Seasonal Variation of Nitrate Concentration and Its Direct Radiative Forcing over East Asia

Chemical Characteristics of Major Inorganic Ions in PM2.5 Based on Year-Long Observations in Guiyang, Southwest China—Implications for Formation Pathways and the Influences of Regional Transport

Sulfate, nitrate and ammonium (SNA) are the dominant components of water-soluble ions (WSIs) in PM2.5, which are of great significance for understanding the sources and transformation mechanisms of PM2.5. In this study, daily PM2.5 samples were collected from September 2017 to August 2018 within the Guiyang urban area and the concentrations of the major WSIs in the PM2.5 samples were characterized. The results showed that the average concentration of SNA (SO42−, NO3−, NH4+) was 15.01 ± 9.35 μg m−3, accounting for 81.05% (48.71–93.76%) of the total WSIs and 45.33% (14.25–82.43%) of the PM2.5 and their possible chemical composition in PM2.5 was (NH4)2SO4 and NH4NO3. The highest SOR (sulfur oxidation ratio) was found in summer, which was mainly due to the higher temperature and O3 concentrations, while the lowest NOR (nitrogen oxidation ratio) found in summer may ascribe to the volatilization of nitrates being accelerated at higher temperature. Furthermore, the nitrate formation was more obvious in NH4+-rich environments so reducing NH3 emissions could effectively control the formation of nitrate. The results of the trajectory cluster analysis suggested that air pollutants can be easily enriched over short air mass trajectories from local emission sources, affecting the chemical composition of PM2.5.

The distributions and direct radiative effects of marine aerosols over East Asia in springtime

The characteristics, distributions, and direct radiative effects (DRE) of marine aerosols in the western Pacific Ocean over East Asia during the period from 17 March to 22 April 2014 were investigated by an online-coupled regional atmospheric chemistry/aerosol-climate model RIEMS-Chem (Regional Integrated Environmental Model System with Chemistry). The emissions and relevant processes of sea salt, marine primary organic aerosol (MPOA), sulfate and Methyl sulfonic acid (MSA) produced from dimethylsulfide (DMS) were parameterized and coupled with RIEMS-Chem. The model results for total aerosol masses (PM₁₀ and PM_2.5), inorganic and carbonaceous aerosols, gas precursors, and aerosol optical depth (AOD) were compared with various observational data sets including a research cruise Dongfanghong II from the Yellow Sea to the open oceans, near-surface aerosol and gas concentrations from the Acid Deposition Monitoring Network in East Asia (EANET) and China National Environmental Monitoring Center (CNEMC), and AOD from the Aerosol Robotic Network (AERONET). Model comparisons demonstrated a generally good skill of the RIEMS-Chem in representing the temporal and spatial variations of these variables. The distributions of marine aerosols were characterized by the maximum sea salt concentration up to 70 μg m^-3 in the ocean northeast of Japan, the maximum concentration of MPOA >2 μg m^-3 in the East China Sea and in portions of the northwest Pacific (NWP) region, and the maximum DMS-produced aerosol concentration >0.3 μg m^-3 in the southern parts of the ocean. It was noteworthy that marine aerosols can be easily transported to the inland areas of south China. The clear-sky DREs by sea salt ranging from -9 to -17 W/m² occurred in the open oceans northeast of Japan, comparable to the DREs of -10 ~ -20 W/m² by anthropogenic aerosols, whereas the DREs by MPOA were strongest (up to -1.3 Wm^-2) in the East China Sea and the oceans northeast of Japan due to active phytoplankton blooms there and comparable in magnitude to the DREs by sea salt (around -3 Wm^-2) in the East China Sea. The maximum DRE by the DMS-produced aerosols was -0.4 Wm^-2 mainly in the northern parts of the South China Sea. Sea salt exhibited an increasing radiative importance from the China marginal seas to the open oceans, accounting for 10% and 33% of the DREs by all aerosols, respectively. Under all-sky conditions, the sum of DREs by all the marine aerosols were estimated to be -2.2 W/m², -3.5 W/m², -2.3 W/m², and -4.3 W/m² averaged over the entire domain, ocean, East China Sea, and the NWP region, accounting for 20%, 27%, 13%, and 36% of the DREs by all aerosols, respectively, which demonstrated the important role of marine aerosols in modulating shortwave radiation in springtime in the western Pacific Ocean which was just downwind of the Asian continent with large amounts of anthropogenic and dust emissions.

Radiative and heterogeneous chemical effects of aerosols on ozone and inorganic aerosols over East Asia

Currently, many challenges are faced in simulating ozone(O₃), sulfate(SO₄^2-), and nitrate(NO₃^-) concentrations over East Asia, particularly the overestimation of surface O₃ and NO₃^- concentrations and underestimation of the SO₄^2- concentration during haze episodes. In this study, we examined the radiative and heterogeneous chemical effects of aerosols by incorporating recently reported mechanisms, including self-amplifying SO₄^2- formation, dinitrogen pentoxide (N₂O₅) hydrolysis, and a heterogeneous reaction converting gaseous nitric acid (HNO₃) to nitric oxide (NO_x), into a Nested Air Quality Prediction Modeling System. Uptakes by aerosols can be computed through a simple parameterization that is dependent on the aerosol core and shell species, shell thickness, and amount of aerosol liquid water. In this study, a 1-year simulation was conducted for 2013. The updated model successfully reproduced the seasonal and daily observations of O₃, fine particulate matter, SO₄^2-, and NO₃^- concentrations in East Asia. Our results revealed that heterogeneous reactions reduced more surface O₃ concentrations (10-20 ppbv) in the polluted regions of East China than did perturbations in photolysis frequencies from aerosols, effectively again improving the comparison between simulations and observations. Oxidation of SO₂ by NO₂ on wet aerosols significantly enhanced SO₄^2- formation, with sulfate covering approximately ~30-60% of total sulfate concentrations in North China Plain during haze days in winter. The uptake of reactive nitrogen species on aerosols effectively reduced NO₃^- concentrations and successfully balanced the NO_x/HNO₃ chemistry in the models. We recommended that larger reductions of gaseous precursors should be considered in China to achieve the national air quality objective. The results show that surface O₃ concentrations over East China will increase if the emission of aerosols is reduced without corresponding reductions in O₃ precursors.

Temporal and spatial variation in major ion chemistry and source identification of secondary inorganic aerosols in Northern Zhejiang Province, China

To investigate the seasonal and spatial variations of ion chemistry of fine particles in Northern Zhejiang Province (NZP), China, one year-long field sampling was conducted at four representative sites (two urban, one suburb, and one rural sites) in both cities of Hangzhou and Ningbo from December 2014 to November 2015. Twelve water soluble inorganic ions (WSII) were characterized in this comprehensive study. The annual average of PM_2.5 concentration in NZP as overall was 66.2 ± 37.7 μg m^-3, and urban sites in NZP were observed with more severe PM_2.5 pollution than the suburban and rural sites. The annual average concentration of total WSII at four sampling sites in NZP was 29.1 ± 19.9 μg m^-3, dominated by SO₄^2- (10.3 μg m^-3), and followed by NO₃^- (8.9 μg m^-3), NH₄⁺ (6.6 μg m^-3), Cl^- (1.3 μg m^-3) and K⁺ (0.7 μg m^-3). Among all cations, NH₄⁺ was the predominant neutralizing ion with the highest neutralization factor (NF), while the remaining cations showed limited neutralization capacity. The highest and lowest sulfur oxidation ratio (SOR) values in this region were found in summer and winter, respectively; while the seasonal patterns for nitrogen oxidation ratio (NOR) were opposite to that of SOR. Principal component analysis (PCA) showed that the significant sources of WSII in NZP were industrial emissions, biomass burning, and formation of secondary inorganic aerosols. In addition, contribution from transboundary transport of polluted aerosols was also confirmed from the assessment through air mass backward trajectory analysis.