Chemistry of reactive nitrogen oxides, NOy, is crucial

for our understanding of composition and properties

of the Earth’s atmosphere. The proof-of-principle

experiments demonstrated that we are able to study the

atmospheric fate of nitrogen oxides that has significant

impact on global climate and hydrological cycle, thus

affecting the likelihood of local floods and acid rain

Hunt, O.R.

King, M.D.

Pfrang, Christian

English

Chemistry of reactive nitrogen oxides, NOy, is crucial for our understanding of composition and properties of the Earth’s atmosphere. The proof-of-principle experiments demonstrated that we are able to study the atmospheric fate of nitrogen oxides that has significant impact on global climate and hydrological cycle, thus affecting the likelihood of local floods and acid rain

University of Birmingham Research Portal

Exploring NOy chemistry in levitated aqueous aerosol droplets

Central Archive at the University of Reading

LASERS FOR SCIENCE FACILITY (LSF) PROGRAMME I Chemistry5177CENTRAL LASER FACILITY Annual Report 2008/2009Exploring NOy chemistry in levitatedaqueous aerosol dropletsC. PfrangDepartment of Chemistry, University of Reading, PO Box 224, Whiteknights, Reading RG6 6AD, UKM. D. King and O. R. HuntDepartment of Earth Sciences, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK Contact c.pfrang@reading.ac.ukIntroductionChemistry of reactive nitrogen oxides, NOy, is crucialfor our understanding of composition and propertiesof the Earth’s atmosphere. The proof-of-principleexperiments demonstrated that we are able to study theatmospheric fate of nitrogen oxides that has significantimpact on global climate and hydrological cycle, thusaffecting the likelihood of local floods and acid rain.Atmospheric aerosolAtmospheric aerosol is a variable, yet criticalcomponent of the climate system and has substantialimpact on climate change. [1-3] Aerosols affect climatedirectly, by scattering and absorbing solar radiation,and indirectly, by acting as cloud condensation nuclei,CCN. Clouds may reduce the incident radiation on thesurface, increase or decrease temperatures, andsuppress rainfall. The chemical composition of aerosolalters the hydrological cycle, which affects quality andavailability of fresh water. The need for an improvedunderstanding of the factors affecting the hydrologicalcycle is well-illustrated by the rapidly increasingfrequency of extreme flood events in the UK. Theeffect of aerosol on radiative forcing and cloudformation has been repeatedly identified [4,5] as thelargest uncertainty in assessing the impact ofparticulate matter on climate change. National andinternational institutions declared climate change to bethe greatest environmental challenge facing the worldtoday. Atmospheric aerosol not only impacts climate,but also directly affects human health: it may suppressdefence mechanisms, respiratory and cardiovascularfunctions and cause allergic and infectious diseases.Aerosol is important for the biosphere throughspreading reproductive materials and micro-organisms.NOy-initiated aerosol ageing Once in the atmosphere, aerosol particles change sizeand composition through chemical reactions,condensation of vapour species, evaporation,coagulation with other particles, or by activation inregions of water supersaturation (relative humidity,RH, above 100%) to become fog and cloud droplets. [6]Chemical ageing of organic atmospheric aerosolproceeds mainly via its interaction with initiators ofatmospheric oxidation such as hydroxyl radicals, OH,ozone, O3, and nitrogen oxides, in particular nitrateradicals, NO3, but also NO2 and N2O5. Reactions ofNO3 are only important at night, since the radical isreadily photolysed during daytime. The mainimportance of N2O5 lies in its heterogeneouschemistry, but several recent studies have investigatedits gas-phase behaviour (e.g.Brown et al. [7] foratmospheric measurements and Pfrang et al. [8] forlaboratory studies). The chemistry of nitrogen oxidesis particularly relevant because of its potential impacton photochemical ozone production, in turnresponsible for smog formation and influencing theformation of secondary organic aerosol. [9] NO2 issomewhat less reactive than either O3 or NO3, but isnevertheless very important in the NOx cycle. Theimportance of photo-enhanced uptake of NO2 onboth solid organic compounds and mineral dustparticles has been highlighted. [10]Humic-like substances, HULISThe term HUmic-LIke Substances, HULIS, describesprobably one of the most intriguing classes ofatmospherically relevant species and attractedsubstantial interest at the Faraday Discussions 137.Several papers were presented investigating theatmospheric importance of HULIS [11] and thediscussion at the meeting demonstrated that researchon HULIS is very timely. HULIS describes organicspecies found in cloud, fog, rain and aerosol thatresemble organic material in river/sea water and soils.HULIS is suggested to be formed within the aerosoland may be present in as much as 20–50% of aerosolparticles. [12] HULIS is very difficult to chemicallycharacterise owing to its complexity, size and non-uniformity. However, consensus is starting to form ona HULIS model structure consisting of an aromaticcore bearing substituted aliphatic chains with –COOH,–CH2OH, –COCH3 groups. Cappiello et al. [13]performed a molecular characterisation of HULIS-type water-soluble organic compounds in fogwater.Their analysis of a range of different air massesshowed common base structures. These structures arethe basis on which we chose the compounds torepresent HULIS in our experimental studies. Sourcesof HULIS include a small terrestrial, marine, biomassburning and secondary aerosol formation(condensation, reaction and oligomerisation). Thepresence or oxidative degradation of HULIS in cloud,fog and aerosol has been found to lead to dropletactivation and enhanced cloud formation. [14] Thereactions of HULIS proxies with initiators ofatmospheric oxidation such as NO2, NO3 and N2O5LASERS FOR SCIENCE FACILITY (LSF) PROGRAMME I ChemistryCENTRAL LASER FACILITY Annual Report 2008/20091785thus need to be studied to determine if they (1) lowerthe critical supersaturation required for cloudformation and (2) lead to the formation of oligomers.The rate for these reactions must be quantified todetermine if they are atmospherically important.ExperimentalAerosols were generated by ultrasonically nebulising asolution of HULIS-type molecules (such as benzoateand fumarate) [15] in water (containing sodium chorideor ammonium sulphate). Optically levitated HULISdroplets typically contained a concentration of~0.1-1M. The aerosol droplets were mixed with ahumidified stream of nitrogen (RH ~95%) and blowninto a newly developed aluminium flow cell on thestage of an optical microscope (Fig. 1). The cellallowed flexible injection through seven different ports,most of them suitable for attachment of an inertPTFE sliding injector. This sliding injector was usedfor introduction of the nitrogen oxides (in a flow ofdry oxygen) after passing through a 500 ml reactionbulb. Four ports also allowed horizontal visualisationusing a fibre-optic borescope. A droplet (3-15 µmdiameter) was trapped in the focus of an argon-ionlaser (λ = 514.5 nm, ~7 mW at the point of focus).The remaining particles were flushed from the cell.The scattered laser radiation was collected andanalyzed on a spectrometer via a notch filter. Spectrawere recorded continuously over 5-10 secondtimescales. The trapped particle was illuminated withan optically filtered microscope light onto a CCD viathe microscope optics to provide an optical image ofthe particle. A still image was recorded every 10-30seconds in order to size the droplet.A typical experiment would monitor size and Ramansignal of the droplet for a few minutes. Subsequently,the dry oxygen flow would be subjected to a Hg pen-ray lamp to produce O3 (ppm range) which thenentered the 500 ml reaction bulb. An additional flowof a NO2 mixture (1000 ppm in air) into this reactionbulb would initiate the NOy generation chemistry. Thereaction was monitored until there was no discernableHULIS remaining in the Raman spectra. Experimentswere also undertaken without oxidants present.Raman spectra were used to detect possible products.In-situ NOy generationA reliable in-situ NOy generation method is vital forthe present experimental studies. To this end, wedeveloped a suitable flow reaction system andconfirmed adequate concentrations in the flow cellin UV-Vis and FTIR spectroscopic studies withinthe Lasers for Science Facility which were alsomatched by a kinetic model (Fig. 2). We chose flowrates and volume of reaction bulb, so that the N2O5-precursor O3 is quantitatively consumed before theoxidants enter the flow cell. We could also vary theNO3/N2O5 ratio to distinguish between reactions ofthese two species.ResultsChanges in chemical composition (Raman) anddroplet size (optical microscopy) could be followedfor uptake of NO2, NO3 and N2O5 on dropletscontaining NaCl or (NH4)2SO4 as well as the HULISproxy sodium fumarate. Detailed analysis to extractkinetic parameters and assign product peaks iscurrently being undertaken. Uptake of nitrogenoxides into the droplet led to the appearance of anitrate peak (at ~1050 cm-1), accompanied bydisappearance of the SO42– peak: the change in pHcaused by N2O5 uptake is likely to lead to conversionof SO42– to HSO4–. Exposure of the HULIS dropletto the nitrogen oxides led to oxidative consumptionof fumarate and we also obtained evidence forformation of Raman-active products.ConclusionsWe performed very successful proof-of-principlestudies opening up a wide range of experimentalsystems for investigation. We demonstrated that thestudy of reactions of nitrogen oxides with opticallylevitated micron-size aqueous droplets containingHULIS-proxies is feasible. Our in-situ method fornitrogen oxide generation in a mobile gas-handlingsystem has for the first time been coupled to a laserRaman tweezers system. We are currently extractingkinetic parameters and identifying functional groupsof reaction products for peer-reviewed publication.Fig. 1. Successful application of a newly developed flowcell with inert sliding injector for study of interaction oflevitated particles with the initiators of atmosphericoxidation NO3 and N2O5. Additional ports allowhorizontal visualisation (e.g. using a borescope).Fig. 2. Gas-phase concentration profiles (kinetic modelconsidering nitrogen oxide reactions). Concentrations ofkey species have been confirmed spectroscopically.LASERS FOR SCIENCE FACILITY (LSF) PROGRAMME I Chemistry5179CENTRAL LASER FACILITY Annual Report 2008/2009Outlook: preliminary tests on probingcharge effects on airborne aerosol Charges impact on the behaviour of atmosphericaerosol: both coalescence efficiencies [16] and thethermodynamic stability of droplets [17] will be affected.This impact of charges on cloud behaviour is not wellunderstood. We performed preliminary tests chargingdroplets levitated in two optical traps (Fig. 3(a)). Aqualitative charging effect could be demonstrated (Fig.3(b)), but quantification would benefit from longer-termaccess to levitation and charge measuring facilities.AcknowledgementsWe are very grateful to the CLF, Rutherford AppletonLaboratory, UK for grant 81025 and for support forthe STFC Strategic Initiative Grant HS30635. Thiswork was in part supported by NERC CASE awardNER/S/U/2006/14253.References1. M. O. Andreae, et al., Science, 303, 1337 (2004).2. U. Pöschl, Angew Chem. Int. Ed., 44, 7520 (2005).3. T. W. Choularton, et al., Faraday Discuss., 137, 205(2008).4. IPCC, Climate Change 2001: The scientific basis,CUP, Cambridge (2001).5. IPCC, Climate Change 2007: The scientific basis,CUP, Cambridge (2007).6. J. Sun and P. A. Ariya, Atmos. Environ., 40, 795(2006).7. S. S. Brown, W. P. Dube, et al., J. Geophys. Res.,112 (D22) (2007).8. C. Pfrang, et al., Atmos. Environ., 41, 1652-1662(2007).9. N. L. Ng, P. S. Chhabra, et al., Atmos. Chem.Phys., 7, 5159 (2007).10. C. George, M. Ndour, et al., NATO Sciences SeriesIV Earth and Environmental Sciences, 219 (2007).11. E. Dinar, A. Abo Riziq, C. Spindler, C. Erlick,G. Kiss and Y. Rudich, Faraday Discuss., 137, 279(2008).12. E. R. Graber and Y. Rudich, Atmos. Chem. Phys.,6, 729 (2006).13. A. Cappiello, E. Desimoni, C. Fiorucci,F. Mangani, P. Palma, H. Trufelli, S. Decesari,M. C. Facchini and S. Fuzzi, Environ. Sci. Tech.,37, 7, 1229 (2003).14. E. Dinar, I. Taraniuk, E. R. Graber, S. Katsman,T. Moise, T. Anttila, T. F. Mentel and Y. Rudich,Atmos. Chem. Phys. Discuss., 6, 1073 (2006).15. M. D. King, K. C. Thompson, A. D. Ward,C. Pfrang and B. R. Hughes, Faraday Discuss.,137, 173 (2008).16. A. Khain, V. Arkhipov, M. Pinsky, Y. Feldman,Y. Ryabov, J. Appl. Meteor., 43, 1513 (2004).17. R. G. Harrison and M. H. Ambaum, Proc. R Soc.A, 464, 2561 (2008).Fig. 3. (a) Preliminary tests using two optical traps tobring two charged ~6 µm droplets to coalescence(arbitrary time steps: 1–6). (b) False-colour image of theinteraction of two charged droplets.

Exploring NOy chemistry in levitated aqueous aerosol droplets

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