High Aerosol Acidity Despite Declining Atmospheric Sulfate .
LETTERSPUBLISHED ONLINE: 22 FEBRUARY 2016 DOI: 10.1038/NGEO2665High aerosol acidity despite declining atmosphericsulfate concentrations over the past 15 yearsRodney J. Weber1*, Hongyu Guo1, Armistead G. Russell2 and Athanasios Nenes1,3,4,5Particle acidity affects aerosol concentrations, chemicalcomposition and toxicity. Sulfate is often the main acidcomponent of aerosols, and largely determines the acidityof fine particles under 2.5 µm in diameter, PM2.5 . Overthe past 15 years, atmospheric sulfate concentrations inthe southeastern United States have decreased by 70%,whereas ammonia concentrations have been steady. Similartrends are occurring in many regions globally. Aerosolammonium nitrate concentrations were assumed to increaseto compensate for decreasing sulfate, which would result fromincreasing neutrality. Here we use observed gas and aerosolcomposition, humidity, and temperature data collected at arural southeastern US site in June and July 2013 (ref. 1), anda thermodynamic model that predicts pH and the gas–particleequilibrium concentrations of inorganic species from theobservations to show that PM2.5 at the site is acidic. pHbuffering by partitioning of ammonia between the gas andparticle phases produced a relatively constant particle pHof 0–2 throughout the 15 years of decreasing atmosphericsulfate concentrations, and little change in particle ammoniumnitrate concentrations. We conclude that the reductions inaerosol acidity widely anticipated from sulfur reductions,and expected acidity-related health and climate benefits, areunlikely to occur until atmospheric sulfate concentrationsreach near pre-anthropogenic levels.Trends of decreasing sulfur dioxide (SO2 ) and sulfate aerosolhave been observed throughout the United States (ref. 2) and arelargely attributable to emission reductions from coal-fired electricalgenerating units through scrubbing and fuel switching. These trendsare expected to endure as additional controls on SO2 are putin place to continue the decline of PM2.5 mass. In contrast, thesource of the main fine-particle acid-neutralizing agent, gas-phaseammonia (NH3 ), is largely linked to agricultural activities, whichhave been relatively steady and are expected to remain so. Thesetrends have led to a long-standing and continuing belief that theaerosol will become increasingly neutral, shifting inorganic aerosolcomposition from ammonium sulfate to ammonium nitrate andminimizing the effectiveness of SO2 reductions on PM2.5 masscontrol3–8 . This has wide-ranging ramifications, from changingthe emphases on what emission sources to control (for example,agricultural) to protect human health9 to effects on aerosol radiativeforcing10 . Other environmental impacts linked to particle pH arealso expected to change. For example, lower pH more effectivelyconverts ubiquitous isoprene emissions by forested regions toPM2.5 through heterogeneous acid-catalysed reactions11 . Low pHincreases solubility of metals associated with mineral dust andanthropogenic sources, which can be either ecosystem nutrients12 ,or have detrimental health impacts through in vivo generationof reactive oxygen species13 . Particle strong acidity has also beendirectly linked to adverse respiratory effects14,15 .Despite large investments in sulfur emission reductions, weshow that the acid/base gas–particle system in the southeasternUnited States is buffered by the partitioning of semivolatile NH3 ,making it insensitive to changing SO2 levels. Counter to expectations, acidic pH effects on air quality will therefore remain largelyunchanged. Although our analysis focuses on the southeasternUnited States, it serves as a model, demonstrating the need fordetailed thermodynamic analyses at locations globally to accuratelyevaluate the effects of sulfate reductions on particle acidity andaerosol composition.To assess if decreasing sulfate leads to substantial changes inaerosol pH, we investigate the sensitivity of pH in the southeasternUnited States to changes in sulfate (SO4 2 ) and gas-phase ammonia(NH3 ) levels, focusing on summertime data from a rural southeastern background site, Centreville (CTR). The historical breadth ofdata collected at CTR, and the detailed observations of key aerosoland gas phase species measured during a recent intensive study atthe site (SOAS), make it ideal for thermodynamic analyses to predictand evaluate pH at high temporal resolution and for comparisons tohistorical trends.A detailed pH calculation that involves both gas and aerosolcomposition data was performed using average conditions from asubset of the CTR SOAS experiment (11 June to 23 June 2013). Thistime period was selected because it excludes periodic precipitationand dust events, providing representative conditions consistent withthe mean summertime aerosol state in the region. ISORROPIA-IIpredicted NH3 agreed with independently measured concentrations(Fig. 1), demonstrating that the thermodynamic analysis accuratelyrepresents the aerosol state, as deviations in predicted pH would leadto large biases in predicted NH3 .Measurements at various SEARCH air-quality monitoring network sites throughout the southeast show that annual mean SO4 2 concentrations have dropped substantially from 1999 to 2014 , withconcentrations going from roughly 6 to 2 µg m 3 (ref. 16). HistoricalNH3 concentrations are not as well known; however, data fromSEARCH sites8 and the Ammonia Monitoring Network (AMoN)(http://nadp.sws.uiuc.edu/amon) show steady overall concentrations going back to 2004. Between 2008 and 2014, CTR meansummer concentration was 0.23 µg m 3 , similar to measurements atCTR during SOAS (mean: 0.36 µg m 3 ; ref. 17). Ammonia at other1 School of Earth and Atmospheric Sciences, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia 30332, USA. 2 School of Civil & EnvironmentalEngineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia 30332, USA. 3 School of Chemical and Biomolecular Engineering, GeorgiaInstitute of Technology, 311 Ferst Drive, Atlanta, Georgia 30332, USA. 4 Institute of Chemical Engineering Sciences, Foundation for Research andTechnology—Hellas, Platani, PO Box 1414, GR-26504 Patras, Greece. 5 Institute for Environmental Research and Sustainable Development, NationalObservatory of Athens, Palea Penteli, GR-15236, Greece. *e-mail: rweber@eas.gatech.eduNATURE GEOSCIENCE ADVANCE ONLINE PUBLICATION www.nature.com/naturegeoscience 2016 Macmillan Publishers Limited. All rights reserved1
NATURE GEOSCIENCE DOI: 10.1038/NGEO2665LETTERS[NH3]pred m [NH3]obs bm 1.02 0.02b 0.03 0.01Predicted NH3 (µg m 3)R2 0.901.00.50.00.50.01.01.5Measured NH3 (µg m 3)Figure 1 Evaluation of the thermodynamic model. Comparison ofmeasured NH3 to ISORROPIA-II-predicted concentrations. Data are fromSOAS (that is, SEARCH CTR site) for measurements between 11 June and23 June 2013. NH3 was measured via a chemical ionization massspectrometer (CIMS; ref. 17). Orthogonal regression and the uncertainty inthe measured NH3 1 h-avg data (10%) are shown. Fit parameteruncertainties are for 95% confidence intervals. The good agreementvalidates the model predictions of pH.sites in the southeast generally ranged between 0.1 and 2 µg m 3 ,with highest levels observed at sites more influenced by agricultural activities (for example, at Yorkville, a rural SEARCH site,mean NH3 is 1.74 µg m 3 ). A mass balance analysis indicates thatNH3 concentrations are directly driven by NH3 emission rates (seeMethods). Given this, and that emissions are relatively steady18 , NH3concentrations have probably been at similar levels even further intothe past.Conceptual modelTo test if these trends imply that the aerosol is becoming less acidicwe first consider a simplified scenario of an isolated ammoniumsulfate aqueous particle. At the average SOAS conditions, for thisparticle the equilibrium NH3 vapour concentration is approximatelyaSensitivity analyses with a full thermodynamic modelWe expand on the historical ranges of both NH3 and SO4 2 formore comprehensive sensitivity analyses. Sulfate and total ammonia(gas particle) were independently varied over two orders ofmagnitude and used as input to ISORROPIA-II. The resultingpredicted equilibrium pH is shown in Fig. 2. As seen above, theseresults indicate a very weak sensitivity of pH to a wide range ofSO4 2 and NH3 , suggesting that the observed decrease in SO4 2 should have little influence on pH. Our predictions are consistentwith the historical summertime observations at CTR. Trends in fineparticle SO4 2 and NH4 and gas-phase NH3 are shown in Fig. 3, andare similar to the general trends of the southeast8,16 , and probablymuch of the eastern United States (ref. 2). pH estimated from theCTR historical aerosol ionic composition data set also demonstratesthat summertime pH has remained remarkably constant and low(between 0 and 1) throughout the past 15 years, similar to the rangeof roughly 0 and 2 predicted in the sensitivity analyses.A further assessment of the thermodynamics can be gained fromammonium-to-sulfate molar ratios, RSO4 (see Methods). 6 82 SO41(µg246 .0pHNH3 (µg m 3)160 µg m 3 (220 ppbv) and pH is near 3 (see Methods for descriptionof calculations). However, for ammonium bisulfate the equilibriumNH3 concentration drops sharply to approximately 0.06 µg m 3(0.08 ppbv), and pH is near 0. Because typical observed NH3concentrations range between 0.1 and 2 µg m 3 , ambient NH3concentrations will rarely ever reach the 160 µg m 3 needed forequilibrium with pure ammonium sulfate, meaning that it willalmost never exist. However, NH3 will always be present in thegas phase, even at very low pH. Furthermore, the sharp dropin equilibrium NH3 when going from (NH4 )2 SO4 to (NH4 )HSO4(160 to 0.06 µg m 3 ) with a pH change of only 3 to 0, independent ofsulfate concentrations, also demonstrates the low sensitivity of pHto NH3 concentrations.Now consider an ammonium sulfate solution that is aerosolizedinto pure air. The aerosol would reach equilibrium by someof the ammonia volatilizing, leaving an aerosol of mixed(NH4 )2 SO4 –NH4 HSO4 with a pH between 0 and 3, and someammonia in the gas phase. If the volume of air were very largecompared to the amount of ammonium sulfate originally present,the resulting aerosol would be predominantly ammonium bisulfate.(Ammonium sulfate molar ratios observed in the southeasternUnited States are in the range of 1.5 to 1.8, which is discussed furtherbelow.) Although conceptually insightful, the full thermodynamicmodel must be run because pure aerosol species in isolation do notexist and a quantified pH is needed to assess the impacts of acidity.NH3 (µg m 3)1.50.1246 82 SO4m 3)1(µg246 810m 3)Figure 2 Sensitivity of PM2.5 pH and RSO4 to gas-phase ammonia (NH3 ) and PM2.5 sulfate (SO4 2 ) concentrations. The results are predictions from athermodynamic analysis assuming equilibrium between the gas and particle phases for typical summer conditions in the southeastern United States. Boxesdefine estimated concentration ranges over the previous 15 years and ranges expected in the future. RSO4 is [(NH4 –NO3 )/SO4 2 ].2NATURE GEOSCIENCE ADVANCE ONLINE PUBLICATION www.nature.com/naturegeoscience 2016 Macmillan Publishers Limited. All rights reserved
NATURE GEOSCIENCE DOI: 10.1038/NGEO2665LETTERS82.5SO42 (µg m 3)2.0r2 0.801.541.020.50NH4 , NO3 , NH3 (µg m 3)Slope 0.31 µg m 3 yr 160.032pHSOAS data102.01.5RSO4r2 0.371.00.5SO42 NH3 CTR0.019982000NH4 NH3 GAS20022004NO3 NH3 OAK2006Year2008201020122014Figure 3 Mean summer (June–August) trends in PM2.5 composition, NH3 , RSO4 and predicted PM2.5 pH at the SEARCH-CTR site. NH3 data are from theSEARCH rural sites Centreville (CTR, Alabama) and Oak Grove (OAK, Mississippi), and AMON Georgia Station (GAS, Georgia) site. SOAS mean data(1 June to 15 July 2013) are also plotted. Error bars represent data ranges (standard errors). pH was estimated with ISORROPIA-II run in the forward modewithout gas-phase species input, resulting in pH systematically low by approximately one unit1 . Uncertainties: ions 15%, molar ratios 26%, NH3 15to 40%.reports RSO4 has ranged roughly between 1.5 and 1.8 at sites inthe southeast, corresponding to a mixture of ammonium sulfateand ammonium bisulfate as the dominant salts. Surprisingly, RSO4decreased by 0.01 to 0.03 units per year over the past 15 years. Thisis counterintuitive, as it shows that, as sulfate is reduced and theNH3 to neutralize it remains constant, the aerosol is increasinglyenriched in acidic ammonium bisulfate, whereas the expectationis that the aerosol should become more neutral over time (that is,RSO4 increasing and approaching 2). CTR RSO4 follows a similardecreasing trend (Fig. 3) and the thermodynamic sensitivity analysis(Fig. 2b) is consistent with these observations.All these results can be explained by the non-volatility of sulfateand semivolatility of ammonia, meaning that only NH3 movesto establish equilibrium between the condensed aqueous and gasphase. For example, decreasing RSO4 occurs because particle-phaseammonium roughly tracks SO4 2 (Fig. 3), so at lower SO4 2 there isless ammonium available when establishing equilibrium with gasphase NH3 . During equilibration, ammonium (NH4 ) is lost to thegas-phase (NH3 ), releasing H in the aerosol and decreasing RSO4 .Because a larger relative loss in ammonium occurs at lower SO4 2 ,RSO4 is lower. The semivolatility of ammonium is also responsible forthe remarkably weak sensitivity of pH to SO4 2 . Further evidence isthat pH begins to rise in the sensitivity analyses (Fig. 2a) for SO4 2 below roughly 0.3 µg m 3 ; the thermodynamic analysis suggests thisis due to association of SO4 2 with other non-volatile cations thatstarts to mitigate the buffering effect of NH3 partitioning, significantly increasing pH (see Supplementary Extended Data Fig. 1).The sensitivity analysis (Fig. 2a) does show a pH increase withincreasing NH3 , but it is relatively weak, a consequence of thebuffering effect of semivolatile NH3 partitioning. For SO4 2 between0.1 and 10 µg m 3 , it is not until NH3 is over 10 µg m 3 (14 ppbv)before pH approaches 2.5; the approximate minimum pH at whichammonium nitrate begins to form for conditions of this study(see Supplementary Extended Data Fig. 2). In summary, for typicalsummer conditions the observed trends in pH and RSO4 are expectedand due to gas–particle thermodynamic equilibrium, along withrelatively constant NH3 levels that have been below a few µg m 3over the past 15 years.Future pHThe thermodynamic sensitivity analyses were continued for SO4 2 down to 0.1 µg m 3 , levels characteristic of remote regions19 . AsNATURE GEOSCIENCE ADVANCE ONLINE PUBLICATION www.nature.com/naturegeoscience 2016 Macmillan Publishers Limited. All rights reserved3
NATURE GEOSCIENCE DOI: 10.1038/NGEO2665LETTERSemissions of NH3 are decoupled from SO4 2 , linked mainlyto agricultural and biogenic activities, in the foreseeable futureone may expect this source to remain largely flat, or modestlyincrease as agricultural production follows population increases.This indicates that future gas-phase NH3 concentrations will notdeviate considerably from current levels8 . The area of expectedfuture conditions in Fig. 2 shows that summertime pH will remainin the 0 to 2 range, similar to current levels and those of the past15 years. The system insensitivity to NH3 implies that it is a poorindicator of fine aerosol pH. Furthermore, if the aerosol is in anaqueous phase the presence of NH3 does not necessarily imply aneutral aerosol, as often assumed20 , as some NH4 always needs tovolatilize to achieve equilibrium.The sensitivity analysis also predicts changes in gas/particleportioning ratios with changing SO4 2 and NH3 . Over the broadSO4 2 range of 0.1 to 10 µg m 3 , the fraction of NH3 in the gasphase relative to total (gas particle) varies from about 90 to 10%,indicting coupling of gas–particle ammonia concentrations acrossthis SO4 2 range. In contrast, nitric acid is predicted to remainlargely in the gas phase, unaffected by SO2 controls owing to thepredicted low pH. Nitrate aerosol is expected only once pH is over 2 to 3, and so minimal particle-phase inorganic nitrate is expectedfor typical summer conditions; neither in the past as SO4 2 hasdecreased, and verified by the historical data (Fig. 3), nor for sometime into the future, given expected pH ranges (Fig. 2). If there areno significant changes in future non-volatile cation levels in the fineaerosol mode (for example, increases in mineral dust due to landuse changes or desertification, discussed in Supplementary Information), summertime PM2.5 in the southeastern United States willremain highly acidic until average sulfate levels approach those ofvery clean remote continental sites ( 0.3 µg m 3 ; refs 19,21) andparticulate nitrate will not become an increasing air-quality issuefor a long time into the future—counter to expectations.Although our analysis focuses on the southeastern United States,it applies to all locations that are impacted by SO2 emissions andwhich lack major sources of non-volatile cations (for example,sea-salt or mineral dust); conditions commonly found in manyregions globally. Although reductions in sulfur dioxide (SO2 ) andoxides of nitrogen (NOx ) will reduce sulfate, nitrate and ammoniumaerosol concentrations, leading to improved air quality, we showthat fine particles are highly acidic and are likely to remain sodespite substantial sulfate reductions. Furthermore, particle pHmust be explicitly considered to accurately gauge its impactson heterogeneous chemical processes, particle composition, massconcentrations and toxicity.MethodsMethods and any associated references are available in the onlineversion of the paper.Received 1 July 2015; accepted 26 January 2016;published online 22 February 2016AcknowledgementsThis research was supported by a grant from the National Science Foundation throughcontract number 1242258, US Environmental Protection Agency Grants RD835410 andRD834799. Its contents are solely the responsibility of the grantee and do not necessarilyrepresent the official views of the US government. Further, the US government does notendorse the purchase of any commercial products or services mentioned in thepublication. Atmospheric Research Associates (ARA) and the SOAS team providedobservational data and logistical support.Author contributionsReferences1. Guo, H. et al. Predicting particle water and pH in the southeastern UnitedStates. Atmos. Chem. Phys. 15, 5211–5228 (2015).2. Hand, J. L., Schichtel, B. A., Malm, W. C. & Pitchford, M. L. Particulate sulfateion concentration and SO2 emission trends in the United States from the early1990s through 2010. Atmos. Chem. Phys. 12, 10353–10365 (2012).3. Pinder, R. W., Gilliland, A. B. & Dennis, R. L. Environmental impact ofatmospheric NH3 emissions under present and future conditions in the easternUnited States. Geophys. Res. Lett. 35, L12808 (2008).4. Pinder, R. W., Adams, P. J. & Pandis, S. N. Ammonia emission controls as acost-effective strategy for reducing atmospheric particulate matter in theeastern United States. Environ. Sci. Technol. 41, 380–386 (2007).45. Heald, C. L. et al. Atmospheric ammonia and particulate inorganic nitrogenover the United States. Atmos. Chem. Phys. 12, 10295–10312 (2012).6. Tsimpidi, A. P., Karydis, V. A. & Pandis, S. N. Response of inorganic fineparticlulate matter to emission changes of sulfur dioxide and ammonia: theeastern United States as a case study. J. Air Waste Manage. Assoc. 57,1489–1498 (2007).7. West, J. J., Ansari, A. S. & Pandis, S. N. Marginal PM2.5 : nonlinear aerosol massresponse to sulfate reductions in the eastern United States. J. Air Waste Manage.Assoc. 49, 1415–1424 (1999).8. Saylor, R., Myles, L., Sibble, D., Caldwell,
Particle acidity a ects aerosol concentrations, chemical composition and toxicity.Sulfate is often the main acid component of aerosols, and largely determines the acidity of fine particles under 2.5 m in diameter, PM2.5. Over the past 15 years, atmospheric sulfate concentrations in the southeastern United States have decreased by 70%,
Cow milk : 0.13 to 0.14% (natural acidity) Buffalo milk : 0.14 to 0.15% The developed or real acidity is due to lactic acid formed as a result of bacterial action on lactose present in milk. Titrable acidity developed natural acidity The acidity and pH of fresh milk varies with species, breed, individuality, stage of lactation & health of .
The State of Acidity in Atmospheric Particles in Clouds Review article in preparation led by Havala Pye (EPA) and Thanos Nenes (EPFL) –focus on aerosol and cloud pH – Definition of pH – Proxies for aerosol pH and assessment of their capabilities – Aqueous-phase chemistry –effects of pH and effects on pH – Observations of aerosol and cloud water pH
atmospheric aerosol. [6] Depending on the relative concentrations of these ions (which vary on the basis of differing oxidation rates, atmospheric aerosols may exhibit acidic properties [Huntzicker et al., 1980]. Such aerosol acidity has been shown to influence
AEROSOL CANS Substances in aerosol cans are under pressure and are usually flammable. Look for alternatives to aerosol cans such as pump sprays. Empty aerosol cans may be disposed of in regular household trash. ALUMINUM Aluminum foil and trays are often used in homes for covering and preparing food. Try to use reusable dishes such as ceramic or
6.4.2.1.1 Storage of Level 2 andAerosol, Level 3 Aerosol, and Plastic Aerosol 3 Products shall be permitted as provided for in 6.4.3. 6.4.2.2.2 Encapsulated storage of uncartoned Level 2 and Level 3 Aerosol Products on slip sheets or in trays shall be permitted. 6.4.2.2.3 Plastic Aerosol 3 products shall not be encapsulated. 6.4.2.2.4 Plastic Aerosol 3 products shall only be packaged in .
The acidity of aqueous atmospheric solutions is a key parameter driving both the partitioning of semi -volatile acidic and basic 20 trace gases and their aqueous -phase chemistry. In addition , the acidity of atmospheric aqueous phases, e.g. deliquesced aerosol particles, cloud and fog droplets, is also dictated by aqueous -phase chemistry.
The acidity (and liquid water content) of atmospheric aerosol are key state parameters that profoundly influence the patterns, fluxes and impacts of atmospheric nutrient deposition to the oceans. These effects are brought about by changes in the gas-aerosol partitioning of nitrogen species and the solubility of iron, phosphorus and trace metals.
Keywords --- algae, o pen ponds, CNG, renewable, methane, anaerobic digestion. I. INTRODUCTION Algae are a diverse group of autotrophic organisms that are naturally growing and renewable. Algae are a good source of energy from which bio -fuel can be profitably extracted [1].Owing to the energy crisis and the fuel prices, we are in an urge to find an alternative fuel that is environmentally .
