Atmospheric trace gas measurements in the tropics

Abstract

Fourier Transform Infrared (FTIR) spectrometry has been used for ground-based solar absorption, laboratory and flux measurements, to study the atmospheric composition, as well as physical and chemical processes in the atmosphere.The solar absorption FTIR measurements have been performed in Paramaribo, Suriname (5.8 N, 55.2 W) between September 2004 and November 2007 and represent the first remote sensing measurements in the inner tropics over severalyears. These measurements are of great importance for a better understanding of global climate and physical and chemical processes of the tropical atmosphere as well as for satellite validations. Vertical profiles of carbon monoxide (CO) and ethane (C2H6) and total columns of methane (CH4), hydrogen cyanide (HCN) and acetylene (C2H2) have been retrieved from the FTIR spectra. The quality of the methane retrieval was limited by the available spectroscopic data. Laboratory cell-based FTIR measurements have been performed to correct the methane spectroscopy in the infrared spectral region, which significantly improved the retrieval of methane from SCIAMACHY and FTIR spectra. The retrieval of methane profiles from near-infrared FTIR spectra by optimal estimation significantly improved the results. The FTIR observations of methane are compared with TM5 model simulations and satellite observations from SCIAMACHY, and are the first validation of the SCIAMACHY retrieval in the tropics using remote sensing techniques. The ratio CH4/CO2, which can be measured directly from SCIAMACHY and FTIR, compares very good, while the column averaged volume mixing ratio (XVMR(CH4)) of SCIAMACHY do not agree with the FTIR observations. Model assumptions areused in the SCIAMACHY retrieval to derive the XVMR(CH4) from the directly measured CH4/CO2 ratio. The worse agreement of SCIAMACHY XVMR(CH4) with FTIR compared to the SCIAMACHY CH4/CO2 ratio with FTIR could be attributed to unrealistic model assumptions used in the SCIAMACHY retrieval that led to wrong time series of the column averaged CH4 VMR. There is a good agreement of the FTIR XVMR(CH4) with the TM5 model. FTIR observations of carbon monoxide agree well with satellite data from the MOPITT instrument for all of the measurement campaigns. Simulations of CO and C2H6 from the MATCH-MPIC model reproduce the mean vertical structure of the FTIR observations. The model is generally not able to reproduce the extreme enhancements seen during the specific biomass burning events by both observation instruments. Nevertheless, the model indicates that beyond the backgroundsource of CO from methane oxidation, the main contributions to the CO mixing ratios are the episodic convective injection of NMHCs and CO from South American biomass burning into the upper troposphere, along with long range transport of African biomass burning CO, particularly during spring. Revised simulation of the MATCH-MPIC model with improved biomass burning emissions still fails toreproduce most of the individual observed pollution events. It generally underestimates the observed concentrations of carbon monoxide and ethane. The revised model is in better agreement with the observations in the upper troposphere, while in the boundary layer and lower troposphere the revised model underestimates the FTIR measurements and results in underestimated total columns. Current generationatmospheric chemistry models underestimate OH is the tropical region and compensate for this part by too low isoprene emissions. It is speculated that, if a mechanism like e.g. isoprene recycling and realistic isoprene emissions would be included in current models, it would result in a powerful CO source in the boundary layer over Suriname from the isoprene oxidation. The CO oxidationrate would also increase due to higher OH concentrations. Because of the complex chemistry and transport processes, it is difficult to predict the exact changes without having done the simulations. The last part of this work presents the development of an advanced flux measurement technique, consisting of a cell-based FTIR analyser and a Relaxed Eddy Accumulation (REA) system, to enable automated and continuous flux measurements of atmospheric trace gases. The combination of the REA technique with the FTIR analyzer was tested successfully in the lab and during a three weeks field campaign. The FTIR-REA technique offers the capacity to measure a range of gases simultaneously under field conditions and enables long-term measurements and monitoring of atmospheric greenhouse gas fluxes.