A175-0011
New techniques for broadband cavity enhanced spectroscopy retrieval algorithms

Tuesday, 15 December 2020
Poster
Caroline Womack1, Rebecca A Washenfelder2, Maurice Roots3, Michael A Robinson4, Jakob Lindaas5, Ilann Bourgeois6, Jeff Peischl4, Jason M St Clair7 and Steve Brown8, (1)CIRES and NOAA Chemical Sciences Laboratory, Boulder, CO, United States, (2)NOAA ESRL, Chemical Sciences Laboratory, Boulder, CO, United States, (3)University of Maryland, Baltimore County, Baltimore, United States, (4)CIRES and NOAA ESRL, Chemical Sciences Laboratory, Boulder, CO, United States, (5)Harvard University, Earth and Planetary Sciences, Cambridge, MA, United States, (6)NOAA Chemical Sciences Division, Boulder, CO, United States, (7)University of Maryland Baltimore County, Baltimore, MD, United States, (8)Chemical Sciences Laboratory, NOAA Earth System Research Laboratories, Boulder, CO, United States
Abstract:
Broadband cavity enhanced spectroscopy (BBCES) uses a high-finesse optical cavity to measure extinction spectra of ambient air samples with high sensitivity. If the air sample contains gas-phase absorbers, the measured absorption spectrum can be used to derive their concentrations. If it contains aerosol particles, the measured spectrum yields information about the particle optical properties. Both strategies require spectral retrieval algorithms. We present here new insights into those algorithms for two BBCES instruments: the NOAA ACES gas-phase spectrometer for NO2, glyoxal and HONO, and the NOAA IBBCEAS aerosol spectrometer.

In gas-phase spectrometers such as ACES, differential optical absorption (DOAS) routines such as DOASIS, Win-DOAS, and Q-DOAS have often been used to extract the number densities of the gas-phase absorbers that contribute to the total absorption spectrum. DOASIS was originally developed for DOAS instruments and includes functionality to account for Fraunhofer lines, the Ring effect, and Mie scattering by aerosols. However, the in situ BBCES instruments do not require these corrections. Therefore, we have developed a simplified standalone version of the non-linear least squares fitting program that makes up core of the DOASIS program, written in Igor Pro. We compare the new version with DOASIS for NO2 concentrations measured on board the NASA DC-8 aircraft during the 2019 FIREX-AQ campaign. We find the retrieval program performs well against two other independent NO2 instruments: the NOAA NOyO3 chemiluminescence instrument and the NASA CANOE LIF instrument.

We have recently implemented ultra-broadband cavity mirrors and a laser-driven light source into the IBBCEAS instrument that extended its wavelength range to 360 – 720 nm. The new cavity mirrors had high variability in transmission and reflectivity as a function of wavelength, which initially resulted in artifacts in the resulting spectra. However, we attribute this to drifts in the optical and light source stability, and can make an empirical correction for these drifts. This process has led to new insights about the nature of broad drifts in other BBCES spectra, typically fit with a fourth order polynomial.

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