We used a single particle mass spectrometry to online detect chemical compositions of individual particles over four seasons in Guangzhou. Number fractions (Nfs) of all the measured particles that contained oxalate were 1.9%, 5.2%, 25.1%, and 15.5%, whereas the Nfs of Fe-containing particles that were internally mixed with oxalate were 8.7%, 23.1%, 45.2%, and 31.2% from spring to winter, respectively. The results provided the first direct field measurements for the enhanced formation of oxalate associated with Fe-containing particles. Other oxidized organic compounds including formate, acetate, methylglyoxal, glyoxylate, purivate, malonate, and succinate were also detected in the Fe-containing particles. It is likely that reactive oxidant species (ROS) via Fenton reactions enhanced the formation of these organic compounds and their oxidation product oxalate. Gas-particle partitioning of oxalic acid followed by coordination with Fe might also partly contribute to the enhanced oxalate. Aerosol water content likely played an important role in the enhanced oxalate formation when the relative humidity is >60%. Interactions with Fe drove the diurnal variation of oxalate in the Fe-containing particles. The study could provide a reference for model simulation to improve understanding on the formation and fate of oxalate, and the evolution and climate impacts of particulate Fe.
Abstract. The heterogeneous reaction of N2O5 with airborne illite and Arizona test dust (ATD) particles was investigated at room temperature and at different relative humidities using an atmospheric pressure aerosol flow tube. N2O5 at concentrations in the range 8 to 24 × 1012 molecule cm−3 was monitored using thermal-dissociation cavity ring-down spectroscopy at 662 nm. At zero relative humidity a large uptake coefficient of N2O5 to illite was obtained, γ(N2O5) = 0.09, which decreased to 0.04 as relative humidity was increased to 67%. In contrast, the uptake coefficient derived for ATD is much lower (~0.006) and displays a weaker (if any) dependence on relative humidity (0–67%). Potential explanations are given for the significant differences between the uptake behaviour for ATD and illite and the results are compared with uptake coefficients for N2O5 on other mineral surfaces.
Abstract. We present an estimation of the uptake coefficient (γ) and yield of nitryl chloride (ClNO2) (f) for the heterogeneous processing of dinitrogen pentoxide (N2O5) using simultaneous measurements of particle and trace gas composition at a semi-rural, non-coastal, mountain site in the summer of 2011. The yield of ClNO2 varied between (0.035 ± 0.027) and (1.38 ± 0.60) with a campaign average of (0.49 ± 0.35). The large variability in f reflects the highly variable chloride content of particles at the site. Uptake coefficients were also highly variable with minimum, maximum and average γ values of 0.004, 0.11 and 0.028 ± 0.029, respectively, with no significant correlation with particle composition, but a weak dependence on relative humidity. The uptake coefficients obtained are compared to existing parameterizations based on laboratory datasets and with other values obtained by analysis of field data.
Abstract. Peroxy radicals were measured by a PeRCA (Peroxy Radical Chemical Amplifier) instrument in the boundary layer during the DOMINO (Diel Oxidant Mechanisms In relation to Nitrogen Oxides) campaign at a coastal, forested site influenced by urban-industrial emissions in southern Spain in late autumn. Total peroxy radicals (RO2* = HO2 + ΣRO2) generally showed a daylight maximum between 10 and 50 pptv at 13:00 UTC, with an average of 18 pptv over the 15 days of measurements. Emissions from the industrial area of Huelva often impacted the measurement site at night during the campaign. The processing of significant levels of anthropogenic organics leads to an intense nocturnal radical chemistry accompanied by formation of organic peroxy radicals at comparable levels to those of summer photochemical conditions with peak events up to 60–80 pptv. The RO2 production initiated by reactions of NO3 with organic trace gases was estimated to be significant, but not sufficient to account for the concentrations of RO2* observed in air masses carrying high pollutant loading. The nocturnal production of peroxy radicals in those periods seems therefore to be dominated by ozonolysis of volatile organic compounds, in particular alkenes of industrial petrochemical origin. RO2* diurnal variations were consistent with HO2 measurements available at the site. HO2/RO2* ratios generally varied between 0.3 and 0.6, though on some occasions this ratio was likely to have been affected by instrumental artifacts (overestimated HO2) associated with high RO2 loads.
Abstract Aerosol deposition is a major source of soluble Fe in open oceans, affecting marine biogeochemistry and primary production. However, Fe fractional solubility, a key parameter in estimating deposition fluxes of soluble aerosol Fe, is still highly uncertain. Abundance and fractional solubility of aerosol Fe in fine (<1 μm) and coarse (>1 μm) particles was measured at Qingdao (a coastal city in northern China) in November‐December 2019. Average concentrations of total and soluble Fe were found to be 798 ± 466 and 7.7 ± 14.5 ng/m 3 in coarse particles, and 801 ± 534 and 7.3 ± 7.6 ng/m 3 in fine particles. Fe solubility was significantly lower in coarse particles (average: 0.80 ± 1.03%) than fine particles (average 1.29 ± 1.41%). Compared to clean days, total Fe concentration was substantially increased during dust and haze days; however, Fe solubility was significantly reduced in dust days and elevated in haze days. Acid processing significantly enhanced Fe solubility in both fine and coarse particles, and the contribution of primary emission to Fe solubility enhancement was important for fine particles but minor for coarse particles. Higher Fe solubility (>1%) in fine and coarse particles was usually observed at high aerosol acidity (pH < 4) and high RH (>60%), suggesting critical roles of aerosol acidity and water content in regulating aerosol Fe solubility.
Abstract. Nighttime mixing ratios of boundary layer N2O5 were determined using cavity-ring-down spectroscopy during the DOMINO campaign. Observation of N2O5 was intermittent, with mixing ratios ranging from below the detection limit (~5 ppt) to ~500 ppt. A steady-state analysis constrained by measured mixing ratios of NO2 and O3 was used to derive NO3 lifetimes and compare them to calculated rates of loss via gas-phase and heterogeneous reactions of both NO3 and N2O5. Three distinct types of air masses were encountered, which were largely marine (Atlantic), continental or urban-industrial in origin. NO3 lifetimes were longest in the Atlantic sector (up to ~30 min) but were very short (a few seconds) in polluted, air masses from the local city and petroleum-related industrial complex of Huelva. Air from the continental sector was an intermediate case. The high reactivity to NO3 of the urban air mass was not accounted for by gas-phase and heterogeneous reactions, rates of which were constrained by measurements of NO, volatile organic species and aerosol surface area. In general, high NO2 mixing ratios resulted in low NO3 lifetimes, though heterogeneous processes (e.g. reaction of N2O5 on aerosol) were generally less important than direct gas-phase losses of NO3. The presence of SO2 at levels above ~2 ppb in the urban air sector was always associated with very low N2O5 mixing ratios indicating either very short NO3 lifetimes in the presence of combustion-related emissions or an important role for reduced sulphur species in urban, nighttime chemistry. High production rates coupled with low lifetimes of NO3 imply an important contribution of nighttime chemistry to removal of both NOx and VOC.
Abstract. Calcium- and magnesium-containing salts are important components for mineral dust and sea salt aerosols, but their physicochemical properties are not well understood yet. In this study, the hygroscopic properties of eight Ca- and Mg-containing salts, including Ca(NO3)2 · 4H2O, Mg(NO3)2 · 6H2O, MgCl2 · 6H2O, CaCl2 · 6H2O, Ca(HCOO)2, Mg(HCOO)2 · 2H2O, Ca(CH3COO)2 · H2O and Mg(CH3COO)2 · 4H2O, were systematically investigated using two complementary techniques. A vapor sorption analyzer was used to measure the change of sample mass with relative humidity (RH) under isotherm conditions, and the deliquescence relative humidities (DRH) for temperature in the range of 5–30 °C as well as water-to-solute ratios as a function of RH at 5 and 25 °C were reported for these eight compounds. DRH values showed a large variation for these compounds; for example, at 25 °C the DRH values were measured to be ~ 28.5 % for CaCl2 · 6H2O and > 95 % for Ca(HCOO)2 and Mg(HCOO)2 · 2H2O. In addition, a humidity-tandem differential analyzer was used to measure the change in mobility diameter with RH (up to 90 %) at room temperature, in order to determine the hygroscopic growth factors of aerosol particles generated by atomizing water solutions of these eight compounds. All the aerosol particles studied in this work, very likely to be amorphous, started to grow at very low RH (as low as 10 %) and showed continuous growth with RH. The hygroscopic growth factors at 90 % RH were found to range from 1.26 ± 0.04 for Ca(HCOO2)2 and 1.79 ± 0.03 for Ca(NO3)2, varying significantly for the eight types of aerosols considered herein. Overall, our work provides a systematical and comprehensive investigation of the hygroscopic properties of these Ca- and Mg-containing salts, largely improving our knowledge in the physicochemical properties of mineral dust and sea salt aerosols.
Abstract. Black carbon (BC) is a crucial component of aerosols in the atmosphere. Understanding the hygroscopicity of BC particles is important for studying their role as cloud condensation nuclei (CCN) and ice nuclei (IN), as well as their chemical behavior and atmospheric lifetime. However, there is still a lack of comprehensive understanding regarding the factors that determine the hygroscopic properties of fresh BC. In this work, the hygroscopic behavior of BC particles generated from different types of fuel and aged with SO2 for varying durations were measured by a vapor sorption analyzer while various characterizations of BC were conducted to understand the key factors that influence the hygroscopic properties of BC. It was found that the presence of water-soluble substances in BC facilitates the completion of monolayer water adsorption at low relative humidity, while also increasing the number of water adsorption layers at high relative humidity. On the other hand, BC prepared by burning organic fuels, which typically lacks water-soluble inorganic ions, primarily exhibits hygroscopicity characteristics influenced by organic carbon (OC) and microstructure. Furthermore, the hygroscopicity of BC can be enhanced by the formation of sulfate ions due to heterogeneous oxidation of SO2. This finding sheds light on the critical factors that affect BC hygroscopicity during water adsorption and allows for estimating the interaction between water molecules and BC particles in a humid atmosphere.
Abstract. In this work, we studied the cloud condensation nuclei (CCN) activity and subsaturated droplet growth of phthalic acid (PTA), isophthalic acid, (IPTA) and terephthalic acid (TPTA), significant benzene polycarboxylic acids and structural isomers found in the atmosphere. Köhler theory (KT) can be effectively applied for hygroscopicity analysis of PTA due to its higher aqueous solubility compared to IPTA and TPTA. As with other hygroscopicity studies of partially water-soluble and effectively water-insoluble species, the supersaturated and subsaturated hygroscopicity derived from KT principles do not agree. To address the disparities in the sub- and supersaturated droplet growth, we developed a new analytical framework called the Hybrid Activity Model (HAM). HAM incorporates the aqueous solubility of a solute within an adsorption-based activation framework. Frenkel–Halsey–Hill (FHH) adsorption theory (FHH-AT) was combined with the aqueous solubility of the compound to develop HAM. Analysis from HAM was validated using laboratory measurements of pure PTA, IPTA, TPTA and PTA–IPTA internal mixtures. Furthermore, the results generated using HAM were tested against traditional KT and FHH-AT to compare their water uptake predictive capabilities. A single hygroscopicity parameter was also developed based on the HAM framework. Results show that the HAM-based hygroscopicity parameter can successfully simulate the water uptake behavior of the pure and internally mixed samples. Results indicate that the HAM framework may be applied to atmospheric aerosols of varying chemical structures and aqueous solubility.
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