DEPARTMENT OF BIOLOGICAL AND ENVIRONMENTAL SCIENCES FORMATION OF ORGANIC NITRATES AND AEROSOLS The case of isoprene oxidation by nitrate radicals Stina Wallgren Degree project for Bachelor of Science with a major in Environmental Science ES1505, Degree project in Environmental Science 1, 15 credits First cycle Semester/year: Spring 2018 Supervisor: Mattias Hallquist, Department of Chemistry and Molecular Biology Co-Supervisor: Epameinondas Tsiligiannis, Department of Chemistry and Molecular Biology Examinator: Johan Boman, Department of Chemistry and Molecular Biology
Transcript
DEPARTMENT OF BIOLOGICAL AND ENVIRONMENTAL SCIENCES
FORMATION OF ORGANIC NITRATES AND AEROSOLS
The case of isoprene oxidation by nitrate radicals
Stina Wallgren
Degree project for Bachelor of Science with a major in Environmental ScienceES1505, Degree project in Environmental Science 1, 15 creditsFirst cycle
Semester/year: Spring 2018Supervisor: Mattias Hallquist, Department of Chemistry and Molecular BiologyCo-Supervisor: Epameinondas Tsiligiannis, Department of Chemistry and Molecular BiologyExaminator: Johan Boman, Department of Chemistry and Molecular Biology
AbstractIsoprene, a Biogenic Volatile Organic Compound (BVOC), is the most common BVOC emittedfrom vegetation. Despite being emitted in daytime, via interaction with sunlight and elevatedtemperature, it can stay in the stratified nocturnal layer. Thus, the nitrate radical (NO3), which ispresent at night-time, can oxidise the isoprene molecule. Since isoprene is volatile, it will stay in thegas-phase after the initial oxidation but can partition into particle-phase after further reactions withoxidants or different radicals. After the oxidation of the first double bond by NO3, an alkyl-radical isproduced, while further oxidation leads to a peroxy radical. Oxidation at night-time seems to bemore effective in production of Secondary Organic Aerosol (SOA) than photo-oxidation. However,to produce SOA, the most effective reactions are with HO2 and RO2 radicals. HO2 is often notconsidered important in chamber experiments but can be abundant in forested areas. Other RO2
radicals can produce dinitrates that could, due to high O:C ratio and molecular weight, effectivelypartition to the particle-phase. To understand the different mechanisms and pathways for theseoxidation products could be important to find solutions for reducing the amount of SOAs in theatmosphere as these affect climate and human health.
SammanfattningIsoprener, ett flyktigt organiskt kolväte, är det kolväte som släpps ut från vegetationen i störstutsträckning. Trots att det släpps ut dagtid, med hjälp av solljus och förhöjd temperatur, kan detansamlas i det laminära gränssiktet nattetid. På så sätt kan nitratradikaler (NO3) som finnstillgängligt nattetid oxidera isoprenmolekylerna. Eftersom isoprener är flyktiga släpps dessa ut igasfasen men kan delta i partikelfasen efter ytterligasre reaktioner med oxidanter och olikaradikaler. Efter den första oxidationen av nitratradikalen, bildas en alkylradikal och efter ytterligareen oxidation bildas en peroxyradikal. Oxidation under nattetid verkar vara mer effektivt förproduktion av sekundära organiska aerosoler (SOA) än vad oxidation under dagtid är. Dessutomerhålls mer SOA när peroxyradikaler reagerar med andra RO2 eller HO2. HO2 är inte alltid anseddsom viktig vid experiment i laboratorium men kan finnas i stor utsträckning i skog där ävenisoprener finns. Både RO2 och HO2 kan bilda dinitrater och dessa kan med hjälp av hög O:C kvotoch molekulär vikt effektivt delta i partikelfasen. Det är viktigt att förstå olika mekanismer ochreaktioner för de oxiderade produkterna för att finna lösningar i att reducera utsläppen av aerosoler iatmosfären. Detta är viktigt då aersoler kan påverka både klimatet och människors hälsa.
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Table of ContentsAbstract.................................................................................................................................................2Sammanfattning....................................................................................................................................21. Introduction......................................................................................................................................42. Background.......................................................................................................................................6
3. Experimental Method - Thesis.........................................................................................................84. Results and Discussion...................................................................................................................10
4.1. General chemistry of isoprene oxidation................................................................................104.2 Mechanisms and pathways for the production of Organic Nitrates.........................................124.3 Formation of Secondary Organic Aerosols..............................................................................15
5. Atmospheric implications...............................................................................................................206. Conclusion and suggestions...........................................................................................................217. Acknowledgements.........................................................................................................................228. References......................................................................................................................................23
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1. IntroductionOrganic nitrates (CxHyOzN) is part of NOy which also includes NOx, NO3, N2O5, HNO2 and HNO3
1.Organic nitrates, e.g. produced from isoprene and oxidants, can go into the particle phase andsubsequently form aerosols2. The aerosols formed can affect the climate with either negative orpositive feedback mechanisms. They can also cause health problems to humans, includingcardiovascular and respiratory problems and allergies3. Organic nitrates are formed when volatileorganic compounds (VOCs) are oxidised in presence of NOx or direct reaction with the NO3 radical.Globally, the majority of VOCs are of biogenic origin with emissions of 1150 Tg/year compared toanthropogenic emissions which are around 142 Tg/year4. The emissions of isoprene are estimated tobe around 500 Tg/year5. This makes isoprene an important precursor to secondary organic aerosols(SOA). Despite its high volatility and relatively small aerosol yield (1-3%) compared to biggerhydrocarbons, the amount of SOA is still relevant due to its abundance6. The global production ofSOA is approximately 50-90 Tg/year7. An estimation of SOA production with isoprene as aprecursor is 13 Tg/year.8 One study, focusing on the SOA formation between isoprene and nitrateradicals, presented a production of 2-3 Tg/year8. Considering that nitrate radicals are producedduring night-time9 whilst isoprene are emitted via interaction with sunlight and elevatedtemperatures10,11 the probability for these to co-exist might be small. However, levels of NO3 can bedetected at day-time in shaded areas under canopies which can subsequently react with isoprene12.As well as that, isoprene have been detected during night-time and its emissions has shown to evenpeak at night when concentration of NO3 radicals are low. This is an indicator that withoutoxidation, there can be an abundance of isoprene at night.13
Some studies have suggested the reaction between isoprene and NO3 is not relevant since theisoprene has already been oxidised/consumed when the concentration of NO3 starts to increase6.Additionally, the reaction rate between oxidised products of isoprene and NO3 is quite slowcompared to OH which is the main daytime oxidant for isoprene6,14. Other studies, however, showsthat there is an abundance of isoprene in the evening in the boundary layer due to the stratificationand less mixing. This could result in intense oxidation of isoprene since the production of NO3 startsat night-time15–18. Both NO3 and O3 are available during night-time but NO3 is most likely the mostimportant night-time oxidant of isoprene because it has a faster reaction rate19. For example, the rateconstant for the reaction between O3 and isoprene is 1.3 × 10−17 cm3 molecule−1 s−1 while the rateconstant for NO3 is 7.0 × 10−13 cm3 molecule−1 s−1
. This means that even if the mixing ratio of NO3 is104 times lower than O3, it is still a relevant oxidant.16
The first-generation product of the oxidation of isoprene by NO3 is an alkyl radical. This will inmost cases then generate a peroxy radical which have different fates that will be described in thisthesis.
SOAs are formed when VOCs are condensed from the gas-phase to the particle-phase. This processmostly occurs through gas-phase oxidation of the VOC but other factors including reactions withexisting particles and clustering of low volatility vapours20. Change of certain properties willincrease the probability of a BVOC to go from gas-phase to particle-phase. The oxidation by NO 3
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could result in higher O:C ratio which consequently increases polarity, hygroscopicity and volatility.The oxidation might also cause higher molecular weight and branching of the molecule. All thesefactors increases the possibility of SOA formation.21,22
The aim for this thesis is to examine the different pathways for potential formation of organicnitrates and subsequently particles from the oxidation of isoprene by NO3 radicals. N2O5 has alsobeen covered since it is a source as well as a sink for NO3. A discussion of different pathways forthe oxidation products and some atmospheric implications of the effect of SOA have been includedas well.
The original plan was to perform a lab experiment but due to different obstacles, including brokeninstruments and unavailability, this was not possible. However, a section of the method used isincluded in the paper as well as a description of the instruments. Even if they were not used, it ishelpful to understand how a chamber experiment could be performed.
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2. Background
2.1. Isoprene
Isoprene, an isoprenoid, is a biogenic volatile organic compound (BVOC) as well as a hydrocarbon.It consists of five carbons and includes two double bonds (See figure 1). It's IUPAC name is 2-methyl-1,3-butandiene23. It is important in the process of formation of tropospheric ozone as itsoxidised product reacts with NO, hence disturbing the destruction of ozone9 and can have an impacton the formation of aerosols24. It is emitted from vegetation and its emission is dependent on light,temperature and in some cases elevated CO2 levels10,23,25. Isoprene is more likely to be emitted inareas with high temperature and sunlight through different mechanisms. The emissions triggered bysunlight seem to have a linkage to the photosynthetic process10,11. Some studies propose that it isreleased as a result of protecting the plant during heat stress10,11 and other studies have shown thatthe emissions can help prevent negative effects from reactive oxygen species (ROS)26. Someisoprene-emitting plants also seem to be protected from the elevated levels of ozone but themechanisms of this is not well understood11. Generally, air pollution does not seem to have mucheffect on the emissions10. More than 1000 Tg of BVOCs are emitted yearly and isoprene account foraround 50 % of those emissions5. Isoprene is mainly emitted from trees rather than crop plants andcommon species are oak and aspen trees. Why some plants emit isoprene and some do not is not yetfully understood. The emission requires energy and thus it is assumed that the benefits for the plantoutweigh the energy cost11. For example, leaves at the top of a canopy will emit around four timesmore isoprene than the leaves on the bottom, supporting the hypothesis of protection from heatstress. This is especially relevant in short periods of high temperature11.
Due to its double bond, isoprene is highly reactive in the atmosphere with many oxidants. Theoxidant, commonly OH, O3 and NO3, will add to one of the double bonds through electrophilicaddition. With isoprene, OH is the most common oxidant in daytime9. However, NO3 is veryimportant in the night-time chemistry of isoprene 1,8,27.
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Figure 2.1 The molecular structure of isoprene
2.2 Nitrate radicals
The nitrate radical, NO3, is formed as follows:
NO2+O3→NO3+O2 (2.1)9
A small amount can also be produce from the reaction between HNO3 and OH:28
HNO3+OH→NO3+H2O (2.2)
Since NO3 absorbs light from the visible spectrum, 620-670 nm, it quickly dissociates throughphotolysis during daytime, hence it can only remain in the atmosphere at dark conditions. Thelifetime of NO3 with overhead sun is 5 seconds29. At highly polluted areas NO3 can persist at fairlyhigh concentrations at day time30. NO3 has also been found at sufficient concentrations undercanopies, indicating that it can remain in places where light cannot penetrate13. NO3 can thermallydecompose and create an NO and NO2 as well as react with water and produce HNO3 but theevidence for this are not quite clear. NO3 can also be removed by reacting with NO and by thisproduce two NO2 molecules.9 In polluted night-time atmospheres NO3 can reach concentrationsabove 400 ppt but a typical concentration of 20 ppt has been suggested29.
N2O5 are produced at night-time via the reaction between NO3 and NO2:
NO3+NO2→N2O5 (2.3)
It is therefore a sink for NO3. In polluted atmospheres, the peak concentration of N2O5 can reach 15ppb. The major loss of N2O5 is through hydrolysis subsequently generating HNO3. It can react withwater in gas phase as well as on surfaces.9 The possibility of N2O5 reacting with alkenes have beendiscussed but not much evidence of this has been presented yet9. N2O5 can thermally dissociate inthe atmosphere and can then re-generate NO3 and NO2
31, thus serving as a reservoir for NO3/NO2
and can be transported to other areas that have low other sources of NOx28.
NO3 can react with alkanes and will then abstract a hydrogen but the reaction is slow due to thestrong C-H bonds. NO3 can also add to a double bond which is typical for most BVOCs. For largealkenes, H abstraction can occur but addition to the double bond and the generation of a nitrooxy-substituted alkyl radical is the most common process29. The initial reaction is always exothermic,meaning that it does not need additional energy for the reaction to start9. The alkyl radical willtypically react with atmospheric O2 and produce a RO2 radical. The radical can later react with HO2,NO2, another RO2 or NO. The latter is unlikely at night-time since NO3 and NO do not coexist.Which reaction that will occur depend on the conditions of the atmosphere and of co-existingpollutants.9 During a 24 h period, NO3 could account for 28% of all the oxidation of VOCs, makingit a very important oxidant in the troposphere.32
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3. Experimental Method - ThesisIn the chamber experiment, the objective was to introduce isoprene and NO3 in a flow reactor andmonitor any potential particle formation after the reaction.
For the synthesis of NO3, NO2 and O3 reacted in a glass vessel and produced N2O5 (See reaction 2.3and 2.4 ). A vial, where the gas was collected, was placed in a thermos containing dry ice andethanol. Due to the low temperature (-78.5°C), the N2O5 sublimed to white solid crystals. Thecrystals were placed in a diffusion vial with a capillary tube where the N2O5 could evaporate andeventually be used in the Gothenburg Flow Reactor for Oxidation Studies at low Temperatures (G-Frost). For a more detailed description, read Faxon et al.33
G-FROST (Figure 3.1), is a temperature-controlled reactor for night-time reactions since there is nolight available. N2 is used as a carrier gas for isoprene and N2O5 is added through two separate lines.The N2O5 will, due to the temperature (+20°C), thermally dissociate to NO3 and O2. However, someN2O5 could remain and react with the isoprene34. Zero air, i.e clean air, is used for dilution of thereactants. The dilution is necessary as the instruments operate in higher inlet flow than the outflowfrom G-FROST. The isoprene and the oxidant are mixed and the reaction happens under laminarflow conditions providing a residence time of 240 s before the central part is sampled via asampling funnel. This is important to avoid interference from the slower flow near the walls of thereactor. The oxidation products will then exit the G-FROST and could either enter a ChemicalIonization Mass Spectrometer (CIMS), a Scanning Mobility Particle Sizer (SMPS) or just aCondensation Particle Counter (CPC), depending on which information that is to be obtained.33
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Figure 3.1: A schematic picture of the G-FROST. Changed by author. From Faxon et al. (2018)33
The SMPS system is used to examine the amount of particles of a given size. It consists of aDifferential Mobility Analyzer (DMA) (See figure 3.2.) and Condensation Particle Counter (CPC). The aim for using the DMA is to go from a polydisperse aerosol to a monodisperse aerosol meaningthat only one size of particle will exit the DMA and enter the CPC. The DMA has a centre rod andan outer tube. The rod is negatively charged whilst the outer tube is electrically grounded, whichcreates an electrical field. Particles with negative charge stick to the outer electrode, whereas neutralparticles are removed with the excess flow. Positively charged particles are carried axiallydownward with the sheath air flow while also being attracted radially toward the center electrodedue to the electric field. Only one particular size, with a narrow range of electrical charge, willreach the end of the rod and exit the DMA and subsequently enter the CPC. When exiting the DMA,the particles will enter the CPC which counts the amount of particles exiting the DMA. In the CPC,butanol is vapourised and saturated, and subsequently condensed onto the particles, creatingdroplets. The droplets are then going through a laser light. As this occurs, the droplets scatter lightand each scatter is counted. By changing the voltage of the DMA, one can scan over many sizes andget a full size distribution, e.g from 10 nm up to <500 nm.
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Figure 3.2. A schematic illustration of the DMA
4. Results and Discussion
4.1. General chemistry of isoprene oxidation
There are three recent publications in this discussions that cover the reaction between isoprene andNO3. These are of interest because they have done experiments both in-situ and ex-situ. They havealso studied the oxidation process after the initial oxidation and consequently the SOA formation.In the study by Rollins et al (2009)1, the ageing process was examined, i.e. which radicals thatreacted with isoprene and further oxidation of its products. In the study by Ng et al. (2008)8, theratio between the isoprene and oxidant was of interest and a comparison was made between slowinjections of NO3 and isoprene to see which generated more aerosols. Both of these were chamberexperiments. In the study by Fry et al (2018)27, the measurements were done in ambient air wherethe NOx were emitted from plumes at an industrial site. The isoprene were naturally emitted fromnearby forest.1,8,27.
The reaction between NO3 and unsaturated compounds could possibly form multifunctionalnitrates1. A molecule with high functionalization will change the properties and reactivity and theaddition of nitrate groups can lower the vapour pressure considerably. After this, it is probable thatthe product has a low enough volatility to partition in the particle phase.2
The NO3 will first attack one of the double bonds of the isoprene molecule. The radical willelecrophilically add to the double bond. There are four possible positions available for the NO 3 toreact with but the the 1-position seems to be the most common (See figure 4.1). The branching ratiofor the 1st and 4th position varies between 3.5 and 7.48,16.
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Figure 4.1. The different positions on the isoprene molecule. From Schwantes et al. (2015)16
After the reaction with NO3, a nitrooxy-alkyl radical will be produced and further oxidation by O2
will create a peroxy radical (RO2) (See figure 4.2 and reaction 4.1 & 4.2).2
RH+NO3→RONO2 (4.1)
RONO2+oxidation→RO2 NO2 (4.2)
The RO2 can then react with NO3, RO2, HO2 or NO. The molecule will become more polar withhigher molecular weight when more nitroxy-groups are added which increases the possibility ofSOA formation. Some observations of small peroxy radicals have given indications that RO2
radicals are more likely to react with other RO2 radicals rather than HO2 and NO3 but other studiesshows that the reaction with HO2 is more common. RO2 reactions with NO3 does not seem to befavoured when there are RO2 available but after further oxidation, and when where is less RO2
available, NO3 becomes more important.8,16,27
The different pathways for the RO2 will be described in the following section.
The peroxy radical could react with NO2:
RO2+NO2→RO2 NO2 (4.3)
→RO+NO3 (4.4)
It could react with NO:
RO2+NO→RO+NO2 (4.5)
→RONO2 (4.6)
It could react with HO2:9,16
RO2+HO2→ROOH +O2 (4.7)
→Carbonyl compound+H 2 O+O2 (4.8)
→ROH+O3 (4,9)
→RO+OH+O2 (4.10)
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Figure 4.2: The oxidation of isoprene and production of a nitroxy-peroxy radical. From Rollins et al (2009)1
It could react with other RO2 radicals:
RO2+RO2→2RO+O2 (4.11)
→ROH+ RCHO+O2 (4.12)
→ROOR+O2 (4.13)
Finally, it could react with NO3 but the mechanism for this is not certain.
4.2 Mechanisms and pathways for the production of OrganicNitrates
4.2.1 Reaction with NO2
Reaction 4.3 can generate PAN-like compounds9. These may thermally decompose so they are morestable under cooler conditions7. Reaction 4.4 has been suggested recently but a connection withperoxy radicals from isoprene is yet to be found19.PAN-like compounds are common sinks for both hydrocarbons and NOx. When the reaction occursin cooler temperatures, the compound act as a sink and can then be transported to other areas. IfNOx and NOy are detected in rural areas far away from emission sources, an assumption can bemade that they have been transported there from other polluted areas.9
4.2.2 Reaction with NO
This is not relevant for this study since NO3 immediately react with NO and form two NO2:
NO3+NO→2 NO2 (4.14)
This means that as long as there is NO, there will be little or no NO3 available. 9
4.2.3 Reaction with HO2
A mechanism for the reaction with HO2 has been suggested by Rollins et al1(See figure 4.3). Themixing ratio of NO3 is often lower than HO2, one measurement showing 4 times lower35. Inchamber experiments for nitrates and isoprene the focus is often on the reactions between RO2 withother RO2 as well as NO3. These do not represent forested areas where there is often an abundanceof HO2. Once a RO2 has been generated from the oxidation of NO3, the reaction with HO2 couldgenerate a yield of organic nitrates as high as 78%16. According to Fry et al, the assumed fate for theRO2 at night-time is reaction with HO2 and the creation of a NO3-ROOH (hydroperoxide) (Seereaction 4.7 and figure 4.6). From this, the reaction rate is the highest with HO2, followed by RO2
and then NO3.
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4.2.4 Reaction with RO2
The main competition for the reaction with NO3 and HO2 are the self-reactions, i.e, reactions withother RO2 radicals (See figure 4.4). This reaction would particularly be relevant for SOA formationnight-time as the dimer formation could be favoured27. The competition is often between RO2 aswell as NO3 and HO2. In the experiment by Rollins et al (2009), high concentrations of isoprenereacted with NO3. The first oxidation products, RO2, predominantly reacted with other RO2
radicals.1
4.2.5 Reaction with NO3
One mechanism for this has been suggested by Rollins et al1. According to this paper, the onlyoxidation product that is reactive with NO3 is the carbonyl nitrate (See figure 4.5).1 The yield oforganic nitrates was significantly higher when NO3 reacted with second generation products, 14%vs 0.7 %.1
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Figure 4.5a. A peroxy radical is reacting with NO3 and generates a carbonyl nitrate. From Rollins et al (2009)1
Figure 4.5b. The carbonyl nitrate reacts with NO3. From Rollins et al (2009)1
Figure 4.3. One mechanism for the peroxy radical when reacting with HO2. From Rollins et al (2009)1
Figure 4.4. A suggested mechanism for the reaction of a peroxy radical with another one. From Rollins et al (2009)1
4.2.6 Production of Organic nitrates
Initially, less than 5% of the RO2 reacted with NO3. Once most of the isoprene had been consumed,the oxidised products reacted to a greater extent with NO3; almost 50%. Generally, the yield ofnitrates was high in the second oxidation, see figure 4.6. Reaction 13 is a C10-molecule which can,due to its molecular weight as well as two nitroxy-groups, quite easily partition in the particlephase. In the study by Fry et al, there was some OH production which could compete with NO 3 forthe isoprene but NO3 is largely dominating the oxidation27,36.
In figure 4.7, the RO2 concentration, loss rates and lifetimes are shown. These are the resultsobtained in the experiment by Fry et al27. The loss rate of RO2 is dominated by HO2, followed byRO2. The lifetime for the RO2 is shown to be longer early in the night and this gives the possibilityfor many different reactions during the night.
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Figure 4.6. The first graph shows the alkyl nitrate production after the first oxidation. The second graph represents the second oxidation, i.e. the other double bond of the isoprene molecule. From Rollins et al (2009)1
Figure 4.7. Simulated peroxy radical concentration (left), loss rates (middle), and lifetime (right) From Fry et al. (2018) 27
In figure 4.8, the reaction rate by RO2 is shown. These are similar to the results by Fry et al,showing that the reaction rate is the highest with HO2.
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In one study by Horowitz et al (2007), it was presented that 50% of the isoprene nitrates werederived from the reaction with NO3 despite that this pathway only represents 6% of all the possiblepathways37. The initial products of NO3 and isoprene, including the organic nitrates, could partiallypartition to the aerosol phase. According to Xu et al38, aerosol consist of 5-12% organic nitrates.First generation nitrates have different fates at night, depending on the NO3 concentrations.According to Rollins et al, if there are NO3 concentrations above 10 ppt, the oxidation products arelikely to react with NO3 and condense into particle-phase. If there are less NO3 available, othersinks for the products are daytime deposition or OH oxidation.1
4.3 Formation of Secondary Organic Aerosols
Monoterpenes generally have larger yields of aerosol particles when they go through oxidation butwith NO3, the reaction with isoprene is much faster than with monoterpenes, 5 × 10 -12 cm3
molecules-1 s-1 for monoterpenes and 6.5 × 10-13 cm3 molecules-1 s-1 for isoprenes and consequentlythe conversion of hydrocarbon to organic nitrates will be dominated by isoprene due to itsabundance.27. The lifetime of the oxidised products of isoprene is higher with NO3 than with O3 andOH29. Rollins et al (2009) estimated a nitrate yield of 70±8% from the reaction between isopreneand NO3. The nitrates could further react with NO3 and produce dinitrates with a yield of 40±20%.The SOA mass yield from the initial reaction is 2% but it could reach 14% yields after the reaction
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Figure 4.8. Shown at the top are the modelled calculations of total peroxy radicals concentrations, and at the bottom are the sum of the rates of all RO2 reactions. From Rollins et al (2009)1
between NO3 and the oxidised isoprene products. According to Fry et al.,27 NO3 accounted for 17%of the oxidation of isoprene at the nocturnal boundary layer at the measurement site. In isoprenerich regions, 40-50% of the nitrate production comes from the reaction between isoprene and NO3
1,39
The ratio between isoprene and radicals can be important to determine the yield of SOA produced 8.In the experiment by Ng et al.,8 two different circumstances were examined. N2O5 were used togenerate NO3 radicals, similar to the one used in this thesis (see method section). Then there wereeither a pre-abundance of isoprene with a slow injection of NO3 radicals (N2O5) or the opposite, i.e.a pre-abundance of NO3 radicals (N2O5) and a slow injection of isoprene. The slow injection ofN2O5 created a higher yield of SOA compared to the other method, almost twice as high (see figure4.9). This seems to be because of the scarce availability of oxidants which causes the RO2 radicalsto react with each other. They could therefore react according to reaction 13 which increases bothfunctionalization and molecular weight. In the other case, the generated RO2 radicals reacted to agreater extent with NO3 but a smaller fraction of the final products participate in the particle phase.
In the experiment, the initial attack of the NO3 was on the C-1 carbon (See figure 4.1) of theisoprene, the most common one as explained earlier. If the initial attack had been on one of theother three carbons, the product would have been different and the yields with RO2 and NO3 mightalso be quite different. Due to the high molecular weight in pathway 13, generating a C10-peroxide,low-volatile products partitioning in the particle-phase can occur. This pathway is possible but notthe most common with small peroxy radicals1,8. This could be more likely after several reactionsbetween the peroxy radicals.
According to Ng et al (2008), the most common first generation product from the oxidation isnitrooxycarbonyl. This is not so effective in its contribution to SOA formation. Considering thatRollins et al. found that NO3 is mostly reactive with carbonyl nitrate, it might explain the smallyields of SOA from NO3 oxidation of RO2. Other, less abundant, products are hydroxynitrates,nitrooxyhydroperoxides, methyl vinyl ketones and methacrolein7. Ng et al.8 found thathydrooxynitrates were the most effective precursor of SOA formation.
Rollins et al (2009)1 presented that as O3 is available during night-time, it could be another possibleoxidant for isoprene and subsequently produce aerosols. However, the correlation between NO3 andSOA formation is stronger than with O3. When the NO3 concentration drastically decreased, so didthe formation of SOA despite the availability of O3. SOA is also more likely to be produced fromthe reaction between oxidised isoprene products and NO3. The conclusion, by looking at figure4.10, was that around 3% of the organic nitrates partitioned to the particle phase.
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Polymerization could also occur, creating SOA with 5-6 isoprene units. The required addition ofNO3 would result in many extra oxygen molecules as well as increased molecular weight. These aretwo important factors for partitioning into particle-phase1,21. An internal isomerisation of the δ-alkoxy radical after the addition of NO3 at the first position via a 6 membered ring, see figure 4.11,could increase the nitrate:organic ratio from 0.93 to 1.2. Addition of nitrates will increase the O:Cratio and this is correlated with SOA formation21. The rate of isomerisation is suggested to be 7times faster than reaction with O2 which could therefore account for a significant amount of SOAformation.1
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Figure 4.9. The mass yield of aerosols created from the two different experiments. From Ng et al. (2008)8
Figure 4.10. The increase of alkyl nitrates and nitrate aerosol. From Rollins et al (2009)1
Longer lifetimes could also lead to more intramolecular reactions. These could generate productswith lower vapour pressure and consequently SOAs. The time needed for this is around 500 seconds40 and this can be the case shown in the study from Fry et al27. This route is highly dependent on thestructure of the RO2, meaning that not just any radical could go through this despite a long lifetime.Internal isomerization of this kind could account for 8-11% of the total reactions of RO 2 radicalsfrom isoprene40.
According to Fry et al (2018)27, the concentrations of SOA from NO3 and isoprene are small infreshly emitted plumes compared to older ones. This is due to the ageing of the aerosols and aresimilar to the results from Rollins et al and Ng et al. When isoprene is emitted it is very volatile andstays in the gas phase but as it goes through further oxidation it get more polar, increasedhygroscopicity, higher molecular weight and a larger ratio between O:C which are all factors thataffects the yields of aerosol21. One oxidation is not enough for the isoprene to go to the particlephase. One example of an initial oxidation of isoprene is the generation of a hydroperoxynitratewhich has a saturation pressure of 14.4 Pascal. When this is oxidised once more, a C5 dihydroxy-dinitrate is created, and the vapour pressure for this compound is 1.1x10-5 Pascal 27. This conclusionhas been made in the other two studies1,8. In the study by Fry et al, the ratio of organic aerosol andorganic nitrate aerosol is around 5 which indicates that the aerosol created might be dominated byRO2 reacting with each other rather than with NO3.27
The yield of SOA from the reaction between isoprene and NO3 are different, depending on thestudy. All the following yields can be compared with photochemical oxidation of isoprene whichgenerates a yield between 1-3%.6 Ng et al observed yields between 4.3-23.8%8 and Rollins et alobtained a yield between 12-14 %1. The study done by Fry et al.,27 showed a 27% higher yield thanprevious studies. This is explained by the more complicated chemistry in the ambient atmospherewhich allows more pathways and ageing than laboratory settings do.
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Figure 4.11. A simplified figure showing the isomerisation as well as decomposition after oxidation of isoprene. From Rollins et al. (2009)1
Hydroperoxides, generated from the reaction between RO2 and HO2 can remain in the atmosphereuntil daytime and then react with OH to create epoxides which could efficiently create SOA throughheterogeneous chemistry with acidic aerosol.27 The effect of the aerosol yield have in several studiesshown to be increased in acidic environments41,42. Highly oxygenated molecules (HOMs) have verylow volatility and will effectively partition into particle-phase. Studies including isoprene and NO3
are not available but oxidation by OH and O3 do not show any significant yields of HOMs.43
One advantage with the experiment by Fry et al27, which could also explain the higher yields, arethe minimal wall losses as well as the amount of ageing that occurred that might not always bepossible in chamber experiments. Additionally, the life-time of a RO2 is not properly represented inchamber experiments as there is a more complicated chemistry of reactions and ageing in theambient atmosphere. Although the advantage with chamber experiments is the ability to isolatecertain reactions that are of interest. Peroxy radicals have a longer lifetime during night-time thandaytime. One chamber experiment by Schwantes et al (2015),16 presented a lifetime of 30 secondswhile the lifetime of the radical in the ambient atmosphere was more than 200 seconds 27.
When measuring organic nitrates, the inorganic nitrate aerosol could interfere with the results.These can be created from the dissolution of HNO3 and ammonium nitrates since these have nitrategroups and are in the particle phase they have similar properties to the organic nitrates. One way todistinguish between these two is to examine the NO2
+/NO+ ratio. A lower ratio is an indication thatthe nitrate molecule is of organic origin44. In Fry et al27, the correlation between the change in NO3
and organic and inorganic nitrates was shown. Even if some inorganic nitrates that originated fromammonium were created, there seemed to be a stronger casual relationship between NO3 and theorganic nitrates.27
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5. Atmospheric implicationsAerosols seems to have an effect on both environment and health but these effects are not yet fullyunderstood. Several studies show that aerosols have a strong impact on climate and human health.The effects depend on several factors including chemical composition, properties, size, thesurrounding environment and their sources45. The climate effects can be expressed in numerousways with negative and positive feedbacks in different areas.
Generally, aerosols originating from anthropogenic emissions have a cooling effect on theatmosphere. Opposite to a green house gas, which absorbs solar radiation and emit it as heat, theycan scatter solar radiation and thus have a cooling effect on its surroundings. They can acts as cloudcondensation nuclei (CCN), meaning that ambient water vapour can condense onto the aerosols andeventually form clouds. These clouds generally have smaller liquid droplets which in turn canreflect more incoming solar radiation.45 Additionally, they have longer lifetimes and are wider thanclean clouds whilst producing less precipitation.46 This could have negative effects, in particular onareas that are already suffering from water scarcity.
There is a consensus that the presence of aerosols have slowed down the warming of theatmosphere, in particular the ones from anthropogenic origin. As more environmental regulationstake place to reduce the emissions that act as precursors to SOAs, this could show the actual effectof global warming.
Particles can have several negative health effects, in particular respiratory problems. Smaller, ultrafine (<1 um) particles could also be inhaled and subsequently enter the blood stream causingpremature death as well as negative cardiovascular effects. At this point, not enough studies havebeen done to make any conclusions of particles containing nitrates. Some studies have includednitrates, organic and inorganic, but any significant effects apart from the actual particles have notbeen found47.
The amount of SOA has drastically increased since pre-industrial times. One calculation states anincrease from 35 Tg/year to 53 Tg/year. The increase in NOx emissions, i.e anthropogenic activitiescould be partly accounted for this48. Mixed results on how to tackle this have been presented. Xie etal (2013)39found that by decreasing the Nox by 50%, a small difference in SOA reduction wasobtained. However, Pye et al (2015)49 presented that a reduction of 25% is enough to reduce theaerosols with 9%. As it would be difficult to reduce isoprene, as well as other VOCs, the NOx wouldbe the option to go for when aiming to reduce the amount of aerosol but the efficiency of this isclearly debated.
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6. Conclusion and suggestionsBecause isoprene is emitted in the daytime, the focus is often on day-time oxidants and thefollowing oxidation products. In this thesis, the night-time chemistry is shown to be of importanceas well. Since isoprene can be accumulated in the stratified boundary layer at night, the oxidationcan occur by NO3 radicals which are only available in the absence of sunlight. The reaction hasproduced yields up to 14% in chamber experiments and even higher in the ambient atmosphere. Asthis has an effect on both climate and human health, it is important to understand the mechanismsand pathways to be able to reduce the production.
Different factors have shown to be relevant concerning aerosol formation. Firstly, the ageingprocess can have a significant influence on different properties such as the O:C ratio and polaritywhich consequently will affect the vapour pressure. When NO3 oxidise isoprene, RO2 radicals areproduced. The reaction between these and other RO2 radicals or HO2 seems to be more effective forSOA formation than further oxidation by NO3. Also, whether the experiment has been done ex-situor in-situ can make a difference for the obtained results. In the ambient atmosphere, the oxidationproducts can react in many different pathways and it would be likely to assume that not all of themhave been suggested at this time. However, important factors such as retention time, environmentalfactors and the chemistry the following day can affect the SOA formation. Additionally, the ratiobetween isoprene and NO3 seems to be of relevance. A lower ratio between oxidant and isoprenehas shown to create more SOA rather than the opposite. This could mean that the transport ofoxidation products, to areas with less oxidants, can affect the SOA yield.
Currently, there is a large amount of isoprene emitted and the yields of SOA at night are not to beunderestimated. Emissions of CO2, which is causing a warmer climate, could result in increasedemissions of isoprene which in turn could produce more aerosols which have a cooling effect.. Thisshows how complex the chemistry is and how there is not one solution to solve several problems.
Further study of isoprene oxidation at night-time is important to get a fuller understanding of itschemistry. Additionally, further study of the surrounding effects enhancing the SOA formation, suchas acidic environment and other radicals, would be very useful. If more knowledge about thereactions is obtained, it will be easier to make suitable environmental regulations and policiesregarding aerosols and emissions of its precursors.
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7. AcknowledgementsI would like to thank my supervisors Mattias Hallquist and Epameinondas Tsiligiannis for makingthis possible and for their thorough supervision throughout this process.
I want to thank Emelie Lindfeldt and Johanna Jildén for being great company and support duringthe whole thesis period.
Finally, I am sending a big hug to my husband Charles and my children for all their help andsupport.
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8. References
1. Rollins, A. W. et al. Isoprene oxidation by nitrate radical: alkyl nitrate and secondary organic
