Changes of water-soluble inorganic sulfate and nitrate during severe dust storm episodes in a coastal city of North China

Dust storms are one of the largest sources of non-exhaust emissions in China, which can adversely affect air quality and human health during long-distance transportation. To study the influence of dust storms on aerosol particle composition, samples of fine aerosol (PM2.5) were collected before, during, and after the severe dust storm episodes in a coastal city of North China. Then the water-soluble inorganic ions in the filters were analyzed. The results showed that the chemical composition varied significantly in different sampling periods. Before the dust storm periods (Phase 1), the weather was characterized by high relative humidity. NO3- was the main water-soluble inorganic ion, accounting for about 1/3 of the total mass of PM2.5, which is very different from the situation a few years ago when sulfate was the dominant. The results indicated that the chemical composition of the atmosphere in China has changed significantly after the implementation of strict air pollution control measures. During the severe dust storm periods (within a few hours after the dust invasion, Phase 2), the proportion of Ca2+ in PM2.5 was high; the sulfate formation was limited due to adiabatic air mass affected by the cold front, and the sulfate content might be mainly from desert soil. However, a small amount of nitrate can be formed during their long-distance transportation. After the dust storm periods (Phase 3), dust plums and local polluted air mass mixed well. The proportion of secondary inorganic ions increased, and nitrate formation was still the main. The changes in the chemical composition from a few years ago during Phase 1 and the sharp changes in different water-soluble inorganic ions during different Phases should be carefully considered to evaluate their implications for air quality and human health.

Emission, transport, deposition, chemical and radiative impacts of mineral dust during severe dust storm periods in March 2021 over East Asia

A Regional Air Quality Model System (named RAQMS) coupled with a developed dust model driven by WRF was applied to synthetically investigate the emission, transport, deposition, budget, and chemical and radiative effects of mineral dust during the severe dust storm periods of 10-31 March 2021. Model results were validated against a variety of ground, vertical and satellite observations, which demonstrated a generally good model ability in reproducing meteorological variables, particulate matter and compositions, and aerosol optical properties. The first dust storm (DS1), which was the severest one since 2010 was originated from the Gobi Desert in southern Mongolia on 14 March, with the dust emission flux reaching 2785 μg m^-2 s^-1 and the maximum dust concentration exceeding 18,000 μg m^-3 in the dust deflation region. This dust storm resulted in remarkably high hourly PM₁₀ observations up to 7506 μg m^-3, 1887 μg m^-3, and 2704 μg m^-3 in Beijing, Tianjin, and Shijiazhuang on 15 March, respectively, and led to a maximum decrease in surface shortwave radiation up to 313.4 W m^-2 (72 %) in Beijing. The second dust storm (DS2) broke out in the deserts of eastern Mongolia, with lower dust emission than the first one. The extinction of shortwave radiation by dust aerosols led to a reduction in photolysis rate and consequently decreases in O₃ and secondary aerosol concentrations over the North China Plain (NCP), whereas total sulfate and nitrate concentrations consistently increased due to heterogeneous reactions on dust surfaces over the middle reaches of the Yellow River and the NCP region during DS1. Sulfate and nitrate formation through heterogeneous reactions were enhanced in the dust backflow on 16-17 March by approximately 18 % and 24 % on average in the NCP. Heterogeneous reactions and photolysis rate reduction by mineral dust jointly led to average changes in sulfate, nitrate, ammonium, and secondary organic aerosol (SOA) concentrations by 13.0 %, 13.5 %, -12.3 %, and -4.4 %, respectively, in the NCP region during DS1, larger than the changes in the Yangtze River Delta (YRD). The maximum dry deposition settled in the 7-11 μm size range in downwind land and ocean areas, while wet deposition peaked in the 4.7-7 μm size range in the entire domain. Wet deposition was approximately twice the dry deposition over mainland China except for dust source regions. During 10-31 March, the total dust emission, dry and wet depositions were estimated to be 31.4 Tg, 13.78 Tg and 4.75 Tg, respectively, with remaining 12.87 Tg of dust aerosols (41 % of the dust emission) suspending in the atmosphere or transporting to other continents and oceans.

Physical and chemical characterization of urban grime: An impact on the NO2 uptake coefficients and N-containing product compounds

Urban grime represents an important environmental surface for heterogeneous reactions in urban environment. Here, we assess the physical and chemical properties of urban grime collected during six consecutive months in downtown of Guangzhou, China. There is a significant variation of the uptake coefficients of NO₂ on the urban grime as a function of the relative humidity (RH). In absence of water molecules (0% RH), the light-induced uptake coefficients of NO₂ on urban grime samples collected during six months are very similar in order of ≈10^-6. At 80% RH, depending on the sampling month the light-induced uptake coefficient of NO₂ can reach one order of magnitude higher values (1.5 × 10^-5, at 80% RH) compared to those uptakes at 0% RH. In presence of 80% RH, there are strong correlations between the measured NO₂ uptakes and the concentrations of the water soluble carbon, soluble anions, polycyclic aromatic hydrocarbons and n-alkanes depicted in the urban grime. These correlations, demonstrate that surface adsorbed water on urban grime play an important role for the uptakes of NO₂. The heterogeneous conversion of NO₂ on two-month old urban grime under sunlight irradiation (68 W m^-2, 300 nm < λ < 400 nm) at 60% RH leads to the formation of unprecedented HONO surface flux of 4.7 × 10¹⁰ molecules cm^-2 s^-1 which is higher than all previously observed HONO fluxes, thereby affecting the oxidation capacity of the urban atmosphere. During the heterogeneous chemistry of NO₂ with urban grime, the unsaturated and N-containing organic compounds are released in the gas phase which can affect the air quality in the urban environment.

Causes of PM2.5 pollution in an air pollution transport channel city of northern China

To develop effective mitigation policies, a comprehensive understanding of the evolution of the chemical composition, formation mechanisms, and the contribution of sources at different pollution levels is required. PM_2.5 samples were collected for 1 year from August 2016 to August 2017 at an urban site in Zibo, then chemical compositions were analyzed. Secondary inorganic aerosols (SNA), anthropogenic minerals (MIN), and organic matter (OM) were the most abundant components of PM_2.5, but only the mass fraction of SNA increased as the pollution evolved, implying that PM_2.5 pollution was caused by the formation of secondary aerosols, especially nitrate. A more intense secondary transformation was found in the heating season (from November 15, 2016, to March 14, 2017), and a faster secondary conversion of nitrate than sulfate was discovered as the pollution level increased. The formation of sulfate was dominated by heterogeneous reactions. High relative humidity (RH) in polluted periods accelerated the formation of sulfate, and high temperature in the non-heating season also promoted the formation of sulfate. Zibo city was under ammonium-rich conditions during polluted periods in both seasons; therefore, nitrate was mainly formed through homogeneous reactions. The liquid water content increased significantly as the pollution levels increased when the RH was above 80%, indicating that the hygroscopic growth of aerosol aggravated the PM_2.5 pollution. Source apportionment showed that PM_2.5 was mainly from secondary aerosol formation, road dust, coal combustion, and vehicle emissions, contributing 36.6%, 16.5%, 14.7%, and 13.1% of PM_2.5 mass, respectively. The contribution of secondary aerosol formation increased remarkably with the deterioration of air quality, especially in the heating season.

Insights into high concentrations of particle-bound imidazoles in the background atmosphere of southern China: Potential sources and influencing factors

Imidazoles are important constituents in atmospheric brown carbon and have gained increasing attention in the past decade. Although imidazoles have been studied widely in laboratories, the sparse field observations severely limit the understanding of imidazole's abundance and sources in the atmosphere. In this study, we measured particle-bound imidazoles and their precursors at a background forest site in the Nanling Mountains of southern China. The average concentration of imidazoles (4.17 ± 3.76 ng/m³) was found to be significantly higher than other background sites worldwide. Further analyses revealed that a majority of imidazoles (59.1%) at the site originated from secondary formation through reactions of dicarbonyls (e.g., glyoxal and methylglyoxal) and reduced nitrogen species, with relatively minor contributions from regional transport (32.8%) and biomass burning (8.1%). In addition, the key factors influencing secondary formation of imidazoles, such as relative humidity, water-soluble inorganic ions, and pH, were analyzed. Our results indicated that the secondary formation of imidazoles can be greatly enhanced under high humidity conditions, particularly during fog events. Overall, this study offers valuable insights into potential sources and influencing factors of ambient imidazoles in background atmospheres.

Exploring dust heterogeneous chemistry over China: Insights from field observation and GEOS-Chem simulation

Dust heterogeneous chemistry plays an important role in tropospheric chemistry, but its parameterization in numerical models is often quite simplified, which hampers accurate prediction of particulate matter and its chemical component. In this study, we investigate the evolution of dust heterogeneous chemical process and its potential impacts on gaseous and aerosol components during a dust pollution episode from March 27 to April 2, 2015 over North China. Based on field measurements, the significant role of relative humidity (RH) in dust heterogeneous chemistry is found and a RH-dependent parameterization for uptake coefficients of HNO₃ and SO₂ is incorporated in GEOS-Chem to reproduce the dust heterogeneous chemical process. During the study period, observed dust sulfate (DSO4) and dust nitrate (DNIT) exhibit maximum concentrations of 9.1 and 22.8 μg m^-3 respectively, accompanied by high RH and gaseous precursor concentrations. DSO4 concentrations are positively related to RH. The observed dust sulfate oxidation ratio (DSOR) is elevated evidently with increased RH, especially when RH is higher than ~40%, implying that enhanced RH could promote heterogeneous oxidation of SO₂ to DSO4. Model simulation shows that when incorporating the RH-dependent parameterization, DNIT and DSO4 are generally well captured and the model performance of total sulfate oxidation ratio (TSOR) and total nitrate oxidation ratio (TNOR) are improved. High contribution of DNIT and DSO4 are found to be located over the regions close to source areas (>60%) and downwind regions (>40%), respectively. Sensitivity results show that SO₂ and HNO₃ reduce by 2-24 μg m^-3 and 1-18 μg m^-3 when considering dust heterogeneous impacts, thus leading to reduction in non-dust sulfate and non-dust nitrate concentrations. As a result, simulated NH₃ increases and ammonium reduces by more than 20%. Our study indicates that the contribution of heterogeneous reactions to sulfate formation is 20-30% over North China.

A new parameterization of uptake coefficients for heterogeneous reactions on multi-component atmospheric aerosols

Based on laboratory studies and field observations, a new parameterization of uptake coefficients for heterogeneous reactions on multi-component aerosols is developed in this work. The equivalent ratio (ER) of inorganic aerosol is used to establish the quantitative relationship between the heterogeneous uptake coefficients and the composition of aerosols. Incorporating the new ER-dependent scheme, the WRF-CUACE model has been applied to simulate sulfate mass concentrations during December 2017 in the Beijing-Tianjin-Hebei region and evaluate the role of aerosol chemical components played in the sulfate formation. Simulated temporal variations and magnitudes of sulfate show good agreement with the observations by using this new scheme. From clean to polluted cases, although both dominant cations and anions increase significantly, the equivalent ratio decreases gradually and is closer to unity, representing the variation of aerosol compositions, which inhibits the heterogeneous uptake of SO₂, with the uptake coefficient decreasing from 1 × 10^-4 to 5.3 × 10^-5. Based on this phenomenon, a self-limitation process for heterogeneous reactions with the increasing secondary inorganic aerosol from clean to polluted cases is proposed.

Heterogeneous reactions of isoprene and ozone on α-Al2O3: The suppression effect of relative humidity

The heterogeneous reactions of α-Al₂O₃ particles with a mixture of ozone (∼50 ppm) and isoprene (∼50 ppm) were studied as a function of relative humidities (RHs). The reactions were monitored in real time through the microscopic Fourier transform infrared (micro-FTIR) spectrometer. The results show that the presence of ozone leads to the rapid conversion of isoprene to carboxylate (COO^-) ions on the surfaces of α-Al₂O₃ particles in the initial stage. The water significantly suppresses the formation of the carboxylate ions. For the isoprene ozonolysis reaction on the α-Al₂O₃ particles, the reactive uptake coefficient is strongly suppressed by over a factor of 8 when the RH increases from 8% to 89%. The negative correlation between RH with the secondary organic aerosol (SOA) produced by isoprene ozonolysis plays a key role in the actual atmospheric environment under high humidity. Our results may provide insight into the ozonolysis process of biogenic alkenes over mineral aerosol surfaces with the influence of RHs.

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.

The influence of relative humidity on the heterogeneous oxidation of sulfur dioxide by ozone on calcium carbonate particles

Heterogeneous reactions of SO₂ and O₃ with CaCO₃ particles were investigated at a series of relative humidity (RH, 1% to 90%) and 298K using a diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The uptake coefficients of SO₂ on CaCO₃ at different RHs were obtained for the first time. Our results proved that high RH could substantially promote the formation of sulfate, for which the highest concentration (80% RH and reaction time of 200min) and highest formation rate in stable stage (85% RH) were 14 times and 43 times that at 1% RH, respectively. The surface products, increment of concentration and formation rate of sulfate changed with RH which were due to the surface adsorbed water (SAW) on the particles. SAW could increase the reactive sites on the particles and thus accelerate the conversion of SO₂ into sulfite, and sulfite could be oxidized rapidly. Liquid-like water layers formed on the particle surface could enhance the ion mobility and promote the aggregation of CaSO₄ hydrates, which could expose more reactive sites and result in additional adsorption of SO₂. Piecewise equations of uptake coefficient with RH were given and could be referred by model simulation. The results are of importance in understanding the explosive growth of sulfate during severe haze episodes accompanied with high RH.

Using X-ray computed tomography and micro-Raman spectrometry to measure individual particle surface area, volume, and morphology towards investigating atmospheric heterogeneous reactions

Heterogeneous reactions on the aerosol particle surface in the atmosphere play important roles in air pollution, climate change, and global biogeochemical cycles. However, the reported uptake coefficients of heterogeneous reactions usually have large variations and may not be relevant to real atmospheric conditions. One of the major reasons for this is the use of bulk samples in laboratory experiments, while particles in the atmosphere are suspended individually. A number of technologies have been developed recently to study heterogeneous reactions on the surfaces of individual particles. Precise measurements on the reactive surface area, volume, and morphology of individual particles are necessary for calculating the uptake coefficient, quantifying reactants and products, and understanding the reaction mechanism better. In this study, for the first time we used synchrotron radiation X-ray computed tomography (XCT) and micro-Raman spectrometry to measure individual CaCO₃ particle morphology, with sizes ranging from 3.5-6.5μm. Particle surface area and volume were calculated using a reconstruction method based on software three-dimensional (3-D) rendering. The XCT was first validated with high-resolution field-emission scanning electron microscopy (FE-SEM) to acquire accurate CaCO₃ particle surface area and volume estimates. Our results showed an average difference of only 6.1% in surface area and 3.2% in volume measured either by micro-Raman spectrometry or X-ray tomography. X-ray tomography and FE-SEM can provide more morphological details of individual CaCO₃ particles than micro-Raman spectrometry. This study demonstrated that X-ray computed tomography and micro-Raman spectrometry can precisely measure the surface area, volume, and morphology of an individual particle.

The effects of coexisting Na2SO4 on heterogeneous uptake of NO2 on CaCO3 particles at various RHs

Atmospheric particles can undergo nucleation, coagulation, chemical-aging, dissolution-precipitation or other atmospheric processes, resulting in complex multicomponent aerosols. The coexisting species have potentially important consequences in the heterogeneous reactions of multicomponent aerosol particles with polluted gases, which are still poorly understood. The effect of coexisting Na₂SO₄ on heterogeneous uptake of NO₂ on CaCO₃ particles is investigated in a broad RH range. The combination of DRIFTS, Raman, SEM and IC provides qualitative and quantitative information about the formation of nitrate and other surface species. Ca(NO₃)₂ and NaNO₃ are generated on mixed CaCO₃-Na₂SO₄ particles under dry condition. Both the amount of NO₃^- formed and the NO₃^- formation rates for the mixtures can be predicted based on the linear addition of those for pure CaCO₃ and Na₂SO₄ particles under dry condition. The further reaction of Ca(NO₃)₂ with Na₂SO₄ could lead to the formation of crystal NaNO₃ and CaSO₄·0.5H₂O at 30% RH. Coagulation between Ca²⁺ and SO₄^2- in surface adsorbed water is observed after part conversion of CaCO₃ to Ca(NO₃)₂, resulting in the formation of CaSO₄·2H₂O at 80% RH. The amount of NO₃^- formed on the mixtures is dramatically enhanced relative to the predictions at 30% and 80% RH. The findings presented here highlight the role of coexisting species in the heterogeneous reactions of trace gases with multicomponent aerosols due to the complexity of atmospheric particles.

Heterogeneous reactions of particulate benzo[b]fluoranthene and benzo[k]fluoranthene with NO(3) radicals

Benzo[b]fluoranthene (B[b]F) and benzo[k]fluoranthene (B[k]F) are widespread priority pollutants of polycyclic aromatic hydrocarbons (PAHs), which can react with atmospheric oxidants during transport in the troposphere and lead to the formation of more toxic compounds. At present, the rates of heterogeneous reactions of B[b]F and B[k]F aerosols with NO3 radicals, an important atmospheric oxidant, are not fully understood. Thus, this study investigated the products and kinetics of heterogeneous reactions of suspended B[b]F and B[k]F particles with NO3 radicals in an aerosol reaction chamber at room temperature (293±2K) under atmospheric pressure. The reactions are monitored online using a vacuum ultraviolet photoionization aerosol time-of-flight mass spectrometer (VUV-ATOFMS) and an atmospheric gas analysis mass spectrometer. The mono-nitro-, di-nitro-, tri-nitro-products, and those products containing both nitro and ketone groups are observed with VUV-ATOFMS. The effective rate constants for heterogeneous reactions of particulate B[b]F and B[k]F with NO3 radicals under the experimental conditions are respectively estimated to (1.2±0.1)×10(-12)cm(3)molecule(-1)s(-1) and (5.8±0.3)×10(-13)cm(3)molecule(-1)s(-1) using a mixed-phase relative rate method, and the corresponding effective uptake coefficients are respectively estimated to 0.22 and 0.65. The lifetimes of particulate B[b]F and B[k]F at a typical concentration of NO3 radicals (5×10(8)moleculecm(-3)) in the lower troposphere during the night are estimated to 3.0 and 3.9h, respectively. The experimental results of these heterogeneous reactions in the aerosol state provide supplementary knowledge for kinetic behaviors of airborne PAHs particles.