution to fine PM (PM2.5) has substantially increased in recent several years. Nitrate aerosols, which are formed via nitric acid (HNO3) to balance inorganic cations in the particle phase, have become a major fraction of PM2.5 during wintertime haze events in the NCP. HNO3 is mainly produced through homogeneous (NO2+OH, NO3+VOCs) and heterogeneous pathways (N2O5 heterogeneous hydrolysis) in the atmosphere, but the contribution of the two pathways to the nitrate formation remains elusive. In this study, the Weather Research and Forecasting model with Chemistry (WRF-Chem) was applied to simulate a heavy haze episode from 16 to December 31, 2016 in the North China Plain, and the source-oriented method (SOM) and brute force method (BFM) were both used to evaluate contributions of the heterogeneous pathway to the nitrate formation. The results demonstrated that the near-surface nitrate contributions of the heterogeneous pathway were about 30.8% based on the BFM, and 51.6% based on the SOM, indicating that the BFM might be subject to underestimating importance of the heterogeneous pathway to the nitrate formation. The SOM simulations further showed that the heterogeneous pathway dominated the nighttime HNO3 production in the planetary boundary layer, with an average contribution of 83.0%. Although N2O5 was photolytically liable during daytime, the heterogeneous N2O5 hydrolysis still contributed 10.1% of HNO3, which was caused by substantial attenuation of incident solar radiation by clouds and high PM2.5 mass loading. Our study highlighted the significantly important role of N2O5 heterogeneous hydrolysis in the nitrate formation during wintertime haze days.

Microscopic Mechanisms of N2O5 Hydrolysis on the Surface of Water Droplets.

Reactions of N2O5, in particular heterogeneous hydrolysis, play a vital role in determining the chemistry of the atmosphere. The N2O5 heterogeneous hydrolysis reaction has been the subject of extensive research for decades, yet the physicochemical details of the mechanism have not been established. In this study, we show that this reaction can occur on the surface of a pure water droplet. We compute a relevant transition state for a nano-size model system and follow its evolution in time by means of ab initio molecular dynamics. This transition state, where N2O5 has a strong ion-pair character, leads directly to HNO3. Both electrophilic and nucleophilic mechanisms take place. It is suggested that corresponding simulations for hydrolysis in the bulk are desirable.

Atmospheric Processing of Loess Particles in a Polluted Urban Area of Northwestern China

AbstractLoess is an important dust component of airborne particles in Northwestern China. Knowledge of the chemical composition, mixing state, and processing of loess particles in urban plumes is still limited. Urban loess particles were characterized using a single‐particle aerosol mass spectrometer. To understand sources and processing of loess particles, source samples from the road, urban background, soil, construction, and biomass burning ash were collected in the urban areas and characterized. Loess particles were determined as a kind of calcium‐silicate‐rich ones, which were internally mixed with calcium, silicates, potassium, elemental carbon, organics, ammonium, sulfate, and nitrate. Road and soil were major sources of loess particles. Among the aged loess particles, the average peak areas of taken‐up nitrate and sulfate were comparable to that of (Fe+Ca+Al). Diurnal uptake profiles of chloride, sulfate, oxalate, and nitrate on loess particles were analyzed. The nocturnal elevation of chloride occurred significantly due to the uptake of HCl (g). Nighttime nitrate formation occurred prevalently under high relative humidity conditions via the heterogeneous hydrolysis of N2O5. The nighttime enrichment of oxalate, which is a marker for aqueous‐formatted secondary organic aerosol, was also found. Besides the nighttime chemistry, the daytime photochemical activities were also a drive for the elevations of sulfate, nitrate, and ammonium. Conclusively, the processing of loess particles in polluted urban plumes significantly altered their chemical composition and mixing state.

Effects of organic coating on the nitrate formation by suppressing the N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;5&amp;lt;/sub&amp;gt; heterogeneous hydrolysis: a case study during wintertime in Beijing–Tianjin–Hebei (BTH)

Abstract. Although stringent emission mitigation strategies have been carried out since 2013 in Beijing–Tianjin–Hebei (BTH), China, heavy haze with high levels of fine particulate matter (PM2.5) still frequently engulfs the region during wintertime and the nitrate contribution to PM2.5 mass has progressively increased. N2O5 heterogeneous hydrolysis is the most important pathway of nitrate formation at nighttime. In the present study, the WRF-Chem model is applied to simulate a heavy haze episode from 10 to 27 February 2014 in BTH to evaluate contributions of N2O5 heterogeneous hydrolysis to nitrate formation and effects of organic coating. The model generally performs reasonably well in simulating meteorological parameters, air pollutants, and aerosol species against observations in BTH. N2O5 heterogeneous hydrolysis with all the secondary organic aerosol assumed to be involved in coating considerably improves the nitrate simulations compared to the measurements in Beijing. On average, organic coating decreases nitrate concentrations by 8.4 % in BTH during an episode, and N2O5 heterogeneous hydrolysis with organic coating contributes about 30.1 % of nitrate concentrations. Additionally, the reaction also plays a considerable role in the heavy haze formation, with a PM2.5 contribution of about 11.6 % in BTH. Sensitivity studies also reveal that future studies need to be conducted to predict the organic aerosol hygroscopicity for accurately representing the organic coating effect on N2O5 heterogeneous hydrolysis.

Summertime fine particulate nitrate pollution in the North China Plain: increasing trends, formation mechanisms and implications for control policy

Abstract. Nitrate aerosol makes up a significant fraction of fine particles and plays a key role in regional air quality and climate. The North China Plain (NCP) is one of the most industrialized and polluted regions in China. To obtain a holistic understanding of the nitrate pollution and its formation mechanisms over the NCP region, intensive field observations were conducted at three sites during summertime in 2014–2015. The measurement sites include an urban site in downtown Jinan – the capital city of Shandong Province –, a rural site downwind of Jinan city, and a remote mountain site at Mt. Tai (1534 m a.s.l.). Elevated nitrate concentrations were observed at all three sites despite distinct temporal and spatial variations. Using historical observations, the nitrate ∕ PM2.5 and nitrate ∕ sulfate ratios have statistically significantly increased in Jinan (2005–2015) and at Mt. Tai (from 2007 to 2014), indicating the worsening situation of regional nitrate pollution. A multiphase chemical box model (RACM–CAPRAM) was deployed and constrained by observations to elucidate the nitrate formation mechanisms. The principal formation route is the partitioning of gaseous HNO3 to the aerosol phase during the day, whilst the nocturnal nitrate formation is dominated by the heterogeneous hydrolysis of N2O5. The daytime nitrate production in the NCP region is mainly limited by the availability of NO2 and to a lesser extent by O3 and NH3. In comparison, the nighttime formation is controlled by both NO2 and O3. The presence of NH3 contributes to the formation of nitrate aerosol during the day, while there is slightly decreasing nitrate formation at night. Our analyses suggest that controlling NOx and O3 is an efficient way, at the moment, to mitigate nitrate pollution in the NCP region, where NH3 is usually in excess in summer. This study provides observational evidence of a rising trend of nitrate aerosol as well as scientific support for formulating effective control strategies for regional haze in China.

Modeling NH4NO3 Over the San Joaquin Valley During the 2013 DISCOVER-AQ Campaign.

The San Joaquin Valley (SJV) of California experiences high concentrations of particulate matter NH4NO3 during episodes of meteorological stagnation in winter. A rich data set of observations related to NH4NO3 formation was acquired during multiple periods of elevated NH4NO3 during the Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) field campaign in SJV in January and February 2013. Here NH4NO3 is simulated during the SJV DISCOVER-AQ study period with the Community Multiscale Air Quality (CMAQ) model, diagnostic model evaluation is performed using the DISCOVER-AQ data set, and integrated reaction rate analysis is used to quantify HNO3 production rates. Simulated NO3- generally agrees well with routine monitoring of 24-hr average NO3-, but comparisons with hourly average NO3- measurements in Fresno revealed differences at higher time resolution. Predictions of gas-particle partitioning of total nitrate (HNO3 + NO3-) and NHx (NH3 + NH4+) generally agree well with measurements in Fresno, although partitioning of total nitrate to HNO3 is sometimes overestimated at low relative humidity in afternoon. Gas-particle partitioning results indicate that NH4NO3 formation is limited by HNO3 availability in both the model and ambient. NH3 mixing ratios are underestimated, particularly in areas with large agricultural activity, and additional work on the spatial allocation of NH3 emissions is warranted. During a period of elevated NH4NO3, the model predicted that the OH + NO2 pathway contributed 46% to total HNO3production in SJV and the N2O5 heterogeneous hydrolysis pathway contributed 54%. The relative importance of the OH + NO2 pathway for HNO3 production is predicted to increase as NOx emissions decrease.

A parameterization of the heterogeneous hydrolysis of N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;5&amp;lt;/sub&amp;gt; for mass-based aerosol models: improvement of particulate nitrate prediction

Abstract. The heterogeneous hydrolysis of N2O5 on the surface of deliquescent aerosol leads to HNO3 formation and acts as a major sink of NOx in the atmosphere during night-time. The reaction constant of this heterogeneous hydrolysis is determined by temperature (T), relative humidity (RH), aerosol particle composition, and the surface area concentration (S). However, these parameters were not comprehensively considered in the parameterization of the heterogeneous hydrolysis of N2O5 in previous mass-based 3-D aerosol modelling studies. In this investigation, we propose a sophisticated parameterization (NewN2O5) of N2O5 heterogeneous hydrolysis with respect to T, RH, aerosol particle compositions, and S based on laboratory experiments. We evaluated closure between NewN2O5 and a state-of-the-art parameterization based on a sectional aerosol treatment. The comparison showed a good linear relationship (R = 0.91) between these two parameterizations. NewN2O5 was incorporated into a 3-D fully online coupled model, COSMO–MUSCAT, with the mass-based aerosol treatment. As a case study, we used the data from the HOPE Melpitz campaign (10–25 September 2013) to validate model performance. Here, we investigated the improvement of nitrate prediction over western and central Europe. The modelled particulate nitrate mass concentrations ([NO3−]) were validated by filter measurements over Germany (Neuglobsow, Schmücke, Zingst, and Melpitz). The modelled [NO3−] was significantly overestimated for this period by a factor of 5–19, with the corrected NH3 emissions (reduced by 50 %) and the original parameterization of N2O5 heterogeneous hydrolysis. The NewN2O5 significantly reduces the overestimation of [NO3−] by ∼ 35 %. Particularly, the overestimation factor was reduced to approximately 1.4 in our case study (12, 17–18 and 25 September 2013) when [NO3−] was dominated by local chemical formations. In our case, the suppression of organic coating was negligible over western and central Europe, with an influence on [NO3−] of less than 2 % on average and 20 % at the most significant moment. To obtain a significant impact of the organic coating effect, N2O5, SOA, and NH3 need to be present when RH is high and T is low. However, those conditions were rarely fulfilled simultaneously over western and central Europe. Hence, the organic coating effect on the reaction probability of N2O5 may not be as significant as expected over western and central Europe.

High N2O5 Concentrations Observed in Urban Beijing: Implications of a Large Nitrate Formation Pathway

The heterogeneous hydrolysis of dinitrogen pentoxide (N2O5) is important to understanding the formation of particulate nitrate (pNO3–). Measurements of N2O5 in the surface layer taken at an urban site in Beijing are presented here. N2O5 was observed with large day-to-day variability. High N2O5 concentrations were determined during pollution episodes with the co-presence of large aerosol loads. The maximum value was 1.3 ppbv (5 s average), associated with an air mass characterized by a high level of O3. N2O5 uptake coefficients were estimated to be in the range of 0.025–0.072 using the steady-state lifetime method. As a consequence, the nocturnal pNO3– formation potential by N2O5 heterogeneous uptake was calculated to be 24–85 μg m–3 per night and, on average, 57 μg m–3 during days with pollution. This was comparable to or even higher than that formed by the partitioning of HNO3. The results highlight that N2O5 heterogeneous hydrolysis is vital in pNO3– formation in Beijing.

Heterogeneous reaction of ClONO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; with TiO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and SiO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; aerosol particles: implications for stratospheric particle injection for climate engineering

Abstract. Deliberate injection of aerosol particles into the stratosphere is a potential climate engineering scheme. Particles injected into the stratosphere would scatter solar radiation back to space, thereby reducing the temperature at the Earth's surface and hence the impacts of global warming. Minerals such as TiO2 or SiO2 are among the potentially suitable aerosol materials for stratospheric particle injection due to their greater light-scattering ability than stratospheric sulfuric acid particles. However, the heterogeneous reactivity of mineral particles towards trace gases important for stratospheric chemistry largely remains unknown, precluding reliable assessment of their impacts on stratospheric ozone, which is of key environmental significance. In this work we have investigated for the first time the heterogeneous hydrolysis of ClONO2 on TiO2 and SiO2 aerosol particles at room temperature and at different relative humidities (RHs), using an aerosol flow tube. The uptake coefficient, γ(ClONO2), on TiO2 was ∼ 1.2 × 10−3 at 7 % RH and remained unchanged at 33 % RH, and increased for SiO2 from ∼ 2 × 10−4 at 7 % RH to ∼ 5 × 10−4 at 35 % RH, reaching a value of ∼ 6 × 10−4 at 59 % RH. We have also examined the impacts of a hypothetical TiO2 injection on stratospheric chemistry using the UKCA (United Kingdom Chemistry and Aerosol) chemistry–climate model, in which heterogeneous hydrolysis of N2O5 and ClONO2 on TiO2 particles is considered. A TiO2 injection scenario with a solar-radiation scattering effect very similar to the eruption of Mt Pinatubo was constructed. It is found that, compared to the eruption of Mt Pinatubo, TiO2 injection causes less ClOx activation and less ozone destruction in the lowermost stratosphere, while reduced depletion of N2O5 and NOx in the middle stratosphere results in decreased ozone levels. Overall, no significant difference in the vertically integrated ozone abundances is found between TiO2 injection and the eruption of Mt Pinatubo. Future work required to further assess the impacts of TiO2 injection on stratospheric chemistry is also discussed.

Evaluating N2O5 heterogeneous hydrolysis parameterizations for CalNex 2010

AbstractNighttime chemistry in the troposphere is closely tied to the dinitrogen pentoxide (N2O5) budget, but high uncertainties remain regarding the model representation of the heterogeneous hydrolysis of N2O5 on aerosol particles. In this study we used the community model WRF‐Chem to simulate a 3‐day period during the California Nexus (CalNex) Campaign in 2010. We extended WRF‐Chem to include the heterogeneous hydrolysis of N2O5 and contrasted the impact of different published parameterizations of N2O5 heterogeneous hydrolysis on the spatial distribution of uptake coefficients and the resulting N2O5 concentrations. For all the cases, modeled N2O5 uptake coefficients showed strong spatial variability, with higher values in the nocturnal boundary layer compared to the residual layer, especially in environments with high relative humidities, such as over the ocean and along the coast. The best agreement of modeled and observed uptake coefficients was obtained using the parameterization by Davis et al. (2008) combined with the treatment of organic coating by Riemer et al. (2009). For this case the temporal evolution of lower boundary layer N2O5 mixing ratios was reproduced well, and the predictions of surface mixing ratios of ozone and NOx were improved. However, the model still overpredicted the uptake coefficients in the residual layer and consequently underpredicted N2O5 concentrations in the residual layer. This study also highlights that environments with low relative humidities pose a challenge for aerosol thermodynamic models in calculating aerosol water uptake, and this impacts N2O5 heterogeneous hydrolysis parameterizations.

Inorganic aerosols responses to emission changes in Yangtze River Delta, China

The new Chinese National Ambient Air Quality standards (CH-NAAQS) published on Feb. 29th, 2012 listed PM2.5 as criteria pollutant for the very first time. In order to probe into PM2.5 pollution over Yangtze River Delta, the integrated MM5/CMAQ modeling system is applied for a full year simulation to examine the PM2.5 concentration and seasonality, and also the inorganic aerosols responses to precursor emission changes. Total PM2.5 concentration over YRD was found to have strong seasonal variation with higher values in winter months (up to 89.9μg/m3 in January) and lower values in summer months (down to 28.8μg/m3 in July). Inorganic aerosols were found to have substantial contribution to PM2.5 over YRD, ranging from 37.1% in November to 52.8% in May. Nocturnal production of nitrate (NO3−) through heterogeneous hydrolysis of N2O5 was found significantly contribute to high NO3− concentration throughout the year. In winter, NO3− was found to increase under nitrogen oxides (NOx) emission reduction due to higher production of N2O5 from the excessive ozone (O3) introduced by attenuated titration, which further lead to increase of ammonium (NH4+) and sulfate (SO42−), while other seasons showed decrease response of NO3−. Sensitivity responses of NO3− under anthropogenic VOC emission reduction was examined and demonstrated that in urban areas over YRD, NO3− formation was actually more sensitive to VOC than NOx due to the O3-involved nighttime chemistry of N2O5, while a reduction of NOx emission may have counter-intuitive effect by increasing concentrations of inorganic aerosols.

Analysis of water-soluble ions and their precursor gases over diurnal cycle

A detailed description of an interactive relationship is made between water-soluble ionic components in PM2.5 and their precursor gases using the hourly resolution data measured at an urban monitoring site in Seoul during the year of 2010. Their diurnal variability was found to be correlated with their precursor gases (HNO3 and NH3). In the case of NO3− and NH4+, a close similarity was seen in their diurnal trends, especially during spring. There were no noticeable differences in the formation pathway of NO3−, SO42−, and NH4+ aerosols between day and night. For NO3− formation, heterogeneous hydrolysis of N2O5 might play a significant role in the nighttime enhancement of the oxidation rate. In addition, the concentrations of major anions (e.g. NO3−, SO42−, and Cl−) exhibited highly diverse seasonal patterns with their maximum values occurring in spring, summer, and winter, respectively. The overall results of this study suggest that the formation pathway of NO3− and SO42− aerosols should be regulated by a competing relationship between the NH3-lean (spring and winter) and NH3-rich conditions (summer and fall).