X. Ren

University of Miami, كورال غيبلز، فلوريدا, Florida, United States

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Publications (64)106.94 Total impact

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    ABSTRACT: The San Joaquin Valley (SJV) experiences some of the worst ozone air quality in the US, frequently exceeding the California 8 h standard of 70.4 ppb. To improve our understanding of trends in the number of ozone violations in the SJV, we analyze observed relationships between organic reactivity, nitrogen oxides (NOx), and daily maximum temperature in the southern SJV using measurements made as part of California at the Nexus of Air Quality and Climate Change in 2010 (CalNex-SJV). We find the daytime speciated organic reactivity with respect to OH during CalNex-SJV has a temperature-independent portion with molecules typically associated with motor vehicles being the major component. At high temperatures, characteristic of days with high ozone, the largest portion of the total organic reactivity increases exponentially with temperature and is dominated by small, oxygenated organics and molecules that are unidentified. We use this simple temperature classification to consider changes in organic emissions over the last and next decade. With the CalNex-SJV observations as constraints, we examine the sensitivity of ozone production (PO3) to future NOx and organic reactivity controls. We find that PO3 is NOx-limited at all temperatures on weekends and on weekdays when daily maximum temperatures are greater than 29 °C. As a~consequence, NOx reductions are the most effective control option for reducing the frequency of future ozone violations in the southern SJV.
    Atmospheric Chemistry and Physics 11/2013; 13(11):28511-28560. · 4.88 Impact Factor
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    ABSTRACT: The understanding of oxidation in forest atmospheres is being challenged by measurements of unexpectedly large amounts of hydroxyl (OH). A significant number of these OH measurements were made by laser-induced fluorescence in low-pressure detection chambers (called Fluorescence Assay with Gas Expansion (FAGE)) using the Penn State Ground-based Tropospheric Hydrogen Oxides Sensor (GTHOS). We deployed a new chemical removal method to measure OH in parallel with the traditional FAGE method in a California forest. The new method gives on average only 40-60% of the OH from the traditional method and this discrepancy is temperature dependent. Evidence indicates that the new method measures atmospheric OH while the traditional method is affected by internally generated OH, possibly from oxidation of biogenic volatile organic compounds. The improved agreement between OH measured by this new technique and modeled OH suggests that oxidation chemistry in at least one forest atmosphere is better understood than previously thought.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 09/2012; 12(17):8009-8020. · 5.51 Impact Factor
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    ABSTRACT: Observations of chemical constituents and meteorological quantities obtained during the two Arctic phases of the airborne campaign ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites) are analyzed using an observationally constrained steady state box model. Measurements of OH and HO2 from the Penn State ATHOS instrument are compared to model predictions. Forty percent of OH measurements below 2 km are at the limit of detection during the spring phase (ARCTAS-A). While the median observed-to-calculated ratio is near one, both the scatter of observations and the model uncertainty for OH are at the magnitude of ambient values. During the summer phase (ARCTAS-B), model predictions of OH are biased low relative to observations and demonstrate a high sensitivity to the level of uncertainty in NO observations. Predictions of HO2 using observed CH2O and H2O2 as model constraints are up to a factor of two larger than observed. A temperature-dependent terminal loss rate of HO2 to aerosol recently proposed in the literature is shown to be insufficient to reconcile these differences. A comparison of ARCTAS-A to the high latitude springtime portion of the 2000 TOPSE campaign (Tropospheric Ozone Production about the Spring Equinox) shows similar meteorological and chemical environments with the exception of peroxides; observations of H2O2 during ARCTAS-A were 2.5 to 3 times larger than those during TOPSE. The cause of this difference in peroxides remains unresolved and has important implications for the Arctic HOx budget. Unconstrained model predictions for both phases indicate photochemistry alone is unable to simultaneously sustain observed levels of CH2O and H2O2; however when the model is constrained with observed CH2O, H2O2 predictions from a range of rainout parameterizations bracket its observations. A mechanism suitable to explain observed concentrations of CH2O is uncertain. Free tropospheric observations of acetaldehyde (CH3CHO) are 2-3 times larger than its predictions, though constraint of the model to those observations is sufficient to account for less than half of the deficit in predicted CH2O. The box model calculates gross O3 formation during spring to maximize from 1-4 km at 0.8 ppbv d-1, in agreement with estimates from TOPSE, and a gross production of 2-4 ppbv d-1 in the boundary layer and upper troposphere during summer. Use of the lower observed levels of HO2 in place of model predictions decreases the gross production by 25-50%. Net O3 production is near zero throughout the ARCTAS-A troposphere, and is 1-2 ppbv in the boundary layer and upper altitudes during ARCTAS-B.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 08/2012; 12(15):6799-6825. · 5.51 Impact Factor
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    ABSTRACT: The hydroxyl (OH) and hydroperoxyl (HO2) radicals, collectively called HOx, play central roles in tropospheric chemistry. Accurate measurements of OH and HO2 are critical to examine our understanding of atmospheric chemistry. Intercomparisons of different techniques for detecting OH and HO2 are vital to evaluate their measurement capabilities. Three instruments that measured OH and/or HO2 radicals were deployed on the NASA DC-8 aircraft throughout Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) in the spring and summer of 2008. One instrument was the Penn State Airborne Tropospheric Hydrogen Oxides Sensor (ATHOS) for OH and HO2 measurements based on Laser-Induced Fluorescence (LIF) spectroscopy. A second instrument was the NCAR Selected-Ion Chemical Ionization Mass Spectrometer (SI-CIMS) for OH measurement. A third instrument was the NCAR Peroxy Radical Chemical Ionization Mass Spectrometer (PeRCIMS) for HO2 measurement. Formal intercomparison of LIF and CIMS was conducted for the first time on a same aircraft platform. The three instruments were calibrated by quantitative photolysis of water vapor by ultraviolet (UV) light at 184.9 nm with three different calibration systems. The absolute accuracies were ±32% (2σ) for the LIF instrument, ±65% (2σ) for the SI-CIMS instrument, and ±50% (2σ) for the PeRCIMS instrument. In general, good agreement was obtained between the CIMS and LIF measurements of both OH and HO2 measurements. Linear regression of the entire data set yields [OH]CIMS = 0.89 × [OH]LIF + 2.8 × 104 cm-3 with a correlation coefficient r2 = 0.72 for OH, and [HO2]CIMS = 0.86 × [HO2]LIF + 3.9 parts per trillion by volume (pptv, equivalent to pmol mol-1) with a correlation coefficient r2 = 0.72 for HO2. In general, the difference between CIMS and LIF instruments for OH and HO2 measurements can be explained by their combined measurement uncertainties. Comparison with box model results shows some similarities for both the CIMS and LIF measurements. First, the observed-to-modeled HO2 ratio increases greatly for higher NO mixing ratios, indicating that the model may not properly account for HOx sources that correlate with NO. Second, the observed-to-modeled OH ratio increases with increasing isoprene mixing ratios, suggesting either incomplete understanding of isoprene chemistry in the model or interferences in the measurements in environments where biogenic emissions dominate ambient volatile organic compounds.
    Atmospheric Measurement Techniques 08/2012; 5(8):2025-2037. · 3.21 Impact Factor
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    ABSTRACT: Observations of chemical constituents and meteorological quantities obtained during the two Arctic phases of the airborne campaign ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites) are analyzed using an observationally constrained steady state box model. Measurements of OH and HO2 from the Penn State ATHOS instrument are compared to model predictions. Forty percent of OH measurements below 2 km are at the limit of detection during the spring phase (ARCTAS-A). While the median observed-to-calculated ratio is near one, both the scatter of observations and the model uncertainty for OH are at the magnitude of ambient values. During the summer phase (ARCTAS-B), model predictions of OH are biased low relative to observations and demonstrate a high sensitivity to the level of uncertainty in NO observations. Predictions of HO2 using observed CH2O and H2O2 as model constraints are up to a factor of two larger than observed. A temperature-dependent terminal loss rate of HO2 to aerosol recently proposed in the literature is shown to be insufficient to reconcile these differences. A comparison of ARCTAS-A to the high latitude springtime portion of the 2000 TOPSE campaign (Tropospheric Ozone Production about the Spring Equinox) shows similar meteorological and chemical environments with the exception of peroxides; observations of H2O2 during ARCTAS-A were 2.5 to 3 times larger than those during TOPSE. The cause of this difference in peroxides remains unresolved and has important implications for the Arctic HOx budget. Unconstrained model predictions for both phases indicate photochemistry alone is unable to simultaneously sustain observed levels of CH2O and H2O2; however when the model is constrained with observed CH2O, H2O2 predictions from a range of rainout parameterizations bracket its observations. A mechanism suitable to explain observed concentrations of CH2O is uncertain. Free tropospheric observations of acetaldehyde (CH3CHO) are 2-3 times larger than its predictions, though constraint of the model to those observations is sufficient to account for less than half of the deficit in predicted CH2O. The box model calculates gross O3 formation during spring to maximize from 1-4 km at 0.8 ppbv d-1, in agreement with estimates from TOPSE, and a gross production of 2-4 ppbv d-1 in the boundary layer and upper troposphere during summer. Use of the lower observed levels of HO2 in place of model predictions decreases the gross production by 25-50%. Net O3 production is near zero throughout the ARCTAS-A troposphere, and is 1-2 ppbv in the boundary layer and upper altitudes during ARCTAS-B.
    Atmospheric Chemistry and Physics 04/2012; 12(4):9377-9450. · 4.88 Impact Factor
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    ABSTRACT: The understanding of oxidation in forest atmospheres is being challenged by measurements of unexpectedly large amounts of hydroxyl (OH). A significant number of these OH measurements were made by laser-induced fluorescence in low-pressure detection chambers (called Fluorescence Assay with Gas Expansion (FAGE)) using the Penn State Ground-based Tropospheric Hydrogen Oxides Sensor (GTHOS). We deployed a new chemical removal method to measure OH in parallel with the traditional FAGE method. The new method gives on average only 40-50% of the OH from the traditional method and this discrepancy is temperature-dependent. Evidence indicates that the new method measures atmospheric OH while the traditional method is affected by internally generated OH, possibly from oxidation of biogenic volatile organic compounds. The agreement between OH measured by this new technique and modeled OH suggests that oxidation chemistry in at least one forest atmosphere is better understood than previously thought.
    Atmospheric Chemistry and Physics 03/2012; 12(3):6715-6744. · 4.88 Impact Factor
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    ABSTRACT: The hydroxyl (OH) and hydroperoxyl (HO2) radicals, collectively called HOx, play central roles in tropospheric chemistry. Accurate measurements of OH and HO2 are critical to examine our understanding of atmospheric chemistry. Intercomparisons of different techniques for detecting OH and HO2 are vital to evaluate their measurement capabilities. Three instruments that measured OH and/or HO2 radicals were deployed on the NASA DC-8 aircraft throughout Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS), in the spring and summer of 2008. One instrument was the Penn State Airborne Tropospheric Hydrogen Oxides Sensor (ATHOS) for OH and HO2 measurements based on Laser-Induced Fluorescence (LIF) spectroscopy. A second instrument was the NCAR Selected-Ion Chemical Ionization Mass Spectrometer (SI-CIMS) for OH measurement. A third instrument was the NCAR Peroxy Radical Chemical Ionization Mass Spectrometer (PeRCIMS) for HO2 measurement. Formal intercomparison of LIF and CIMS was conducted for the first time on a same aircraft platform. The three instruments were calibrated by quantitative photolysis of water vapor by UV light at 184.9 nm with three different calibration systems. The absolute accuracies were ±32% (2σ) for the LIF instrument, ±65% (2σ) for the SI-CIMS instrument, and ±50% (2σ) for the PeRCIMS instrument. In general, good agreement was obtained between the CIMS and LIF measurements of both OH and HO2 measurements. Linear regression of the entire data set yields [OH]CIMS = 0.89 × [OH]LIF + 2.8 × 105 cm-3 with a correlation coefficient, r2 = 0.72 for OH and [HO2]CIMS = 0.86 × [HO2]LIF + 3.9 parts per trillion by volume (pptv, equivalent to pmol mol-1) with a correlation coefficient, r2 = 0.72 for HO2. In general, the difference between CIMS and LIF instruments for OH and HO2 measurements can be explained by their combined measurement uncertainties. Comparison with box model results shows some similarities for both the CIMS and LIF measurements. First, the observed-to-modeled HO2 ratio increases greatly for higher NO mixing ratios, indicating that the model may not properly account for HOx sources that correlate with NO. Second, the observed-to-modeled OH ratio increases with increasing isoprene mixing ratios, suggesting either incomplete understanding of isoprene chemistry in the model or interferences in the measurements in environments where biogenic emissions dominate ambient volatile organic compounds.
    Atmospheric Measurement Techniques Discussions. 03/2012; 5(2):2529-2565.
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    ABSTRACT: We investigate the impact of NOx reductions on ozone production in the Southern San Joaquin Valley using a large suite of radical and trace gas measurements collected during CalNex-2010 in Bakersfield, California (May 15-June 28) combined with the historical record of O3, nitrogen oxides and temperature from CARB monitoring sites in the region. We calculate the instantaneous ozone production rate (PO3) by radical balance and investigate relationships between PO3 and NOx abundance; finding temperature to be a useful proxy for VOC reactivity. We show Bakersfield photochemistry is at peak PO3 and therefore at a minimum with respect to the effectiveness of NOx controls indicating: (1) more than 30% reductions from present day are required before sizable decreases in ozone will occur and (2) reduction from the lower weekend baseline NOx concentrations will result in weekend PO3 decreases with continued NOx controls at high temperatures when VOC reactivity is largest.
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: Aircraft observations of constituents and meteorological quantities observed during the two seasonal Arctic phases of ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites) and during the 2000 TOPSE (Tropospheric Ozone Production About the Spring Equinox) are analyzed using an observationally-constrained steady state box model. An examination of the springtime Arctic portion of the 2000 TOPSE program shows a highly similar meteorological background and chemical composition relative to ARCTAS-A, with the exception of peroxides. Concentrations of H2O2 observed during ARCTAS-A were 2-3 times larger than those during TOPSE. The cause of this discrepancy is unresolved, and it will be shown to have important implications for conclusions related to the Arctic HOx budget. Measurements of HOx from the Penn State ATHOS instrument are available during ARCTAS and are compared to box model predictions. Model predictions show striking inconsistencies during both phases of ARCTAS between observed concentrations of HO2 and of HOx precursors, primarily H2O2 and CH2O. Using observations of precursors in the box model results in predictions of HO2 that are up to nearly a factor of 2 larger than observed. An estimated temperature-dependent terminal loss rate of HO2 to aerosol [Mao et al., 2010] was shown to be insufficient to reconcile model predictions and observations of HO2. When the terminal losses from GEOS-Chem are directly inserted into the fully constrained boxmodel, predictions of upper tropospheric HO2 decrease by no more than 15-25%. Steady state predictions of upper tropospheric CH2O are lower than observations by factors of 2-4 during both phases of ARCTAS. Likewise, steady state predictions of H2O2 are lower than observations by factors of 2-3, and are similar to concentrations measured during TOPSE. Global models suggest that there is an important transport component to the Arctic H2O2 budget not captured by steady state models. An examination of back-trajectories and observations from ARCTAS for in-situ evidence of transport on Arctic peroxide concentrations does not find a widespread persistent signal from transport, although short-lived transient features are evident. Additional possible explanations for the inconsistencies between HO2 and precursor observations are explored. A comparison of calculated and observed OH during the spring (ARCTAS-A) phase shows that median observed-to-calculated ratios are near one, but have large scatter. 40% of OH measurements below 2 km were at the limit of detection (LOD) during the spring, and analysis indicates that the scatter of raw observations at these very low concentrations is larger than the ambient variability of OH, limiting the practicality of further analysis such as finding observational evidence of the impact of halogens on the HO2/OH ratio. Alternately, during ARCTAS-B, model predictions of OH were persistently lower than observations.
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: CalNex 2010 provided the opportunity to study the oxidation photochemistry of Bakersfield CA, an area known for its substantial air quality problems. During the six-week (May 15 - June 28) field campaign hydroxyl, hydroperoxyl, and OH reactivity were measured. OH was measured by two methods: the usual method of alternating the laser wavelength on and off the OH absorption (called OHwave) and a second method of periodically adding an OH reactant (called OHchem), akin to the method used by the chemical ionization/mass spectrometry technique. During the day, OHchem was .85 of OH wave and about 5x106 cm-3. However at night, OHchem was 0.5 of OHwave and about 5x105 cm-3. Laboratory studies indicate that OHchem is actually OH and that OH wave includes an interference that has not been identified. Hydroperyxl (HO2) was typically 10-12 pptv during the day and a few pptv at night, lower than in other urban areas. OH reactivity was typically less than 10 s-1 during the day, but rose to 10 s-1 at night. In a cold, rainy period early in the campaign, OH reactivity was about 5 s-1 with little diurnal variation, about as low as we have ever observed in an urban area. These measurements will be compared to a model to test the understanding of the oxidation photochemistry in this region.
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: Detailed comparisons of airborne CH2O measurements acquired by tunable diode laser absorption spectroscopy with steady state box model calculations were carried out using data from the 2006 INTEX-B campaign in order to improve our understanding of hydrocarbon oxidation processing. Select previous comparisons in other campaigns have highlighted some locations in the boundary layer where steady state box models have tended to underpredict CH2O, suggesting that standard steady state modeling assumptions might be unsuitable under these conditions, and pointing to a possible role for unmeasured hydrocarbons and/or additional primary emission sources of CH2O. Employing an improved instrument, more detailed measurement-model comparisons with better temporal overlap, up to date measurement and model precision estimates, up to date rate constants, and additional modeling tools based on both Lagrangian and Master Chemical Mechanism (MCM) runs, we have explained much of the disagreement between observed and predicted CH2O as resulting from non-steady-state atmospheric conditions in the vicinity of large pollution sources, and have quantified the disagreement as a function of plume lifetime (processing time). We show that in the near-field (within ~4 to 6 h of the source), steady-state models can either over-or-underestimate observations, depending on the predominant non-steady-state influence. In addition, we show that even far field processes (10-40 h) can be influenced by non-steady-state conditions which can be responsible for CH2O model underestimations by as much as a factor of 2. At the longer processing times in the 10 to 40 h range during Mexico City outflow events, MCM model calculations, using assumptions about emissions of high-order NMHCs, further indicate the potential importance of CH2O produced from unmeasured and multi-generation hydrocarbon oxidation processing, particularly methylglyoxal and 3-hyroxypropanal.
    Atmospheric Chemistry and Physics 03/2011; 11(3):9887-9957. · 4.88 Impact Factor
  • Atmospheric Chemistry and Physics 01/2011; 11(22):11867-11894. · 5.51 Impact Factor
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    ABSTRACT: Nitrous Acid (HONO) plays an important role in tropospheric chemistry as a precursor of the hydroxyl radical (OH), the most important oxidizing agent in the atmosphere. Nevertheless, the formation mechanisms of HONO are still not completely understood. Recent field observations found unexpectedly high daytime HONO concentrations in both urban and rural areas, which point to unrecognized, most likely photolytically enhanced HONO sources. Several gas-phase, aerosol, and ground surface chemistry mechanisms have been proposed to explain elevated daytime HONO, but atmospheric evidence to favor one over the others is still weak. New information on whether the HONO formation occurs in the gas-phase, on aerosol, or at the ground may be derived from observations of the vertical distribution of HONO and its precursor nitrogen dioxide, NO2, as well as its dependence on solar radiation or actinic flux. Here we present field observations of HONO, NO2 and other trace gases in three altitude intervals (30-70 m, 70-130 m and 130-300 m) using UCLA's long path DOAS instrument, as well as in situ measurements of OH, NO, photolysis frequencies and solar irradiance, made in Houston, TX, during the Study of Houston Atmospheric Radical Precursor (SHARP) experiment from 20 April to 30 May 2009. The observed HONO mixing ratios were often ten times larger than the expected photostationary state with OH and NO. Larger HONO mixing ratios observed near the ground than aloft, imply, but do not clearly prove, that the daytime source of HONO was located at or near the ground. Using a pseudo steady-state (PSS) approach, we calculated the missing daytime HONO formation rates, Punknown, on four sunny days. The NO2-normalized Punknown, Pnorm, showed a clear symmetrical diurnal variation with a maximum around noontime, which was well correlated with actinic flux (NO2 photolysis) and solar irradiance. This behavior, which was found on all clear days in Houston, is a strong indication of a photolytic HONO source. [HONO]/[NO2] ratios also showed a clear diurnal profile with maxima of 2-3 % around noon. PSS calculations show that this behavior cannot be explained by the proposed NO2-->NO2* photolysis or any other gas-phase or aerosol photolytic process occurring at similar or longer wavelengths than that of HONO photolysis. HONO formation by aerosol nitrate photolysis in the UV also seems to be unlikely. Pnorm correlated better with solar irradiance (average R2 = 0.85/0.87 for visible/UV) than with actinic flux (R2 = 0.76) on the four sunny days, clearly pointing to a HONO formation at the ground rather than the aerosol or the gas-phase. In addition, the observed [HONO]/[NO2] diurnal variation can be explained if the formation of HONO depends on solar irradiance but not if it depends on the actinic flux. The vertical mixing ratio profiles together with the stronger correlation of solar irradiance vs. actinic flux support the idea that photolytically enhanced NO2 to HONO conversion on the ground was the dominant source of HONO in Houston.
    Atmospheric Chemistry and Physics 01/2011; 11:24365-24411. · 4.88 Impact Factor
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    ABSTRACT: Ozone production rates, P(O3), were measured directly using the Penn State Measurement of Ozone Production Sensor (MOPS) during the Study of Houston Atmospheric Radical Precursors (SHARP 2009). Measured P(O3) peaked in the late morning, with values between 15 ppbv h-1 and 100 ppbv h-1, although values of 40-80 ppbv h-1 were typical for higher ozone days. These measurements were compared against ozone production rates calculated using measurements of hydroperoxyl (HO2), hydroxyl (OH), and nitric oxide (NO) radicals, called "calculated P(O3)". The same comparison was done using modeled radicals obtained from a box model with the RACM2 mechanism, called "modeled P(O3)". Measured and calculated P(O3) had similar peak values but the calculated P(O3) tended to peak earlier in the morning when NO values were higher. Measured and modeled P(O3) had a similar dependence on NO, but the modeled P(O3) was only half the measured P(O3). This difference indicates possible missing radical sources in the box model with the RACM2 mechanism and thus has implications for the ability of air quality models to accurately predict ozone production rates.
    Atmospheric Chemistry and Physics 01/2011; 11:27521-27546. · 4.88 Impact Factor
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    ABSTRACT: A relaxed eddy accumulation (REA) system combined with a nitrous acid (HONO) analyzer was developed to measure atmosperhic HONO vertical fluxes. The system consists of three major components: (1) a fast-response sonic anemometer measuring vertical wind velocity and air temperature, (2) a fast-response controlling unit separating air motions into updraft and downdraft samplers by the sign of vertical wind velocity, and (3) a highly sensitive HONO analyzer based on aqueous long path absorption photometry measuring HONO concentations in these updrafts and downdrafts. A dynamic velocity threshold (±0.5sigmaw, where sigmaw is a standard deviation of the vertical wind velocity) was used for valve switching determined by the running means and standard deviations of the vertical wind velocity. Using measured temperature as a tracer and the average values from two field deployments, the flux proportionality coefficient, beta, was determined to be 0.42 ± 0.02, in good agreement with the theoretical estimation. The REA system was deployed in two ground-based field studies. In the California Research at the Nexus of Air Quality and Climate Change (CalNex) study in Bakersfield, California in summer 2010, measured HONO fluxes appeared to be upward during the day and were close to zero at night. The upward HONO flux was highly correlated to the product of NO2 and solar radiation. During the Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX 2009) at Blodgett Forest, California in July 2009, the overall HONO fluxes were small in magnitude and were close to zero. Causes for the differences in HONO fluxes in the two different environments are briefly discussed.
    Atmospheric Measurement Techniques Discussions. 01/2011;
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    ABSTRACT: Rates of ozone production were measured directly during the Study of Houston Atmospheric Radical Precursors (SHARP) with a new Penn State instrument. The set of data contains low ozone production days and high ozone production days that developed under different meteorological conditions. Among the data collected, there are three verified episodes of high ozone in Houston. The large amount of VOCs combined with NOx emissions from point and mobile sources intensify the production of ozone in this urban center. Interesting correlations between measured ozone production and ambient nitrogen oxides have been found and will be presented. In addition, relationships between known organic radical precursors and measured ozone production will be analyzed. This discussion will be performed within the context of the specific meteorological conditions that favored the accumulation of ozone in Houston in the month of May, 2009.
    AGU Fall Meeting Abstracts. 12/2010;
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    ABSTRACT: The 2010 California Research at the Nexus of Air Quality and Climate Change (CalNex) field campaign supersite in Bakersfield, CA, presented an opportunity to investigate the gas-particle partitioning of trace atmospheric species in a low-sulphur high-NH3 environment. With the demand for cleaner energy, atmospheric S levels are expected to continue to decrease across North America, while restrictions on NH3 emissions are still in preliminary stages. Despite the absence of significant sulphur emissions contributing to particulate matter (PM) formation, California's San Joaquin Valley still experiences some of the worst air quality in the continental US. Partitioning of other trace gases may become more important in low sulphur regions; here we report our observations of HONO, HCl and oxalic acid partitioning. Observations of the water-soluble composition of atmospheric gases and fine particulate matter (PM2.5) were made with an Ambient Ion Monitor - Ion Chromatography (AIM-IC) system from University Research Glassware (Chapel Hill, NC). The AIM-IC was fitted with a newly designed impactor inlet that minimizes gas and particle losses prior to collection by a wet wall parallel plate denuder and super saturated steam condensation chamber, respectively, situated at a height of 4.5 m for this study. Observations of NH3 indicated ambient levels typically in excess of 10 ppb, up to 60 ppb, and all observed inorganic aerosol was found to be completely neutralized throughout the campaign by NH4+. Mixing ratios of HONO showed excellent agreement with concurrent measurements made by a LOPAP instrument from University of Miami, maximizing during the night at values in the range of 1 - 1.5 ppb. On several nights, particulate NO2- mass loadings in the range of 0.2 mug m-3 (equivalent to 0.1 ppb HONO) were observed by the AIM-IC. The diurnal pattern of HCl showed maximum mixing ratios of 0.2 - 0.3 ppb occurring near 13:00 PST, and near zero values at night. Intermittent bursts of Cl- were also detected, however the Cl- present in PM2.5 was significantly less than that present as HCl. These online observations are some of the few to date describing simultaneous measurements of both gas and particle phase reservoirs of chlorine as HCl/Cl-. The diurnal pattern of oxalic acid showed maximum mixing ratios less than 0.1 ppb near 16:00 PST and near zero values at night. The presence of oxalate in PM2.5 was observed throughout the campaign with mass loadings in the range of 0 - 0.2 mug m-3 also maximizing during the day.
    AGU Fall Meeting Abstracts. 12/2010;
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    ABSTRACT: Approximately 3-20% of NOy in the flame front of biomass burning, as determined from field and lab studies, is HONO. Because HONO is readily photolyzed, it serves as a short-term reservoir for HOx and NOx. As the HONO is lost to photolysis, it affects NOy partitioning as well as O3 formation/loss. Using data from the NASA DC-8 payload during the summer 2008 ARCTAS campaign for model initialization, we have previously implemented a gas phase chemical model to evaluate the impact of HONO on the chemical evolution of one plume on 1 July 2008. We present results incorporating the ARCTAS particle phase data with the Aerosol Simulation Program and a Lagrangian parcel model to more completely represent the plume evolution.
    AGU Fall Meeting Abstracts. 12/2010;
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    ABSTRACT: The chemistry of atmospheric radicals, especially the hydroxyl radical (OH) and hydroperoxyl radical (HO2), together called HOx, is deeply involved in the formation of the secondary pollutants ozone and fine particles. Radical precursors such as nitrous acid (HONO) and formaldehyde (HCHO) significantly affects HOx budget in urban environments like Houston. These chemical processes connect surface emissions, both human and natural, to local and regional pollution, and climate change. Using the data collected during the Study of Houston Atmospheric Radical Precursors (SHARP) in Houston, TX in spring 2009, we test our understanding of photochemistry through the analysis of the radical budget and ozone production. A numerical box model was used to simulate the oxidation processes and observed OH and HO2 during this study. Using the model results, we calculate the radical budget and analyze the sensitivity of ozone production to nitrogen oxides (NOx) and volatile organic compounds (VOCs). The radical budget shows that the photolysis of HONO and the photolysis of HCHO were significant HOx sources in this urban environment. We also compare the observed OH reactivity and ozone production rate to the model calculations. In general, ozone production rate was VOC limited in the morning and NOx limited in the afternoon. This relationship results from the ratio of VOCs to NOx in Houston. Results from this study provide additional support for regulatory actions to reduce reactive VOCs in Houston in order to reduce ozone and other secondary pollutants.
    AGU Fall Meeting Abstracts. 12/2010;
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    ABSTRACT: CalNex 2010 was an opportunity to study the air chemistry of Bakersfield CA, which has some of the worst air quality in California. During the six-week (May 15 - June 28) field campaign, we measured OH and HO2 by laser-induced fluorescence. OH reactivity, the inverse of the OH lifetime, was also measured. OH measurements were performed two ways - the usual method of alternating the laser wavelength on and off the OH absorption and a second method of periodically adding an OH reactant, akin to the method used by the chemical ionization / mass spectrometry technique. While the OH measurements from these two methods are often very similar, they are not always the same. In addition, OH by both methods is often greater than 105 cm3 at night. There are significant differences in these measurements between a cooler period (May 18 - May 29), when OH, HO2, and OH reactivity were very low, and a warmer period (May 30 - June 28). We discuss these results in the context of OH production and loss and ozone production.
    AGU Fall Meeting Abstracts. 12/2010;

Publication Stats

443 Citations
106.94 Total Impact Points

Institutions

  • 2007–2013
    • University of Miami
      • Rosenstiel School of Marine and Atmospheric Science
      كورال غيبلز، فلوريدا, Florida, United States
  • 2009
    • William Penn University
      Worcester, Massachusetts, United States
  • 2004–2006
    • Pennsylvania State University
      • Department of Meteorology
      State College, PA, United States
    • University of California, Davis
      Davis, California, United States