C. Thomas McElroy

The ACE-MAESTRO instrument on SCISAT: description, performance, and preliminary results

The Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation (MAESTRO) instrument on the SCISAT satellite is a simple, compact spectrophotometer for the measurement of atmospheric extinction, ozone, nitrogen dioxide, and other trace gases in the stratosphere and upper troposphere as part of the Atmospheric Chemistry Experiment (ACE) mission. We provide an overview of the instrument from requirements to realization, including optical design, prelaunch and on-orbit performance, and a preliminary examination of retrievals of ozone and NO(2).

Initial comparison of ozone and NO2 profiles from ACE‐MAESTRO with balloon and satellite data

[1] Atmospheric retrievals of ozone and NO 2 by the Measurements of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation (MAESTRO) instrument which is part of the Atmospheric Chemistry Experiment (ACE) satellite aboard SCISAT are compared statistically with coincident measurements by ozonesondes, the Fourier Transform Spectrometer (ACE-FTS) also aboard SCISAT, the SAGE III and the POAM III instruments. The ozone mixing ratio profiles from MAESTRO and ozonesondes agree within about 5-10% from 16-30 km in the northern middle and high latitudes. Further, ACE-FTS and MAESTRO ozone profiles agree within ∼5-15% from 16-50 km. MAESTRO ozone profiles show a systematic bias which is opposite for sunrise (SR) and sunset (SS) events and was also seen in comparisons with SAGE III and POAM III ozone. MAESTRO SS ozone profiles mostly agree within 5-10% from 16-40 km with either SAGE III or POAM III SR or SS retrievals, but show a significant high bias from 40-55 km, reaching a maximum of ∼20-30%. MAESTRO SR ozone profiles show a low bias of ∼5-15% from 20-50 km, as compared to SAGE III and POAM III SR or SS measurements. The NO 2 profiles agree within about 10-15% between ACE-FTS and MAESTRO from 15-40 km for the SR and 22-35 km for the SS measurements. Further, MAESTRO NO 2 profiles agree with SAGE III NO 2 mostly within 10% from 25-40 km. MAESTRO NO 2 profiles agree with POAM III SR profiles within 5-10% from 25-42 km. However, compared to POAM III SS profiles, MAESTRO NO 2 profiles show a low bias between 20 and 25 km (∼30-50%), a high MAESTRO bias between 25 and 32 km (10-30%), and again a low bias above 33 km that increases with altitude to 50-60%.

A sensitivity study of photolysis rate coefficients during POLARIS

Recent improvements in the agreement between observation-derived and modeled photolysis rate coefficients (j-values) have allowed for the close examination of the sensitivity of j-values to variations in physical parameters influencing their magnitude and temporal(spatial variability. Altitude and solar zenith angle profiles of j-values for two photolytic processes, NO2 → NO + O(3P) and O3 → O2 + O(1D), are modeled, varying surface albedo, atmospheric baseheight, total column ozone, and ozone and temperature altitude profiles over the ranges observed during the NASA Photochemistry of Ozone Loss in the Arctic Region In Summer (POLARIS) high-altitude ER-2 aircraft campaign. The effect of atmospheric refraction at high solar zenith angles is also addressed. Modeled j-values using measured ozone/albedo input from the Composition and Photodissociative Flux Measurement (CPFM) spectroradiometer on board the ER-2 exceed those derived from CPFM flux measurements by 6% for jNO2 and 14% for jO3, within experimental uncertainties. The individual effects of albedo, baseheight, and ozone on j-values along specific ER-2 flight tracks are modeled and related to the temporal and spatial variability observed. For jNO2, surface albedo has the greatest effect; for jO3, the ozone above the aircraft and surface albedo are the most important.

Ground‐based measurements of ozone and NO2 during MANTRA 1998 using a Zenith‐sky spectrometer

Abstract A portable ground‐based instrument has been constructed for the automated measurement of vertical column abundances of a number of gases pertinent to stratospheric ozone chemistry. The instrumentation is described in this paper and results are presented from the first set of field measurements, made during the Middle Atmosphere Nitrogen TRend Assessment (MANTRA) 1998 field campaign at Vanscoy, Saskatchewan, Canada. Zenith‐sky spectra in the near ultraviolet and visible wavelength regions were recorded for a period of seven days, prior to and following the launch of the MANTRA balloon on 24 August 1998. The spectra were then analysed using the differential optical absorption spectroscopy (DOAS) technique in conjunction with a radiative transfer model to determine vertical column amounts of ozone and NO2. Ozone measurements compared favourably with concurrent observations by ozonesondes, a Brewer spectrophotometer, and satellite instruments. Vertical NO2 columns were in broad agreement with those determined by the Global Ozone Monitoring Experiment (GOME) satellite instrument.

Intercomparison of total ozone observations at Fairbanks, Alaska, during POLARIS

The pattern of seasonal ozone loss over Fairbanks, Alaska (AK), during the NASA Photochemistry of Ozone Loss in the Arctic Region In Summer (POLARIS) campaign in the spring and summer of 1997 is defined. Five independent data sets of total ozone observations at Fairbanks are presented, from the Earth Probe and ADEOS Total Ozone Mapping Spectrometer (TOMS) satellite instruments, balloon-borne electrochemical concentration cell ozonesondes, and ground-based (Brewer spectroradiometer, Dobson spectrophotometer, and the Jet Propulsion Laboratory MkIV infrared interferometer) instruments. The excellent agreement between different observational techniques lends confidence to the observed rate of summertime loss of total ozone at high latitudes. In addition, the small offsets between the data sets are well understood.

University of Toronto’s balloon-borne Fourier transform spectrometer

A commercial ABB-Bomem DA5 Fourier transform spectrometer (FTS) was refitted with new software and electronics to create a FTS that is appropriate for both ground-based and balloon-based measurements. Nearly all the electronics were replaced, and new control software was written that allows the instrument to run remotely, provides access to all housekeeping information, and permits considerable freedom in data processing approaches. A “delta” tracker was used for fine tracking of the sun over a small tracking range, using the main gondola pointing system for coarse azimuth tracking. This facilitated a simple, effective method of instrument integration onto the payload. The new design reduced the mass of the FTS from 90to55kg and reduced the power consumption from 145to65W.

The concentration profile of nitric acid and other species over Saskatchewan in August 1998: Retrieval from data recorded by thermal-emission radiometry

Abstract We present vertical mixing ratio profiles for nitric acid (HNO3) recorded during the Middle Atmosphere Nitrogen TRend Assessment (MANTRA) 1998 balloon flight over Saskatchewan, Canada. The profiles are based on radiance spectra containing HNO3 thermal‐emission features and were collected during balloon ascent and over a latitude and longitude interval (52°± 0.2°N, 106.6° ± 0.5°W) between 3:30 and 6:30 am local time (9:30 and 12:30 UTC), 24 August 1998. The spectra were simultaneously recorded by two radiometer instruments in the 715–1250 cm−1 atmospheric window at an approximate instrument resolution of 20 cm−1. Profiles of CFC‐11, CFC‐12, ozone (O3), methane (CH4) and nitrous oxide (N2O) based on emission features in the same observation window are also presented. Raw radiance measurements are analysed using a forward estimation technique to recover multiple gas profiles from very low‐spectral resolution measurements of atmospheric radiance. The technique uses detailed atmosphere and instrument models and a least‐mean‐squares estimator to iterate maximum‐likelihood volume mixing ratio (VMR) from 7 to 30 km on a 2‐km grid. The analysis approach described is adaptable to other retrievals of multiple constituents from low‐resolution spectra recorded by lower‐cost, robust instrumentation developed for balloon and space flight. Averaging kernels and an error analysis are included in order to illustrate instrument sensitivity to vertical composition and expected accuracy. Results from each instrument compare favourably and show close agreement with HNO3 climatology results based on satellite observations made by the Microwave Limb Sounding (MLS) instrument, 1992–94.

The Effect of Instrumental Stray Light on Brewer and Dobson Total Ozone Measurements

The article is interesting as the effect of the stray light on the ozone cross section calculation were not studied on the past.I have a few comments which I would the authors to answer, pending those I support the publication of the manuscript. The principal comment i repeat from my first evaluation is why they don’t use the Serdyuchenko et al. (2014) cross section in his calculations , when is now the recommended ozone cross section for Brewer and Dobson. Moreover some of the discussions 5 of the paper like the AD/CD ozone difference in the Dobson measurements and the Brewer/Dobson differences are also affected with the change of cross section (Redondas et al., 2014). The discussion (Section 3.1 ) is still difficult to follow especially the Dobson section (see specific comments). The second point to mention is the ETC calculation in section 3.2, is not clear how is calculated, in particular how is related from Chance and Kurucz (table 2). 10 As suggested by the referee Julian Groebner on the discussion on the paper also part of this special number ((Redondas et al., 2018)). The difference between the use of trapezoid slit and a triangular slit is about 0.7% in a double brewer. Can you include this case in your calculations ?. The error introduced by the assumption of the fixed air-mass is also showed but could be more illustrative to show the difference in ozone rather than in airmass. The effect on the Dobson record at South Pole was also studied by (Bernhard et al.) 15 a comparison with his results could be also illustrative.

Comparison of ground-based and satellite measurements of water vapour vertical profiles over Ellesmere Island, Nunavut

Improving measurements of water vapour in the upper troposphere and lower stratosphere (UTLS) is a priority for the atmospheric science community. In this work, UTLS water vapour profiles derived from Atmospheric Chemistry Experiment (ACE) satellite measurements are assessed with coincident ground-based measurements taken at a high Arctic observatory at Eureka, Nunavut, Canada. Additional comparisons to satellite measurements taken by the Atmospheric Infrared Sounder (AIRS), Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), Microwave Limb Sounder (MLS), Scanning Imaging Absorption Spectrometer for Atmospheric CHartography (SCIAMACHY), and Tropospheric Emission Spectrometer (TES) are included to put the ACE Fourier transform spectrometer (ACE-FTS) and ACE Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation (ACE-MAESTRO) results in context. Measurements of water vapour profiles at Eureka are made using a Bruker 125HR solar absorption Fourier transform infrared spectrometer at the Polar Environment Atmospheric Research Laboratory (PEARL) and radiosondes launched from the Eureka Weather Station. Radiosonde measurements used in this study were processed with software developed by the Global Climate Observing System (GCOS) Reference Upper-Air Network (GRUAN) to account for known biases and calculate uncertainties in a well-documented and consistent manner. ACE-FTS measurements were within 11 ppmv (parts per million by volume; 13 %) of 125HR measurements between 6 and 14 km. Between 8 and 14 km ACE-FTS profiles showed a small wet bias of approximately 8 % relative to the 125HR. ACE-FTS water vapour profiles had mean differences of 13 ppmv (32 %) or better when compared to coincident radiosonde profiles at altitudes between 6 and 14 km; mean differences were within 6 ppmv (12 %) between 7 and 11 km. ACE-MAESTRO profiles showed a small dry bias relative to the 125HR of approximately 7 % between 6 and 9 km and 10 % between 10 and 14 km. ACE-MAESTRO profiles agreed within 30 ppmv (36 %) of the radiosondes between 7 and 14 km. ACE-FTS and ACE-MAESTRO comparison results show closer agreement with the radiosondes and PEARL 125HR overall than other satellite datasets – except Published by Copernicus Publications on behalf of the European Geosciences Union. 4040 D. Weaver et al.: Comparison of ground-based and satellite measurements of water vapour for AIRS. Close agreement was observed between AIRS and the 125HR and radiosonde measurements, with mean differences within 5 % and correlation coefficients above 0.83 in the troposphere between 1 and 7 km. Comparisons to MLS at altitudes around 10 km showed a dry bias, e.g. mean differences between MLS and radiosondes were − 25.6 %. SCIAMACHY comparisons were very limited due to minimal overlap between the vertical extent of the measurements. TES had no temporal overlap with the radiosonde dataset used in this study. Comparisons between TES and the 125HR showed a wet bias of approximately 25 % in the UTLS and mean differences within 14 % below 5 km.

Science commissioning of the atmospheric chemistry experiment (ACE)

The Atmospheric Chemistry Experiment (ACE) was launched in August 2003 on board the Canadian scientific satellite SciSat-1. The ACE payload consists of two instruments: ACE-FTS, a high resolution (0.02 cm-1) Fourier transform infrared spectrometer and MAESTRO (Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation), a dual UV-visible-NIR spectrograph. Primarily, the two instruments use a solar occultation technique to make measurements of trace gases, temperature, pressure and atmospheric extinction. It will also be possible to make near-nadir observations with the ACE instruments. The on-orbit commissioning of the instruments and spacecraft were undertaken in the months following launch. At the end of this period, a series of science-oriented commissioning activities were undertaken. These activities had two aims: the first was to verify and extend the measurement results obtained during the pre-launch Science Calibration Test campaign and the second was to confirm appropriate parameters and establish procedures for operational measurements (occultation and near-nadir observations and exo-atmospheric calibration measurements). One of the most important activities was to determine the relative location of each instrument field of view and optimize the pointing of the sun-tracker to provide the best viewing for both instruments.