Spectroscopic Studies on the Photochemical transformation of Tannic Acid as a Model for HULIS in Atmospheric Aerosols
Little is known about the photochemical transformation of humic like substances (HULIS) in atmospheric aerosols, and the role of adsorbed water and photosensitizers in this photochemistry. We present in-situ and surface-sensitive spectroscopic studies on the photochemical transformation of tannic acid in the absence and presence of sodium nitrite salt. Tannic acid was chosen as a synthetic proxy for HULIS because it has a defined molecular structure with elemental composition similar to that determined for atmospheric HULIS. Photochemical studies were conducted using diffuse reflectance infrared spectroscopy (DRIFTS) as a function of time (3 hrs), relative humidity (5-50%) and total irradiance (7-100 Wm-2). DRIFTS has proven to be a powerful tool for studying the heterogeneous chemistry of model components in aerosols. Preliminary spectral results show loss and growth features suggesting bonds breakage and formation of carboxylate or organic nitrogen species. The growth of the band assigned to the bending mode of water with irradiation time was also observed and suggests an increase in the hydrophilicity of tannic acid. The structure of water adsorbed on tannic acid resembles that of water at the interface with organic solvents. The implication of these results to the field of aerosols aging due to photochemistry will be discussed.
Measurements of Total Alkyl and Peroxy Nitrate Abundance by Thermal Dissociation Cavity Ring-Down Spectroscopy (TD-CRDS)
The oxides of nitrogen (NOx = NO + NO2) regulate many atmospheric processes, including production and destruction of tropospheric ozone. Processes that affect NOx abundance are hence of interest. Organic nitrates in the form of peroxy nitrates, RO2NO2, and alkyl nitrates, RONO2, may act as NOx reservoirs but can also act as NOx sinks. Many aspects of organic nitrate chemistry in ambient air remain uncertain, warranting further research. Organic nitrates are frequently quantified by gas chromatography (GC); the main disadvantage of this technique is its low acquisition speed. Recently, high- temporal resolution measurements of total peroxy (ΣPN) and alkyl nitrate (ΣAN) abundances were made by thermal dissociation to NO2, which was quantified by laser-induced fluorescence (TD-LIF). Here, we describe a Thermal Dissociation Cavity Ring-Down Spectrometer (TD-CRDS) for measurements of ΣPN and ΣAN. The thermal dissociation product NO2 is quantified by optical absorption at 532 nm using a Nd:YAG laser pulsed at 20 Hz repetition rate. The organic nitrates are quantified by difference relative to NO2 measured in a reference channel at room temperature. The inlet temperatures for ΣPN and ΣAN were set to 264 C and 480 C, respectively. Under these conditions, conversion of organic nitrates to NO2 was quantitative for a variety of laboratory-generated samples and over a wide range of mixing ratios. The conversion efficiency was verified by simultaneous measurements of NOy (= NOx + NO3 + 2N2O5 + ΣPN + ΣAN + HNO3 + ...) using a commercial NO-O3 chemiluminescence detector. Unlike TD-LIF, TD-CRDS may thus not need to rely on external calibration to quantify organic nitrates. At present, the ΣAN and ΣPN measurement precision of the TD-CRDS is ± 100 pptv (1σ) in a 1 s integration time.
Measurements of Nitrous Acid in Southern Ontario, Canada
A Novel Experimental Setup for Determination of Atmospheric Ammonia Fluxes Using a Tunable Diode Laser Absorption Spectrometer.
Characterizing area-source volatilization of ammonia has presented many challenges using fast-response techniques such as eddy covariance due to the adhesive and reactive nature of NH3 within the measuring system. A series of laboratory experiments were conducted to determine the optimal setup using a tunable diode laser absorption spectrometer (TDLAS). The series of experiments were performed concomitantly between the TDLAS and a quantum cascade tunable infrared laser differential absorption spectrometer and results are presented in a companion paper. These experiments consisted of a range of standard additions (10-1000ppbv) using both perfluoroalkoxy (PFA) and polyethylene (PE) inlet tubing ranging in lengths between 3.9 and 8.9m. To address the issue of NH3 adsorption, a test using a heated (40oC) 5-m PE sample line was used in one test series. The standard NH3 additions were mixed with either pre-purified N2 or ambient room air to mimic ambient field conditions. A novel sample inlet, provided by University of Toronto and based on the design of Aerodyne Inc., was employed for the test duration. This inlet was designed to relinquish the use of a filter on the inlet, which may pose attenuation and sample flow issues. The responses to concentration changes using these various configurations demonstrated that the response to the [NH3] changes exhibited a double exponential decay. On average, the primary decay curve represented 88% of the total change in concentration and the average decay coefficient was 0.24s. However, the secondary decay coefficient was much larger (35.2s). The optimal response of the TDLAS was obtained using the shortest length of PFA tubing (3.9m) where the primary decay responses were all greater than 90% of the total change in 0.17s on average and the remaining decay occurred over a period of 0.12s. Surprisingly, the test using the heated PE tubing did not produce any discernible improvements to the instrument response. The optimal configuration proved to be a viable setup of the instrumentation for measuring NH3 fluxes over agricultural landscapes.
Ground-based Tropospheric Measurements of CH4, C2H6, N2O and CO over Toronto and Comparisons with the GEOS-Chem Model
We present recent tropospheric trace gas columns retrieved using a ground-based Fourier Transform Infrared
(FTIR) spectrometer at the University of Toronto Atmospheric Observatory (TAO). TAO is located in downtown
Toronto (43.66 N, 79.40 W, 174 m above sea level), and has been recording high-resolution solar absorption
spectra on a regular basis since May 2002. The trace gas retrievals are performed using the optimal
estimation method implemented semi-empirically with the SFIT2 algorithm.
The time series of the total columns from May 2002 to December 2008 are presented here for tropospheric species CH4, C2H6, N2O and CO. The retrievals used the suggested spectral microwindows from the "Upper Free Troposphere Observations from a European Ground-Based FTIR Network" (UFTIR). The time series are compared to the GEOS-Chem model, which is a global 3-D chemical transport model of atmospheric composition driven by assimilated meteorological fields from the Goddard Earth Observing System (GEOS). We assess both the measurement retrieval parameters and the model emissions inventories, and interpret the measurements in terms of atmospheric processes.
Retrieval of CO Mixing Ratios From the Atmospheric Emission Spectra of Down-Welling Middle IR Radiance in Oklahoma.
CO abundances are retrieved from spectra of atmospheric down-welling radiation measured by the Atmospheric Emitted Radiance Interferometer (AERI) at the Southern Great Plains (SGP) observatory of the Atmospheric Radiation Measurements (ARM) program sponsored by the United States Department of Energy (DOE). These spectra measured every 8 minutes are publicly available for the period between 1997 and the present. The R-branch of the CO fundamental vibration-rotational band between 2135 and 2200 cm-1 is chosen for analysis. Radiative transfer calculations are performed using the k-Compressed Atmospheric Radiative Transfer Algorithm (kCARTA). Atmospheric temperature and water vapor profiles were retrieved from other parts of the AERI spectra using standard software developed by the University of Wisconsin and constrained with SGP Microwave Radiometer total precipitable water retrievals. An a priori constant CO mixing ratio profile is perturbed to minimize the spectral residuals due to CO. A correction for the scattered solar radiance during day-time is applied. Independent measurements of CO boundary layer mixing ratios by the Lawrence Berkeley National Laboratory (LBNL) and free tropospheric CO by the Earth System Research Laboratory (ESRL) of the Global Monitoring Division (GMD) , NOAA provide validation for the AERI retrieval technique.
Compound Specific Concentration and Stable Isotope Ratio Measurements of Atmospheric Particulate Organic Matter and Gas Phase Nitrophenols
Atmospheric particulate organic matter (POM) adversely affects health and climate. One of the still poorly understood sources of secondary organic matter (SOM) is the formation of secondary POM from the photo- oxidation of atmospheric volatile organic compounds (VOC). Nitrophenols, which are toxic semi-volatile compounds, are formed in the atmosphere by OH-radical initiated photo-oxidation of aromatic hydrocarbons, such as toluene. A method was developed to determine concentrations and stable carbon isotope ratios of particulate methyl nitrophenols in the atmosphere. This method has been used to quantify methyl nitrophenols, specifically 2-methyl-4-nitrophenol and 4-methyl-2-nitrophenol, found in atmospheric PM samples in trace quantities. Using this method, we conducted measurements of methyl nitrophenols in atmospheric PM in rural and suburban areas in Southern Ontario. The results of these measurements showed that the concentration of methyl nitrophenols in atmospheric PM is much lower than expected from the extrapolation of laboratory experiments and measured atmospheric toluene concentrations. In order to better understand the reasons for these findings, an analytical method for the analysis of nitrophenols in the gas phase is currently being developed. Similarly, the measurement technique is modified to allow analysis of other phenolic products of the oxidation of aromatic hydrocarbons in PM as well as in the gas phase. In this poster, sampling techniques for collection and GC-MS analysis of nitrophenols in gas phase and PM will be presented along with preliminary results from summer 2008 and spring 2009 studies.
Sampling Method and Experimental Procedure for Stable Carbon Isotope Ratio Measurements of VOC
Stable carbon isotopic composition measurement has proved to be a reliable analytical technique to study the atmospheric chemistry of Volatile Organic Compounds (VOC). It can be used to determine the photochemical age of atmospheric VOC and to differentiate between chemical loss due to processing of individual VOC and atmospheric mixing. Significant amounts of secondary organic pollutants are formed as a result of photochemical oxidation of VOC in the atmosphere. While some of the resulting semi- and non-volatile products contribute significant mass to atmospheric particulate matter impacting physical and chemical properties of aerosols, products remaining in the gas phase may impact regional and global air quality. Despite numerous studies, the formation processes for atmospheric particular organic matter (POM) are still not well understood. Most laboratory investigations of POM formation have been conducted at pollutant concentrations several orders of magnitude higher than typical ambient levels. Therefore, there is a substantial uncertainty in the application of these results to the atmospheric conditions. It is expected that including isotope ratio measurements of selected VOC and specific components of secondary POM will allow to better understand the relation between the amount of precursor oxidized and the concentration of secondary pollutants formed during this process. In this poster we will present methods for efficient collection of ambient VOC and subsequent measurements of compound specific stable carbon isotope ratios. VOC are collected in adsorbent-filled cartridges, thermally desorbed using a two stage preconcentration system, and analyzed by gas chromatography coupled on-line to isotope ratio mass spectrometry via a combustion interface. Stable carbon isotope composition and concentrations of several VOC in ambient samples will be presented and discussed.
A New Technology for Elimination of Isobaric Interferences in Ultra-Sensitive Isotope Measurements
Accelerator Mass Spectrometry (AMS) is a technique used for ultra-sensitive abundance ratio measurements. Applications in the earth sciences typically involve the measurement of 10Be, 14C, 26Al, 36Cl, 41Ca and 129I, for example in exposure dating and ground water studies, sometimes yielding ratios of unstable to stable isotope at 10-15 or less. AMS is most effective when the stable atomic isobar does not form a stable negative ion. When this is not the case larger accelerators have been required. Here we report preliminary tests of a prototype radio-frequency quadrupole collision cell system for the removal of isobaric interferences for Accelerator Mass Spectrometry (AMS) and for studies of anion-gas interactions at low energy. A fully developed ISA could allow smaller AMS systems to handle a wider range of samples. This system, known as an Isobar Separator for Anions (ISA), decelerates a mass-analyzed beam of anions from an energy typically generated by an AMS ion source (∼20 keV) down to < 10 eV. Radiofrequency quardrupoles and electrostatic lenses then guide the ions through the collision cell where ion-gas reactions attenuate most of the unwanted isobars and ion-gas elastic collisions reduce the ion energy and energy spread of the ion beam (cool the ions). The anions are then reaccelerated to their original energy for injection into the rest of the AMS system. With the ISA installed on a full 3MV AMS system, attenuations of 32S-, 12C3-, and 39K- by six, seven, and > ten orders of magnitude respectively have been achieved using NO2 gas in the collision cell. Transmission of a non-reactive anion is approximately 10-20% through the ISA. Further measurements of four 36Cl standards (4 x 10-13 < 36Cl/Cl < 4 x 10-11) and an estimate of the attenuation of the interfering isobars 36S-and 12C3-is also described.