Abstract
Abstract. In complex and urban environments, atmospheric trace gas composition is highly variable in time and space. Point measurement techniques for trace gases with in situ instruments are well established and accurate, but do not provide spatial averaging to compare against developing high-resolution atmospheric models of composition and meteorology with resolutions of the order of a kilometre. Open-path measurement techniques provide path average concentrations and spatial averaging which, if sufficiently accurate, may be better suited to assessment and interpretation with such models. Open-path Fourier transform spectroscopy (FTS) in the mid-infrared region, and differential optical absorption spectroscopy (DOAS) in the UV and visible, have been used for many years for open-path spectroscopic measurements of selected species in both clean air and in polluted environments. Near infrared instrumentation allows measurements over longer paths than mid-infrared FTS for species such as greenhouse gases which are not easily accessible to DOAS.In this pilot study we present the first open-path near-infrared (4000–10 000 cm−1, 1.0–2.5 µm) FTS measurements of CO2, CH4, O2, H2O and HDO over a 1.5 km path in urban Heidelberg, Germany. We describe the construction of the open-path FTS system, the analysis of the collected spectra, several measures of precision and accuracy of the measurements, and the results a four-month trial measurement period in July–November 2014. The open-path measurements are compared to calibrated in situ measurements made at one end of the open path. We observe significant differences of the order of a few ppm for CO2 and a few tens of ppb for CH4 between the open-path and point measurements which are 2 to 4 times the measurement repeatability, but we cannot unequivocally assign the differences to specific local sources or sinks. We conclude that open-path FTS may provide a valuable new tool for investigations of atmospheric trace gas composition in complex, small-scale environments such as cities.
Highlights
The cycling of carbon between Earth’s surface and the atmosphere is dominated by carbon dioxide (CO2) and methane (CH4), which are the two most important anthropogenically influenced greenhouse gases
The positive spikes observed regularly near 18:00–19:00 local time on clear sunny days are due to direct sunlight scattered into the Fourier transform spectroscopy (FTS) and detector as described in the previous section – when the solar beam path is from the west at low elevation and approximately aligned with the optical path (Fig. 2), solar radiation is back-reflected from the retroreflectors and captured by the telescope
We have introduced a long open-path Fourier transform spectrometer operating in the near-infrared over a 3.1 km return path in open air
Summary
The cycling of carbon between Earth’s surface and the atmosphere is dominated by carbon dioxide (CO2) and methane (CH4), which are the two most important anthropogenically influenced greenhouse gases. The steady increases in CO2 and CH4 concentrations in the global atmosphere since industrialisation have been well documented by the global network of surface in situ point measurements (e.g. GLOBAL-VIEW-CO2, 2009). Such point-based in situ measurements in clean baseline air are well suited to monitoring long term global changes in atmospheric greenhouse gases ( including nitrous oxide (N2O) and other minor species), and have provided most of the data from which long term global trends have been assessed. To characterise and quantify individual sources and sinks of greenhouse gases, measurements in regional, urban, agricultural and industrial environments located near the sources and sinks, combined with fine-resolution local and regional-scale atmospheric transport modelling, are required. Lee et al (2017) trialled a network of five mobile CO2 sensors in the Vancouver urban area combined with an aerodynamic model to calculate fluxes
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