Carbon monoxide mixing ratios over Oklahoma between 2002 and 2009 retrieved from Atmospheric Emitted Radiance Interferometer spectra

Abstract

Abstract. CO mixing ratios weighted over the bottom 2-km thick atmospheric layer between 2002 and 2009 were retrieved from downwelling infrared (IR) radiance spectra of the clear sky measured by a zenith-viewing Atmospheric Emitted Radiance Interferometer (AERI) deployed at the Southern Great Plains (SGP) observatory of the Atmospheric Radiation Measurements (ARM) Program near Lamont, Oklahoma. A version of the algorithm proposed by He at al. (2001) was significantly improved and validated. Essentially, the new algorithm retrieves a CO mixing ratio that is determined by the convolution of the a priori profile (assumed to be constant with altitude), the true profile, and the averaging kernel which maximizes near the surface. Approximately 70% of the CO signal comes from the boundary layer and the remaining 30% come from the lower part of the free troposphere. Archived temperature and water vapor profiles retrieved from the same AERI spectra through automated ARM processing were used as input data for the CO retrievals. We found the archived water vapor profiles required additional constraint using SGP Microwave Radiometer retrievals of total precipitable water vapor. Additionally, a correction for scattered solar light was developed. The retrieved CO was validated using simultaneous independently measured CO profiles. An aircraft supplied in situ CO measurements at altitudes up to 4572 m above sea level once or twice a week between March 2006 and December 2008. The aircraft measurements were supplemented with ground-based CO measurements at the SGP and retrievals from the Atmospheric IR Sounder (AIRS) above 5 km to create full tropospheric CO profiles. Comparison of the convolved profiles to the AERI CO retrievals found a squared correlation coefficient of 0.57, a standard deviation of ±11.7 ppbv, a bias of 16 ppbv, and a slope of 0.92. Averaged seasonal and diurnal cycles measured by AERI are compared with those measured continuously in situ at the SGP in the boundary layer. Monthly mean CO values measured by AERI between 2002 and 2009 are compared with those measured by AIRS over North America, the Northern Hemisphere mid-latitudes, and over the tropics.