Florian Nichitiu

Atmospheric Chemistry Experiment (ACE): Mission overview

SCISAT-1, also known as the Atmospheric Chemistry Experiment (ACE), is a Canadian satellite mission for remote sensing of the Earths atmosphere. It was launched into low Earth circular orbit (altitude 650 km, inclination 74°) on 12 Aug. 2003. The primary ACE instrument is a high spectral resolution (0.02 cm-1) Fourier Transform Spectrometer (FTS) operating from 2.2 to 13.3 μm (750-4400 cm-1). The satellite also features a dual spectrophotometer known as MAESTRO with wavelength coverage of 285-1030 nm and spectral resolution of 1-2 nm. A pair of filtered CMOS detector arrays records images of the Sun at 0.525 and 1.02 μm. Working primarily in solar occultation, the satellite provides altitude profile information (typically 10-100 km) for temperature, pressure, and the volume mixing ratios for several dozen molecules of atmospheric interest, as well as atmospheric extinction profiles over the latitudes 85°N to 85°S. This paper presents a mission overview and some of the first scientific results. Copyright 2005 by the American Geophysical Union.

Evidence of vertical transport of carbon monoxide from Measurements of Pollution in the Troposphere (MOPITT)

[1]xa0Vertical profiles of carbon monoxide (CO) mixing ratio retrieved from MOPITT measurements have been analyzed. We find that variations in the vertical structure of CO can be detected in the MOPITT data. The Asian summer monsoon plume in CO is observed for the first time as a strong enhancement of CO in the upper troposphere (UT) over India and southern China indicating the effect of deep convective transport. Similarly, zonal mean height latitude cross-sections for the months of September–December, 2002 indicate deep convective transport of CO from biomass burning in the southern tropics. These findings show that MOPITT CO can provide valuable information on vertical transport phenomena in the troposphere.

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).

Measurement of low‐altitude CO over the Indian subcontinent by MOPITT

We show that the dayside MOPITT retrievals in the lower troposphere can provide useful information on surface sources of atmospheric CO over the Indian subcontinent. We find that MOPITT retrievals at 850 hPa show localized enhancements over the Indian subcontinent, which correlate with similar enhancements seen in the tropospheric NO2 columns from the SCIAMACHY instrument. In particular, high concentrations of CO over the Indo-Gangetic basin and some prominent cities are captured in the lower-tropospheric retrievals in spring. MOPITT averaging kernels (normalized to take into account the absorber amounts in the layers) indicate that the retrievals are sensitive to CO in the lower troposphere. In winter, MOPITT retrievals at 850 hPa can detect the strongest source areas over the eastern states of Bihar and West Bengal, thus confirming the so-called “Bihar pollution pool,” which was detected earlier in the aerosol measurements by the multiangle imaging spectroradiometer (MISR) aboard Terra. The pollution features are consistent with the spatial distribution of CO emissions in India, as reflected in the GEOS-Chem simulation of CO. Furthermore, these lower-tropospheric features in the simulation are still present after smoothing the modeled fields using the MOPITT averaging kernels and a priori profile, demonstrating that the retrievals do have sensitivity in the lower troposphere. This work indicates that although MOPITT retrievals are often most sensitive to CO in the middle and upper troposphere, they do provide information on lower-tropospheric CO in selected continental regions with strong thermal contrast and could be useful for pollution studies.

Large horizontal gradients in atmospheric CO at the synoptic scale as seen by spaceborne Measurements of Pollution in the Troposphere

[1]xa0We have examined the influence of synoptic processes on the distribution of atmospheric CO as observed by the Measurements of Pollution in the Troposphere (MOPITT) satellite instrument. In the MOPITT data, large horizontal gradients in CO, coherent at the synoptic scale, have been observed. The concentration of CO varies rapidly by as much as 50–100% across distances of ∼100 km, forming distinct boundaries in the CO distribution. These can last one to several days and span horizontal distances of 600–1000 km. On average, such events were observed in the MOPITT CO daily images once every 3–4 days over North America in spring and summer 2000. We focused on three case studies over North America in August 2000 to understand the mechanisms responsible for the large gradients in CO. Through an analysis of meteorological data from the National Centers for Environmental Prediction/National Center for Atmospheric Research Reanalysis, parcel trajectory modeling, and global three-dimensional chemical transport modeling, we found that the large horizontal gradients typically reflect the differential vertical and horizontal transport of air with different chemical signatures. In the first case, the large gradients in CO over North Dakota resulted from the lifting ahead of a cold front that transported boundary layer air enriched with CO from forest fires in Montana, combined with the descent of clean air from the Canadian Prairies behind the front. In the second case, the large gradients over northeastern Texas were produced by the convective lifting over Arkansas of air with high concentrations of CO from the oxidation of volatile organic compounds and the onshore transport of clean air from the Gulf of Mexico. In the third case, we examined an example of outflow of surface pollution from North America by a cyclone. The largest gradients in this case were observed along the boundary between the boundary layer air transported by the warm conveyor belt ahead of the cold front and the clean air transported from the Atlantic by the semipermanent high-pressure system in the central Atlantic. Our results demonstrate that MOPITT can capture the influence of synoptic processes on the horizontal and vertical distribution of CO. The large gradients in CO observed on synoptic scales represent valuable information that can be exploited to improve our understanding of atmospheric CO. In particular, these results suggest that the MOPITT observations provide a useful data set with which to address a range of issues from air quality on local/regional scales to long-range transport of pollution on continental/global scales.

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%.

Carbon monoxide (CO) maximum over the Zagros mountains in the Middle East: Signature of mountain venting?

[1]xa0We report an intriguing feature observed in daytime measurements of CO over the Middle East, in spring and summer, by the Measurements of Pollution in the Troposphere (MOPITT) satellite instrument. Enhanced CO is observed over the Zagros mountains of Iran, following the local topography over this region (25–40N, 40–60E). The MOPITT averaging kernels do not seem to indicate any data artifacts in this area. We argue that this feature likely forms by the process of mountain venting by thermal winds caused by strong daytime differential heating. This is consistent with an analysis of vertical velocity in the NCEP reanalysis data in this region. The phenomenon was observed in all the years of available MOPITT measurements and may have implications for the pollution episodes in the region and the Middle East ozone maximum that has been observed earlier.

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.

MOPITT flight operations

This paper will describe the day-to-day control of the Measurements Of Pollution In The Troposphere (MOPITT) instrument. The MOPITT onboard software is designed to make maintenance of the instrument fairly simple in nature. Very few commands are required during normal operations, and the instrument, once set up, can be run for several months with no additional intervention. The ground system that controls the Terra spacecraft, of which MOPITT is one instrument, allows both real time commanding and scheduled commanding. The scheduled commanding can be planned far in advance and uses pre-tested blocks of command sequences to do more complicated instrument functions, such as long calibration events. MOPITT commands are arranged in a hierarchy that allows top level routine commanding to be carried out efficiently, but also permits bottom level commanding to deal with unforeseen conditions. The extensive use of tables, and the ability to update the permanent memory on-orbit, all contribute to a simple yet powerful, control capability. Real time commanding is directed by MOPITT IOT (Instrument Operations Team) members at the University of Toronto (UoT) on a voice line that is permanently connected to the Earth Observing System (EOS) Operations Center (EOC) in Greenbelt, Maryland. A data link permits real-time displays of the instrument status to be viewed by the Toronto personnel. EOC personnel send instrument commands after confirmation from UoT personnel. This method of operation is extremely reliable and has been used extensively to do routine maintenance and configuration changes of the MOPITT instrument.

The calibration of the MOPITT instrument

The MOPITT (Measurements Of Pollution In The Troposphere) instrument aboard the Terra Spacecraft was launched on Dec. 18, 1999 and has operated successfully since then. Instrument radiances are calculated from a total of 8 channels, which are combined in a retrieval scheme to measure the carbon monoxide (CO) profile and methane (CH/sub 4/) column in the troposphere. The instrument gain and offset, which are the key parameters to utilize the instrument measurements and to evaluate performance, are determined through an in-flight 2-point calibration scheme. Fluctuations and trends in the gain and offset on various time scales can be understood in terms of the instrument design, its performance, and the thermal environment. Some techniques for optimizing the noise levels as well as alternative methods of data processing, such as are required to cope with instrument anomalies, will be discussed.