scholarly journals Aircraft measurements of BrO, IO, glyoxal, NO<sub>2</sub>, H<sub>2</sub>O, O<sub>2</sub>–O<sub>2</sub> and aerosol extinction profiles in the tropics: comparison with aircraft-/ship-based in situ and lidar measurements

2015 ◽  
Vol 8 (5) ◽  
pp. 2121-2148 ◽  
Author(s):  
R. Volkamer ◽  
S. Baidar ◽  
T. L. Campos ◽  
S. Coburn ◽  
J. P. DiGangi ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Column Density ◽  
Tropospheric Chemistry ◽  
Aerosol Extinction ◽  
Reference Spectrum ◽  
Free Troposphere ◽  
Near Surface ◽  
Oxygenated Hydrocarbons

Abstract. Tropospheric chemistry of halogens and organic carbon over tropical oceans modifies ozone and atmospheric aerosols, yet atmospheric models remain largely untested for lack of vertically resolved measurements of bromine monoxide (BrO), iodine monoxide (IO) and small oxygenated hydrocarbons like glyoxal (CHOCHO) in the tropical troposphere. BrO, IO, glyoxal, nitrogen dioxide (NO2), water vapor (H2O) and O2–O2 collision complexes (O4) were measured by the University of Colorado Airborne Multi-AXis Differential Optical Absorption Spectroscopy (CU AMAX-DOAS) instrument, aerosol extinction by high spectral resolution lidar (HSRL), in situ aerosol size distributions by an ultra high sensitivity aerosol spectrometer (UHSAS) and in situ H2O by vertical-cavity surface-emitting laser (VCSEL) hygrometer. Data are presented from two research flights (RF12, RF17) aboard the National Science Foundation/National Center for Atmospheric Research Gulfstream V aircraft over the tropical Eastern Pacific Ocean (tEPO) as part of the "Tropical Ocean tRoposphere Exchange of Reactive halogens and Oxygenated hydrocarbons" (TORERO) project (January/February 2012). We assess the accuracy of O4 slant column density (SCD) measurements in the presence and absence of aerosols. Our O4-inferred aerosol extinction profiles at 477 nm agree within 6% with HSRL in the boundary layer and closely resemble the renormalized profile shape of Mie calculations constrained by UHSAS at low (sub-Rayleigh) aerosol extinction in the free troposphere. CU AMAX-DOAS provides a flexible choice of geometry, which we exploit to minimize the SCD in the reference spectrum (SCDREF, maximize signal-to-noise ratio) and to test the robustness of BrO, IO and glyoxal differential SCDs. The RF12 case study was conducted in pristine marine and free tropospheric air. The RF17 case study was conducted above the NOAA RV Ka'imimoana (TORERO cruise, KA-12-01) and provides independent validation data from ship-based in situ cavity-enhanced DOAS and MAX-DOAS. Inside the marine boundary layer (MBL) no BrO was detected (smaller than 0.5 pptv), and 0.2–0.55 pptv IO and 32–36 pptv glyoxal were observed. The near-surface concentrations agree within 30% (IO) and 10% (glyoxal) between ship and aircraft. The BrO concentration strongly increased with altitude to 3.0 pptv at 14.5 km (RF12, 9.1 to 8.6° N; 101.2 to 97.4° W). At 14.5 km, 5–10 pptv NO2 agree with model predictions and demonstrate good control over separating tropospheric from stratospheric absorbers (NO2 and BrO). Our profile retrievals have 12–20 degrees of freedom (DoF) and up to 500 m vertical resolution. The tropospheric BrO vertical column density (VCD) was 1.5 × 1013 molec cm−2 (RF12) and at least 0.5 × 1013 molec cm−2 (RF17, 0–10 km, lower limit). Tropospheric IO VCDs correspond to 2.1 × 1012 molec cm−2 (RF12) and 2.5 × 1012 molec cm−2 (RF17) and glyoxal VCDs of 2.6 × 1014 molec cm−2 (RF12) and 2.7 × 1014 molec cm−2 (RF17). Surprisingly, essentially all BrO as well as the dominant IO and glyoxal VCD fraction was located above 2 km (IO: 58 ± 5%, 0.1–0.2 pptv; glyoxal: 52 ± 5%, 3–20 pptv). To our knowledge there are no previous vertically resolved measurements of BrO and glyoxal from aircraft in the tropical free troposphere. The atmospheric implications are briefly discussed. Future studies are necessary to better understand the sources and impacts of free tropospheric halogens and oxygenated hydrocarbons on tropospheric ozone, aerosols, mercury oxidation and the oxidation capacity of the atmosphere.

scholarly journals Aircraft measurements of bromine monoxide, iodine monoxide, and glyoxal profiles in the tropics: comparison with ship-based and in situ measurements

2015 ◽  
Vol 8 (1) ◽  
pp. 623-687 ◽  
Author(s):  
R. Volkamer ◽  
S. Baidar ◽  
T. L. Campos ◽  
S. Coburn ◽  
J. P. DiGangi ◽  
...  
Keyword(s):  
Tropospheric Chemistry ◽  
Optical Absorption Spectroscopy ◽  
Reference Spectrum ◽  
Validation Data ◽  
Tropical Ocean ◽  
Near Surface ◽  
Oxygenated Hydrocarbons ◽  
Iodine Monoxide

Abstract. Tropospheric chemistry of halogens and organic carbon over tropical oceans modifies ozone and atmospheric aerosols, yet atmospheric models remain largely untested for lack of vertically resolved measurements of bromine monoxide (BrO), iodine monoxide (IO), and small oxygenated hydrocarbons like glyoxal (CHOCHO) in the tropical troposphere. BrO, IO, glyoxal, nitrogen dioxide (NO2), water vapor (H2O) and O2-O2 collision complexes (O4) were measured by the CU Airborne Multi AXis Differential Optical Absorption Spectroscopy (CU AMAX-DOAS) instrument, in situ aerosol size distributions by an Ultra High Sensitivity Aerosol Spectrometer (UHSAS), and in situ H2O by Vertical-Cavity Surface-Emitting Laser hygrometer (VCSEL). Data are presented from two research flights (RF12, RF17) aboard the NSF/NCAR GV aircraft over the tropical Eastern Pacific Ocean (tEPO) as part of the "Tropical Ocean tRoposphere Exchange of Reactive halogens and Oxygenated hydrocarbons" (TORERO) project. We assess the accuracy of O4 slant column density (SCD) measurements in the presence and absence of aerosols, and find O4-inferred aerosol extinction profiles at 477 nm agree within 5% with Mie calculations of extinction profiles constrained by UHSAS. CU AMAX-DOAS provides a flexible choice of geometry which we exploit to minimize the SCD in the reference spectrum (SCDREF, maximize signal-to-noise), and to test the robustness of BrO, IO, and glyoxal differential SCDs. The RF12 case study was conducted in pristine marine and free tropospheric air. The RF17 case study was conducted above the NOAA RV Ka'imimoana (TORERO cruise, KA-12-01), and provides independent validation data from ship-based in situ Cavity Enhanced- and MAX-DOAS. Inside the marine boundary layer (MBL) no BrO was detected (smaller than 0.5 pptv), and 0.2–0.55 pptv IO and 32–36 pptv glyoxal were observed. The near surface concentrations agree within 20% (IO) and 10% (glyoxal) between ship and aircraft. The BrO concentration strongly increased with altitude to 3.0 pptv at 14.5 km (RF12, 9.1 to 8.6° N; 101.2 to 97.4° W). At 14.5 km 5–10 pptv NO2 agree with model predictions, and demonstrate good control over separating tropospheric from stratospheric absorbers (NO2 and BrO). Our profile retrievals have 12–20 degrees of freedom (DoF), and up to 500 m vertical resolution. The tropospheric BrO VCD was 1.5 × 1013 molec cm−2 (RF12), and at least 0.5 × 1013 molec cm−2 (RF17, 0–10 km, lower limit). Tropospheric IO VCDs correspond to 2.1 × 1012 molec cm−2 (RF12) and 2.5 × 1012 molec cm−2 (RF17), and glyoxal VCDs of 2.6 × 1014 molec cm−2 (RF12) and 2.7 × 1014 molec cm−2 (RF17). Surprisingly, essentially all BrO, and the dominant IO and glyoxal VCD fraction was located above 2 km (IO: 58 ± 5%, 0.1–0.2 pptv; glyoxal: 52 ± 5%, 3–20 pptv). To our knowledge there are no previous vertically resolved measurements of BrO and glyoxal from aircraft in the tropical free troposphere.

scholarly journals Mercury oxidation from bromine chemistry in the free troposphere over the southeastern US

2016 ◽  
Vol 16 (6) ◽  
pp. 3743-3760 ◽  
Author(s):  
Sean Coburn ◽  
Barbara Dix ◽  
Eric Edgerton ◽  
Christopher D. Holmes ◽  
Douglas Kinnison ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Column Density ◽  
Atmospheric Mercury ◽  
Water Soluble ◽  
Reference Spectrum ◽  
Free Troposphere ◽  
Mercury Oxidation ◽  
Oxidation Rates ◽  
Southeastern Us

Abstract. The elevated deposition of atmospheric mercury over the southeastern United States is currently not well understood. Here we measure partial columns and vertical profiles of bromine monoxide (BrO) radicals, a key component of mercury oxidation chemistry, to better understand the processes and altitudes at which mercury is being oxidized in the atmosphere. We use data from a ground-based MAX-DOAS instrument located at a coastal site ∼  1 km from the Gulf of Mexico in Gulf Breeze, FL, where we had previously detected tropospheric BrO (Coburn et al., 2011). Our profile retrieval assimilates information about stratospheric BrO from the WACCM chemical transport model (CTM), and uses only measurements at moderately low solar zenith angles (SZAs) to estimate the BrO slant column density contained in the reference spectrum (SCDRef). The approach has 2.6 degrees of freedom, and avoids spectroscopic complications that arise at high SZA; knowledge about SCDRef further helps to maximize sensitivity in the free troposphere (FT). A cloud-free case study day with low aerosol load (9 April 2010) provided optimal conditions for distinguishing marine boundary layer (MBL: 0–1 km) and free-tropospheric (FT: 1–15 km) BrO from the ground. The average daytime tropospheric BrO vertical column density (VCD) of ∼  2.3  ×  1013 molec cm−2 (SZA  <  70°) is consistent with our earlier reports on other days. The vertical profile locates essentially all tropospheric BrO above 4 km, and shows no evidence for BrO inside the MBL (detection limit  <  0.5 pptv). BrO increases to  ∼  3.5 pptv at 10–15 km altitude, consistent with recent aircraft observations. Our case study day is consistent with recent aircraft studies, in that the oxidation of gaseous elemental mercury (GEM) by bromine radicals to form gaseous oxidized mercury (GOM) is the dominant pathway for GEM oxidation throughout the troposphere above Gulf Breeze. The column integral oxidation rates are about 3.6  × 105 molec cm−2 s−1 for bromine, while the contribution from ozone (O3) is 0.8  ×  105 molec cm−2 s−1. Chlorine-induced oxidation is estimated to add  <  5 % to these mercury oxidation rates. The GOM formation rate is sensitive to recently proposed atmospheric scavenging reactions of the HgBr adduct by nitrogen dioxide (NO2), and to a lesser extent also HO2 radicals. Using a 3-D CTM, we find that surface GOM variations are also typical of other days, and are mainly derived from the FT. Bromine chemistry is active in the FT over Gulf Breeze, where it forms water-soluble GOM that is subsequently available for wet scavenging by thunderstorms or transport to the boundary layer.

scholarly journals Near-surface and path-averaged mixing ratios of NO<sub>2</sub> derived from car DOAS zenith-sky and tower DOAS off-axis measurements in Vienna: a case study

2019 ◽  
Vol 19 (9) ◽  
pp. 5853-5879 ◽  
Author(s):  
Stefan F. Schreier ◽  
Andreas Richter ◽  
John P. Burrows
Keyword(s):  
Boundary Layer ◽  
Planetary Boundary Layer ◽  
Linear Regression Analysis ◽  
Optical Absorption Spectroscopy ◽  
Near Surface ◽  
Pollution Levels ◽  
Mixing Ratios ◽  
Tropospheric No2

Abstract. Nitrogen dioxide (NO2), produced as a result of fossil fuel combustion, biomass burning, lightning, and soil emissions, is a key urban and rural tropospheric pollutant. In this case study, ground-based remote sensing has been coupled with the in situ network in Vienna, Austria, to investigate NO2 distributions in the planetary boundary layer. Near-surface and path-averaged NO2 mixing ratios within the metropolitan area of Vienna are estimated from car DOAS (differential optical absorption spectroscopy) zenith-sky and tower DOAS horizon observations. The latter configuration is innovative in the sense that it obtains horizontal measurements at more than a hundred different azimuthal angles – within a 360∘ rotation taking less than half an hour. Spectral measurements were made with a DOAS instrument on nine days in April, September, October, and November 2015 in the zenith-sky mode and on five days in April and May 2016 in the off-axis mode. The analysis of tropospheric NO2 columns from the car measurements and O4 normalized NO2 path averages from the tower observations provide interesting insights into the spatial and temporal NO2 distribution over Vienna. Integrated column amounts of NO2 from both DOAS-type measurements are converted into mixing ratios by different methods. The estimation of near-surface NO2 mixing ratios from car DOAS tropospheric NO2 vertical columns is based on a linear regression analysis including mixing height and other meteorological parameters that affect the dilution and reactivity in the planetary boundary layer – a new approach for such conversion. Path-averaged NO2 mixing ratios are calculated from tower DOAS NO2 slant column densities by taking into account topography and geometry. Overall, lap averages of near-surface NO2 mixing ratios obtained from car DOAS zenith-sky measurements, around a circuit in Vienna, are in the range of 3.8 to 26.1 ppb and in good agreement with values obtained from in situ NO2 measurements for days with wind from the southeast. Path-averaged NO2 mixing ratios at 160 m above the ground as derived from the tower DOAS measurements are between 2.5 and 9 ppb on two selected days with different wind conditions and pollution levels and show similar spatial distribution as seen in the car DOAS zenith-sky observations. We conclude that the application of the two methods to obtain near-surface and path-averaged NO2 mixing ratios is promising for this case study.

scholarly journals Near-surface and path-averaged mixing ratios of NO<sub>2</sub> derived from car DOAS zenith-sky and tower DOAS off-axis measurements in Vienna: a case study

2018 ◽  
Author(s):  
Stefan F. Schreier ◽  
Andreas Richter ◽  
John P. Burrows
Keyword(s):  
Boundary Layer ◽  
Planetary Boundary Layer ◽  
Linear Regression Analysis ◽  
Optical Absorption Spectroscopy ◽  
Near Surface ◽  
Pollution Levels ◽  
Mixing Ratios ◽  
Tropospheric No2

Abstract. Nitrogen dioxide (NO2), produced as a result of fossil fuel combustion, biomass burning, lightning, and soil emissions, is a key urban and rural tropospheric pollutant. In this case study, ground-based remote sensing has been coupled with the in situ network in Vienna, Austria, to investigate NO2 distributions in the planetary boundary layer. Near-surface and path-averaged NO2 mixing ratios within the metropolitan area of Vienna are estimated from car DOAS (Differential Optical Absorption Spectroscopy) zenith-sky and tower DOAS horizon observations. The latter configuration is innovative in the sense that it obtains horizontal measurements at more than hundred different azimuthal angles – within a 360° rotation taking less than half an hour. Spectral measurements were made with a DOAS instrument on nine days in April, September, October, and November 2015 in the zenith-sky mode and on five days in April and May 2016 in the off-axis mode. The analysis of tropospheric NO2 columns from the car measurements and O4 normalized NO2 path averages from the tower observations provide interesting insights into the spatial and temporal NO2 distribution over Vienna. Integrated column amounts of NO2 from both DOAS-type measurements are converted into mixing ratios by different methods. The estimation of near-surface NO2 mixing ratios from car DOAS tropospheric NO2 vertical columns is based on a linear regression analysis including mixing-height and other meteorological parameters that affect the dilution and reactivity in the planetary boundary layer – a new approach for such conversion. Path-averaged NO2 mixing ratios are calculated from tower DOAS NO2 slant column densities by taking into account topography and geometry. Overall, lap averages of near-surface NO2 mixing ratios obtained from car DOAS zenith-sky measurements, around a circuit in Vienna, are in the range of 3.8 to 26.2 ppb and in good agreement with values obtained from in situ NO2 measurements for days with wind from the Southeast. Path-averaged NO2 mixing ratios at 160 m above the ground as derived from the tower DOAS measurements are between 2.5 and 9 ppb on two selected days with different wind conditions and pollution levels and show similar spatial distribution as seen in the car DOAS zenith-sky observations. We conclude that the application of the two methods to obtain near-surface and path-averaged NO2 mixing ratios is promising for this case study.

scholarly journals Mercury oxidation from bromine chemistry in the free troposphere over the southeastern US

2015 ◽  
Vol 15 (20) ◽  
pp. 28317-28360 ◽  
Author(s):  
S. Coburn ◽  
B. Dix ◽  
E. Edgerton ◽  
C. D. Holmes ◽  
D. Kinnison ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Column Density ◽  
Transport Model ◽  
Chemical Transport ◽  
Water Soluble ◽  
Reference Spectrum ◽  
Free Troposphere ◽  
Chemical Transport Model ◽  
Mercury Oxidation ◽  
Southeastern Us

Abstract. The elevated deposition of atmospheric mercury over the Southeastern United States is currently not well understood. Here we measure partial columns and vertical profiles of bromine monoxide (BrO) radicals, a key component of mercury oxidation chemistry, to better understand the processes and altitudes at which mercury is being oxidized in the atmosphere. We use the data from a ground-based MAX-DOAS instrument located at a coastal site ~ 1 km from the Gulf of Mexico in Gulf Breeze, FL, where we had previously detected tropospheric BrO (Coburn et al., 2011). Our profile retrieval assimilates information about stratospheric BrO from the WACCM chemical transport model, and uses only measurements at moderately low solar zenith angles (SZA) to estimate the BrO slant column density contained in the reference spectrum (SCDRef). The approach has 2.6 degrees of freedom, and avoids spectroscopic complications that arise at high SZA; knowledge about SCDRef helps to maximize sensitivity in the free troposphere (FT). A cloud-free case study day with low aerosol load (9 April 2010) provided optimal conditions for distinguishing marine boundary layer (MBL: 0–1 km) and free tropospheric (FT: 1–15 km) BrO from the ground. The average daytime tropospheric BrO vertical column density (VCD) of ~ 2.3 × 1013 molec cm−2 (SZA < 70°) is consistent with our earlier reports on other days. The vertical profile locates essentially all tropospheric BrO above 4 km, and shows no evidence for BrO inside the MBL (detection limit < 0.5 pptv). BrO increases in the FT. The average FT-BrO mixing ratio was ~ 0.9 pptv between 1–15 km, consistent with recent aircraft observations. We find that the oxidation of gaseous elemental mercury (GEM) by bromine radicals to form gaseous oxidized mercury (GOM) is the dominant pathway for GEM oxidation throughout the troposphere above Gulf Breeze. The column integral oxidation rates range from 3.0–3.4 × 105 molec cm−2 s−1 for bromine, while contributions from ozone (O3) and chlorine (Cl) were 0.9 × 105 and 0.2 × 105 molec cm−2 s−1, respectively. The GOM formation rate is sensitive to recently proposed atmospheric scavenging reactions of the HgBr adduct by nitrogen dioxide (NO2), and to a lesser extent also HO2 radicals. Using a 3-D chemical transport model, we find that surface GOM variations are typical also of other days, and are mainly derived from the free troposphere. Bromine chemistry is active in the FT over Gulf Breeze, where it forms water-soluble GOM that is subsequently available for wet scavenging by thunderstorms or transport to the boundary layer.

scholarly journals Aircraft validation of Aura Tropospheric Emission Spectrometer retrievals of HDO / H<sub>2</sub>O

2014 ◽  
Vol 7 (9) ◽  
pp. 3127-3138 ◽  
Author(s):  
R. L. Herman ◽  
J. E. Cherry ◽  
J. Young ◽  
J. M. Welker ◽  
D. Noone ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Water Vapor ◽  
Standard Deviation ◽  
Lower Troposphere ◽  
Temporal Variations ◽  
Observation Error ◽  
Free Troposphere ◽  
Data Set ◽  
Airborne Measurements

Abstract. The EOS (Earth Observing System) Aura Tropospheric Emission Spectrometer (TES) retrieves the atmospheric HDO / H2O ratio in the mid-to-lower troposphere as well as the planetary boundary layer. TES observations of water vapor and the HDO isotopologue have been compared with nearly coincident in situ airborne measurements for direct validation of the TES products. The field measurements were made with a commercially available Picarro L1115-i isotopic water analyzer on aircraft over the Alaskan interior boreal forest during the three summers of 2011 to 2013. TES special observations were utilized in these comparisons. The TES averaging kernels and a priori constraints have been applied to the in situ data, using version 5 (V005) of the TES data. TES calculated errors are compared with the standard deviation (1σ) of scan-to-scan variability to check consistency with the TES observation error. Spatial and temporal variations are assessed from the in situ aircraft measurements. It is found that the standard deviation of scan-to-scan variability of TES δD is ±34.1‰ in the boundary layer and ± 26.5‰ in the free troposphere. This scan-to-scan variability is consistent with the TES estimated error (observation error) of 10–18‰ after accounting for the atmospheric variations along the TES track of ±16‰ in the boundary layer, increasing to ±30‰ in the free troposphere observed by the aircraft in situ measurements. We estimate that TES V005 δD is biased high by an amount that decreases with pressure: approximately +123‰ at 1000 hPa, +98‰ in the boundary layer and +37‰ in the free troposphere. The uncertainty in this bias estimate is ±20‰. A correction for this bias has been applied to the TES HDO Lite Product data set. After bias correction, we show that TES has accurate sensitivity to water vapor isotopologues in the boundary layer.

scholarly journals Automated ground-based remote sensing measurements of greenhouse gases at the Białystok site in comparison with collocated in situ measurements and model data

2012 ◽  
Vol 12 (15) ◽  
pp. 6741-6755 ◽  
Author(s):  
J. Messerschmidt ◽  
H. Chen ◽  
N. M. Deutscher ◽  
C. Gerbig ◽  
P. Grupe ◽  
...  
Keyword(s):  
Remote Sensing ◽  
Boundary Layer ◽  
Lower Troposphere ◽  
Fourier Transform Spectrometer ◽  
Model Data ◽  
Solar Absorption ◽  
Near Surface ◽  
Tall Tower ◽  
Absorption Measurements

Abstract. The in situ boundary layer measurement site in Białystok (Poland) has been upgraded with a fully automated observatory for total greenhouse gas column measurements. The automated Fourier Transform Spectrometer (FTS) complements the on-site in situ facilities and FTS solar absorption measurements have been recorded nearly continuously in clear and partially cloudy conditions since March 2009. Here, the FTS measurements are compared with the collocated tall tower data. Additionally, simulations of the Jena CO2 inversion model are evaluated with the Białystok measurement facilities. The simulated seasonal CO2 cycle is slightly overestimated by a mean difference of 1.2 ppm ± 0.9 ppm (1σ) in comparison with the FTS measurements. CO2 concentrations at the surface, measured at the tall tower (5 m, 90 m, 300 m), are slightly underestimated by −1.5 ppm, −1.6 ppm, and −0.7 ppm respectively during the day and by −9.1 ppm, −5.9 ppm, and −1.3 ppm during the night. The comparison of the simulated CO2 profiles with low aircraft profiles shows a slight overestimation of the lower troposphere (by up to 1 ppm) and an underestimation in near-surface heights until 800 m (by up to 2.5 ppm). In an appendix the automated FTS observatory, including the hardware components and the automation software, is described in its basics.

scholarly journals Difficulties with phase spectrum unwrapping in spectral analysis of surface waves nondestructive testing of pavements

1992 ◽  
Vol 29 (3) ◽  
pp. 506-511 ◽  
Author(s):  
M. O. Al-Hunaidi
Keyword(s):  
Spectral Analysis ◽  
Nondestructive Testing ◽  
Surface Waves ◽  
Phase Spectrum ◽  
Correction Procedure ◽  
Near Surface ◽  
Surface Profiling ◽  
Usual Procedure

Spectral analysis of surface waves (SASW) is a nondestructive and in situ method for determining the stiffness profiles of soil and pavement sites. This method involves the generation and measurement of surface Rayleigh waves. By exploiting the dispersive characteristic of these waves in layered systems, the SASW method provides information on the variation of stiffness with depth. This paper presents the results of a case study for near-surface profiling of a pavement site using the SASW method. In this study, inconsistencies were observed in the dispersion curve of the site when the usual procedure of unfolding the relative phase spectrum was followed. A correction procedure to eliminate these inconsistencies is suggested and discussed. The thickness and wave velocities of the various layers obtained with the SASW method, after applying the correction procedure, matched closely those determined from cored samples and cross-hole tests. Key words : nondestructive testing, pavement, layered media, Rayleigh wave, spectral analysis, shear wave velocity, wave propagation.

scholarly journals The CU Airborne MAX-DOAS instrument: vertical profiling of aerosol extinction and trace gases

2013 ◽  
Vol 6 (3) ◽  
pp. 719-739 ◽  
Author(s):  
S. Baidar ◽  
H. Oetjen ◽  
S. Coburn ◽  
B. Dix ◽  
I. Ortega ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Air Quality ◽  
Spatial Scales ◽  
Trace Gases ◽  
Stray Light ◽  
Aerosol Extinction ◽  
Vertical Profiles ◽  
Maximum Information ◽  
Extinction Coefficients

Abstract. The University of Colorado Airborne Multi-Axis Differential Optical Absorption Spectroscopy (CU AMAX-DOAS) instrument uses solar stray light to detect and quantify multiple trace gases, including nitrogen dioxide (NO2), glyoxal (CHOCHO), formaldehyde (HCHO), water vapor (H2O), nitrous acid (HONO), iodine monoxide (IO), bromine monoxide (BrO), and oxygen dimers (O4) at multiple wavelengths (absorption bands at 360, 477, 577, 632 nm) simultaneously in the open atmosphere. The instrument is unique as it (1) features a motion compensation system that decouples the telescope field of view from aircraft movements in real time (<0.35° accuracy), and (2) includes measurements of solar stray light photons from nadir, zenith, and multiple elevation angles forward and below the plane by the same spectrometer/detector system. Sets of solar stray light spectra collected from nadir to zenith scans provide some vertical profile information within 2 km above and below the aircraft altitude, and the vertical column density (VCD) below the aircraft is measured in nadir view. Maximum information about vertical profiles is derived simultaneously for trace gas concentrations and aerosol extinction coefficients over similar spatial scales and with a vertical resolution of typically 250 m during aircraft ascent/descent. The instrument is described, and data from flights over California during the CalNex (California Research at the Nexus of Air Quality and Climate Change) and CARES (Carbonaceous Aerosols and Radiative Effects Study) air quality field campaigns is presented. Horizontal distributions of NO2 VCD (below the aircraft) maps are sampled with typically 1 km resolution, and show good agreement with two ground-based MAX-DOAS instruments (slope = 0.95 ± 0.09, R2 = 0.86). As a case study vertical profiles of NO2, CHOCHO, HCHO, and H2O concentrations and aerosol extinction coefficients, ε, at 477 nm calculated from O4 measurements from a low approach at Brackett airfield inside the South Coast Air Basin (SCAB) are presented. These profiles contain ~12 degrees of freedom (DOF) over a 3.5 km altitude range, an independent information approximately every 250 m. The boundary layer NO2 concentration, and the integral aerosol extinction over height (aerosol optical depth, AOD) agrees well with nearby ground-based in situ NO2 measurement, and AERONET station. The detection limits of NO2, CHOCHO, HCHO, H2O442, &amp;varepsilon;360, &amp;varepsilon;477 for 30 s integration time spectra recorded forward of the plane are 5 ppt, 3 ppt, 100 ppt, 42 ppm, 0.004 km−1, 0.002 km−1 in the free troposphere (FT), and 30 ppt, 16 ppt, 540 ppt, 252 ppm, 0.012 km−1, 0.006 km−1 inside the boundary layer (BL), respectively. Mobile column observations of trace gases and aerosols are complimentary to in situ observations, and help bridge the spatial scales that are probed by satellites and ground-based observations, and predicted by atmospheric models.

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