Model Simulations and Predictions of Hydroxymethanesulfonate (HMS) in the Beijing-Tianjin-Hebei Region, China: Roles of Aqueous Aerosols and Atmospheric Acidity

Hydroxymethanesulfonate (HMS) has been found to be an abundant organosulfur aerosol compound in the Beijing-Tianjin-Hebei (BTH) region with a measured maximum daily mean concentration of up to 10 μg per cubic meter in winter. However, the production medium of HMS in aerosols is controversial, and it is unknown whether chemical transport models are able to capture the variations of HMS during individual haze events. In this work, we modify the parametrization of HMS chemistry in the nested-grid GEOS-Chem chemical transport model, whose simulations provide a good account of the field measurements during winter haze episodes. We find the contribution of the aqueous aerosol pathway to total HMS is about 36% in winter in Beijing, due primarily to the enhancement effect of the ionic strength on the rate constants of the reaction between dissolved formaldehyde and sulfite. Our simulations suggest that the HMS-to-inorganic sulfate ratio will increase from the baseline of 7% to 13% in the near future, given the ambitious clean air and climate mitigation policies for the BTH region. The more rapid reductions in emissions of SO2 and NOx compared to NH3 alter the atmospheric acidity, which is a critical factor leading to the rising importance of HMS in particulate sulfur species.

Aqueous ·OH Oxidation of Highly Substituted Phenols as a Source of Secondary Organic Aerosol

Biomass burning (BB) releases large quantities of phenols (ArOH), which can partition into cloud/fog drops and aerosol liquid water (ALW), react, and form aqueous secondary organic aerosol (aqSOA). While simple phenols are too volatile to significantly partition into particle water, highly substituted ArOH partition more strongly and might be important sources of aqSOA in ALW. To investigate this, we measured the ^·OH oxidation kinetics and aqSOA yields for six highly substituted ArOH from BB. Second-order rate constants are high, in the range (1.9-14) × 10⁹ M^-1 s^-1 at pH 2 and (14-25) × 10⁹ M^-1 s^-1 at pH 5 and 6. Mass yields of aqSOA are also high, with an average (±1σ) value of 82 (±12)%. ALW solutes have a range of impacts on phenol oxidation by ^·OH: a BB sugar and some inorganic salts suppress oxidation, while a nitrate salt and transition metals enhance oxidation. Finally, we estimated rates of aqueous- and gas-phase formation of SOA from a single highly substituted phenol as a function of liquid water content (LWC), from conditions of cloud/fog (0.1 g-H₂O m^-3) to ALW (10 μg-H₂O m^-3). Formation of aqSOA is significant across the LWC range, although gas-phase ^·OH becomes dominant under ALW conditions. We also see a generally large discrepancy between measured and modeled aqueous ^·OH concentrations across the LWC range.

A light-driven burst of hydroxyl radicals dominates oxidation chemistry in newly activated cloud droplets

Aerosol particles and their interactions with clouds are one of the most uncertain aspects of the climate system. Aerosol processing by clouds contributes to this uncertainty, altering size distributions, chemical composition, and radiative properties. Many changes are limited by the availability of hydroxyl radicals in the droplets. We suggest an unrecognized potentially substantial source of OH formation in cloud droplets. During the first few minutes following cloud droplet formation, the material in aerosols produces a near-UV light-dependent burst of hydroxyl radicals, resulting in concentrations of 0.1 to 3.5 micromolar aqueous OH ([OH]_aq). The source of this burst is previously unrecognized chemistry between iron(II) and peracids. The contribution of the "OH burst" to total OH in droplets varies widely, but it ranges up to a factor of 5 larger than previously known sources. Thus, this new process will substantially enhance the impact of clouds on aerosol properties.

Iron-Catalyzed Oxidation of Sulfite: From Established Results to a New Understanding

A survey is made of the iron-catalyzed oxidation of sulfite describing a conceptual framework to explain the key processes involved, with a focus on kinetics. Perhaps most importantly, the incorporation of the HSO5- + Fe(II) step into the regeneration of catalytically active ferric ions which does not deplete its role over the iron redox cycle. The radical-radical recombination SO5-• + SO5-•, which terminates the cycling between ferric and ferrous ions, represents a gross but not a net loss of the chain-carriers, because nearly all of them are reformed through the branching step HSO5- + Fe(II) → Fe2+ + H2O + SO4-•, [Formula: see text] in just a few seconds or somewhat longer. A branching mechanism is thus the only possible means of allowing the catalytic process to reach a stationary state. Observations that may be considered as evidence (fingerprints) of rate variations in sulfite depletion due to the branching mechanism are explored in detail, and the related dynamics of the chain-carriers and metal ions cycles are discussed. In particular, the most important is found to be the aspect related to the intrinsic limitation of the cycle of metal ions. This limitation governs the extent of the oxidative/reducing potential of sulfite solutions with respect to the Fe(III/II) couple, thereby governing the quasi-state partioning between ferric and ferrous ions. Such a view enables examination of those conditions under which the limitation to the rate of the catalytic reaction is controlled by the reduction or re-oxidation of ferric ions. Readily applicable kinetic criteria and kinetic diagrams to delimit the conditions are given. In such a framework, the majority of known anomalies of the catalytic reaction receive an explanation.

Study of Cloud Water Samples Collected over Northern Poland

The paper gives the results of the first studies on the chemistry of cloud water collected during 3 mo (Aug.-Oct. 2010) in the free atmosphere over the area to the south of the Tri-City (Gdansk-Sopot-Gdynia) conurbation on the Gulf of Gdansk, Poland. Taken from cumulus, stratus, and stratocumulus clouds by means of an aircraft-mounted collector, the water samples were analyzed for the following contaminants: anions (chlorides, fluorides, nitrates, sulfates, and phosphates), cations (lithium, sodium, potassium, ammonium, calcium, and magnesium), and trace metals. In addition, pH values were measured, and the type and composition of suspended particulate matter was determined. We discuss the relationship between the concentration of inorganic ions and the type of cloud from which water was sampled. The chemistry is also likely related to the circulation pattern and inflow of clean air masses from the Baltic Sea. Moreover, a relationship was found between the composition of the samples examined and the location of pollutant emission sources.

A general scavenging rate constant for reaction of hydroxyl radical with organic carbon in atmospheric waters

Hydroxyl radical (OH) is an important oxidant in atmospheric aqueous phases such as cloud and fog drops and water-containing aerosol particles. We find that numerical models nearly always overestimate aqueous hydroxyl radical concentrations because they overpredict its rate of formation and, more significantly, underpredict its sinks. To address this latter point, we examined OH sinks in atmospheric drops and aqueous particles using both new samples and an analysis of published data. Although the molecular composition of organic carbon, the dominant sink of OH, is extremely complex and poorly constrained, this sink behaves very similarly in different atmospheric waters and even in surface waters. Thus, the sink for aqueous OH can be estimated as the concentration of dissolved organic carbon multiplied by a general scavenging rate constant [kC,OH = (3.8 ± 1.9) × 10(8) L (mol C)(-1) s(-1)], a simple process that should significantly improve estimates of OH concentrations in atmospheric drops and aqueous particles.

Metals in air pollution particles decrease whole-blood coagulation time

The mechanism underlying procoagulative effects of air pollution particle exposure is not known. The authors tested the postulate that (1) the water-soluble components of an air pollution particle could affect whole-blood coagulation time and (2) metals included in this fraction were responsible for this effect. Exposure to the water-soluble fraction of particulate matter (PM), at doses as low as 50 ng/ml original particle, significantly diminished the whole-blood coagulation time. Inclusion of deferoxamine prolonged coagulation time following the exposures to the water-soluble fraction, whereas equivalent doses of ferroxamine had no effect. Except for nickel, all metal sulfates shortened the whole-blood coagulation time. Iron and zinc were two metals with the greatest capacity to reduce the coagulation time, with an effect observed at 10 ng/ml. Finally, in contrast to the anticoagulants citrate and EDTA, their iron complexes were found to be procoagulative. The authors conclude that metals in the water-soluble fraction of air pollution particles decrease whole-blood coagulation time. These metals can potentially contribute to procoagulative effects observed following human exposures to air pollution particles.

Impact of radical versus non-radical pathway in the Fenton chemistry on the iron redox cycle in clouds

Modeling studies have shown that the Fenton reaction of iron(II) with H2O2 can contribute, in a significant amount, to OH radicals production in cloud droplets. However, the destruction mechanism of hydrogen peroxide by iron(II) is still uncertain. Two reaction pathways for the first step of Fenton chemistry have been advanced: a radical pathway which considers an OH radical production and a non-radical pathway considering ferryl ion production. The aim of this work is to evaluate the impact of these two possible reaction pathways for Fenton chemistry on the iron redox cycle in cloud droplets. For this purpose, the numerical model of multiphase chemistry M2C2 has been applied to a rural chemical scenario representative of continental conditions. This study highlights that the iron redox cycling is driven by Fenton reaction whatever Fenton mechanism is considered. The ferryl ion chemistry becomes significant in the iron redox cycling when this species is considered as an active intermediate in Fenton chemistry and under night time conditions the iron redox chemistry is controlled by the ferryl ion reactivity. The partitioning of iron between its two main oxidation states (+II and +III) in cloud droplets, which is the indicator of the iron oxido-reduction potential, does not change significantly between the two cases. However, for the non-radical case, the ferryl ion concentration is up to four orders of magnitude higher than the OH concentration highlighting its potential role in oxidative capacity of cloud droplets.

Sulfate content correlates with iron concentrations in ambient air pollution particles

Current levels of air pollution particles in American cities can increase human mortality. Both the mechanism of injury and the responsible components are not known. We have postulated that injury following air pollution particle exposure is produced through a generation of oxygen-based free radicals catalyzed by metals present in the particles. As a result of its abundance in the atmosphere, sulfate appears to potentially be the most successful ligand to complex metal cations. We tested the hypothesis that (1) some portion of iron in ambient air pollution particles is present as sulfate and (2) this relationship between iron and sulfate results from the capacity of the latter to function as a ligand to mobilize the metal from the oxide. Concentrations of sulfate and iron in acid extracts of 20 filters (total suspended particles) from Utah were measured using inductively coupled plasma emission spectroscopy. In vitro oxidant generation was also measured using thiobarbituric acid-reactive products of deoxyribose. There were significant correlations between sulfate content, iron concentrations, and oxidant generation. Agitation of calcium sulfate with iron(III) oxide produced concentrations of water-soluble, catalytically active iron. We conclude that some portion of iron in the atmosphere is present as a sulfate. This relationship between sulfate and iron concentrations is likely the product of SO42- functioning as a ligand for the meal after its mobilization from an oxide by photoreduction. There were also associations between sulfate content, iron concentrations, and oxidant generation. However, sulfates had no capacity to support electron transport unless they were present with iron.

Cellular and biochemical response of the human lung after intrapulmonary instillation of ferric oxide particles

Bronchoalveolar lavage (BAL) was used to sample lung cells and biochemical components in the lung air spaces at various times from 1 to 91 d after intrapulmonary instillation of 2.6 microm-diameter iron oxide particles in human subjects. The instillation of particles induced transient acute inflammation during the first day post instillation (PI), characterized by increased numbers of neutrophils and alveolar macrophages as well as increased amounts of protein, lactate dehydrogenase, and interleukin-8 in BAL fluids. This response was subclinical and was resolved within 4 d PI. A similar dose-dependent response was seen in rats 1 d after intratracheal instillation of the same particles. The particles contained small amounts of soluble iron (240 ng/mg) and possessed the capacity to catalyze oxidant generation in vitro. Our findings indicate that the acute inflammation after particle exposure may, at least partially, be the result of oxidant generation catalyzed by the presence of residual amounts of ferric ion, ferric hydroxides, or oxyhydroxides associated with the particles. These findings may have relevance to the acute health effects associated with increased levels of ambient particulate air pollutants.

REGIONAL PHOTOCHEMICAL AIR QUALITY MODELING:Model Formulations, History, and State of the Science

▪ Abstract  Regional air quality models have been used for scientific investigation of trace species dynamics for over two decades and are now beginning to take a central position in air quality management. In particular, they have been used for studying the transport and fate of atmospheric acids, photochemical oxidants (e.g. ozone), and more recently, aerosols. Such models are based on numerically solving the mass conservation equations for a chemically interacting system of species and are applied to horizontal domains of 1000s of kms. Primary applications include assessing the response of pollutant concentrations to emissions controls, quantifying the flux of pollutants across and out of a region, and understanding the impact of specific processes on pollutant concentrations.