John Smith Defense
Exploration of a whole atmosphere lidar concept for whole atmosphere science: Advances in resonance Doppler lidar technologies
Candidate: John A. Smith
Adviser: Prof. Xinzhao Chu
Although resonance Doppler lidars have made substantial contributions to our understanding of the mesosphere and lower thermosphere (MLT), today's whole atmosphere models demand a more complete picture which will require mobile instruments with superior resolution and coverage (0-200+ km). Higher resolution is needed to capture smaller scale impacts on transport of energy and momentum in the MLT and greater coverage to understand the ways in which disturbances originating in the lower atmospheres can govern both large and small scale dynamics in the MLT and higher into the thermosphere. A whole atmosphere lidar is needed. Efforts to extend the coverage of wind and temperature in the lower atmosphere with resonance lidars have so far been of limited scientific use since a large gap in coverage still exists where scattering by the Cabannes-Mie/Rayleigh process is too weak and unbound meteoric metals do not extend low enough. Many direct detection techniques suffer from measurement contamination from aerosols also and a measurement sensitivity which can vary depending on the temperature and pressure of the scattering volume, making measurements unreliable. We have a two-pronged approach to address the need for a whole atmosphere instrument in the community.
First, by improving receiver efficiencies of resonance lidars we can reduce photon noise and resolve smaller scale features at higher temporal resolutions. A critical gap in temperatures derived by the Rayleigh lidar technique can be filled by increasing the available signal between where the Rayleigh signal diminishes and where the scattering from resonant species such as Fe picks up in the MLT region. Higher signal levels will also lower the detection threshold for tenuous layers of metal in the thermosphere and enable studies of thermospheric Fe and Na up to 200 km with far better detail. We show how optical design methods and certain alignment considerations were used to improve the sensitivity of existing resonance-fluorescence lidars by several factors. Cases of the implementation of this technique at Boulder, Colorado and Cerro Pachón, Chile show that our procedures in the design and alignment of resonance receivers has improved signal levels by as much as a factor of five - sufficient to directly resolve seasonal vertical eddy flux in the MLT region for the first time and detect extremely tenuous layers of Na in the thermosphere above Cerro Pachón which were previously unstudied.
Second, a novel receiver design concept using a field-widened Mach-Zehnder interferometer (MZI) is investigated which will enable a resonance lidar to conduct wind measurement below the MLT region. The MZI has emerged as a promising direct detection wind technique since it avoids the problem of contamination from aerosols and has been shown to have the best theoretical performance among molecular direct detection techniques. When used in the receiver of an Fe-Doppler lidar system, we show that a particular ratio formulated from the signals in each channel and frequency can be made nearly as sensitive to a unit Doppler shift as the most sensitive implementations of the MZI, yet is also insensitive to aerosol backscatter ratio and temperature and pressure of the scatter volume. Summation of the two channels results in a signal which is independent of the intermediate MZI and can therefore be used harmoniously with the existing resonance Doppler 3-frequency technique, thereby enabling routine coverage of wind from the near surface to more than 115 km with a single lidar. The implications for such measurements are an improved understanding of, for example, gravity wave spectra propagation, Brewer-Dobson circulation and the 'cold pole' problem and the tracing of inertia gravity waves to their source regions.
Analytical Chemistry Seminar: Weiwei Hu and Zhe Peng
Jointly sponsored by the Department of Chemistry and Biochemistry, CIRES, and the Environmental Program
Characterization of a Real-Time Tracer for Isoprene Epoxydiols-Derived Secondary Organic Aerosol (IEPOX-SOA) from Aerosol Mass Spectrometer Measurements
Weiwei Hu et al. - Cooperative Institute for Research in Environmental Sciences, and Department of Chemistry and Biochemistry, University of Colorado, Boulder
Secondary organic aerosol (SOA) can be formed from isoprene epoxydiols (IEPOX), compounds that are produced from isoprene oxidation under low-NO conditions. IEPOX-SOA can account for a substantial fraction of organic aerosol (OA) in biogenic-influenced areas. In this study, IEPOX-SOA was identified from measurements at the forested Southeast U.S. supersite (Centreville, AL) during the Southern Oxidant and Aerosol Study (SOAS) using Positive Matrix Factorization (PMF) of aerosol mass spectrometer (AMS) measurements. The SOAS results clearly show that the fraction of OA measured at C5H6O+ (fC5H6O) in AMS spectra is a good tracer for IEPOX-SOA and correlates with the well-known methyltetrol tracers of this chemistry. Other field and chamber studies are included in the analysis to investigate the robustness of fC5H6O as an IEPOX-SOA tracer in AMS data. We observed clearly higher fC5H6O (3x10-3 - 25x10-3) in OA from regions with strong isoprene emissions vs those from urban and biomass-burning plumes (0 - 3.5x10-3 with an average of 1.75x10-3). In isoprene-influenced areas, fC5H6O in ambient OA positively correlates with the relative contribution of IEPOX-SOA to OA and decreases with OA aging. The kOH for destruction of IEPOX-SOA via heterogeneous oxidation in aerosol phase is approximately 5.3x10-13 cm3 molec－1 s－1, with life time is around 14.5 days (assuming average OH concentration=1.5x106 molecular cm-3). Volatility analysis using a thermal denuder shows that IEPOX-SOA is not more volatile than bulk SOA, contrasting with the high volatility of the tracers identified. Finally, we develop a simplified method to estimate ambient IEPOX-SOA mass concentrations as a function of fC5H6O, which is shown to perform well compared to the full PMF method. When only unit mass resolution data is available as in ACSM data, the method performs less well because of increased interferences from other ions at m/z 82. Estimated IEPOX-SOA concentrations in the southeastern U.S. from an aircraft campaign (SEAC4RS)correlate well with the IEPOX-related species detected by other techniques over a wide range of conditions, which confirms the usefulness of our method. IEPOX-SOA accounts for 14 - 17% of the OA in the SE U.S. during the summer according to both the SEAC4RS and SOAS results.
Zhe Peng - Cooperative Institute for Research in Environmental Sciences, and Department of Chemistry and Biochemistry, University of Colorado, Boulder
Oxidation flow reactors (OFRs) using OH produced from low-pressure Hg lamps at 254 nm(OFR254) or both 185 and 254 nm (OFR185) are commonly used in atmospheric chemistry and other fields. OFR254 requires the addition of externally formed O3 since OH is formed from O3 photolysis, while OFR185 does not since O3 is formed in the reactor and OH can also be formed from H2O photolysis. In this study, we use a plug-flow kinetic model to investigate OFR properties under a very wide range of conditions applicable to both field and laboratory studies. We show that the radical chemistry in OFRs can be characterized as a function of UV light intensity, H2O concentration, and total external OH reactivity (e.g., from VOCs, NOx, and SO2). In OFR185, OH exposure is more sensitive to external OH reactivity than in OFR254, because injected O3 in OFR254 promotes the recycling of HO2 to OH, making external perturbations to theradical chemistry less significant. There has been some speculation in the literature about whether 'non-tropospheric chemistry' (photolysis at 185 or 254 nm, and/or reactions with O(1D) and O(3P)) may play an important role in these OFRs. For field studies in forested regions or the urban area of Los Angeles, reactants of atmospheric interest are predominantly consumed by OH. Nontropospheric oxidants contribute to the degradation of some species under conditions of low H2O concentration and/or very high external OH reactivity. This appears to have been a problem in some laboratory and source studies, but can be avoided in future studies by experimental planning based on our findings. Some biogenic VOCs can have substantial contributions of reaction with O3 under some operating conditions, especially for OFR254. NO3 may have played an unexpectedly significant role in some past laboratory studies. RO2 fate is similar to that in the atmosphere under low-NOx conditions. A comparison of OFRs with typical environmental chamber studies using UV blacklights and with the atmosphere is presented. OFRs' key advantages are their short experimental time scales, portability to field sites, enabling a direct connection of field and laboratory studies, and controllable and predictable radical chemistry. This study further establishes the usefulness of these reactors and enables better experiment planning and interpretation.