Rodrigo Jiménez

Airborne measurements of HCHO and HCOOH during the New England Air Quality Study 2004 using a pulsed quantum cascade laser spectrometer

[1] Atmospheric mixing ratios of formaldehyde and formic acid have been measured from the NOAA WP-3 aircraft during the New England Air Quality Study (NEAQS) of July and August 2004 using a newly developed quantum cascade laser (QCL) spectrometer operating at a wavelength of 5.6 mm. The laser operates in pulsed mode with thermoelectric cooling. The detection is based on direct absorption in a compact 76-m multiple pass absorption cell. The laser is swept over a 0.5 cm 1 spectral region containing multiple lines of both HCHO and HCOOH. Absolute concentrations are retrieved by simultaneous spectral fitting routines with a detection limit (2s) for HCHO of 0.3 parts in 10 9 (ppbv) with an averaging time of 60 s under stable flight conditions. HCHO mixing ratios in the range from 0.3 to 5 ppb were encountered during flight conditions. Some of the highest mixing ratios of HCHO were observed over heavily vegetated areas of Florida during the test flights where the ratios of HCHO to methacrolein and methyl vinyl ketone, measured by proton transfer mass spectroscopy, are consistent with formaldehyde production by isoprene oxidation. The highest mixing ratios of HCOOH, up to 10 ppbv, were observed in an aged forest fire plume encountered over northern Canada, in which ratios of HCOOH/CO are greater than previous observations, while the ratios of HCHO/CO are less than previous reports from forest fire plumes. Observations of HCHO/CO and HCOOH/CO in urban plumes are indicative of a mixture of sources from direct emissions and secondary oxidation of anthropogenic and biogenic hydrocarbons. The ability to measure both HCHO and HCOOH simultaneously is of value in assessing the oxidation mechanisms of atmospheric hydrocarbons and secondary organic aerosol formation and oxidation.

A high precision pulsed quantum cascade laser spectrometer for measurements of stable isotopes of carbon dioxide

We describe a prototype instrument using a Peltier cooled quantum cascade laser for precise measurement of stable carbon (13C/12C) isotopologue ratios in atmospheric CO2. Using novel optics and signal processing techniques in a compact instrument, we are able to detect the difference between sample and reference with a precision of 0.1‰ (2σ standard error of mean of 11 samples) in 10min of analysis time. The standard deviation of 0.18‰ for individual 30 s measurements shows that this prototype instrument already approaches the best reported literature values using continuous wave lead alloy tunable diode lasers. The application of pulsed near room-temperature quantum cascade lasers to this demanding problem opens the possibility of field worthy rapid response isotopic instrumentation and attests to the maturity of these lasers as spectroscopic sources.

Atmospheric trace gas measurements using a dual quantum-cascade laser mid-infrared absorption spectrometer

We present an overview of the dual QC laser spectrometer developed at Aerodyne Research and various examples of its application for atmospheric trace gas detection. The instrument incorporates two pulsed QC lasers, a compact 76-m (or 56-m) multipass absorption cell, a dual HgCdTe detector, and a sophisticated signal generation, data acquisition and processing system. Recent findings and hardware innovations are highlighted. Our results show that the precision and minimal detectable absorbance obtainable with pulsed QC lasers are comparable to those achieved with cryogenically cooled CW Pb-salt lasers in spite of the broader laser linewidths inherent to pulsed operation. This is demonstrated through in situ measurements of several trace gases, including methane, nitrous oxide, carbon monoxide, formaldehyde, formic acid, nitrous acid and ethylene. Recent measurements of HCHO and HCOOH on board a NOAA aircraft are presented. The precision, stability and intrinsic accuracy of the instrument were assessed through inter-comparisons measuring CH4 and CO. These measurements were made either comparing two QC lasers sweeping over different transitions or comparing the dual QCL spectrometer and a standard instrument (NDIR CO). The absorbance precision achieved is typically 2x10-5 Hz-1/2. For long-lived species, such as CH4 and N2O, this implies 1-Hz fractional precisions of 0.1% or better, which fulfill the requirement for meaningful measurements from aircraft platforms. Spectroscopically derived mixing ratios are accurate within 5% or better. The spectrometer is equipped to perform automatic, periodic calibrations with zero and span gases whenever higher accuracy is required.

Infrared QC laser applications to field measurements of atmospheric trace gas sources and sinks in environmental research: enhanced capabilities using continuous wave QCLs

The advent of continuous wave quantum cascade lasers operating at near room temperature has greatly expanded the capability of spectroscopic detection of atmospheric trace gases using infrared absorption at wavelengths from 4 to 12 μm. The high optical power, narrow line width, and high degree of single mode purity result in minimal fractional absorptions of 5x10-6 Hz-1/2 detectable in direct absorption with path lengths up to 210 meters. The Allan plot minima correspond to a fractional absorbance of 1x10-6 or a minimum absorption per unit path length 5x10-11 cm-1 in 50s. This allows trace gas mixing ratio detection limits in the low part-per-trillion (1 ppt = 10-12) range for many trace gases of atmospheric interest. We present ambient measurements of NO2 with detection precision of 10 ppt Hz-1/2. The detection precision for the methane isotopologue 13CH4 is 25 ppt Hz-1/2 which allows direct measurements of ambient ratios of 13CH4/12CH4 with a precision of 0.5‰ in 100 s without pre-concentration. Projections are given for detection limits for other gases including COS, HONO and HCHO as CWRT lasers become available at appropriate wavelengths.

Development of a Quantum Cascade Laser-Based Detector for Ammonia and Nitric Acid

We have developed a compact, robust, atmospheric trace gas detector based on mid-infrared absorption spectroscopy using pulsed quantum cascade (QC) lasers. The spectrometer is suitable for airborne measurements of ammonia, nitric acid, formaldehyde, formic acid, methane, nitrous oxide, carbon monoxide, nitrogen dioxide and other gases that have line-resolved absorption spectra in the mid-infrared spectral region. The QC laser light source operates near room temperature with thermal electric cooling instead of liquid nitrogen which has been previously required for semiconductor lasers in the mid-infrared spectral region. The QC lasers have sufficient output power so that thermal electric cooled detectors may be used in many applications with lower precision requirements. The instrument developed in this program has been used in several field campaigns from both the Aerodyne Mobile Laboratory and from the NOAA WP3 aircraft. The Phase II program has resulted in more than 10 archival publications describing the technology and its applications. Over 12 instruments based on this design have been sold to research groups in Europe and the United States making the program both a commercial as well as a technological success. Anticipated Benefits The development of a sensitive, cryogen-free, mid-infrared absorption method for atmospheric trace gas detection will have wide benefits for atmospheric and environmental research and broader potential commercial applications in areas such as medical diagnostic and industrial process monitoring of gaseous compounds. Examples include air pollution monitoring, breath analysis, combustion exhaust diagnostics, and plasma diagnostics for semi-conductor fabrication. The substitution of near-room temperature QC lasers for cryogenic lead salt TDLs and the resulting simplifications in instrument design and operation will greatly expand the range of applications.

NO2 Vertical Column Density at the Marambio Antarctic Station as Retrieved by DOAS

A number of chemical species present in the stratosphere in very small concentrations (parts per billion and even smaller) contribute significantly to its chemical balance. One of the main stratospheric trace gases is nitrogen dioxide (NO2). This species acts as a restrictive agent for stratospheric ozone destruction (due to the chlorine monoxide), hence the importance of its study. We present a preliminary analysis of passive remote sensing measurements carry out at the Marambio Argentinean Antarctic Base (64.233° S; 56.616° W; 197 m amsl) during the months of January—February of 2008. The spectroscopy system consists of an optical fiber (400 μm core diameter and 6 m of longitude) and a portable spectral analyzer (spectrometer HR4000, Ocean Optics). The device analyzes diffuse solar spectral irradiance in the UV‐visible range (290–650 nm), collected and transferred by a zenith‐pointing optical fiber. The NO2 vertical column density (VCD) is derived from the radiance spectra using the DOAS (Differential O...