R-3 Repository :: Browsing by Author "Griffin, Robert J."

Browsing by Author "Griffin, Robert J."

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A compact mid-infrared dual-gas CH4/C2H6 sensor using a single interband cascade laser and custom electronics
(SPIE, 2017) Ye, Weilin; Zheng, Chuantao; Tittel, Frank K.; Sanchez, Nancy P.; Gluszek, Aleksander K.; Hudzikowski, Arkadiusz J.; Lou, Minhan; Dong, Lei; Griffin, Robert J.
A compact mid-infrared (MIR) dual-gas sensor system was demonstrated for simultaneous detection of methane (CH4) and ethane (C2H6) using a single continuous-wave (CW) interband cascade laser (ICL) based on tunable laser absorption spectroscopy (TDLAS) and wavelength modulation spectroscopy (WMS). Ultracompact custom electronics were developed, including a laser current driver, a temperature controller and a lock-in amplifier. These custom electronics reduce the size and weight of the sensor system as compared with a previous version based on commercial electronics. A multipass gas cell with an effective optical length of 54.6 m was employed to enhance the absorption signal. A 3337 nm ICL was capable of targeting a C2H6 absorption line at 2996.88 cm-1 and a CH4 line at 2999.06 cm-1. Dual-gas detection was realized by scanning both the CH4 and C2H6 absorption lines. Based on an Allan deviation analysis, the 1 σ minimum detection limit (MDL) was 17.4 ppbv for CH4 and 2.4 ppbv for C2H6 with an integration time of 4.3 s. TDLAS based sensor measurements for both indoor and outdoor mixing ratios of CH4 and C2H6 were conducted. The reported single ICL based dual-gas sensor system has the advantages of reduced size and cost without influencing the midinfrared sensor detection sensitivity, selectivity and reliability.
A zero-dimensional view of atmospheric degradation of levoglucosan (LEVCHEM_v1) using numerical chamber simulations
(Copernicus Publications, 2021) Suciu, Loredana G.; Griffin, Robert J.; Masiello, Caroline A.
Here, we developed a zero-dimensional (0-D) modeling framework (LEVCHEM_v1) to provide insights into the atmospheric degradation of a key tracer emitted during biomass burning – levoglucosan (LEV), while additionally exploring its effects on the dynamics of secondary organic aerosols (SOA) and other gases. For this, we updated existing chemical mechanisms (homogeneous gas-phase chemistry and heterogeneous chemistry) in the BOXMOXv1.7 model to include the chemical degradation of LEV and its intermediary degradation products in both phases (gas and aerosol). In addition, we added a gas-particle partitioning mechanism to the model to account for the effect of evaporation and condensation on the phase-specific concentrations of LEV and its degradation products. Comparison of simulation results with measurements from various chamber experiments (spanning summer and winter conditions) show that the degradation timescale of LEV varied by phase, with gas-phase degradation occurring over ∼1.5–5 d and aerosol-phase degradation occurring over ∼8–36 h. These relatively short timescales suggest that most of the initial LEV concentration can be lost chemically or deposited locally before being transported regionally. We varied the heterogeneous reaction rate constant in a sensitivity analysis (for summer conditions only) and found that longer degradation timescales of LEV are possible, particularly in the aerosol phase (7 d), implying that some LEV may be transported regionally. The multiphase chemical degradation of LEV has effects on SOA and other gases. Several first- or second-generation products resulted from its degradation; most of the products include one or two carbonyl groups, one product contains a nitrate group, and a few products show the cleavage of C−C bonds. The relative importance of the products varies depending on the phase and the timing of the maximum concentration achieved during the simulation. Our estimated secondary organic aerosol SOA yields (4 %–32 %) reveal that conversion of LEV to secondary products is significant and occurs rapidly in the studied scenarios. LEV degradation affected other gases by increasing the concentrations of radicals and decreasing those of reactive nitrogen species. Decreases of the mixing ratios of nitrogen oxides appear to drive a more rapid increase in ozone compared with changes in volatile organic compounds levels. An important next step to confirm longer degradation timescales will be to extend the evaluation of the modeled LEV degradation beyond 3–6 h by using more extensive data from chambers and, possibly, from fire plumes. The mechanism developed here can be used in chemical transport models applied to fire plumes to trace LEV and its degradation products from source to deposition, to assess their atmospheric implications and to answer questions relevant to fire tracing, carbon and nitrogen cycling, and climate.
An omnipresent diversity and variability in the chemical composition of atmospheric functionalized organic aerosol
(Springer Nature, 2018) Ditto, Jenna C.; Barnes, Emily B.; Khare, Peeyush; Takeuchi, Masayuki; Joo, Taekyu; Bui, Alexander A.T.; Lee-Taylor, Julia; Eris, Gamze; Chen, Yunle; Aumont, Bernard; Jimenez, Jose L.; Ng, Nga Lee; Griffin, Robert J.; Gentner, Drew R.
The atmospheric evolution of organic compounds encompasses many thousands of compounds with varying volatility, polarity, and water solubility. The molecular-level chemical composition of this mixture plays a major, yet uncertain, role in its transformations and impacts. Here we perform a non-targeted molecular-level intercomparison of functionalized organic aerosol from three diverse field sites and a chamber. Despite similar bulk composition, we report large molecular-level variability between multi-hour organic aerosol samples at each site, with 66 ± 13% of functionalized compounds differing between consecutive samples. Single precursor environmental laboratory chamber experiments and fully chemically-explicit modeling confirm this variability is due to changes in emitted precursors, chemical age, and/or oxidation conditions. These molecular-level results demonstrate greater compositional variability than is typically observed in less-speciated measurements, such as bulk elemental composition, which tend to show less daily variability. These observations should inform future field and laboratory studies, including assessments of the effects of variability on aerosol properties and ultimately the development of strategic organic aerosol parameterizations for air quality and climate models.
Analysis and Prediction of Rainfall and Storm Surge Interactions in the Clear Creek Watershed using Unsteady-State HEC-RAS Hydraulic Modeling
(2012-09-05) Winter, Heather; Bedient, Philip B.; Griffin, Robert J.; Raun, Loren H.
This study presents an unsteady-state hydraulic model analysis of hurricane storm surge and rainfall-runoff interactions in the Clear Creek Watershed, a basin draining into Galveston Bay and vulnerable to flooding from both intense local rainfalls and storm surge. Storm surge and rainfall-runoff have historically been modeled separately, and thus the linkage and interactions between the two during a hurricane are not completely understood. This study simulates the two processes simultaneously by using storm surge stage hydrographs as boundary conditions in the Hydrologic Engineering Center’s – River Analysis System (HEC-RAS) hydraulic model. Storm surge hydrographs for a severe hurricane were generated in the Advanced Circulation Model for Oceanic, Coastal, and Estuarine Waters (ADCIRC) model to predict the flooding that could be caused by a worst-case scenario. Using this scenario, zones have been identified to represent areas in the Clear Creek Watershed vulnerable to flooding from storm surge, rainfall, or both.
Apportioned primary and secondary organic aerosol during pollution events of DISCOVER-AQ Houston
(Elsevier, 2021) Yoon, Subin; Ortiz, Stephanie M.; Clark, Adelaide E.; Barrett, Tate E.; Usenko, Sascha; Duvall, Rachelle M.; Ruiz, Lea Hildebrandt; Bean, Jeffrey K.; Faxon, Cameron B.; Flynn, James H.; Lefer, Barry L.; Leong, Yu Jun; Griffin, Robert J.; Sheesley, Rebecca J.
Understanding the drivers for high ozone (O3) and atmospheric particulate matter (PM) concentrations is a pressing issue in urban air quality, as this understanding informs decisions for control and mitigation of these key pollutants. The Houston, TX metropolitan area is an ideal location for studying the intersection between O3 and atmospheric secondary organic carbon (SOC) production due to the diversity of source types (urban, industrial, and biogenic) and the on- and off-shore cycling of air masses over Galveston Bay, TX. Detailed characterization of filter-based samples collected during Deriving Information on Surface Conditions from Column and VERtically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) Houston field experiment in September 2013 were used to investigate sources and composition of organic carbon (OC) and potential relationships between daily maximum 8 h average O3 and PM. The current study employed a novel combination of chemical mass balance modeling defining primary (i.e. POC) versus secondary (i.e. SOC) organic carbon and radiocarbon (14C) for apportionment of contemporary and fossil carbon. The apportioned sources include contemporary POC (biomass burning [BB], vegetative detritus), fossil POC (motor vehicle exhaust), biogenic SOC and fossil SOC. The filter-based results were then compared with real-time measurements by aerosol mass spectrometry. With these methods, a consistent urban background of contemporary carbon and motor vehicle exhaust was observed in the Houston metropolitan area. Real-time and filter-based characterization both showed that carbonaceous aerosols in Houston was highly impacted by SOC or oxidized OC, with much higher contributions from biogenic than fossil sources. However, fossil SOC concentration and fractional contribution had a stronger correlation with daily maximum 8 h average O3, peaking during high PM and O3 events. The results indicate that point source emissions processed by on- and off-shore wind cycles likely contribute to peak events for both PM and O3 in the greater Houston metropolitan area.
Atmospheric ammonia measurements and implications for particulate matter formation in urban and suburban areas of Texas
(2013-09-16) Gong, Longwen; Griffin, Robert J.; Cohan, Daniel S.; Tittel, Frank K.
In order to improve the current understanding of the dynamics of ammonia (NH3) in the Greater Houston and Dallas-Fort Worth (DFW) areas and to examine the effects of NH3 on local and regional air quality with respect to particulate matter formation, intensive field investigations were made. A 10.4-μm external cavity quantum cascade laser based-sensor employing conventional photo-acoustic spectroscopy was used to conduct real-time and continuous measurements of atmospheric NH3 in this work. Results from the Houston campaign indicate that the mixing ratio of NH3 ranged from 0.1 to 8.7 ppb with a mean of 2.4±1.2 (1σ) ppb in winter and ranged from 0.2 to 27.1 ppb with a mean of 3.1±2.9 ppb in summer. The larger levels in summer probably are due to higher ambient temperature. A notable morning increase and a mid-day decrease were observed in the diurnal profile of NH3 mixing ratios. Motor vehicles were found to be major contributors to the elevated levels during morning rush hours in winter. However, changes in vehicular catalytic converter performance and other local or regional emission sources from different wind directions governed the behavior of NH3 during morning rush hours in summer. There was a large amount of variability, particularly in summer, with several episodes of elevated NH3 mixing ratios that could be linked to industrial facilities. A considerable discrepancy in NH3 mixing ratios existed between weekdays and weekends. During the simultaneous high-time-resolution measurements of gaseous and aerosol species in summer, elevated NH3 levels occurred around mid-day when NH4+ (0.5 ± 1.0 μg m-3) and SO42- (4.5 ± 4.3 μg m-3) also increased considerably, indicating that NH3 likely influenced aerosol particle mass. NH4+ mainly existed in the form of (NH4)2SO4 and NH4HSO4; by contrast, the formation of NH4NO3 and NH4Cl was suppressed. Power plant plumes were found to be potential contributors to the enhancements in NH3 at the urban sampling site under favorable meteorological conditions. Increased particle number concentrations were predicted by the SAM-TOMAS model downwind of a large coal-fired power plant when NH3 emissions (based on these measurements) were included, highlighting the potential importance of NH3 with respect to particle number concentration. Measurements also show the role of NH3 in new particle formation in Houston under low-sulfur conditions. Results from the DFW campaign indicate that the mixing ratio of NH3 ranged from 0.1 to 10.1 ppb, with a mean of 2.7 ± 1.7 ppb. The diurnal profile of NH3 exhibited a daytime increase, likely due to increasing temperatures affecting temperature-dependent sources in the study region. Automobiles might be potential sources of NH3 on Sundays based on the Pearson’s correlation coefficient between NH3 and carbon monoxide, but the relationship did not exist on weekdays and Saturdays, probably due to decreased traffic volume and different traffic composition. According to the results from the EPA PMF 3.0 model, biogenic (primarily vegetation and soil) emissions were major contributors to gas-phase NH3 levels measured at the suburban site during the campaign. In addition, agriculture (especially livestock-related activities) also was expected to be a potentially significant source of NH3 based on the nature of the region. Inorganic aerosol components of submicron particles (PM1) (4.41 ± 2.13 μg m-3) were dominated by SO42- (1.25 ± 0.66 μg m-3), followed by NH4+ (0.44 ± 0.24 μg m-3) and NO3- (0.12 ± 0.11 μg m-3). Pearson’s correlation coefficients between NH4+, SO42-, and NO3- imply that particulate NH4+ mainly existed as (NH4)2SO4 and that NH4NO3 was not formed during most of the study period, likely due to high temperatures (30.15 ± 4.12 oC) over the entire campaign. Ambient aerosols tended to be nearly neutral. Theoretical calculations of thermodynamic equilibrium were performed to consider the formation of NH4NO3 and NH4Cl. When relative humidity (RH) was lower than deliquescence relative humidity (DRH), the partial pressure products of PNH3PHNO3 and PNH3PHCl were smaller than the associated equilibrium constants, indicating the lack of NH4NO3 and NH4Cl formation. When RH was above DRH, higher levels of NO3- often were observed. A strong relationship between NO3- and SO42- at higher RH suggests that NH4NO3 might be formed on the moist surface of pre-existing sulfate aerosols. In the particle mixture, (NH4)2SO4 reduces the equilibrium constant, making the aqueous system a more favorable medium for NH4NO3 formation. In addition, measured particle number size distributions showed that an aerosol nucleation and growth event was coincident with humid periods characterized by substantially increased concentrations of particulate NH4+, NO3-, and SO42-. Excess NH4+ also was found to be correlated closely with NO3- during this episode when elevated PM1 levels imply aqueous NH4NO3 formation.
Captive Aerosol Growth and Evolution (CAGE) chamber system to investigate particle growth due to secondary aerosol formation
(Copernicus Publications, 2021) Sirmollo, Candice L.; Collins, Don R.; McCormick, Jordan M.; Milan, Cassandra F.; Erickson, Matthew H.; Flynn, James H.; Sheesley, Rebecca J.; Usenko, Sascha; Wallace, Henry W.; Bui, Alexander A.T.; Griffin, Robert J.; Tezak, Matthew; Kinahan, Sean M.; Santarpia, Joshua L.
Environmental chambers are a commonly used tool for studying the production and processing of aerosols in the atmosphere. Most are located indoors and most are filled with air having prescribed concentrations of a small number of reactive gas species. Here we describe portable chambers that are used outdoors and filled with mostly ambient air. Each all-Teflon® 1 m3 Captive Aerosol Growth and Evolution (CAGE) chamber has a cylindrical shape that rotates along its horizontal axis. A gas-permeable membrane allows exchange of gas-phase species between the chamber and surrounding ambient air with an exchange time constant of approximately 0.5 h. The membrane is non-permeable to particles, and those that are injected into or nucleate in the chamber are exposed to the ambient-mirroring environment until being sampled or lost to the walls. The chamber and surrounding enclosure are made of materials that are highly transmitting across the solar ultraviolet and visible wavelength spectrum. Steps taken in the design and operation of the chambers to maximize particle lifetime resulted in averages of 6.0, 8.2, and 3.9 h for ∼ 0.06, ∼ 0.3, and ∼ 2.5 µm diameter particles, respectively. Two of the newly developed CAGE chamber systems were characterized using data acquired during a 2-month field study in 2016 in a forested area north of Houston, TX, USA. Estimations of measured and unmeasured gas-phase species and of secondary aerosol production in the chambers were made using a zero-dimensional model that treats chemical reactions in the chamber and the continuous exchange of gases with the surrounding air. Concentrations of NO, NO2, NOy, O3, and several organic compounds measured in the chamber were found to be in close agreement with those calculated from the model, with all having near 1.0 best fit slopes and high r2 values. The growth rates of particles in the chambers were quantified by tracking the narrow modes that resulted from injection of monodisperse particles and from occasional new particle formation bursts. Size distributions in the two chambers were measured intermittently 24 h d−1. A bimodal diel particle growth rate pattern was observed, with maxima of about 6 nm h−1 in the late morning and early evening and minima of less than 1 nm h−1 shortly before sunrise and sunset. A pattern change was observed for hourly averaged growth rates between late summer and early fall.
Characterization of Atmospheric Aerosols in Kathmandu, New Hampshire, and Texas: Carbonaceous, Isotopic, and Water-soluble Organic Composition
(2011) Shakya, Kabindra Man; Griffin, Robert J.
To improve the understanding of aerosol composition, sources, and spatial and temporal variations, atmospheric aerosols were characterized in three locations. Ambient aerosols were characterized using 24-hour samples collected from Kathmandu, Nepal (urban), New Hampshire (semi-rural) and Houston (urban). Results are reported in the main chapters. Chamber studies of secondary organic aerosols (SOA) formation from polycyclic aromatic hydrocarbons (PAHs) and the effects of in-situ SOA formation on atmospheric mercury oxidation are described in the appendices. Carbonaceous, ionic, and isotopic species in aerosols from Kathmandu identified local primary emissions, most likely vehicular exhaust as the most important aerosol sources. Carbonaceous aerosols collected in Kathmandu (24.5 μg C m -3 ) were much larger than those in New Hampshire (3.74 μg C m -3 ) during winter. Stable carbon isotope in aerosols of Kathmandu and New Hampshire were similar (Δδ 13 C ∠ 0.5[per thousand]) while stable nitrogen isotope were much lower in aerosols of Kathmandu (Δδ 15 N = 8.3[per thousand]). Aerosols in New Hampshire exhibited a large seasonal variation for carbonaceous aerosols, stable nitrogen isotope, and the aromatic fraction of water-soluble organic carbon (WSOC). Pure aliphatics (H-C) were the dominant functional group in WSOC. Results illustrate the importance of secondary aerosol sources throughout the year, with enhanced importance of primary sources during winter. Stable carbon isotope values suggest a consistent isotopic signature of carbonaceous aerosol sources, while the nitrogen isotope values indicate the variable nitrogenous sources and the strong influence of meteorological parameters (temperature and relative humidity) on nitrogen isotope fractionation. Characteristics of methoxyphenols (lignin macropolymers) in the ambient aerosols are reported for the first time using CuO oxidation method. The study illustrates the use of lignin oxidation products (LOPs) in aerosols as potential tracers of primary biological aerosol particles (PBAP). The methoxyphenols identified soil organic matter and altered woody angiosperms, with minor influence from soft tissues and gymnosperms as the important PBAP sources in mainly coarse particles in Houston atmosphere. Solvent-extracted methoxyphenols (lignin monomers) and anhydrosugars (levoglucosan, mannosan, and galactosan) in aerosols were either absent or very small, suggesting very limited biomass burning influence with any trace-level presence originating from long-range transport.
Characterization of simple saccharides and other organic compounds in atmospheric particulate matter and source apportionment using positive matrix factorization
(2010) Jia, Yuling; Fraser, Matthew P.; Griffin, Robert J.
Ambient particulate matter samples were collected at various sites in Texas, Arizona, and Austria from 2005 to 2009 to characterize the organic compositions and local PM sources. The primary biologically derived carbon sources, specifically the atmospheric entrainment of soil and associated biota and primary biological aerosol particles (PBAPs), are major sources contributing to ambient PM. This dissertation work proposes simple saccharides as well-suited tracers to characterize the contribution to ambient PM from these primary biologically derived carbon sources. Saccharide concentrations in ambient PM were determined from various locations and various seasons. Aerosol saccharide compounds displayed seasonal variations, inter-correlations, and size fractionations (fine vs. coarse) that were consistent between samples and that can be used to determine sources. The difference in aerosol saccharide concentrations and relative species abundances was reflective of different climate patterns and ecosystems. Selected saccharide compounds including an established marker (levoglucosan) and novel markers (glucose, sucrose, trehalose, mannitol, and arabitol) were used along with other markers to model the major source contributions to ambient PM using a positive matrix factorization (PMF) model. Major local PM sources were resolved at three Texas sites (San Augustine, Dallas, and Big Bend National Park) and one Arizona site (Higley), with two source factors enriched in the proposed novel saccharide markers that can be related to the primary biologically derived carbon sources. The contribution to PM from the saccharide-rich primary biological sources was estimated to range from 16% (remote area) to 36% (rural and suburban area) at the four sampling sites studied. Other PM sources identified by PMF included motor vehicles, secondary aerosol formation, meat cooking, biogenic wax, sea salt, crustal material, and road dust. To further characterize the primary biologically derived carbon sources, different soil and source samples representing PBAPs (plants and spores) were collected at Higley (AZ) to study their saccharide compositions in particle sizes equivalent to PM2.5 and PM10. It was found that the total measured non-levoglucosan saccharide content relative to PM mass in ambient aerosols (0.2% on average in PM2.5 and 0.11% in PM10) was much higher than the soil samples (<0.02% in both PM2.5 and PM10) but much lower than in the PBAP source samples (2% on average in plant PBAP samples and 16% in spore PBAP samples). The measured PBAP samples contained a concentration of sucrose and glucose that is consistent with the saccharide-rich source profiles resolved from ambient aerosol data analyzed by PMF while the measured soil samples did not. This can be interpreted as confirmation that PBAPs are an important PM source in additional to soil and associate biota at Higley, AZ. However, the saccharide levels in the measured PBAP samples were several orders of magnitude higher than the PMF results, suggesting that the ambient aerosol samples are a combination of high saccharide concentration PBAPs and lower saccharide concentration soils at Higley, AZ.
Compact CH4 sensor system based on a continuous-wave, low power consumption, room temperature interband cascade laser
(AIP Publishing, 2016) Dong, Lei; Li, Chunguang; Sanchez, Nancy P.; Gluszek, Aleksander K.; Griffin, Robert J.; Tittel, Frank K.
A tunable diode laser absorption spectroscopy-based methane sensor, employing a dense-pattern multi-pass gas cell and a 3.3 μm, CW, DFB, room temperature interband cascade laser (ICL), is reported. The optical integration based on an advanced folded optical path design and an efficient ICL control system with appropriate electrical power management resulted in a CH4 sensor with a small footprint (32 × 20 × 17 cm3) and low-power consumption (6 W). Polynomial and least-squares fit algorithms are employed to remove the baseline of the spectral scan and retrieve CH4 concentrations, respectively. An Allan-Werle deviation analysis shows that the measurement precision can reach 1.4 ppb for a 60 s averaging time. Continuous measurements covering a seven-day period were performed to demonstrate the stability and robustness of the reported CH4 sensor system.
Compact TDLAS based sensor design using interband cascade lasers for mid-IR trace gas sensing
(The Optical Society, 2016) Dong, Lei; Tittel, Frank K.; Li, Chunguang; Sanchez, Nancy P.; Wu, Hongpeng; Zheng, Chuantao; Yu, Yajun; Sampaolo, Angelo; Griffin, Robert J.
Two compact TDLAS sensor systems based on different structural optical cores were developed. The two optical cores combine two recent developments, gallium antimonide (GaSb)-based ICL and a compact multipass gas cell (MPGC) with the goal to create compact TDLAS based sensors for the mid-IR gas detection with high detection sensitivity and low power consumption. The sensors achieved minimum detection limits of ~5 ppbv and ~8 ppbv, respectively, for CH4 and C2H6 concentration measurements with a 3.7-W power consumption.
Comparison of estimates of airmass aging using particle and other measurements near Fort Worth, TX
(2013-06-05) Karakurt Cevik, Basak; Griffin, Robert J.; Cohan, Daniel S.; Raun, Loren H.
The composition, concentration, and size of submicron aerosols were measured with a time resolution of five minutes by an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) at a rural location northwest of the Dallas-Fort Worth, TX, area for the month of June 2011. A TSI, Inc., Model AE51 aethalometer using an optical absorption technique also was deployed to measure black carbon (BC) concentrations. The total measured PM1 mass concentration ranged between 1.0 µg/m3 and 17.1 µg/m3, with a mean and standard deviation of 4.6± 2.7 µg/m3. Significant variability is observed in the time series of total PM1 and of all four HR-ToF-AMS species, particularly between June 21 and 25. The average aerosol composition was dominated by organic matter (52.1 ± 14.8%) and sulfate (28.8 ± 11.8%). Organic aerosol concentrations were positively correlated with tracers of combustion carbon monoxide (CO) and BC, the coefficients of determination were r2=064 and r2=0.48, respectively. Because of the large influence of organics on total aerosol concentration, organic data were analyzed in the context of ΔOA/ΔCO, which typically is used to investigate the relative importance of secondary organic aerosol. The average ∆OA/∆CO for the data used was 64.0 ± 26.9 µg/ (m3 ppmv), which is typical of an aged air mass. Other metrics of age include the ratio of OOAI (more oxidized) to total oxidized organic aerosol (OOA), the ratio of sulfate to total sulfur, the ratio of its oxidation products to isoprene, and the ratio of nitrogen oxides to total reactive nitrogen. All metrics point to aged air masses, but variations in these age matrices, particularly during one period of enhanced ΔOA/ΔCO, help elucidate the contributions of various precursors and processes to organic aerosols at the site.
Composition and Sources of Particulate Matter Measured near Houston, TX: Anthropogenic-Biogenic Interactions
(MDPI, 2016) Bean, Jeffrey K.; Faxon, Cameron B.; Leong, Yu Jun; Wallace, Henry William; Cevik, Basak Karakurt; Ortiz, Stephanie; Canagaratna, Manjula R.; Usenko, Sascha; Sheesley, Rebecca J.; Griffin, Robert J.; Ruiz, Lea Hildebrandt
Particulate matter was measured in Conroe, Texas (~60 km north of downtown Houston, Texas) during the September 2013 DISCOVER-AQ campaign to determine the sources of particulate matter in the region. The measurement site is influenced by high biogenic emission rates as well as transport of anthropogenic pollutants from the Houston metropolitan area and is therefore an ideal location to study anthropogenic-biogenic interactions. Data from an Aerosol Chemical Speciation Monitor (ACSM) suggest that on average 64 percent of non-refractory PM1 was organic material, including a high fraction (27%–41%) of organic nitrates. There was little diurnal variation in the concentrations of ammonium sulfate; however, concentrations of organic and organic nitrate aerosol were consistently higher at night than during the day. Potential explanations for the higher organic aerosol loadings at night include changing boundary layer height, increased partitioning to the particle phase at lower temperatures, and differences between daytime and nighttime chemical processes such as nitrate radical chemistry. Positive matrix factorization was applied to the organic aerosol mass spectra measured by the ACSM and three factors were resolved—two factors representing oxygenated organic aerosol and one factor representing hydrocarbon-like organic aerosol. The factors suggest that the measured aerosol was well mixed and highly processed, consistent with the distance from the site to major aerosol sources, as well as the high photochemical activity.
Computational Simulation of Secondary Organic Aerosol (SOA) Formation from Toluene Oxidation
(2012-09-05) Liu, Ying; Griffin, Robert J.; Cohan, Daniel S.; Wong, Michael S.
Toluene is one of the most prevalent aromatic volatile organic compounds (VOCs) in the atmosphere and has large secondary organic aerosol (SOA) yields compared to many other aromatic VOCs. Recent photo-oxidation studies highlight that toluene oxidation produces more SOA than observed previously, particularly at low levels of nitrogen oxides (NOx). This study focuses on: 1.) the development of a gas-phase chemical mechanism describing toluene oxidation by hydroxyl radicals (OH); 2.) the prediction of SOA formation from toluene oxidation products; and 3.) the impact of NOx level on SOA formation. The oxidation mechanism, which includes multiple pathways after the initial OH attack, has been incorporated into the Caltech Atmospheric Chemistry Mechanism (CACM). Toluene concentrations simulated in chamber experiments by the updated CACM as a function of time are typically within 5% of observed values for most experiments. Predicted ozone and NO2 concentrations are typically within 15% of the experimental values. The gas-phase mechanism indicates the importance of bicyclic peroxy radical reactions in determining the product distribution and thus the likelihood of SOA formation. A gas-aerosol partitioning model is used in conjunction with the gas-phase mechanism to simulate SOA formation. Predicted SOA concentrations are typically within 15% of the experimental values. Under low NOx conditions, simulation shows that more than 98% of SOA mass is contributed by bicyclic products from reactions between bicyclic peroxy radicals and other peroxy radicals. Increasing NOx levels cause bicyclic peroxy radicals to react with NO or nitrate radical, leading to fragmentation products that are less likely to form SOA. SOA yield dropped from 19.26% with zero initial NOx to 13.27% with 100 ppb initial NO because of the change in the amount of toluene consumed. Composition of NOx also has an impact on SOA yield and formation, showing that NO has a greater impact on SOA yield and formation than NO2.
CW DFB-QCL and EC-QCL based sensor for simultaneous NO and NO2 measurements via frequency modulation multiplexing using multi-pass absorption spectroscopy
(SPIE, 2017) Yu, Yajun; Sanchez, Nancy P.; Lou, Minhan; Zheng, Chuantao; Wu, Hongpeng; Głuszek, Aleksander K.; Hudzikowski, Arkadiusz J.; Griffin, Robert J.; Tittel, Frank K.
Nitrogen oxides (NOx), including nitric oxide (NO) and nitrogen dioxide (NO2) play important roles in determining the photochemistry of the ambient atmosphere, controlling the production of tropospheric ozone, affecting the concentration levels of the hydroxyl radical, and forming acid precipitation. A sensor system capable of simultaneous measurements of NO and NO2 by using a commercial 76 m astigmatic multi-pass gas cell (MPGC) was developed in order to enable fast-response NOx detection. A continuous wave (CW), distributed-feedback (DFB) quantum cascade laser (QCL) and a CW external-cavity (EC) QCL were employed for targeting a NO absorption doublet at 1900.075 cm-1 and a NO2 absorption line at 1630.33 cm-1, respectively. Both laser beams were combined and transmitted through the MPGC in an identical optical path and subsequently detected by a single mid-infrared detector. A frequency modulation multiplexing scheme was implemented by modulating the DFB-QCL and EC-QCL at different frequencies and demodulating the detector signal with two Labview software based lock-in amplifiers to extract the corresponding second-harmonic (2f) components. Continuous monitoring of NO and NO2 concentration levels was achieved by locking the laser frequencies to the selected absorption lines utilizing a reference cell filled with high concentrations of NO and NO2. The experimental results indicate minor performance degradation associated with frequency modulation multiplexing and no cross talk between the two multiplexed detection channels. The performance of the reported sensor system was evaluated for real time, sensitive and precise detection of NO and NO2 simultaneously.
CW EC-QCL-based sensor for simultaneous detection of H2O, HDO, N2O and CH4ﾠusing multi-pass absorption spectroscopy
(The Optical Society, 2016) Yu, Yajun; Sanchez, Nancy P.; Griffin, Robert J.; Tittel, Frank K.
A sensor system based on a continuous wave, external-cavity quantum-cascade laser (CW EC-QCL) was demonstrated for simultaneous detection of atmospheric H2O, HDO, N2O and CH4using a compact, dense pattern multi-pass gas cell with an effective path-length of 57.6 m. The EC-QCL with a mode-hop-free spectral range of 1225-1285 cm−1 operating at ~7.8 µm was scanned covering four neighboring absorption lines, for H2O at 1281.161 cm−1, HDO at 1281.455 cm−1, N2O at 1281.53 cm−1 and CH4 at 1281.61 cm−1. A first-harmonic-normalized wavelength modulation spectroscopy with second-harmonic detection (WMS-2f/1f) strategy was employed for data processing. An Allan-Werle deviation analysis indicated that minimum detection limits of 1.77 ppmv for H2O, 3.92 ppbv for HDO, 1.43 ppbv for N2O, and 2.2 ppbv for CH4 were achieved with integration times of 50-s, 50-s, 100-s and 129-s, respectively. Experimental measurements of ambient air are also reported.
Detecting plumes in mobile air quality monitoring time series with density-based spatial clustering of applications with noise
(Copernicus Publications, 2023) Actkinson, Blake; Griffin, Robert J.
Mobile monitoring is becoming an increasingly popular technique to assess air pollution on fine spatial scales, but methods to determine specific source contributions to measured pollutants are sorely needed. One approach is to isolate plumes from mobile monitoring time series and analyze them separately, but methods that are suitable for large mobile monitoring time series are lacking. Here we discuss a novel method used to detect and isolate plumes from an extensive mobile monitoring data set. The new method relies on density-based spatial clustering of applications with noise (DBSCAN), an unsupervised machine learning technique. The new method systematically runs DBSCAN on mobile monitoring time series by day and identifies a subset of points as anomalies for further analysis. When applied to a mobile monitoring data set collected in Houston, Texas, analyzed anomalies reveal patterns associated with different types of vehicle emission profiles. We observe spatial differences in these patterns and reveal striking disparities by census tract. These results can be used to inform stakeholders of spatial variations in emission profiles not obvious using data from stationary monitors alone.
Development of aroCACM/MPMPO 1.0: a model to simulate secondary organic aerosol from aromatic precursors in regional models
(Copernicus Publications, 2016) Dawson, Matthew L.; Xu, Jialu; Griffin, Robert J.; Dabdub, Donald
The atmospheric oxidation of aromatic compounds is an important source of secondary organic aerosol (SOA) in urban areas. The oxidation of aromatics depends strongly on the levels of nitrogen oxides (NOx). However, details of the mechanisms by which oxidation occurs have only recently been elucidated. Xu et al. (2015) developed an updated version of the gas-phase Caltech Atmospheric Chemistry Mechanism (CACM) designed to simulate toluene and m-xylene oxidation in chamber experiments over a range of NOx conditions. The output from such a mechanism can be used in thermodynamic predictions of gas–particle partitioning leading to SOA. The current work reports the development of a model for SOA formation that combines the gas-phase mechanism of Xu et al. (2015) with an updated lumped SOA-partitioning scheme (Model to Predict the Multi-phase Partitioning of Organics, MPMPO) that allows partitioning to multiple aerosol phases and that is designed for use in larger-scale three-dimensional models. The resulting model is termed aroCACM/MPMPO 1.0. The model is integrated into the University of California, Irvine – California Institute of Technology (UCI-CIT) Airshed Model, which simulates the South Coast Air Basin (SoCAB) of California. Simulations using 2012 emissions indicate that “low-NOx” pathways to SOA formation from aromatic oxidation play an important role, even in regions that typically exhibit high-NOx concentrations.
Differences in BVOC oxidation and SOA formation above and below the forest canopy
(Copernicus Publications on behalf of the European Geosciences Union, 2017) Schulze, Benjamin C.; Wallace, Henry W.; Flynn, James H.; Lefer, Barry L.; Erickson, Matt H.; Jobson, B. Tom; Dusanter, Sebastien; Griffith, Stephen M.; Hansen, Robert F.; Stevens, Philip S.; VanReken, Timothy; Griffin, Robert J.
Gas-phase biogenic volatile organic compounds (BVOCs) are oxidized in the troposphere to produce secondary pollutants such as ozone (O3), organic nitrates (RONO2), and secondary organic aerosol (SOA). Two coupled zero-dimensional models have been used to investigate differences in oxidation and SOA production from isoprene and α-pinene, especially with respect to the nitrate radical (NO3), above and below a forest canopy in rural Michigan. In both modeled environments (above and below the canopy), NO3 mixing ratios are relatively small (< 0.5 pptv); however, daytime (08:00–20:00 LT) mixing ratios below the canopy are 2 to 3 times larger than those above. As a result of this difference, NO3 contributes 12 % of total daytime α-pinene oxidation below the canopy while only contributing 4 % above. Increasing background pollutant levels to simulate a more polluted suburban or peri-urban forest environment increases the average contribution of NO3 to daytime below-canopy α-pinene oxidation to 32 %. Gas-phase RONO2 produced through NO3 oxidation undergoes net transport upward from the below-canopy environment during the day, and this transport contributes up to 30 % of total NO3-derived RONO2 production above the canopy in the morning (∼ 07:00). Modeled SOA mass loadings above and below the canopy ultimately differ by less than 0.5 µg m−3, and extremely low-volatility organic compounds dominate SOA composition. Lower temperatures below the canopy cause increased partitioning of semi-volatile gas-phase products to the particle phase and up to 35 % larger SOA mass loadings of these products relative to above the canopy in the model. Including transport between above- and below-canopy environments increases above-canopy NO3-derived α-pinene RONO2 SOA mass by as much as 45 %, suggesting that below-canopy chemical processes substantially influence above-canopy SOA mass loadings, especially with regard to monoterpene-derived RONO2.
Disparities in air quality downscaler model uncertainty across socioeconomic and demographic indicators in North Carolina
(Elsevier, 2022) Zhou, Shan; Griffin, Robert J.; Bui, Alexander; Lilienfeld Asbun, Aaron; Bravo, Mercedes A.; Osgood, Claire; Miranda, Marie Lynn
Studies increasingly use output from the Environmental Protection Agency's Fused Air Quality Surface Downscaler (“downscaler”) model, which provides spatial predictions of daily concentrations of fine particulate matter (PM2.5) and ozone (O3) at the census tract level, to study the health and societal impacts of exposure to air pollution. Downscaler outputs have been used to show that lower income and higher minority neighborhoods are exposed to higher levels of PM2.5 and lower levels of O3. However, the uncertainty of the downscaler estimates remains poorly characterized, and it is not known if all subpopulations are benefiting equally from reliable predictions. We examined how the percent errors (PEs) of daily concentrations of PM2.5 and O3 between 2002 and 2016 at the 2010 census tract centroids across North Carolina were associated with measures of racial and educational isolation, neighborhood disadvantage, and urbanicity. Results suggest that there were socioeconomic and demographic disparities in surface concentrations of PM2.5 and O3, as well as their prediction uncertainties. Neighborhoods characterized by less reliable downscaler predictions (i.e., higher PEPM2.5 and PEO3) exhibited greater levels of aerial deprivation as well as educational isolation, and were often non-urban areas (i.e., suburban, or rural). Between 2002 and 2016, predicted PM2.5 and O3 levels decreased and O3 predictions became more reliable. However, the predictive uncertainty for PM2.5 has increased since 2010. Substantial spatial variability was observed in the temporal changes in the predictive uncertainties; educational isolation and neighborhood deprivation levels were associated with smaller increases in predictive uncertainty of PM2.5. In contrast, racial isolation was associated with a greater decline in the reliability of PM2.5 predictions between 2002 and 2016; it was associated with a greater improvement in the predictive reliability of O3 within the same time frame.

Browsing by Author "Griffin, Robert J."

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