Open path incoherent broad band cavity enhanced absorption spectroscopy for atmospheric trace gas detection and aerosol extinction measurements

Abstract

The earth s atmosphere consists of gases like nitrogen, oxygen, water newlinevapour, carbon dioxide and other trace gases and aerosol in small quantities. Trace newlinegases like nitrogen dioxide (NO 2 ), nitrous acid (HONO) etc. and radicals like nitrate newline(NO 3 ) and hydroxyl (OH) in polluted environments cause chemical reactions leading to newlinethe formation of toxic secondary pollutants. Accurate measurement methods for fast in- newlinesitu monitoring of these pollutants are important. In this study, incoherent broadband newlinecavity-enhanced absorption spectroscopy in the open-path configuration (OP- newlineIBBCEAS) was explored employing long optical paths for non-invasive in-situ newlinedetection of nocturnal NO 3 and other trace pollutants. The instrument designed and newlinedeveloped consisted of a transmitter (source) unit and a receiver (detector) unit, each newlinehousing one of the cavity mirrors and separated by open air. While most extractive newlineinstruments try to avoid interferences arising from aerosol extinction, the open-path newlineconfiguration has advantages owing to its ability to detect trace gases even in the newlinepresence of aerosol loading. Instrument control and automation software was developed newlinein LabVIEW platform. Spectral analysis algorithms were developed based on Singular newlineValue Decomposition (SVD) based linear least square fit. The analysis codes were newlinewritten in MATALB and were embedded inside the LabVIEW-based automation newlinesoftware. After laboratory calibrations, the spectrometer was taken for field newlineobservations of trace gases, primarily targeting nocturnal NO 3 in the 658 668 nm newlinespectral range. newlineFirst field application was the observation of the NO 3 in a polluted summer newlineenvironment in continental China. The instrument was installed 17 m above the ground newlineat the top of a residential complex near the CAREBeijing NCP 2014 site in Wangdu, newlineHebei province, about 200 km southwest of Beijing, P R China, over the period 28 30 newlineJune 2014. newline