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Determining the Optical Properties of Secondary Organic Aerosols using UV-Vis Spectroscopy

A senior thesis submitted to The Department of Math-Science

College of Theology, Arts, & Sciences

In partial fulfillment of the requirements for a Bachelor of Arts degree in Chemistry

by

Vanessa Selimovic

Faculty Supervisor_____________________________________ ____________ Dr. Matthew Wise Date

Department Chair______________________________________ ____________ Dr. Sergei Polozov Date Dean, College of Theology, Arts, & Sciences_________________________________ ____________ Rev. Dr. David Kluth Date Provost_________________________________________________ ____________ Dr. Mark Wahlers Date

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Concordia University Portland, Oregon

April 2014

I, _______________________________________, do hereby irrevocably consent to and authorize the Concordia University Library to catalog and file the thesis entitled ________________________________________________________________________ ________________________________________________________________________ ____________ and to make the thesis available in both paper and electronic formats for use, circulation, and limited reproduction by users at the Concordia University Library. I do not, however, relinquish my copyrights. ____________________________________________________ (Signature) ________________ (Date)

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Acknowledgements

First and foremost, I would like to thank my thesis advisor Dr. Matthew Wise, for

not only giving me knowledge, guidance, and insight, but also for teaching me that

science doesn’t always have to be so cut and dry, and that it can actually be enjoyable

(both when it works and doesn’t work). Thank you for your kindness, patience, and for

being there when I had questions that needed an answer, figures that needed an

explanation, and food poisoning that needed Gatorade. Most importantly, thank you for

your friendship, and for always being there when I needed someone to talk to.

Second, I’d like to thank my close friends Jenny Smith and Ruthie Nelson for

keeping me in check, keeping me sane, and for sticking it out with me through both thick

and thin. Thank you for the wild nights, endless laughs, countless conversations and for

what it truly means to be a friend. If the science library could talk, we would be in big

trouble. Taco Bell trips without you two are not Taco Bell trips at all.

Additionally, I would like to extend many thanks the members of my thesis

committee for always providing excellent insight and for their aid in my journey. I’d also

like to thank Pacific Northwest National Laboratory, the College of Theology, Arts, and

Sciences, and Concordia University for allowing me the opportunity to grow and develop

into someone that I am proud to be.

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“It is good to have an end to journey toward;

But it is the journey that matters, in the end.”

- Ernest Hemingway

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Table of Contents

I. Abstract…………………………………………………..7.

II. Background…………………………………………8.

a. Layers of the Atmosphere………………………9.

b. Reactivity of Aerosols with the Atmosphere…..10.

c. Types of Aerosols……………………………...13.

d. Light Interactions……………………………....15.

e. MAC and k……………………………………..20.

f. Light-Absorbing Aerosol……………………….21.

g. Brown Carbon…………………………………..22.

III. Hypothesis………………………………………….24.

IV. Materials and Methods……………………………..27.

a. Optimization of Procedure………………………27.

b. Generation of SOA……………………………. 28.

c. Measurement of SOA……………………………30.

V. Results………………………………………………31.

a. Optimization/Fulvic Acid………………………31.

b. SOA Spectra……………………………………36.

c. Conclusions/Future Work……………………...41.

VI. References………………………………………….43.

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Abstract

Atmospheric light-absorbing carbon (LAC) particles, which include black carbon

(BC) and brown carbon (BrC) particles play an important role in global climate change.

Currently, optical properties of BC are well defined, with studies showing absorption of

light by BC at wavelengths above ~500 nm. At lower wavelengths of the visible light

spectrum (300-500 nm), light absorption by BrC may be substantial. However, the extent

of absorption is dependent on accurate knowledge of the optical properties of BrC, which

are currently not well established. Optical properties of secondary organic aerosols

(SOA), a variant of BrC, have been determined, but only for a limited subset of source

types and atmospheric conditions. Furthermore, the studies have not quantified optical

properties such as mass-specific cross sections (MAC) and complex refractive indices (k).

As a result, the extent of their influence in the atmosphere remains poorly understood. In

this study, Fulvic Acid was used as a model to validate the experimental procedure for

the analysis of several different samples of varying concentrations and compositions of

SOA generated by the oxidation of volatile organic compounds (VOC), specifically

1,2,4-trimethylbenzene. SOA were dissolved in water and analyzed using UV/Vis

spectroscopy. Absorption values determined using the UV/Vis spectra were then

incorporated into the calculation for MAC and k.

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Background

Layers of the Atmosphere

Atmospheric aerosol particles are solids or liquids dispersed in the layers of the

atmosphere. Aerosol particles are primarily located in the first layer of the atmosphere,

known as the troposphere (Chang, 2005). This layer of the atmosphere extends from the

ground to approximately 10-11 km in altitude, and consists of approximately 80% of the

total mass of the air and practically all of the water vapor in the Earth’s atmosphere. It is

the thinnest layer of the atmosphere and all weather happens in this location.

Furthermore, temperature in this layer decreases in a near linear fashion as you increase

in altitude (Chang). Other layers include the stratosphere, mesosphere, and thermosphere

(Fig. 1).

Figure 1. Earth’s layers of the atmosphere, show as a function of altitude and temperature. (Encyclopedia Britannica, 2012).

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The stratosphere consists of mostly of nitrogen, oxygen, and ozone. (Finlayson-

Pitts, 2000). Unlike the troposphere, the temperature in the stratosphere rises with

increasing altitude. This is a series of reactions that are triggered by UV radiation. One of

the products of these reactions is ozone (O3), which plays an important part in preventing

harmful UV rays from reaching the surface of the Earth (Chang).

Above the troposphere and the stratosphere, are the mesosphere and the

thermosphere. In the mesosphere, concentration of ozone and gases are low, and

temperature decreases with increasing altitude. In the thermosphere, temperature rises as

a result of an abundance of nitrogen and oxygen reacting with electrons and protons from

the sun (Chang).

Our planet is unique in comparison to other planets in our solar system due to the

fact that it has an atmosphere that is very chemically active and rich in oxygen

(Finlayson-Pitts). For example, Mars has an atmosphere that is very thin and about 90%

carbon dioxide, while Saturn is approximately 75% hydrogen and 25% helium, with

traces of other substances (Chang). Table 1 shows the composition of Earth’s atmosphere

at sea level.

Gas Composition (%) N2 78.03 O2 20.99 Ar 0.94

CO2 0.033 Ne 0.0015 He 0.000524 Kr 0.00014 Xe 0.000006

Table 1. Composition of dry air at sea level.

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Reactivity of Aerosols with the Atmosphere

The effects of aerosol particles and their chemical interactions with the

atmosphere have recently been a large topic of discussion related to climate and health

concerns. Aerosol particles are emitted from a variety of different sources and range in

size from less than a nanometer to tens of micrometers (Ziemann and Atkinson, 2012).

Ziemann and Atkinson also found that aerosols come from a variety of different sources

and can vary in chemical composition depending on their origins. Large, coarse particles

(greater than 2.5 micrometers in diameter) consist primarily of components such as soil

dust, sea salt, and plant debris. Smaller particles (less than or equal to 2.5 micrometers in

diameter) originate mostly from incomplete combustion reactions or are formed in the

atmosphere when aerosols interact with one another. Size and composition of particles

helps to determine other important characteristics, such as phase (e.g. solid or liquid),

optical properties, and chemical reactivity.

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Aerosol particles can have a direct effect on the environment by scattering and

absorbing sunlight (Fig 2, AR5). Additionally, aerosols can indirectly affect the climate

by serving as cloud condensation nuclei and ice nuclei. In both instances, aerosols have

the ability to modify the Earth’s radiation balance (Kirchstetter, Novakov, and Hobbs,

2004). This radiation balance takes into account all incoming and outgoing radiation.

These types of radiation include shortwave and longwave radiation (Chen & Bond,

2010). Shortwave radiation enters the atmosphere from the sun and is high in energy

(ultraviolet and visible), whereas longwave radiation (infrared) is primarily emitted from

the Earth’s surface and is lower in energy. (“Longwave and Shortwave Radiation,”

2009). Alteration of the natural balance of incoming and outgoing radiation is known as

anthropogenic climate forcing and is a factor in global climate change. (National Oceanic

and Atmospheric Administration, 2007).

 

Figure 2. Interactions between aerosols and the atmosphere. Aerosols that scatter incoming radiation (a and b) lead to an overall cooling of the surrounding environment, while aerosols that absorb radiation (c and d) lead to an overall warming. These effects are primarily due to the physical and chemical properties of the

aerosol. Figure Source: Second Order Draft of the IPCC 5th Assessment Report (AR5).

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The impact of aerosol particles on climate and atmospheric chemistry depends, in

part, on their phase (e.g. solid or aqueous solution) and size, which in turn depends on

atmospheric relative humidity (RH) and particle composition (Ziemann and Atkinson).

Large variations in the physical properties of different aerosol particles allow them to

scatter or absorb radiation to varying degrees. How long the aerosols remain in the

atmosphere also determines a large part of how they contribute to global climate. Because

so many factors go into determining exactly how a specific aerosol particle will react

with the atmosphere, their effect on the global radiation budget still remains poorly

understood (Haywood and Schulz, 2007) (Fig 3). What we aim to do in this study is

alleviate some of that uncertainty by pinpointing a single factor in a specific aerosol (the

interaction of an aerosol with incoming radiation), to obtain a more comprehensive view

of not only properties of that aerosol but how these properties might affect its role in the

surrounding environment.

 

Figure 3. Agents of radiative forcing and their contribution of forcing on the climate. High uncertainty still lies in understanding total contribution of aerosols. Figure Source: Second Order Draft of the IPCC 5th

Assessment Report (AR5).

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Types of Aerosols

Three major types of aerosols significantly affect the Earth’s climate. The first are

known as volcanic aerosols. The volcanic aerosol layer forms in the stratosphere after

major volcanic eruptions. The aerosol layer is formed by sulfur dioxide gas, which is

converted to droplets of sulfuric acid over the course of a week to a few months after

eruption. Global air circulation contributes to the spread of these aerosols across the

globe (Fig. 4). Once these aerosols are formed, they stay in the atmosphere, where they

reflect sunlight, consequently reducing the amount of energy that reaches the lower

atmosphere and the earth’s surface. This phenomenon results in a cooler climate (Dunbar,

2004).

 

Figure 4. Influence of volcanic aerosols on climate. The dispersal of volcanic aerosols has a drastic effect on the Earth's atmosphere. After an eruption, large amounts of sulfur dioxide (SO2), hydrochloric acid

(HCL) and ash are spewed into the Earth's atmosphere and react with the surrounding environment. Specifically, SO2 transforms into H2SO4, condenses, and lingers in the atmosphere. The interaction of

these new sulfate aerosols with surrounding nitrogen and chloric compounds (heterogeneous chemistry), is a prime contributor to ozone depletion. Figure Source: NASA.gov  

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Dunbar also states that the second type of aerosol that has a significant impact on

the Earth’s radiation budget is particles created by the mechanical action of wind over

deserts. Because the particles are composed of a variety of minerals, they can absorb

sunlight as well as scatter and reflect it (“Aerosols and Incoming Sunlight”). If the

particles absorb sunlight, they warm the layer of the atmosphere where they reside. Those

that scatter sunlight have the reverse effect, and subsequently cool the atmosphere

(“Aerosols and Incoming Sunlight”). However, they are relatively large for aerosol

particles and normally fall out of the atmosphere (Dunbar).

Finally, Dunbar states that the third and an increasingly prevalent type of aerosol

is one that originates from human activities. While a large portion of these aerosols arise

in the form of smoke from burning tropical forests, even larger portions arise in the form

of sulfate aerosols, created by the burning of coal and oil. The concentration of these

aerosols has grown rapidly since the dawn of the industrial revolution. These sulfate

aerosols absorb no sunlight, but they do reflect it, subsequently reducing the amount of

sunlight capable of reaching the Earth’s surface. Furthermore, these aerosols can act as

cloud condensation nuclei. If they are good cloud condensation nuclei, and are present in

large numbers, they will increase the number of cloud droplets, thus making the droplets

smaller. This net effect contributes to the clouds reflecting more sunlight than they

would without the presence of sulfate aerosols, due to the increased surface area of the

newly formed clouds.

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Additionally, a substantial fraction of the anthropogenic aerosol mass is in the

form of carbonaceous particles (aerosol particles that contain carbon) originating from

fossil fuel and biomass burning (Chen and Bond). Prevalent in urban areas, carbonaceous

aerosols are composed of black carbon (BC), which comes from anthropogenic sources,

and varying compositions of organic carbon (OC), that comes from biological sources.

(Bergstrom and Bond, 2006) coined the term “light absorbing carbon (LAC)” particles

for those containing BC. Absorption of light, a component of light extinction (loss of

light), is an important component in Earth’s atmosphere because it contributes to the

change in Earth’s radiation budget (Chen and Bond).

Light Interactions

The atom is the source of all forms of electromagnetic radiation, both visible and

invisible. High energy forms of radiation, such as a gamma rays and X-rays are produced

when the nuclear stability of the atom is disturbed (Molecular Expressions Microscopy

Primer: Light and Color – Electromagnetic Radiation). Ultraviolet (UV) radiation is the

section of electromagnetic radiation that ranges from 100 to below 400 nm, visible light

is the section of electromagnetic radiation that ranges from 400 nm to 700 nm and is

visible to the human eye, and infrared (IR) radiation is the section that ranges from

approximately 700 nm to 1 mm (Elert, 2010) (Fig. 5).

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Figure 5. The electromagnetic spectrum of radiation. (Electromagnetic spectrum, 2012).

These types of energy corresponds to the phenomenon that electrons, which

surround the nucleus of an atom, are arranged in different energy levels (“Electron

Energy Levels”, 2006) When the electrons interact with energy from a different source,

the electrons are promoted from their ground state energy level to a higher energy level

(Reusch, 2013) (Fig 6).

 

Figure 6. Electron excited state. An electron interacting with an outside source of radiation, in this case, a photon. The electron interacts with the photon, and is moved to an excited state, as the energy of the photon

matches the vibrational energy of the electron.

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Chang (2005) states, a number of things can happen when electromagnetic

radiation strikes an object. It can be absorbed, reflected, or transmitted. When radiation is

absorbed, the electrons become excited and are moved from their ground state energy

level (from an electron orbital which often contributes to the bonding), or HOMO

(highest occupied molecular orbital), to a higher energy anti-bonding orbital, or LUMO

(lowest unoccupied molecular orbital), which does not directly contribute to bonding (Fig

7). Chang goes on to say, each jump between levels takes some amount of energy, which

is obtained from the radiation absorbed. If the particular amount of energy is just the right

amount to make the jump from one energy state to the next, the radiation is absorbed.

Figure 7. HOMO and LUMO. Depiction of the jump between different energy levels. The labels σ, π, n, π*, and σ* all refer to different molecular orbitals. These are areas where electrons can be found. Electrons found in the bonding orbitals contribute to bonding, while electrons that are found in the anti-bonding orbitals are said to be in an excited state.

A larger jump in energy requires a higher frequency of radiation. Higher

frequencies are correlated with shorter wavelengths. Conversely, a smaller jump in

energy requires a lower frequency of light, and is thus correlated with longer wavelengths

(Reusch). This is shown in Equation 1, where c is the speed of light in a vacuum, ν is the

frequency, and λ is the wavelength.

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𝜆 =   !! (1)

Additionally, absorption of infrared radiation can produce heat. This is due to

molecular vibrations (“Interaction of Radiation with Matter”). We can think of bonds

between atoms as springs, which have the ability to vibrate back and forth. All of these

“springs” have a natural frequency at which they tend to vibrate. When a wave of

radiation with the same natural frequency as the molecular vibration of an atom interacts

with the atom, the electrons of the atom are set into a vibrationally excited state (Jensen,

2000). Moreover, during these vibrations, the electrons can interact with other atoms in

their environment and convert the vibrational energy into thermal energy (“Interaction of

Radiation with Matter”).

On the other hand, reflection and transmission occur when the frequencies of the

incident radiation do not match the absorption profile of vibration of the objects. Instead

of promoting electrons from one energy level to another, the radiation, because it does

not interact well with the different energy levels, is reflected off the object and reemitted

as a wave of light (“Light Absorption, Reflection, and Transmission.”) During

transmittance, the light waves are passed through the object, and instead of interacting

with the different vibrational levels, they are reemitted on the other side of the object.

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1. Conjugated Systems

The more easily excited the electrons of a particular molecule can be, the longer the

wavelength of light it can absorb. The structure of a molecule contributes to how a

molecule will interact with light (Jensen, 2000). Atoms can be arranged in a variety of

different ways with one another, including being bonded to one another in a system of

single bonds (referred to as a sigma bonds), double bonds, (referred to as a pi bonds), or

triple bonds (which include one sigma bond and two pi bonds). In a conjugated system,

compounds with alternating single and multiple bonds contain a set of connected p-

orbitals, with delocalized electrons. Increased conjugation brings the HOMO and LUMO

orbitals closer together (Chang, 2005) (Fig 8) .

 

Figure 8. (A) Depicts a conjugated system and a non-conjugated system involving pi bonds. (B) Shows the delocalization of the p-orbitals among different atoms. That is, the electrons that are delocalized are free to

move around the atom and are not confined to one specific orbital. (Conjugated system, 2014).

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Delocalized electrons are not associated with just one single atom or bond, and can

contribute to the overall stability of a molecule (Pauling, 1960) Conjugated systems are

significant when discussing the absorption of light because they absorb strongly in the

UV and visible spectrum of electromagnetic radiation (Chang, 2005). This is due to the

fact that in conjugated systems, the excited and ground states are much closer in energy

than in non-conjugated systems. The less conjugated the system is, the more energy is

required to promote the electrons from one energy state to another, and thus the more

unwilling it is to absorb UV-light. Absorption needs less energy as the amount of

delocalization increases, as there is less of an energy gap between the bonding and anti-

bonding orbitals. Here, the jump from the HOMO to the LUMO is a smaller jump, and

thus requires a lower energy of light, and subsequently a lower frequency. (“Ultraviolet

and Visible Light”).

Mass Specific-Absorption Cross Section and Complex Index of Refraction

In addition to structure and vibrational frequencies, there are several parameters

that govern the absorption of light by particles in the atmosphere. One parameter is the

Mass-Specific Absorption Cross-Section, or MAC. Babin and Stramski, (2004) state the

MAC is a measure for the probability of an absorption process. Furthermore, the MAC is

used to quantify the probability of a certain particle to particle interaction, specifically the

interaction of a molecule with a photon, and the probability that the molecule will absorb

light. The MAC value (in units of m2/g) is calculated using Equation 2:

𝑀𝐴𝐶 =   !!   ! !

(2)

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Where A is the measured UV/Vis absorbance, L is the path length (m) of the

spectrometer and [X]e is the mass concentration (g/m3) of aerosol in solution. An

additional parameter is the complex refractive index of the solution k. When k > 0, the

substance is said to absorb light, when k = 0, light travels forever without loss, and when

k < 0, the light is said to be amplified, due to the angle of incidence of radiation (Born

and Wolf, 2005). While a refractive index can be used to describe the way radiation

propagates through a medium, a complex refractive index takes into account the portion

of this radiation that is absorbed as it passes through said medium (“The Complex Index

of Refraction”). It is calculated using the MAC value and Equation 3:

𝑘 = (!"#)(!")!!

(3)

where MAC is the MAC value obtained from equation 2, 𝜆  is the wavelength, and 𝜌 is

density.

Light Absorbing Aerosols

A few types of aerosols, specifically ones that contain carbon and mineral dust,

absorb radiation. Visible light absorption is important for direct radiative forcing, since

47% of solar energy is distributed within the visible range of 400 nm and 700 nm.

Specifically, it is known that BC significantly absorbs light at wavelengths greater than

500 nm (Bergstrom and Bond, 2006), and to date, is the strongest knownlight-absorbing

component of particulate matter.

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A second type of LAC that has recently been gathering attention is “brown”

carbon (BrC). BrC is a subset of OC and is commonly referred to as the light absorbing

part of organic aerosol particles (Feng, et al., 2013). It is thought that BrC has different

optical properties and different sources compared to BC. Recently, optical and thermal

analysis from laboratory and field experiments have provided strong evidence for the

existence of organic carbon with light absorbing properties. Thatcher and Kirchstetter,

(2012), calculated that 14 % of the solar radiation (300-2500 nm) absorbed by wood

smoke was attributed to OC and 86 % was attributed to BC. Furthermore, they calculated

that at wavelengths shorter than 400 nm, the radiation absorbed by the OC increased to

49%.

In many modeling studies, organic compounds are treated as if they scatter and

reflect radiation. These studies have not included the absorption of radiation due to BrC,

which challenges the notion that the combination of BC and OC in aerosol particles leads

to a cooling effect in the Earth’s atmosphere. Futhermore, although the absorption of

UV/Vis light does not necessarily always contribute to heating, it can have a strong

contribution in what role the particles play in different reactions, specifically ones that

involve UV light.

Brown Carbon

BrC can originate from several sources including biomass burning, fuel-

combustion, and can be formed in a variety of atmospheric chemical reactions. Secondary

organic aerosols (SOA), a subset of BrC, are particles formed in the atmosphere from the

oxidation of gas phase organic compounds emitted from either natural or anthropogenic

sources (Lambe. et. al., 2012) When the gas phase compounds are oxidized, their vapor

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pressures decrease, and thus transfer the compounds from the gas phase to the particle

phase (Kokkola et. al.,) (Fig 9). Therefore it is reasonable to assume that BrC can be a

component of SOA.

 

Figure 9. Process of formation of secondary organic aerosols. This study primarily focuses on the middle process, where VOC (volitile organic compounds), are emitted into the atmosphere, are oxidized to form Secondary Volitile Organic Compounds (SVOC), and then go through a series of additional reactions to

form Secondary Organic Compounds (SOA). Figure Source: Pohlker, et al. (2012).

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Lambe et al. (2013) studied the relationship between the level of oxidation and the

optical properties of different types of SOA. It was found that the SOA created in the

study could contribute significantly to the amount of BrC in the atmosphere. The various

types of SOA were created in a flow reactor by the hydroxyl (OH) oxidation of gas phase

precursors which modeled man-made biomass burning and biological volatile organic

compounds (VOC). The resulting SOA were analyzed to determine their MAC and k

values at 405 and 532 nm. The results of the study lend experimental credibility to the

notion that BrC is a component of SOA. Subsequently, because of their results, climate

models that do not include the influence of SOA in their calculations could be drastically

underestimating their warming potential.

Hypothesis

A thorough understanding of the properties of aerosols and accurate techniques

for their determination are essential. This is true not only for comprehending how organic

aerosols contribute to radiative and climate forcing, but understanding where they fit in

future global climate models.

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Wyche et al. (2009) published a study predicting the possible mechanisms and

proposed structures for the formation of SOA by the oxidation of a VOC known as 1,3,5-

trimethylbenzene (Fig 10). Proposed structures include products that contain some

conjugated systems. Although we used 1,2,4-trimethylbenezne (Fig 11). in this study, we

predict that our products will be similar enough to the products proposed by Wyche et al.

(Fig 12), in that they too will contain conjugated systems to some degree. This being said,

we expect to see the SOA produced in this experiment absorb light in the UV and

possibly blue range, thus significantly allowing for a better understanding of the optical

properties of SOA and their true contribution to global climate models.

                                                                                         

Figure 10. 1,3,5-Trimethylbenzene. Figure 11. 1,2,4-Trimethylbenzene.

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Figure 12. Products and mechanisms predicted for the oxidation of 1,3,5-Trimethybenzene. Figure Source: (Wyche, et. al) 2013.

 

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Materials & Methods:

Optimization of Experimental Procedure.

Solutions consisting of various concentrations of Fulvic Acid (FA) (Fig 13)   in

water were analyzed before analyzing SOA samples to ensure the instrumentation was

working properly. FA was used as a model as there have been several studies confirming

the optical properties (including MAC and k values) of FA. Also, we wanted to optimize

our procedure before we ran any of the SOA samples, as they were much more

“valuable” than the FA samples, as they took a long time to collect.    

 

Figure 13. Fulvic Acid.

First, a stock solution of FA in water was made by adding 0.0125 g of FA to 100

mL of water. This produced a 125 mg/L solution that could be diluted to various

concentrations. Next, a dilute FA solution was placed in a 10 mL syringe and injected

into a 100-cm path length liquid waveguide capillary flow cell (World Precision

Instruments model LWCC-3100), at 3 mL/hr using a syringe pump (New Era Pump

Systems Inc., model NE-300).

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The absorbance of the solutions, at wavelengths ranging from 200-700 nm, were

measured using a high resolution UV/Vis spectrometer (Ocean Optics, Inc. Jaz model

UX-83500-20) coupled with a miniature Deuterium-Tungsten Halogen light source

(Ocean Optics Inc. model DT-MINI-2-GS). With the data generated, MAC and k values

were calculated and compared with literature values (Lambe et al., 2013) to determine if

the experimental procedure was working correctly.

Generation of SOA Particles

SOA particles were produced in a temperature controlled 10.6 m3 Teflon reaction

chamber at Pacific Northwest National Laboratory (Richland, WA, USA) (Liu, et al.,

2012). The chamber consists of two 10’ x 5’ x 7’ Teflon reaction bags enclosed in a

single temperature controlled room (15 – 45 °C). Both chambers can be operated

independently and may operate either in the traditional batch mode or in a continuous-

flow mode (Shilling, et al., 2009). Constant light flux is provided by 110 UVA-340

fluorescent lamps which surround the chamber. VOC(s) are typically injected into a

gently heated glass bulb with a syringe. Vapors are transferred to the chamber via a flow

of air. A variety of oxidants may be used to initiate SOA formation from the vapors.

(Table 2). Ozone is generated by passing pure air through a mercury lamp. OH is

generated by photolysis of either H2O2 or HONO (depending on whether experiments are

run under high-NOx or low-NOx conditions). Additional NO is sometimes injected from a

pre-mixed gas cylinder to further control the NOx concentrations, which determines the

fate of generated organic peroxy radicals. A suite of analytical instrumentation analyzes

the particle and gas phases in real time including an Aerodyne HR-ToF-AMS (High

Resolution Time-of-Flight Aerosol Mass Spectrometer), an Ionicon PTR-MS (Proton

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Transfer Mass Spectrometer), and a TSI SMPS (Scanning Mobility Particle Sizer) to

name a few (Fig 14).

Date Oxidant [TMB] [NO] [NOx] [ozone] 05/21/2013 (A) H2O2 170.4 1.94 2.1 21.22

05/22/2013 (B) HONO+O3+RH 189.7 22 1673.7 5.85

06/07/2013 (C) HONO+RH(naifon) 173 642.2 854.7 3.08

06/13/2013 (D) HONO+RH+seed 158.9 575.5 799.2 3.08

Table 2. Oxidants used on differing days of SOA production. [TMB] is concentration of 1,2,4-Trimethylbenzene, [NO] is concentration of nitrogen oxide, [NOx] is the concentration of NO, or NO2. Concentrations are all given in units of

parts per billion (ppb).

 

Figure 14. Schematic of the chamber where SOA is produced. The chamber is connected to several instruments which monitor production and collection of SOA, as well as instruments that collect the SOA

for further analyzation. Note: Adapted from Niskordov, et. al. (2011). Adapted with permission.

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Measurement of the Optical Properties of SOA particles.

SOA samples (generated using the procedure outlines above) were collected on

47 mm Teflon filters. Collection times ranged from 5 to 9 hours. After collection, the

filters were cut in half with a razor blade, and the SOA were extracted from the filter via

ultrasonication in 15 mL of water. After sonicating one side of the filter for 5 minutes, the

filter was flipped and sonicated for another 5 minutes. The absorbance of the resulting

SOA solution was measured using the procedure outlined previously for the FA

solutions, and MAC and k values were calculated.

Determining Concentrations of SOA

Concentrations of SOA were determined using raw data obtained from the TSI

SMPS. Raw output of particles under specific reaction conditions were given in the

number of particles per volume of sample collected, and the volume of particles per

volume of sample collected (#/cm3 and nm3/cm3). Using the latter, we were able to

calculate their concentration in mg/L. First, we looked at how long the sample was

running for, which averaged out to be nine hours for each different day/reaction

condition. Sampling onto the filter however was only being calculated one third of the

time. Here, we incorporated the sample flow rate of 9 L/min. Because we knew that the

entire process was taking about nine hours (540 minutes), and that samples were being

collected a third of the time, we estimated that entire sampling occurred for 180 minutes.

We then took the average volume of particles for each run, and multiplied by the total

time sampled (180 minutes), to get a true concentration in nm3/cm3. We then converted

this value to give us units of mg/L. Furthermore, we assumed that the particle density was

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1.4g/cm3, and that the particles were spherical. It is also important to note that because

we only used half of the filter that the samples were collected on, we made sure to divide

our assumed final concentration by two, and then by 0.015 L, to account for the amount

of solution in which the particles were dissolved. The following equation shows exactly

how the calculation was done.

(𝐴𝑣𝑒𝑟𝑎𝑔𝑒  𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒  𝑣𝑜𝑙𝑢𝑚𝑒 !"!

!"!)(!  !"!

!  !")(!"""  !"

!  !)( !  !!!"#

)(180min)

𝑉𝑜𝑙𝑢𝑚𝑒  𝑛𝑚! 1  𝑚1 ∗ 10!𝑛𝑚

! 100  𝑐𝑚1  𝑚

! 1.4  𝑔1𝑐𝑚!

1000  𝑚𝑔1  𝑔

2 ∗1

0.015  𝐿

Results:

Optimization of Procedure/Fulvic Acid

A calibration curve of the absorbance at a specific wavelength was made using

varying concentrations of FA. This curve was then compared with a calibration curve of

absorbance of FA at a specific wavelength done by a similar study (Ghabbour and

Davies, 2009), to determine the accuracy of the procedure (Fig 15). In addition to

creating a calibration curve of FA, the spectra of differing concentrations of FA were

plotted on the same graph to get a better glimpse at the differences in magnitude of

absorption and to provide a visual comparison (Fig 16). Small differences were found

between the experimentally derived calibration curve and the calibration curve from

literature, but we speculate that this can be attributed to the fact that concentrations of FA

used by the study were 100 times greater in concentration than those used in this study.

Additionally, path length for the UV-Vis in the literature value was 1 cm, where as in this

study, it was 100 cm. While technical differences in set-up and procedure do exist, it is

32    

also much more probable that the differences between the two lie purely in human error

and failure to completely optimize our methods.

Figure 15. Comparison of calibration curve done in this study with that of a literature value.(Ghabbour and Davies, 2009). 1F, 2F, and 3F all refer to different standards of Fulvic Acid provided by the IHSS (International Humic Substance Society). 1F resembles our sample most closely, as it is Suwanee River Fulvic Acid. Both calibration curves measured and plotted absorbance of different concentrations of Fulvic Acid at 370 nm. The difference in slopes is the study done by Ghabbour and Davies is due to different variations of Fulvic Acid. Discrepancy between the slope of the curve in this study and the slope of the curve of the literature is due to the fact that this study used a concentration of Fulvic Acid that was 100 times more dilute.

 

Figure 16. Absorbance spectra of different concentrations of Fulvic Acid all plotted on the same graph. At 500 nm for clarity, and left at the original 780 nm (insert) for comparison.

33    

Several samples of the same concentration of FA were analyzed to ensure that our

experimental method was not only reliable but could also be repeated. The absorbance

spectra of both trials were measured and plotted on the same graph, along with a measure

of the percent difference between the two (Fig. 17). While the graph remains consistent

throughout, with little to no real differences between the two spectra, a large difference

between the two happens at around 750 nm. This can be ignored, as the instrument is

incredibly sensitive and will pick up a large amount of “noise” when the particles are no

longer absorbing in that range.

 

Figure 17. Absorbance spectra of the same concentration of Fulvic Acid repeated, to ensure repeatability of procedure. Percent difference between the two is shown in the insert. Large variations around 750 nm are

due to "background noise" picked up by the instrument, when particles are no longer absorbing in that range.

34    

Finally, to further optimize our procedure, we placed 1 mL of .498 mg/L FA onto

a Teflon filter to ensure that only thing coming off of the filter when it underwent

sonication was in fact our sample and nothing more. We left this filter to dry until all of

the FA had been soaked up by the filter. After it had dried, we analyzed and plotted it on

our calibration curve to ensure that both our methods and the instrument were working

properly. Figure (18) shows the new calibration curve, with the new dried concentration

of FA. The fact that the dried Fulvic Acid lined up directly on the curve suggested to us

that our methods were in fact accurate, and that our procedure was stable enough to begin

analyzing the SOA.

 

Figure 18. Original calibration curve, with the addition of the absorbance of .498 mg/L FA that was placed on a Teflon filter and left to dry (green).

35    

In addition to obtaining spectra for FA, we also obtained MAC and k values. Figure 19

shows values of MAC vs. wavelength for different concentrations of FA, while figure 20

shows values of k vs. wavelength for different concentrations of FA.

Figure 19. MAC values of different concentrations of FA as a function of wavelength

 

Figure 20. k values of different concentrations of FA as a function of wavelength.

36    

Figure 19 shows that as wavelength increases, MAC values subsequently go

down. This is because the amount and intensity of absorbance decreases as you increase

in wavelength. Similarly, Figure 20 shows that k as a function of wavelength follows the

same pattern. FA also seems to follow a uniform distribution in the plot of both their

MAC and k values. We expect this to be the case because they are all the same substance,

and vary only in their respective concentration.

SOA Spectra

Spectra of varying concentrations and composition of SOA were plotted on

separate graphs (Fig 21), and together on one (Fig 22). Peak absorbances of SOA all

occurred around 280 nm, with varying intensities of absorbance.

 

Figure 21. Different concentrations of SOA plotted on their respective graphs. (A) is SOA collected on 05/21/13, at a concentration of 6.06 mg/L. (B) is SOA collected on 05/22/13, at a concentration of .331 mg/L. (C) is a SOA collected on 06/07/13, at a concentration of .961 mg/L, and (D) is SOA collected on

06/13/13, at a concentration of .890 mg/L. Spectra are different despite higher and lower variations in concentration because of different reaction conditions leading to different SOA products, which have

different molar absorptivities.

37    

 

Figure 22. Absorbance spectra of all concentrations of SOA plotted on one graph for comparison.

We expect the absorbance spectra to follow Beer’s Law, where absorbance is

proportional to the product of path length, concentration, and molar absorptivity. Because

concentration does not play a significant part in how the SOA absorb radiation, we can

assume that differences in molar absorptivity must be why higher concentrations of one

SOA absorb lower than a lower concentration of a different SOA. Due to the different

conditions under which TMB was oxidized, we can assume that we had four different

SOA particles to work with, and possibly even a combination of several.

38    

Because we did not have any means of analyzing the structure of SOA samples

we obtained from PNNL, we are not 100% certain of their chemical compositions. Our

best guess is to assume that the SOA produced in this experiment might be a variant of

the products predicted in the study done by Wyche et al, with the main difference being

that in the original study, 1,3,5-trimethylbenzene was used, and the methyl groups on

1,3,5-trimethylbenzne directed functional groups to the 2,4 and 6 carbon positions, but

the methyl groups in 1,2,4-trimethylbenzene would direct them to solely the carbon 5

position. In addition to assuming the final SOA product contains conjugated systems, we

found that the TMB oxidized by HONO and exposed to high levels of NO and NOx,

absorbed the highest. It could be possible that the addition of nitro groups contributes to

not only absorption itself, but the intensity of absorption. However, because the oxidants

are not exactly the same for each day the samples were collected, it is difficult to tell

whether or not this would certainly be the case.

There also remains uncertainty in whether or not the SOA collected throughout

sampling remained the same when it was dissolved in water, or whether it interacted with

it, for example, undergoing different solvent reactions that might protonate or

deprotonate groups on the sample. Due to time constraints, we were only able to first

dissolve SOA in water. Our plan initially included dissolving the SOA in a nonpolar

solvent and looking to see if there was any difference between the two spectra, and

whether or not solvent had an impact on the structure of SOA.

Once absorbance spectra were obtained, MAC (Fig 23) and k values (Fig 24) for

different concentrations of SOA were also plotted. All of the determined MAC and k

values from the SOA in the experiment were compared to MAC and k values derived

from different SOA in separate literature, done by Lambe et al. (Table 3).

39    

 

Figure 23. MAC values for different concentrations and compositions of SOA as a function of wavelength.

 

Figure 24. k value of different concentrations and compositions of SOA as a function of wavelength.

 

The case is the same for SOA as it is for FA in terms of pattern. Figure 23 MAC

shows that as wavelength increases, MAC values subsequently go down. Similarly, k as a

40    

function of wavelength follows the same pattern. However, there is no uniform

distribution of MAC and k values across the spectra as there are in the spectra of FA. This

further suggests that this is due to the SOA not being of the same composition.

While SOA “B” is only second in terms of intensity of absorbance, it is highest in values

of MAC and k values, across the board. Additionally, SOA “D” is highest in absorbance,

but second highest in values of MAC and k. Using the equation for MAC and the data

obtained, we can show that MAC is not only strongly dependent on concentration but on

absorbance as well, with absorbance being largely influenced by the composition of the

molecule. Here, we can see that the more oxidized the conditions are, the higher the

values of MAC and k seem to be. Moreover, parameters limiting the accuracy of

absorbance spectra for SOA should also be applied to MAC and k values, as both values

are derived from initial absorbance of the compound.

 

Experimental SOA Literature SOA

MAC Values <0.001 to 0.527 <0.001 to 0.088

k Values .0001 to .0148 .0001 to .025

Table 3. Comparison of experimental SOA values (done in this study) to the values derived from the study done by Lambe et al. (2013). Although the values between the two remain in the same range, differences

can be attributed to not only the solvents used (NaOH as opposed to water), but to the production, collection, and analysis of SOA products.

Differences in our MAC and k values can be attributed to the use of different

solvents, and to perhaps an inaccuracy in our methods. The study we compared our

values to used NaOH as a solvent, whereas in our experiment, water was used.

Furthermore, this study and the study done by Lambe used different methods to

completely produce, collect and analyze the SOA. However, qualitative conclusions and

41    

speculations of this study and in the study produced by Lambe were found to be similar.

It appears that as oxidation levels of VOC increase in the production of SOA, the optical

properties of the SOA change in such a way that MAC and k values also increase.

Conclusion and Future Work

As it stands, we can assume that the SOA produced in the lab at PNNL absorbs

radiation, specifically very strongly in the UV region of the electromagnetic spectrum.

Very minimal absorbance is present above 400 nm. Currently, we do not know the

structure of the SOA, but can make the assumption that based on the absorbance of the

SOA, that it contains some conjugation, although not a substantial amount. Furthermore,

due to the findings in this study, and the comparison of the study done by Lambe, we can

make the assumption that although under typical conditions the absorbance of SOA does

not appear to be very large, SOA can be a potential contributor to Brown Carbon and

atmospheric radiative forcing, with the magnitude of contribution dependent on not only

its precursor (origination), but also oxidation level. This is to say, further neglect of SOA

in climate models may provide inaccurate predictions as to what future patterns in

climate may look like. To the extent that SOA does absorb some amount of radiation, it

may be a non-negligible contributor to climate change.

Our plan initially included dissolving the SOA in a nonpolar solvent and looking

to see if there was any difference between the two spectra, and whether or not solvent had

an impact on the structure of SOA. However, we were not able to get to it due to the time

constraint. For future studies, we will plan to dissolve the SOA in a more nonpolar

42    

solvent, specifically by using solvents suggested in other studies, and building up from

there.

Furthermore, in future studies we might hold the oxidants constant, while the

amount of NO and NOx is changed as we did not have a control for this study. This

would hopefully give us accurate information on whether or not oxidation level of SOA

is truly at fault for differences in optical properties, as opposed to just making a scientific

assumption.

43    

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