Chemistry and Biochemistry

Jaron C. Hansen

Jaron Hansen

Office: C307 BNSN
Office Phone: 801-422-4066
Lab Room: C394 BNSN
Lab Phone: 801-422-8980
Office Hours


BS, Utah State University (1997)

Ph.D., Purdue University (2002)

Postdoctoral Scholar, California Institute of Technology/Jet Propulsion Laboratory (2002-2005)


The Hansen Lab utilizes both computational and experimental tools to investigate a variety of environmental and atmospheric chemistry issues. 

The Hansen Lab couples together high level ab initio computational studies with experimental studies designed to investigate the kinetics and spectroscopy of important atmospheric species and reactions. Hansen Lab studies are complemented by in situ air sampling campaigns designed to investigate source apportionment and general air quality issues. Hansen researchers utilize a human subject exposure chamber/environmental chamber to aid in the interpretation of air sampling campaign studies. Hansen researchers also have an active research element in their group that studies the conversion of biomass into energy. 

Kinetics and Spectroscopy of Atmospherically Important Molecules

The research objective of this element in Hansen Lab efforts is to improve the understanding of atmospheric chemical processes through focused laboratory and computational studies.  

Aerosol Formation 

Aerosols are solid or liquid particles suspended in air. They affect visibility, human health, and climate.1 Primary aerosols are released directly into the atmosphere from both biogenic and anthropogenic sources.1 Secondary aerosols form in the atmosphere via physical and chemical processes. The formation of secondary aerosol particles is frequently modeled with classical nucleation theory (CNT),2, 3 as outlined in Figure 1. The first step in CNT is nucleation, in which molecular clusters form and then grow in size until they reach the critical cluster size. The critical cluster size is defined as the cluster size with the maximum Gibbs free energy. The second step in CNT is the growth of the critical cluster through coagulation or condensation. Current CNT predictions of atmospheric aerosol contents underestimate the measured concentrations of aerosols.4-11 

Figure 1: Graphical representation of the steps involved in classical nucleation theory (CNT).

Sulfuric acid (H2SO4) has been extensively studied and serves as a model for particle formation.4, 12-14 Curtius et al.,2 Becker and Doring et al.,15 and others16-18 have proposed particle formation via CNT in which H2SO4 interacts with water vapor to form a nucleating seed (i.e., H2SO4–H2O complex). Additional water molecules, volatile organic compounds, semivolatile secondary organic gases, or other substituents in the atmosphere can adhere to the complex to overcome the surface free energy barrier, resulting in the formation of the critical cluster. Once the critical cluster size (~1 nm) has been reached, cluster growth becomes thermodynamically favorable. 

 Recent studies have demonstrated that the inclusion of ppb concentrations of amines in a gaseous mixture of H2SO4 and water vapor increases the rate of particle formation by 10–1,000 times.19-24 When modeled by CNT, new particle formation requires a high relative humidity for particle growth to occur. In a surprise finding, the introduction of ppb concentrations of amines into a reaction vessel containing H2SO4 coupled with relative humidities as low as 20% resulted in particle formation.19 These findings suggest that amines can play a critical role in new particle formation under otherwise unfavorable conditions. 

Amines are the most abundant bases emitted into the atmosphere, with estimated global emissions in excess of 5,000 Gg∙N∙yr−1 and concentrations measured in the atmosphere ranging from 1 ppt to 10 ppb.25, 26 The many biogenic and anthropogenic sources of amines include the decomposition of plants, animal husbandry, and biomass burning. Trimethylamine (TMA) and ammonia (NH3) are the most abundant amines in the atmosphere, with emission rates of 108 and 5,000 Gg∙N∙yr−1, respectively.25, 26 

Carboxylic acids have been detected in particulates from various parts of the world.5, 27 Formic acid is the most abundant carboxylic acid in the atmosphere. Its strong binding energy with water makes it an ideal candidate for studying its ability to serve as a nucleating seed along with water vapor for particle formation.   Recent work by Shaw et al.29 demonstrated that acetaldehyde can undergo photo-tautomerization into formic acid. Figure 2 shows the influence of this previously unknown reaction on global concentrations of formic acid, which we hypothesize may serve as nucleating seeds for particle formation. The higher formic acid concentrations, indicated by warm colors in Figure 2, are typically centered over the oceans and indicate that between 39% and 50% of the formic acid found in these regions can be attributed to the photo-tautomerization of acetaldehyde.

Figure 2: Fraction of formic acid produced from the photo-tautomerization of acetaldehyde. Figure from Ref.29.

In a collaboration with the Francisco group at the University of Pennsylvania, the thermodynamics of new particle formation by formic acid, water vapor, and TMA were investigated using high-level ab initio and Born–Oppenheimer molecular dynamics (BOMD) simulations.30 These computational studies were done in support of experimental work performed in the Hansen Laboratory. The results of the ab initio calculations indicate that a strongly hydrogen-bonded complex forms between formic acid, water vapor, and TMA with a binding energy of 77.9 kJ∙mol−1 (relative to the energy of the separated monomers, see the geometry illustrated in Figure 3).30

Figure 3: Optimized geometry of formic acid-water-trimethylamine complex. The principle hydrogen bonds in the complex are shown as dashed lines. Bond lengths are reported in Angstroms.

BOMD simulations provide useful information about the time scale and molecular mechanism of ion-pair formation along with the dynamic behavior of the ion pair formed on the aqueous surface. We performed BOMD simulations to probe the nature of the interaction between HCOOH and N(CH3)3 on a water cluster containing 191 H2O molecules. The HCOOH⋅⋅N(CH3)3 interaction was found to follow a typical trajectory of acid–base chemistry and involves proton transfer between HCOOH and N(CH3)3 without the direct involvement of surface water molecules. This process, shown in Figure 4, results in the formation of a HCOO⋅⋅⋅NH(CH3)3 + ion pair on a picosecond (ps) time scale. The water cluster stabilizes the ion-pair particle by forming a hydration shell around it. These findings are consistent with field measurements conducted in Riverside, California31 and the Central Valley region of California,32 which indicated that ammonium salts can form in aged organic carbon particles. A laboratory study of the reactive uptake of NH3 onto slightly soluble organic acid particles also found that this process can significantly enhance the cloud condensation nuclei activity and hygroscopic growth of these particles.33 For the reaction between HCOOH and N(CH3)3, a transition state-like complex is formed at 5.18 ps. In this complex, the hydroxyl proton of HCOOH is partially dissociated and transferred toward N(CH3)3; the resulting O1–H1 bond length is 1.33 Å, whereas the H1–N1 bond length is 1.29 Å. This complex is converted into the HCOO⋅⋅⋅NH(CH3)3 + ion pair at 5.23 ps. In this ion pair, the O1–H1 bond is lengthened to 1.70 Å, indicative of a hydrogen-bonding interaction, whereas the H1–N1 bond has become a true covalent bond (H1–N = 1.06 Å).

Figure 4: Snapshots of structures from the BOMD simulations of the reaction of formic acid with TMA [N(CH3)3], which illustrate the formation of a HCOO-...NH(CH3)3+ ion pair on a water droplet.

The Hansen lab has investigated the kinetics of particle formation using a slow-flow reaction cell equipped with a sliding injector and coupled to two mobility particle sizers as detectors for measuring the size distribution and absolute number of particles formed (Figure 5).  Gaseous mixtures of formic acid, water vapor, and TMA were introduced into the cell, and the particle size distribution and total number of particles were measured as a function of reaction time. One new element of this study was the introduction of trace concentrations of TMA into the gaseous formic acid/water vapor reaction mixture. The rate of particle formation was observed to increase by a factor of 2 when ppb quantities of TMA were incorporated into the formic acid/water vapor mixtures. Figure 6 shows the total particle concentration and size distribution for reaction times of 8 and 48 s. In the absence of TMA, flowing formic acid and water vapor resulted in a total of ~3.5×106 particles∙cm−3 with a size distribution of 0.2–75 nm. Adding 200 ppb TMA to the reaction mixture resulted in the generation of ~4×106 particles∙cm−3 (8 s reaction time) with a size distribution of 14–200 nm. Upon increasing the reaction time to 48 s, the total concentration of particles increased to ~7.5×106 particles∙cm−3, and the size distribution shifted toward larger particle diameter. Based on these results, we hypothesize that the HC(O)OH–H2O–N(CH3)3 complex (along with other carboxylic acid–water–amine complexes) can serve as nucleating seeds for particle formation. 

Figure 5: Schematic of the slow-flow reactor/SMPS instrument for aerosol generation and detection.

The strong hydrogen-bonding interactions between carboxylic acids, water vapor, and amines may play an important role in controlling the abundance and chemical reactivity of particles in the atmosphere.34 Our previous computational work30, 35, 36 demonstrated that in the presence of amine, formic and acetic acids react to form an ion pair solvated by a cage of water molecules. Due to the strong polarizing effect of the carboxylic acid–amine ion pair in water clusters, we theorize that carboxylic acids may serve as nucleating agents for particle formation in the presence of trace amounts of amines. 

Figure 6: Comparison of particle formation initiated from 140 ppm formic acid, 630 ppm water vapor, and 200 ppb TMA after 8 and 48 s of reaction time.

In the Hansen lab we experimentally test the above hypothesis using the method/setup described previously. The apparatus shown in Figure 5 allows us to probe the effects of temperature, concentrations of carboxylic acid, water vapor, and amine on aerosol particle growth in terms of size distribution and concentration. 

Air Sampling Campaigns and Human Exposure/Environmental Chamber

The most frequent cause of death among adults in the United States is disease of the heart (principally heart attacks), followed by cancer, and cerebrovascular diseases (stroke). Two of the three main causes are related to the function of the cardiovascular system. Long term exposure to elevated levels of particulate matter (PM) pollution have been implicated in the increased risk of the onset of ischemic heart disease and sub-clinical chronic inflammatory lung injury and atherosclerosis. A proposed mechanism for the effects of PM exposure on the cardiovascular system is via an inflammatory response of the endothelium. The exposure of heritable hyperlipidemic rabbits and mice to elevated, environmentally relevant PM concentrations has been shown to accelerate the progression of atherosclerotic plaques and vascular inflammation. Short term exposure to elevated and acute levels of PM was found to cause an increase in fibrogen and inflammatory markers in pulmonary and respiratory system of humans.

The connection of short term PM exposure and the onset of myocardial infarction has been observed in general population studies. Additionally, a cross-over study of 12,865 patients living in Utah showed that short-term exposure to high PM levels contributed to acute coronary disease, especially among the individuals predisposed or with current coronary condition. Acute vasoconstriction was observed in healthy adults after short term exposure to levels of fine particulate pollution and ozone common in urban areas. Although the effects of exposure to ambient pollution in humans have been studied, and the effects of exposure of experimental animals to concentrated ambient particulate material, CAPS, has been reported in several studies, a study of the endothelial function effects of direct, short-term exposure of humans to PM in laboratory conditions have not yet been performed.

Several designs for controlled human exposure have been developed. These include full-body exposure chambers, hoods and masks. Particulate matter generation in these systems employs either on-board production of pollution via previously obtained powder samples (wheat flour, dust, etc.) or the use of the particulate pollution directly extracted and concentrated from ambient atmospheric conditions. The Hansen Lab Group has designed and characterizes the performance of a two-stage PM exposure chamber/environmental chamber for human subjects developed to study the effects of short-term PM exposure as well as serve as an environmental chamber (Figure 7). 

Figure 7: 30 m3 Teflon environmental chamber with bank of UV and black lights located above the inflatable bag.

The design of this chamber allows for the measurement of: 1) the concentrations of non-volatile and semi-volatile PM using semi-continuous monitors, 2) time-dependent size distribution, and 3) the concentrations of environmentally relevant gases, including CO, CO2, NOx, and O3. Additionally, the current design allows for the pretreatment (photochemically aging) of PM or atmospheric gases mixtures. The system allows scientists to investigate the influence of various conditions on ozone and PM production. 

The Hansen Lab also uses a human exposure/environmental chamber to test and validate new semi-continuous instruments designed to measure PM and its components. These instruments are ultimately placed into the field where they are used in air sampling campaigns designed to investigate source apportionment. The Hansen Lab Group operates a sampling site in Lindon, Utah (Figure 8).  Recently, the Hansen Lab investigated the sources of elevated concentrations of formaldehyde (CH2O) observed in the Bountiful, Utah. Formaldehyde was measured using a newly designed and built Broad Band Cavity Enhanced Absorption Spectrometer.

The U.S. Environmental Protection Agency (EPA) National Air Toxics Trends Station (NATTS) Network has been in place since 2003 and was developed to provide long-term monitoring of hazardous air pollutants (HAPs).[1] Since 2003, the Bountiful, Utah monitoring site has served as one location in the NATTS network. The U.S. EPA has set guidelines for a range of HAPs, and most of these pollutants have been detected in low concentrations in Utah. The Utah Division of Air Quality (DAQ) has sponsored or collaborated on several studies to measure the concentrations of various HAPs in Utah, including formaldehyde. Formaldehyde is a ubiquitous trace compound in the atmosphere.  Inhalation of formaldehyde can be irritating to the upper respiratory tract and eyes.  Animal studies have shown that inhalation can affect the lungs and impair learning and change behavior.[2]   Formaldehyde has been classified as a probable human carcinogen (Group B1) by the U.S. Environmental Protection Agency (EPA) and carcinogen by the International Agency for Research on Cancer (IARC).[3,4]  Of 187 compounds that have been identified as HAPs, HCHO contributes over half the total cancer risk and 9% of noncancer risk in the United States (U.S.).[1,5,6]  Over 12,000 people year-1 are estimated to develop cancer based on ambient formaldehyde exposure in the US.[7].

Formaldehyde is a volatile organic compound (VOC) that plays a vital role in ozone formation in urban areas.  Its photolysis is a source of both OH and HO2 radicals, which both serve to drive tropospheric O3 formation.  As a result of HCHO’s carcinogenic nature and role in tropospheric ozone formation, a wealth of research has been done to better elucidate sources of formaldehyde.[8-12] HCHO can be directly emitted into the atmosphere from both anthropogenic and biogenic sources.  Secondary production of HCHO occurs during the photooxidation of almost every VOC albeit with varying efficiencies and rates.  

Starting in February 2019, an eight-week intensive campaign was started to measure HCHO at the Bountiful, Utah site on a two-hour averaged basis.  The components expected to be important to understanding the sources of formaldehyde including benzene, ethylbenzene, toluene, and xylenes (BTEX) were also measured.  In addition, the concentrations of NOX (NO, NO2) and O3, were also measured on a two-hour averaged basis. Figure 8 shows the location of the Bountiful sampling site, the five oil refineries located between 2-5 miles to the south, southwest of the sampling site as well as the I-15 interstate and the location of other DAQ permitted VOC emitting point sources.  In 2017, the annual average daily traffic count for vehicles passing through the section of I-15 that runs parallel to the sampling site was 168,000.  A total of 84% of this traffic was cars, 9.3% was single unit trucks (i.e. vehicle on a single frame including box trucks, camping and recreation vehicles and motor homes) and 6.7% was combination unit trucks (i.e. truck-tractors units traveling with a trailer or multiple trailers).[19]

Figure 8: The locations of various VOC emission sources, oil refineries (red crosses) and industrial (blue circles) with emission strengths (tons year-1) located to the SSW of the Bountiful NATTS sampling site (black star).Green color represents mountains with forest area.  Emission strengths are taken from permits issued by the State of Utah Department of Air Quality. Also included is the location of major roadways and forested areas in the area.  

In collaboration with researchers from the University of Utah and the Utah Department of Air Quality a positive matrix factorization (PMF) analysis was done using historical data (2004-2015) to better understand the sources of formaldehyde in the region.  The historical data set measurements collected every sixth day on a 24-hour basis.  The data set collected in 2019 includes two-hour averaged measurements of formaldehyde and some of its possible precursors.  The more rapid data collection method used in the 2019 study allows for additional conclusions to be made about sources of formaldehyde in the Bountiful region.  To better understand the possible variety of formaldehyde emissions, corresponding backtrajectory wind calculations for selected time periods are presented to aid in the understanding of the effects BTEX emission sources on the secondary formation of formaldehyde.

Formaldehyde and NO2 were measured using a newly designed and built Broadband Cavity Enhanced Absorption Spectrometer (BBCEAS) instrument.   The BBCEAS leverages long path lengths (1-5 km) by use of multi-reflections in a short instrument footprint (1-2 m).[25]  A cage system constructed of carbon-fiber tubes was employed to obtain optical alignment, with structural parts being 3-D printed (laser-sintering or extruded PLA, depending on the function of the part). The instrument has a base path of 98.5 cm and 5 cm diameter highly reflective mirrors from Advanced Thin Films (ATFilms) centered at 365 nm, with a second cavity centered at 455 nm. Light was produced by LEDEngin (blue) and Thorlabs (M340D3) LEDs centered at 450 and 340 nm, respectively, and collected at the rear of the cavity onto optical fibers. An Andor Shamrock SR-303i spectrograph with gated, intensified CCD was used as a detector in the UV region (310-400 nm range, ~0.5 nm FWHM).  In the visible region, an Avantes AvaSpec-2048L was used as a detector.  Figure 9 shows the instrument.  

Figure 9: Illustration of an open-cavity BBCEAS instrument. Not shown is the fiber optic to spectrometer and CCD connection.

The Hansen Lab collaborates with Dr. Ryan Thalman (Snow College) and Dr. Matt Asplund (BYU) to optimize the performance of this new instrument for measurement of a variety of trace gases commonly found in the atmosphere.    

Biofuel/Alternative Energy

Transformative advancement in renewable energy production by anaerobic digestion (AD) of waste streams requires an inexpensive, simple, and scalable pretreatment to increase the conversion of organic wastes into biogas. (Zamri et al. 2021, Atelge et al. 2020) Production of biogas by AD offers a proven, readily-scalable, and well-understood mechanism for energy production and disposal of organic wastes. However, inefficient conversion of waste into biogas, typically 30-40% in mesophilic digesters without pretreatment (Liu et al. 2021, Atelge et al. 2020, Tabatabaei et al. 2020, Rico et al. 2011, Nasir et al. 2012), makes it difficult for AD to be an economically viable source of renewable energy. Improving the economic viability of AD in the renewable energy market therefore requires a low-cost, efficient pretreatment that consistently and significantly increases the fraction of biomass converted into biogas. (Sevillano et al. 2021, Atelge et al. 2020, Cheah et al. 2020, Anukam and Berghel 2020, Carrere et al. 2016)

Pretreatment of organic wastes prior to AD by physical (e.g., mechanical pulverization, cavitation, and limited pyrolysis), physicochemical (e.g., steam explosion and ammonia fiber explosion), chemical (e.g., acid hydrolysis, alkaline hydrolysis, high temperature organic solvent pretreatment, and oxidative delignification), biological (e.g., lignin degradation by white- and soft-rot fungi), and electrical methods, and various combinations thereof, have existed for several decades, but are energy inefficient and are often not economically viable (Atelge et al. 2020, Anukam and Berghel 2020; Vyas et al. 2017; Kumar and Sharma 2017; Lee et al. 2016). To date, the only economically successful pretreatment method for increasing degradation and biogas production is the thermal hydrolysis process (THP), in which the influent is heated to 130-180°C for 30-60 minutes. THP of sewage sludges increases biogas yield by 50%, decreases viscosity, allowing higher loading rates, decreases effluent chemical oxygen demand (COD) by 50%, improves dewatering, and provides sterilized, odor-free compost. (Liao et al. 2014) 

The optimum system for waste pretreatment depends on the physical and chemical characteristics of the waste being treated, and for some wastes, a pretreatment that uses a thermophilic biological component may provide many of the same advantages as THP at less cost. A biological pre-digestion process is more energy efficient then THP because it operates at lower temperature and pressure. However, for some wastes, the optimum pretreatment may be to add thermophilic biology post-THP, which could be done with no additional energy cost because the influent is already heated. Such a combination of compatible pretreatments may provide a significant increase in performance over THP or biological pre-digestion alone for some wastes.

Many wastes are recalcitrant for AD because the organic solids are large, polymeric molecules, e.g. lignocellulose, that are not directly accessible to methanogens (Atelge et al. 2020, Sayara and Sanchez 2019). Hydrolysis of these polymeric materials into small, soluble molecules or ions makes them readily accessible for methanogenesis and improves the rate and efficiency of conversion of the substrate into biogas by AD. 

Figure 10 shows a schematic diagram of how a biological pre-digestion would be implemented in a commercial plant for producing biogas from an organic waste stream.  

Figure 10: The proposed commercial process occurs in three steps: First, feedstock is mixed and heated in a hydrolysis tank to drive off O2 and reach the requisite temperature and pH for growth of C. bescii. Second, feedstock is pre-digested in an anaerobic secretome bioreactor (ASB). And third, the predigested feedstock is anaerobically digested to produce biogas in a conventional anaerobic digestion vessel.

In the first stage or tank, an organic waste containing polymeric organic materials is suspended in water in a mixing-hydrolysis tank at 75°C or higher where partial hydrolysis of the substrate occurs, O2 is removed by decreased solubility and reaction with the organic material, and the suspension is pasteurized. Note that this hydrolysis tank could be a THP tank which may be advantageous for some wastes. Pre-digestion takes place in a second stage or tank (termed an anaerobic secretome bioreactor, ASB) at 75°C and pH 7-8.   The temperature in the ASB is high enough to provide relatively fast reactions and short retention times, but not so high as to require special materials or designs for tanks, pumps and fittings or to incur excessive heating costs. In the last phase, AD takes place in a third vessel that could be thermophilic or mesophilic. Thermophilic digestion may be advantageous since the energy cost of heating the influent has already been incurred during the pre-digestion phase. The hypothesis being tested in the Hansen Lab in collaboration with Dr. Zach Aanderud (BYU) is that biological pre-digestion of will significantly increase the amount of VS destroyed and therefore increase the yield of biogas and methane.

The Hansen Lab Group has, to date, filed 3 patents (patent pending) technologies. This technology has been licensed from Brigham Young University to a start-up company called Verde LLC.   The pretreatment technology has been successfully used to pretreat and digest algae, human waste, manure, green waste (leaves and grass clippings), and sawdust.

Research Group


1.  Hansen, Jaron C.; Li, Yumin; Francisco, Joseph S.; Li, Zhuangjie, “On the Mechanism of the BrO + CH2O Reaction” Journal of Physical Chemistry, A, 1999, 103, 8543-8546.


2.  Good, David A.; Hansen, Jaron; Francisco, Joseph S.; Li, Zhuangjie; Jeong, Gill-Ran, “Kinetics and Reaction Mechanism of Hydroxyl Radical Reaction with Methyl Formate,” Journal of Physical Chemistry, A, 1999, 103, 10893-10898.


3.  Hansen, Jaron C.; Li, Yumin; Li, Zhuangjie; Francisco, Joseph S., “On the Mechanism of the BrO + HBr Reaction,” Chemical Physical Letters, 1999, 314, 341-346.


4.  Good, David A.; Hansen, Jaron; Kamboures, Mike; Santiono, Randy; Francisco,  Joseph S., “An Experimental and Computational Study of the Kinetics and Mechanism of the Reaction of Methyl Formate with Cl Atoms,”  Journal of Physical Chemistry, A, 2000, 104,1505-1511.


5.  Li, Zhuangjie; Tao, Ahining; Naik, Vaishali; Good, David A.; Hansen, Jaron C.; Jeong, Gill-Ran; Francisco, Joseph S.; Atul K.; Wuebbles, Donald J., “Global Warming Potential Assessment for CF3OCF=CF2,Journal of Geophysical Research, 2000, 105, 4019-4029.


6.  Hansen, Jaron C.; Li, Yumin; Francisco, Joseph S.; Szente, Joseph J.; Maricq, M. Matti, “An Experimental and Computational Study of the Methyl Formate Radical  Ultraviolet Spectrum,” Journal of Chemical Physics, 2000, 113, 6465-6468.


7.  Li,Z.; Jeong, G.-R.; Hansen, J.C.; Good, D.A.; Francisco, J.S., “Rate constant for the reactions of CF3OCHFCF3 with OH and Cl.” Chemical Physics Letters, 2000, 320, 70-76.


8.  Jain, Atul K.; Li, Zhuangjie; Naik, Vaishali; Wuebbles, Donald J.; Good, David A.; Hansen, Jaron C.; Francisco, Joseph S., “Evaluation of the Atmospheric Lifetime and Radiative Forcing on Climate for 1,2,2,2-Tetrafluoroethyl Trifluoromethyl Ether (CF3CHFOCF3),” Journal of Geophysical Research, 2001, 106, 12615-12618.


9.  Kamboures, Mike; Hansen, Jaron C.; Francisco, Joseph S., “A Study of the Kinetics and Mechanism Involved in the Atmospheric Degradation of Bromoform by Atomic Chlorine,Chemical Physics Letters, 2002, 353, 335-344.


10.  Hansen, Jaron C.;Francisco, J.S.; Szente, Joseph J.; Maricq, M. Matti,  “An   Experimental Study of the Mechanism and Branching Ratio for the Reaction between Methyl Formate and Chlorine Atoms,” Chemical Physics Letters, 2002, 365, 267-278.       


11.  Xu, Dadong; Huang, Jianhua; Francisco, Joseph S.;  Hansen, Jaron C.; Jackson, William M., “Photodissociation of carbonic dibromide at 267nm: observation of three-body dissociation and molecular elimination of Br2,” Journal of Chemical Physics, 2002, 117, 7483-7490.


12.  Hansen, Jaron C. and Francisco, Joseph S., “Radical-molecule complexes: changing our perspective on the molecular mechanisms of radical-molecule reactions and their impact on atmospheric chemistry,” ChemPhysChem, 2002, 3, 833-840.


13.  Hansen, Jaron C.; Li, Yumin; Rosado-Reyes, Claudette M.; Francisco, Joseph S.; Szente, Joseph  J.; Maricq, M. Matti, “Theoretical and Experimental Investigation of the UV Cross Section and Kinetics of the Methyl Formate Peroxy Radical,” Journal of Physical Chemistry, 2003, 107, 5306-5316.


14.  Christensen, Lance E.; Okumura, Mitchio; Hansen, Jaron C.; Sander, Stanley P.; Francisco, Joseph S., “Experimental and ab Initio Study of the HO2-CH3OH Complex: Thermodynamics and Kinetics of Formation,” Journal of Physical Chemistry A, 2006, 110, 6948-6959.


15.  Hansen, Jaron C.; Flowers, Bradley A.; Stanton, John F., “Computational study of the vibrational and electronic spectroscopy of a HO2-H2O2 complex,” Journal of Molecular Structure: THEOCHEM, 2006, 768, 111-118.


16.  Pope, Francis D.; Hansen, Jaron C.; Bayes, Kyle D.; Friedl, Randall R.; Sander, Stanley P.” Ultraviolet Absorption Spectrum of Chlorine Peroxide, ClOOCl,” Journal of Physical Chemistry A., 2007, 111, 4322-4332.


17.  Clark, Jared; English, Alecia M.; Francisco, Joseph S.; Hansen, Jaron C. “Computational Study of the Existence of Organic Peroxy Radical-Water Complexes (RO2-H2O),“ Journal of Physical Chemistry A, 2008, 112, 1587-1595.


18.  English, Alecia J.; Szente, Joseph J.; Maricq, Matti M.; Hansen, Jaron C., “The Effects of Water Vapor on the CH3O2 Self-Reaction and Reaction with HO2,“ Journal of Physical Chemistry A., 2008, 112, 9220-9228.


19.  Hansen, Jaron C.; Friedl, Randall R.; Sander, Stanley P. “Kinetics of the OH + ClOOCl and OH + Cl2O Reactions:  Experiment and Theory,” Journal of Physical Chemistry A, 2008, 112, 9229-9237.

20.  Hansen, Jaron C.; Woolwine III, Woods R.; Bates, Brittney L.; Clark, Jared M.; Kuprov, Roman Y.; Mukherjee, Puspak; Murray, Jacolin A.; Simmons, Michael A.; Waite, Mark F.; Eatough, Norman L.; Eatough, Delbert J.; Long, Russell; Grove, Brett D. “Semi-Continuous PM2.5 and PM10 Mass and Composition Measurements in Lindon, Utah During Winter 2007,“ Journal of the Air and Waste Management Association, 2010, 60, 346-355.


21.  Clark, Jared; Call, Seth T.; Austin, Daniel; Hansen, Jaron C. “Computational Study of Isoprene Hydroxyalkyl Peroxy Radical-Water Complexes (C5H8(OH)O2-H2O),” Journal of Physical Chemistry A, 2010, 114, 6534-6541.


22.  Hansen, Clifford W.; Hansen, Lee D.; Nicholson, Allen D.; Thomas, Nathan; Clark, Jared; Chilton, Marie C.; Hansen, Jaron C.; “Correction for Instrument Time Constant in Determination of Reaction Kinetics” International Journal of Chemical Kinetics, 2010, 43, 53-60.


23.  Cline, Taylor; Thomas, Nathan; Shumway, Logan; Yeung, Irene; Hansen, Conly L.; Hansen, Lee D.; Hansen, Jaron C. “ Method for Evaluating Anaerobic Digester Performance,” Bioresource Technology, 2010, 101, 8623-8626.


24.  Riffault, Veronique; Hansen, Jaron C.; Clark, Jared; Ravishankara, A.R.; Burkholder, James B. “Temperature-Dependent Rate Coefficients and Theoretical Calculations for the OH + Cl2O Reaction” ChemPhysChem, 2010, 11, 4060-4068.


25.  Kuprov, Roman; Buck, David; Eatough, Delbert J.; Hansen, Jaron C.; “Design and Characterization of a Two-stage Human Subject Exposure Chamber,” Journal of the Air and Waste Management Association, 2011, 61(8), 864-871.


26.  Mayo, MacKenzie H.; Nicholson, Allen D.; Hansen, Lee D.; Hansen, Jaron C. “Chemical Treatment of Algae to Facilitate Biogas Production by Anaerobic Digestion,” American Society of Agricultural and Biological Engineers, 2011, 54, 1547-1550.


27.  Pope, Arden C. III; Hansen, Jaron C.; Kuprov, Roman; Sanders, Matthew D.; Anderson, Michael N.; Eatough, Delbert J. “ Vascular Function and Short-Term Exposure to Fine Particulate Air Pollution”, Journal of the Air and Waste Management Association, 2011, 61(8), 858-863.


28.  Burrell, Emily; Clark, Jared M.; Snow, Mathew; Dumais, Heidi; Lee, Seong-Cheol; Nielson, Brad J.; Osborne, Derek; Cardona-Salamanca, Lucia; Zemp, Logan; DaBell, Ryan S.; Hansen, Jaron C. “Computational Study of Hexanal Peroxy Radical-Water Complexes” International Journal of Quantum Chemistry, 2011, 112, 1936-1944. 


29.  Clark, Jared; Kumbhani, Sambhav; Francisco, Joseph S.; Hansen, Jaron C. “HNO3-NHx, H2SO4-NHx, CH(O)OH-NHx, and CH3C(O)OH-NHx complexes and their role in the formation of condensation nuclei”, The Journal of Chemical Physics, 2011, 135, 244305.


30.  Cropper, Paul M.; Hansen, Jaron C.; Eatough, Delbert J. “Measurement of Scattering in an Urban Area Using a Nephelometer and PM2.5 FDMS TEOM Monitor: Accounting for the Effect of Water”, Journal of the Air and Waste Management Association, 2013, 63(9), 1004-1011


31.  Kuprov, Roman; Eatough, Delbert J.; Cruickshank, Tyler; Olson, Neal; Cropper, Paul; Hansen, Jaron C. “Composition and Secondary Formation of Fine Particulate Material in the Salt Lake Valley: Winter 2009”, Journal of the Air and Waste Management Association, 2014, 64(8), 957-969.


32. Khan, M.A.H.; Cooke, M.C.; Utembe, S.R.; Archibald, A.T.; Derwent, R.G.; Jenkin, M.E.; Morris, W.C.; South, N.; Hansen, J.C.; Francisco, J.S.; Percival, C.J.; Shallcross, D.E. “Global Analysis of Peroxy Radicals and Peroxy Radical-Water Complexation using STOCHEM-CRI Global Chemistry and Transport Model”, Atmospheric Environment, 2015, 106, 278-287.


33.  Kumbhani, Sambhav; Cline, Taylor S.; Killian, Marie C.; Clark, Jared; Hansen, Lee D.; Shirts, Randall B.; Robichaud, David J.; Hansen, Jaron C. “ Water Vapor Enhancement of Peroxy Radical Reactions”, International Journal of Chemical Kinetics, 2015, 47(6), 395-409


34. Cropper, Paul; Goates, Steve R.; Hansen, Jaron C. “A Compact Gas Chromatograph and Pre-Column Concentration System for Enhanced In-field Separation of Levoglucosan and other Polar Organic Compounds”, Journal of Chromatography A, 2015, 1417, 73-78


35. Kumbhani, Sambhav ; Cline, Taylor  S.; Killian, Marie C.; Clark, Jared; Keeton, William; Hansen, Lee D.; Shirts, Randall B.; Robichaud, David J.; Hansen, Jaron C. “Response to the Comment on Paper” Water vapor Enhancement of Rates of Peroxy Radical Reactions”, International Journal of Chemical Kinetics, 2015, 48(7), 399-401


36. Cropper, Paul M.; Cary, Robert; Eatough, Delbert; Hansen, Jaron  C. “Development of the GC-MS Organic Aerosol Monitor (OAM) for In-field Detection of Particulate Organic Compounds”, Analytical Environment, 2017, 169, 258-266


37.  Cropper, Paul M.; Eatough, Delbert J.; Overson, Devon K.; Caka, Fern; Cary, Robert A.; Hansen, Jaron C. “Use of a GC-MS Organic Aerosol Monitor for In-Field Detection of Fine Particulate Organic Compounds in Source Apportionment”, Journal of the Air and Waste Management Association, 2017, In Press


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