A53G-01 INVITED
Remote Sensing of Cloud Properties: Beyond the A-Train
The combination of active and passive sensors on the A-Train satellites provides the best data set to date for studying cloud and aerosol properties from space. Lidar and mm-wavelength radar provide an unprecedented global view of aerosol and cloud vertical structure. Multi-spectral imaging and thermal wavelength interferometry add spatial context and complementary information on cloud properties and atmospheric state. Science applications of this rich data set are now becoming available. The lifetime of the A-Train constellation is uncertain but the planned duration of the active sensors is only a few years. The planned joint European-Japanese EarthCARE mission with a scheduled launch in 2013 should supply a somewhat similar dataset. The question for the cloud-aerosol community is whether these systems are adequate for our science needs and, if not, what are the measurement needs? The current systems are largely focused on producing global datasets of aerosol and cloud properties. They are less useful for addressing process studies, particularly the impacts of aerosol on cloud properties. Here we examine the requirements of a mission that would target the aerosol-cloud problem and describe a satellite constellation of instruments that can meet these requirements.
A53G-02 INVITED
A Portable FTIR Analyser for Field Measurements of Trace Gases and their Isotopologues: CO2, CH4, N2O, CO, del13C in CO2 and delD in water vapour
We describe a portable Fourier Transform InfraRed (FTIR) analyser capable of simultaneous high precision analysis of CO2, CH4, N2O and CO in air, as well as δ13C in CO2 and δD in water vapour. The instrument is based on a commercial 1 cm-1 resolution FTIR spectrometer fitted with a mid-IR globar source, 26 m multipass White cell and thermoelectrically-cooled MCT detector operating between 2000 and 7500 cm-1. Air is passed through the cell and analysed in real time without any pre-treatment except for (optional) drying. An inlet selection manifold allows automated sequential analysis of samples from one or more inlet lines, with typical measurement times of 1-10 minutes per sample. The spectrometer, inlet sampling sequence, real-time quantitative spectrum analysis, data logging and display are all under the control of a single program running on a laptop PC, and can be left unattended for continuous measurements over periods of weeks to months. Selected spectral regions of typically 100-200 cm-1 width are analysed by a least squares fitting technique to retrieve concentrations of trace gases, 13CO2 and HDO. Typical precision is better than 0.1% without the need for calibration gases. Accuracy is similar if measurements are referenced to calibration standard gases. δ13C precision is typically around 0.1‰, and for δD it is 1‰. Applications of the analyser include clean and polluted air monitoring, tower-based flux measurements such as flux gradient or integrated horizontal flux measurements, automated soil chambers, and field-based measurements of isotopic fractionation in soil-plant-atmosphere systems. The simultaneous multi–component advantages can be exploited in tracer-type emission measurements, for example of CH4 from livestock using a co-released tracer gas and downwind measurement. We have also developed an open path variant especially suited to tracer release studies and measurements of NH3 emissions from agricultural sources. An illustrative selection of applications will be presented.
A53G-03
Aero3X: Fast, Accurate Measurement of Aerosol Optical Properties for Climate and Air Quality Studies
Aerosols (particulate matter) have a dramatic effect on radiative forcing of the climate, in some cases cooling and in other cases warming. The Fourth Assessment Report of the IPCC estimates that direct radiative forcing due to all aerosols is a cooling of -0.50 W m-2 with absorbing aerosol (black carbon) responsible for a warming of +0.22 W m-2, but the uncertainties associated with these numbers are very large. Better measurements of the optical properties of aerosols, especially absorption coefficient and asymmetry parameter, and their spatial and temporal distribution are required to reduce these uncertainties and improve the ability of models to predict climate change. Aero3X was designed to provide such measurements. It is a light weight (11 kg), compact (0.25 x 0.30 x 0.6 m), and fast (1 Hz sample rate) instrument intended for use on Unmanned Aerial System (UAS) but suitable for flight on other aircraft and for surface measurements. Aero3X uses an off-axis cavity ring-down technique to measure extinction coefficient and a reciprical nephelometry technique for measurement of total-, forward- and back-scatter coefficients at wavelengths of 405 nm and 675 nm. Its outstanding precision (0.1 Mm-1) and sensitivity (0.2 Mm- 1) allow the determination of absorption coefficient, single-scattering albedo, estimates of backscatter to extinction ratio and asymmetry parameter at both wavelengths, and Angstrom exponent. Together with its humidification system for measurement of the dependence of aerosol optical properties on relative humidity, these represent a complete set of the aerosol optical properties important to climate and air quality. Aero3X was designed to operate in pollution plumes where NO2 may cause interference with the measurement, therefore, a measurement of NO2 mixing ratio is also made. The paper will cover the main points of instrument design and present performance data obtained in Ames Research Center labs with calibration aerosol and at Lawrence Berkeley Laboratory with black carbon aerosol. A comparison with more standard instrumentation will be presented. A theoretical study showing that the combination of measurements made by Aero3X can give a rough idea of aerosol composition will also be presented.
A53G-04
Long-Term Continuous Isotopic Ratio Measurements of Atmospheric CO2 Using a 4.3 Micron Pulsed Quantum Cascade Laser
Real time methods to monitor the stable isotopic ratios of carbon dioxide are needed to quantify the sources and sinks of this centrally important greenhouse gas. This is an extreme instrumental challenge since the ratios need to be measured with an accuracy of at least one part in ten thousand or 0.1 ‰. We use tunable infrared laser differential absorption spectroscopy with pulsed QC lasers (QC-TILDAS) to address this challenge. QC lasers are attractive optical sources due to: 1) absence of cryogenic fluids, 2) stable single mode spectral output, 3) optical simplicity. Our optical system employs a dual-cell arrangement with ambient air flowing through the sample cell and a reference gas flowing through the reference cell. Both 13C/12C and 18O/16O ratios can be obtained in a single spectral window near 2310 cm-1. Spectral analysis is applied to the ratio of the sample and reference spectra, canceling correlated noise components. Because the absorption lines of the isotopic species have different temperature dependences, temperature stability is critical. The optical system is temperature controlled and the two multipass cells are thermally coupled and individually monitored with a precision of 1 mK. The ratio technique compensates for temperature sensitivity, as well as for drifts in laser line width, frequency, tuning rate and power variation. The long term stability of the thermally stabilized dual-cell QCL system has been evaluated using the Allan variance technique. The Allan plot shows a 1-sec RMS noise of 0.2 per mil, and a minimum RMS noise of 0.03 per mil after 300 sec integration using a liquid nitrogen cooled detector. We have used this instrument to perform long term (more than 6 months) monitoring of ambient air from the roof top of our laboratory. The data show variations in R13 and R18 that follow diurnal cycles. Winter data show clear correlations with morning and evening signatures of local automobile traffic. Data collected during spring are distinctly different and seem to show evidence of plant respiration.
A53G-05
Precise Measurement of Carbon Dioxide Column by Passive Ground Based Sensor
Over the past four years we have developed a family of differential radiometers based upon the Fabry-Perot interferometers that exhibit very great sensitivity to changes in the atmospheric column of carbon dioxide, oxygen, and water vapor. Our instruments employ a solid Fabry-Perot etalon that is tuned to the proper wavelength by changing its temperature. The thickness of the etalon has been selected so that its multiple pass bands align with regularly space absorption features of the molecule under investigation. Using multiple absorption features improves the optical throughput of the instrument and improves the stability of the instrument response with respect to environmental changes. We are presently working to extend this technique to the carbon 13 isotope of carbon dioxide and to methane. Our instruments are intrinsically rugged and can be fabricated in a small package at relatively low cost.. As such they hold promise for widespread use in ground based networks for calibration and validation of satellite instruments such as OCO and GOSAT. Results will be presented for long term ground based operations of these systems. The effects of atmospheric scattering, pointing errors, pressure broadening and temperature effects will be discussed with regard to achieving precision better than .5% required for validation of carbon dioxide column measured from space. Finally we will outline the approach for extension of this methodology to additional molecular species of interest.
A53G-06
Measuring 35S of Aerosol Sulfate: Techniques and First Results
On a global and regional level, the cycling of sulfur in the environment has consequences for air quality, human health, and may contribute to global climate change. Due to its multiple oxidation states, the sulfur cycle is very complex and poorly understood. Stable isotopes are currently used to understand reaction pathways as well as sources and sinks of sulfurous compounds in the environment. Sulfur also has one short lived (τ1/2 ~87 d) radioactive isotope (35S) which is continuously made in the atmosphere by the cosmic ray spallation of argon, is then quickly oxidized to 35SO2 and enters the atmospheric sulfur cycle. The short-lived radioactive nature of this isotope of sulfur provides us with potentially powerful tracer for understanding the time scales at which sulfur is oxidized, deposited, and transported in the atmosphere and the deposition of atmospheric sulfate into rivers and water catchments. However, despite its potential, the use of 35S as a tracer of aerosol chemistry has not been fully exploited, Here we present details of instrumental set up for measuring 35S in aerosol sulfate and some preliminary results of measurements of 35S abundances in aerosols from Riverside (inland) and La Jolla (coastal) CA and discuss the sensitivity and limitations of the measurements in providing insights into day/night aerosol chemistry (Riverside) as well as the uptake of SO2 pollution in coastal environments by sea-salt aerosols. Also, we present preliminary results from measurement of sulfate in river water in Ecuador before and after precipitation events.