Sulfate Aerosol Production Research Articles

How well are aerosol–cloud interactions represented in climate models? – Part 1: Understanding the sulfate aerosol production from the 2014–15 Holuhraun eruption

Abstract. For over 6 months, the 2014–2015 effusive eruption at Holuhraun, Iceland, injected considerable amounts of sulfur dioxide (SO2) into the lower troposphere with a daily rate of up to one-third of the global emission rate, causing extensive air pollution across Europe. The large injection of SO2, which oxidises to form sulfate aerosol (SO42-), provides a natural experiment offering an ideal opportunity to scrutinise state-of-the-art general circulation models' (GCMs) representation of aerosol–cloud interactions (ACIs). Here we present Part 1 of a two-part model inter-comparison using the Holuhraun eruption as a framework to analyse ACIs. We use SO2 retrievals from the Infrared Atmospheric Sounding Interferometer (IASI) instrument and ground-based measurements of SO2 and SO42- mass concentrations across Europe, in conjunction with a trajectory analysis using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model, to assess the spatial and chemical evolution of the volcanic plume as simulated by five GCMs and a chemical transport model (CTM). IASI retrievals of plume altitude and SO2 column load reveal that the volcanic perturbation is largely contained within the lower troposphere. Compared to the satellite observations, the models capture the spatial evolution and vertical variability of the plume reasonably well, although the models often overestimate the plume altitude. HYSPLIT trajectories are used to attribute to Holuhraun emissions 111 instances of elevated sulfurous surface mass concentrations recorded at European Monitoring and Evaluation Programme (EMEP) stations during September and October 2014. Comparisons with the simulated concentrations show that the modelled ratio of SO2 to SO42- during these pollution episodes is often underestimated and overestimated for the young and mature plume, respectively. Models with finer vertical resolutions near the surface are found to better capture these elevated sulfurous ground-level concentrations. Using an exponential function to describe the decay of observed surface mass concentration ratios of SO2 to SO42- with plume age, the in-plume oxidation rate constant is estimated as 0.032 ± 0.002 h−1 (1.30 ± 0.08 d e-folding time), with a near-vent ratio of 25 ± 5 (µg m−3 of SO2 / µg m−3 of SO42-). The majority of the corresponding derived modelled oxidation rate constants are lower than the observed estimate. This suggests that the representation of the oxidation pathway/s in the simulated plumes is too slow. Overall, despite their coarse spatial resolutions, the six models show reasonable skill in capturing the spatial and chemical evolution of the Holuhraun plume. This capable representation of the underlying aerosol perturbation is essential to enable the investigation of the eruption's impact on ACIs in the second part of this study.

The effect of temperature and salinity on DMSP production in Gephyrocapsa oceanica (Isochrysidales, Coccolithophyceae)

ABSTRACT A subtropical clone of Gephyrocapsa oceanica was grown over the temperature and salinity range 10–30°C and 20‰–45‰ respectively. Cellular DMSP increased with increasing salinity, compatible with the hypothesis that DMSP is a compatible osmolyte. Cellular DMSP content was highest at colder temperatures and decreased as temperature increased. Net DMSP production rate also depended on cell size and growth rate was greatest about 2°C below the optimum growth temperature of 20°C for this clone. This resulted in a unimodal response of net DMSP production to increasing temperature: net DMSP production increased with increasing temperature when the cells were growing at temperatures below optimum for growth. At and above optimum growth temperature, further warming decreased net DMSP production. For the effect of temperature alone, in the subtropical oceans, where G. oceanica is growing at or above its optimum, further warming due to climate change will result in decreased net DMSP production and so a probable decrease in the flux of DMS to the atmosphere and sulphate aerosol production. Inasmuch as these aerosols modulate cloud albedo and longevity then these too will both decrease, resulting in a positive feedback response for temperature. The reverse effect may occur in higher latitude oceans where growth temperature is below optimum. The exact response in both regions is complicated because warming will also enhance water column stratification and may reduce mixed layer depths, affecting both nutrient and light regimes, as well as possible species succession effects. Further work is required to investigate these other indirect temperature effects.

Formation of H3O+ and OH by CO2 and N2O trace gases in the atmospheric environment.

The impact of cosmic rays' energetic subatomic particles on climate and global warming is still controversial and under debate. Cosmic rays produce ions that can trigger fast reactions affecting chemical networks in the troposphere and stratosphere especially when a large amount of relevant trace gases such as carbon dioxide, methane, sulfur dioxide and water are injected by volcanic eruptions. This work focuses on synchrotron experiments and an ab initio theoretical study of the ion chemistry of carbon dioxide and nitrous oxide radical cations reacting with water. These molecules catalyze a fast exothermic formation of hydronium ions H3O+ and the hydroxyl radical OH, the main oxidant in the atmosphere. Moreover, theoretical calculations demonstrate that at the end of the catalytic cycle, CO2 and N2O are produced vibrationally excited and subsequently they quench in the microsecond time scale by collision with the surrounding atmospheric molecules at the pressure and temperature of the upper-troposphere/stratosphere. The chemistry involved in these reactions has a strong impact on the oxidant capacity of the atmosphere, on the sulfate aerosol production, on the cloud formation and eventually on the chemical networks controlling climate and global warming models.

Measurement report: On the contribution of long-distance transport to the secondary aerosol formation and aging

Abstract. To investigate the physio-chemical properties of aerosol transported from major pollution regions in China, observations were conducted ∼ 200 m above the ground at the junction location of the North China Plain and Fenwei Basin, which are two regions of top priority for China's Blue Sky Campaign. We identified three pollution transport sectors including those from Beijing–Tianjin–Hebei (BTH), urban Guanzhong Basin (GZB) and northern China and one clean transport sector from the rural Guanzhong Basin region. Secondary inorganic aerosol (SIA) constituted a major fraction (39 %–46 %) in all pollution transport sectors, with a high sulfur oxidation ratio (0.44–0.58) and a high nitrogen oxidation ratio (0.24–0.29), suggesting efficient formation of secondary inorganic aerosol during regional transport. More oxidized oxygenated organic aerosol (MO-OOA) played a dominant role in the source of organic aerosol in all sectors including the clean one, accounting for 42 %–58 % of total organic aerosol. Elemental analysis (O and C) shows that aerosol particles at this receptor site were much more oxidized than in urban regions, pointing that long-range transport contributed markedly to the organic aerosol oxidation and aging. Case studies of pollution events with high sulfate, nitrate and more-oxidized oxygenated organic aerosol production rate indicate the strong formation efficiency of secondary aerosol during regional transport in the Beijing–Tianjin–Hebei transport sector.

Targeting Atmospheric Oxidants Can Better Reduce Sulfate Aerosol in China: H2O2 Aqueous Oxidation Pathway Dominates Sulfate Formation in Haze.

Particulate sulfate is one of the most important components in the atmosphere. The observation of rapid sulfate aerosol production during haze events provoked scientific interest in the multiphase oxidation of SO2 in aqueous aerosol particles. Diverse oxidation pathways can be enhanced or suppressed under different aerosol acidity levels and high ionic strength conditions of atmospheric aerosol. The importance of ionic strength to sulfate multiphase chemistry has been verified under laboratory conditions, though studies in the actual atmosphere are still limited. By utilizing online observations and developing an improved solute strength-dependent chemical thermodynamics and kinetics model (EF-T&K model, EF is the enhancement factor that represents the effect of ionic strength on an aerosol aqueous-phase reaction), we provided quantitative evidence that the H2O2 pathway was enhanced nearly 100 times and dominated sulfate formation for entire years (66%) in Tianjin (a northern city in China). TMI (oxygen catalyzed by transition-metal ions) (14%) and NO2 (14%) pathways got the second-highest contributions. Machine learning supported the result that aerosol sulfate production was more affected by the H2O2 pathway. The collaborative effects of atmospheric oxidants and SO2 on sulfate aerosol production were further investigated using the improved EF-T&K model. Our findings highlight the effectiveness of adopting target oxidant control as a new direction for sustainable mitigation of sulfate, given the already low SO2 concentrations in China.

Rapid Sulfate Formation via Uncatalyzed Autoxidation of Sulfur Dioxide in Aerosol Microdroplets.

Severe winter haze events in Beijing and North China Plain are characterized by rapid production of sulfate aerosols with unresolved mechanisms. Oxidation of SO2 by O2 in the absence of metal catalysts (uncatalyzed autoxidation) represents the most ubiquitous SO2 conversion pathway in the atmosphere. However, this reaction has long been regarded as too slow to be atmospherically meaningful. This traditional view was based on the kinetic studies conducted in bulk dilute solutions that mimic cloudwater but deviate from urban aerosols. Here, we directly measure the sulfate formation rate via uncatalyzed SO2 autoxidation in single (NH4)2SO4 microdroplets, by using an aerosol optical tweezer coupled with a cavity-enhanced Raman spectroscopy technique. We find that the aqueous reaction of uncatalyzed SO2 autoxidation is accelerated by two orders of magnitude at the high ionic strength (∼36 molal) conditions in the supersaturated aerosol water. Furthermore, at acidic conditions (pH 3.5-4.5), uncatalyzed autoxidation predominately occurs on droplet surface, with a reaction rate unconstrained by SO2 solubility. With these rate enhancements, we estimate that the uncatalyzed SO2 autoxidation in aerosols can produce sulfate at a rate up to 0.20 μg m-3 hr-1, under the winter air pollution condition in Beijing.

An Experimental Assessment of the Importance of S(IV) Oxidation by Hypohalous Acids in the Marine Atmosphere

AbstractHypohalous acids (HOCl + HOBr) play an important but highly uncertain role in sulfate aerosol production in the marine atmosphere due to the unmeasured reaction rate constants (k) of HOCl/HOBr and HSO3−, the dominant S(IV) species in acidified sea salt aerosols and cloudwater. We directly determined by investigating the kinetics of aqueous oxidation of dissolved SO2 by HOCl in low pH aerosol particles in a flow tube. Further, we predicted using a novel kinetics model developed from the HOCl/HSO3− results. and were estimated to be 2 and 3 orders of magnitude lower, respectively, than the values used in atmospheric models. We assess the importance of halogen‐driven S(IV) oxidation in the marine atmosphere with this new fundamental understanding and find that in‐cloud sulfate production via HOBr oxidation remains an important atmospheric process, but the HOCl oxidation pathway in acidified sea salt aerosols is unlikely to be important.

The sensitivity of Southern Ocean aerosols and cloud microphysics to sea spray and sulfate aerosol production in the HadGEM3-GA7.1 chemistry–climate model

Abstract. With low concentrations of tropospheric aerosol, the Southern Ocean offers a “natural laboratory” for studies of aerosol–cloud interactions. Aerosols over the Southern Ocean are produced from biogenic activity in the ocean, which generates sulfate aerosol via dimethylsulfide (DMS) oxidation, and from strong winds and waves that lead to bubble bursting and sea spray emission. Here, we evaluate the representation of Southern Ocean aerosols in the Hadley Centre Global Environmental Model version 3, Global Atmosphere 7.1 (HadGEM3-GA7.1) chemistry–climate model. Compared with aerosol optical depth (AOD) observations from two satellite instruments (the Moderate Resolution Imaging Spectroradiometer, MODIS-Aqua c6.1, and the Multi-angle Imaging Spectroradiometer, MISR), the model simulates too-high AOD during winter and too-low AOD during summer. By switching off DMS emission in the model, we show that sea spray aerosol is the dominant contributor to AOD during winter. In turn, the simulated sea spray aerosol flux depends on near-surface wind speed. By examining MODIS AOD as a function of wind speed from the ERA-Interim reanalysis and comparing it with the model, we show that the sea spray aerosol source function in HadGEM3-GA7.1 overestimates the wind speed dependency. We test a recently developed sea spray aerosol source function derived from measurements made on a Southern Ocean research voyage in 2018. In this source function, the wind speed dependency of the sea spray aerosol flux is less than in the formulation currently implemented in HadGEM3-GA7.1. The new source function leads to good agreement between simulated and observed wintertime AODs over the Southern Ocean; however, it reveals partially compensating errors in DMS-derived AOD. While previous work has tested assumptions regarding the seawater climatology or sea–air flux of DMS, we test the sensitivity of simulated AOD, cloud condensation nuclei and cloud droplet number concentration to three atmospheric sulfate chemistry schemes. The first scheme adds DMS oxidation by halogens and the other two test a recently developed sulfate chemistry scheme for the marine troposphere; one tests gas-phase chemistry only, while the second adds extra aqueous-phase sulfate reactions. We show how simulated sulfur dioxide and sulfuric acid profiles over the Southern Ocean change as a result and how the number concentration and particle size of the soluble Aitken, accumulation and coarse aerosol modes are affected. The new DMS chemistry scheme leads to a 20 % increase in the number concentration of cloud condensation nuclei and cloud droplets, which improves agreement with observations. Our results highlight the importance of atmospheric chemistry for simulating aerosols and clouds accurately over the Southern Ocean.

Model simulation of ammonium and nitrate aerosols distribution in the Euro-Mediterranean region and their radiative and climatic effects over 1979–2016

Abstract. Aerosols play an important role in Europe and the Mediterranean area where different sources of natural and anthropogenic particles are present. Among them ammonium and nitrate (A&N) aerosols may have a growing impact on regional climate. In this study, their representation in coarse and fine modes has been introduced in the prognostic aerosol scheme of the ALADIN-Climate regional model. This new aerosol scheme is evaluated over Europe and the Mediterranean Sea, using two twin simulations over the period 1979–2016 with and without A&N aerosols. This evaluation is performed at local and regional scales, using surface stations and satellite measurements. Despite an overestimate of the surface nitrate concentration, the model is able to reproduce its spatial pattern including local maxima (Benelux, Po Valley). Concerning the simulated aerosol optical depth (AOD), the inclusion of A&N aerosols significantly reduces the model bias compared to both AERONET stations and satellite data. Our results indicate that A&N aerosols can contribute up to 40 % of the total AOD550 over Europe, with an average of 0.07 (550 nm) over the period 2001–2016. Sensitivity studies suggest that biases still present are related to uncertainties associated with the annual cycle of A&N aerosol precursors (ammonia and nitric acid). The decrease in sulfate aerosol production over Europe since 1980 produces more free ammonia in the atmosphere leading to an increase in A&N concentrations over the studied period. Analyses of the different aerosol trends have shown for the first time to our knowledge that, since 2005 over Europe, A&N AOD550 and A&N shortwave (SW) direct radiative forcing (DRF) are found to be higher than sulfate and organics, making these the species with the highest AOD and the highest DRF. On average over the period 1979–2016, the A&N DRF is found to be about −1.7 W m−2 at the surface and −1.4 W m−2 at the top of the atmosphere (TOA) in all sky conditions over Europe, with regional maxima located at the surface over the Po Valley (−5 W m−2). Finally, the dimming effect of A&N aerosols is responsible for a cooling of about −0.2∘ C over Europe (summer), with a maximum of −0.4 ∘C over the Po Valley. Concerning precipitation, no significant impact of A&N aerosols has been found.

Fireworks: A major source of inorganic and organic aerosols during Christmas and New Year in Mexico city

Abstract The fireworks activity in Christmas and New Year is the culprit of a substantial increase in the particles pollution during those festivity dates in Mexico City. The fireworks' thrilling explosions fill the air with small particles full of metals and organic and inorganic species, and noxious gases such as sulfur dioxide, nitrogen dioxide, carbon monoxide and nitrous acid (HONO). The stagnant atmosphere at night accumulates the fireworks haze under a shallow boundary layer. By sunrise, once the fireworks activity has stopped, the photolysis of the abundant HONO accumulated throughout the night, leads to a rapid production of nitrate (NO3−), ammonium (NH4+), sulfate (SO42−) and low volatility secondary organic aerosols. This was the pattern observed during the winter of 2013–2014, when online measurements of water-soluble inorganic ions in particles smaller than 2.5 μm (PM2.5), non-refractory ions and organics in particles smaller than 1 μm (PM1), equivalent black carbon, and above-mentioned gases were conducted. The geometric average of the sum of the three major inorganic ions, NO3−, NH4+ and SO42− accounted for 25% (28%) and 34% (37%) of the total PM2.5 mass during the fireworks activity at night and the following morning on Christmas Day (New Year's Day), respectively. Similarly, their sum in the non-refractory fraction accounted for 37% (49%) and 47% (53%) of the total NR-PM1 mass concentration, while the organic fraction accounted for 56% (44%) and 49% (44%). Biomass burning organic aerosols and semi-volatile oxygenated organic aerosols (BBOA and SV-OOA, respectively) dominated the organic fraction during the fireworks period at night, while the low-volatility oxygenated organic aerosols (LV-OOA) increased substantially their concentration along the post-firework hours the following morning. Concentrations of PM2.5 > 100 μg m−3 and PM1 > 50 μg m−3 were observed on both days, with maximums of 157 and 137 μg m−3 for PM2.5 and PM1, respectively. The meteorological conditions helped to reduce the fireworks impact by late afternoon on both days, limiting the firework episodes to 22 h. The SO42− and NO3− related to the fireworks were not able to offset the deficit of NH4+, and thus the acidity nature of the particles during the mild winter of Mexico City did not change.

Examination of in-cloud sulfate chemistry using a different model initialization

We first examine characteristics of in-cloud sulfate chemistry of a mid-latitude convective cloud using a different model initialization. Specifically, we studied the relative importance of convection on sulfate aerosol transport and redistribution. The analysis is focused on 4 April 2000, when considerable dust transport is evidenced with high concentration and wet deposition of sulfate aerosol measured at the European Monitoring Environmental Program (EMEP) station located in the western mountain part of the Republic of Macedonia. The cloud-resolving model coupled with aqueous sulfate chemistry module has been employed to simulate the sulfate aerosol aqueous chemistry and wet deposition. The modeled results and sensitivities under various initializations are evaluated and compared with a ground-based measurements and laboratory analysis. The results indicate that the local environmental profiles derived from 2.5 km-scale WRF model provides a better cloud model initialization, capable of realistic representation of sub-grid-scale processes which are not adequately resolved by the WRF model with a coarser grid resolution. The method also shows a relatively good performance in simulation of cloud physical processes which take place in-cloud and the near cloud environment, cloud-chemistry interactions, relevant to sulfate acidification and sulfate deposition, which allows a more accurate quantitative assessment of sulfate concentration and pH values. Analysis also indicated that scavenging and oxidation are the principal processes affecting sulfate production, participating with 33 and 46%, respectively. Turning off the ice-phase processes leads to overprediction of sulfate aerosol production for about 8% relative to the base run. The verification analysis indicates that the numerical simulation initialized with a 2.5-km-scale WRF model shows a higher correlation coefficient with observations compared to other runs. This approach of initialization based on WRF conditions provides a scientific contribution by evaluating simulations of convective clouds in Macedonia against ground-based meteorological and chemical data, as well as by using the model to understand the driving processes affecting sulfate production and wet deposition.

MIPAS observations of volcanic sulfate aerosol and sulfur dioxide in the stratosphere

Abstract. Volcanic eruptions can increase the stratospheric sulfur loading by orders of magnitude above the background level and are the most important source of variability in stratospheric sulfur. We present a set of vertical profiles of sulfate aerosol volume densities and derived liquid-phase H2SO4 (sulfuric acid) mole fractions for 2005–2012, retrieved from infrared limb emission measurements performed with the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on board of the Environmental Satellite (Envisat). Relative to balloon-borne in situ measurements of aerosol at Laramie, Wyoming, the MIPAS aerosol data have a positive bias that has been corrected, based on the observed differences to the in situ data. We investigate the production of stratospheric sulfate aerosol from volcanically emitted SO2 for two case studies: the eruptions of Kasatochi in 2008 and Sarychev in 2009, which both occurred in the Northern Hemisphere midlatitudes during boreal summer. With the help of chemical transport model (CTM) simulations for the two volcanic eruptions we show that the MIPAS sulfate aerosol and SO2 data are qualitatively and quantitatively consistent with each other. Further, we demonstrate that the lifetime of SO2 is explained well by its oxidation by hydroxyl radicals (OH). While the sedimentation of sulfate aerosol plays a role, we find that the long-term decay of stratospheric sulfur after these volcanic eruptions in midlatitudes is mainly controlled by transport via the Brewer–Dobson circulation. Sulfur emitted by the two midlatitude volcanoes resides mostly north of 30∘ N at altitudes of ∼ 10–16 km, while at higher altitudes (∼ 18–22 km) part of the volcanic sulfur is transported towards the Equator where it is lifted into the stratospheric “overworld” and can further be transported into both hemispheres.

Primary sulfur isotope signatures preserved in high-grade Archean barite deposits of the Sargur Group, Dharwar Craton, India

We report in situ and bulk S-isotope analyses of the 3.2Ga old barite deposit of the Sargur Group, Dharwar Craton, India. Compared with the two other coeval barite deposits in Western Australia (Pilbara Craton, Warrawoona and Sulfur Springs Group) and South Africa (Kaapvaal Craton, Onverwacht and Fig Tree Group), the rocks of the Sargur Group have experienced pervasive ductile deformation and equilibrated under high-grade metamorphic conditions, which could have caused partial S-isotope re-equilibration. However, in situ techniques revealed high spatial heterogeneities in δ34S of up to ∼18‰ in barite-bearing pyrites, as well as similar Δ33S anomalies compared to the associated sulfate indicating bacterial sulfate reduction isotope effects and a minor impact of the pervasive ductile deformation and metamorphism on the primary/diagenetic signatures. Our data in associated silicate rocks define consistent trends in Δ33S/δ34S and Δ36S/Δ33S reflecting metamorphic reworking of primary photolytic signatures with sulfide deriving from thermal reduction of the barite sulfate at >400°C. Our results match the previous data obtained on other Early Archean barite deposits. This strongly suggests that similar sulfur sources were at play over the three continents during the production and preservation of atmospheric sulfate and elemental sulfur aerosols. This also confirms that the different Early Archean barite deposits known on Earth formed during periods of intense subaerial volcanism and record volcanic photolytic signatures different from those common in the Archean.

Light absorption enhancement of black carbon from urban haze in Northern China winter

Atmospheric black carbon (BC) is an important pollutant for both air quality and Earth's energy balance. Estimates of BC climate forcing remain highly uncertain, e.g., due to the mixing with non-absorbing components. Non-absorbing aerosols create a coating on BC and may thereby act as a lens which may enhance the light absorption. However, this absorption enhancement is poorly constrained. To this end a two-step solvent dissolution protocol was employed to remove both organic and inorganic coatings, and then investigate their effects on BC light absorption. Samples were collected at a severely polluted urban area, Jinan, in the North China Plain (NCP) during February 2014. The BC mass absorption cross-section (MAC) was measured for the aerosol samples before and after the solvent-decoating treatment, and the enhancement of MAC (EMAC) from the coating effect was defined as the ratio. A distinct diurnal pattern for the enhancement was observed, with EMAC 1.3 ± 0.3 (1 S.D.) in the morning, increasing to 2.2 ± 1.0 in the afternoon, after that dropping to 1.5 ± 0.8 in the evening-night. The BC absorption enhancement primarily was associated with urban-scale photochemical production of nitrate and sulfate aerosols. In addition to that, regional-scale haze plume with increasing sulfate levels strengthened the absorption enhancement. These observations offer direct evidence for an increased absorption enhancement of BC due to severe air pollution in China.

Oxidation of Gas-Phase SO2 on the Surfaces of Acidic Microdroplets: Implications for Sulfate and Sulfate Radical Anion Formation in the Atmospheric Liquid Phase.

The oxidation of SO2(g) on the interfacial layers of microdroplet surfaces was investigated using a spray-chamber reactor coupled to an electrospray ionization mass spectrometer. Four major ions, HSO3(-), SO3(•-), SO4(•-) and HSO4(-), were observed as the SO2(g)/N2(g) gas-mixture was passed through a suspended microdroplet flow, where the residence time in the dynamic reaction zone was limited to a few hundred microseconds. The relatively high signal intensities of SO3(•-), SO4(•-), and HSO4(-) compared to those of HSO3(-) as observed at pH < 3 without addition of oxidants other than oxygen suggests an efficient oxidation pathway via sulfite and sulfate radical anions on droplets possibly via the direct interfacial electron transfer from HSO3(-) to O2. The concentrations of HSO3(-) in the aqueous aerosol as a function of pH were controlled by the deprotonation of hydrated sulfur dioxide, SO2·H2O, which is also affected by the pH dependent uptake coefficient. When H2O2(g) was introduced into the spray chamber simultaneously with SO2(g), HSO3(-) is rapidly oxidized to form bisulfate in the pH range of 3 to 5. Conversion to sulfate was less at pH < 3 due to relatively low HSO3(-) concentration caused by the fast interfacial reactions. The rapid oxidation of SO2(g) on the acidic microdroplets was estimated as 1.5 × 10(6) [S(IV)] (M s(-1)) at pH ≤ 3. In the presence of acidic aerosols, this oxidation rate is approximately 2 orders of magnitude higher than the rate of oxidation with H2O2(g) at a typical atmospheric H2O2(g) concentration of 1 ppb. This finding highlights the relative importance of the acidic surfaces for SO2 oxidation in the atmosphere. Surface chemical reactions on aquated aerosol surfaces, as observed in this study, are overlooked in most atmospheric chemistry models. These reaction pathways may contribute to the rapid production of sulfate aerosols that is often observed in regions impacted by acidic haze aerosol such as Beijing and other megacities around the world.

Two-Years PM2.5 Observations at Four Urban Sites along the Coast of Southeastern China

PM2.5 samples from four coastal urban sites (Fuzhou, Putian, Quanzhou, and Xiamen) in Southeastern China were collected to analyze their major chemical composition including inorganic ions, organic carbon (OC), elemental carbon (EC), and inorganic elements. Organic matter (OM = 1.6 × OC) was the largest contributor, accounting for 30.2% of PM2.5 mass concentration, and followed by sulfate (19.8%), ammonium (10.6%), and nitrate (9.7%), with minor contribution from trace species (5.1%), crustal species (4.7%) and EC (2.9%). Sea-salt and biomass burning potassium together contributed less than 2%. At the four sites, higher PM2.5 and its major components were observed in the northeast monsoon season while lower levels were found in the southwest monsoon season. The periodic PM2.5 cycle was observed and influenced mainly by rain wash out. However, regular diurnal variations of PM2.5 with high concentration during daytime were only observed in summer due to the greater production of sulfate and organic aerosols in spite of the fact that the vertical mixing coefficients were lower during nighttime. The relative contributions of secondary inorganic aerosols (sulfate, nitrate, and ammonium) to PM2.5 increased rapidly while the contribution of OM decreased during the haze episodes. The reconstructed visibility using revised IMPROVE method correlated well with the measured values. At Xiamen and Fuzhou sites, the major contributors of light extinction coefficient were ammonium sulfate, ammonium nitrate, OM and coarse mass, and accounted for more than 80% of the light extinction coefficient on average.

Surface and free tropospheric sources of methanesulfonic acid over the tropical Pacific Ocean

AbstractThe production of sulfate aerosols through sulfur chemistry in marine environments is critical to the tropical climate system. However, not all sulfur compounds have been studied in detail. One such compound is methanesulfonic acid (MSA). In this study, we use a one‐dimensional chemical transport model to analyze the observed vertical profiles of gas phase MSA during the Pacific Atmospheric Sulfur Experiment. The observed sharp decrease in MSA from the surface to 600 m implies a surface source of 4.0 × 107 molecules/cm2/s. Evidence suggests that this source is photolytically enhanced in daytime. We also find that the observed large increase of MSA from the boundary layer into the lower free troposphere (1000–2000 m) results mainly from the degassing of MSA from dehydrated aerosols. We estimate a source of 1.2 × 107 molecules/cm2/s to the free troposphere through this pathway. This source of soluble MSA could potentially provide an important precursor for new particle formation in the free troposphere over the tropics, affecting the climate system through aerosol‐cloud interactions.

Earth system models: an overview

AbstractEarth System models (ESMs) are global climate models with the added capability to explicitly represent biogeochemical processes that interact with the physical climate and so alter its response to forcing such as that associated with human‐caused emissions of greenhouse gases. Representing the global carbon cycle allows for feedbacks between the physical climate and the biological and chemical processes in the ocean and on land that take up some of the emitted carbon dioxide and so act to reduce warming. The sulfur cycle is also important in that both natural and human emissions of sulfur contribute to the production of sulfate aerosols which reflect incoming solar radiation (a direct cooling effect) and alter cloud properties (an indirect cooling effect). Other components such as ozone are also being incorporated into some ESMs. Evaluating the physical component of an ESM is becoming increasingly comprehensive and sophisticated, but the evaluation of the biogeochemical components suffer somewhat from a lack of comprehensive global‐scale observational data. Nevertheless, such models provide valuable insight into climate variability and change, and the role of human activities and possible mitigation actions on future climate change. Internationally coordinated experiments are increasingly important in providing a multimodel ensemble of climate simulations, thereby taking advantage of some ‘cancellation of errors’ and allowing better quantification of uncertainty. WIREs Clim Change 2011, 2:783–800. doi: 10.1002/wcc.148This article is categorized under: Climate Models and Modeling &gt; Earth System Models

A regional scale modeling analysis of aerosol and trace gas distributions over the eastern Pacific during the INTEX-B field campaign

Abstract. The Sulfur Transport and dEposition Model (STEM) is applied to the analysis of observations obtained during the Intercontinental Chemical Transport Experiment-Phase B (INTEX-B), conducted over the eastern Pacific Ocean during spring 2006. Predicted trace gas and aerosol distributions over the Pacific are presented and discussed in terms of transport and source region contributions. Trace species distributions show a strong west (high) to east (low) gradient, with the bulk of the pollutant transport over the central Pacific occurring between ~20° N and 50° N in the 2–6 km altitude range. These distributions are evaluated in the eastern Pacific by comparison with the NASA DC-8 and NSF/NCAR C-130 airborne measurements along with observations from the Mt. Bachelor (MBO) surface site. Thirty different meteorological, trace gas and aerosol parameters are compared. In general the meteorological fields are better predicted than gas phase species, which in turn are better predicted than aerosol quantities. PAN is found to be significantly overpredicted over the eastern Pacific, which is attributed to uncertainties in the chemical reaction mechanisms used in current atmospheric chemistry models in general and to the specifically high PAN production in the SAPRC-99 mechanism used in the regional model. A systematic underprediction of the elevated sulfate layer in the eastern Pacific observed by the C-130 is another issue that is identified and discussed. Results from source region tagged CO simulations are used to estimate how the different source regions around the Pacific contribute to the trace gas species distributions. During this period the largest contributions were from China and from fires in South/Southeast and North Asia. For the C-130 flights, which operated off the coast of the Northwest US, the regional CO contributions range as follows: China (35%), South/Southeast Asia fires (35%), North America anthropogenic (20%), and North Asia fires (10%). The transport of pollution into the western US is studied at MBO and a variety of events with elevated Asian dust, and periods with contributions from China and fires from both Asia and North America are discussed. The role of heterogeneous chemistry on the composition over the eastern Pacific is also studied. The impacts of heterogeneous reactions at specific times can be significant, increasing sulfate and nitrate aerosol production and reducing gas phase nitric acid levels appreciably (~50%).

The impact of dust on sulfate aerosol, CN and CCN during an East Asian dust storm

Abstract. A global model of aerosol microphysics is used to simulate a large East Asian dust storm during the ACE-Asia experiment. We use the model together with size resolved measurements of aerosol number concentration and composition to examine how dust modified the production of sulfate aerosol and the particle size distribution in East Asian outflow. Simulated size distributions and mass concentrations of dust, sub- and super-micron sulfate agree well with observations from the C-130 aircraft. Modeled mass concentrations of fine sulfate (Dp&lt;1.3 μm) decrease by ~10% due to uptake of sulfur species onto super-micron dust. We estimate that dust enhanced the mass concentration of coarse sulfate (Dp&gt;1.0 μm) by more than an order of magnitude, but total sulfate concentrations increase by less than 2% because decreases in fine sulfate have a compensating effect. Our analysis shows that the sulfate associated with dust can be explained largely by the uptake of H2SO4 rather than reaction of SO2 on the dust surface, which we assume is suppressed once the particles are coated in sulfate. We suggest that many previous model investigations significantly overestimated SO2 oxidation on East Asian dust, possibly due to the neglect of surface saturation effects. We extend previous model experiments by examining how dust modified existing particle concentrations in Asian outflow. Total particle concentrations (condensation nuclei, CN) modeled in the dust-pollution plume are reduced by up to 20%, but we predict that dust led to less than 10% depletion in particles large enough to act as cloud condensation nuclei (CCN). Our analysis suggests that E. Asian dust storms have only a minor impact on sulfate particles present at climate-relevant sizes.