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Downsizing and compatibility with MEMS silicon foundries is an attractive path towards a large diffusion of photoacoustic trace gas sensors. As the photoacoustic signal scales inversely with the chamber volume, a trend to miniaturization has been followed by several teams. We review in this article the approach initiated several years ago in our laboratory. Three generations of components, namely a 40 mm3 3D-printed cell, a 3.7 mm3 silicon cell, and a 2.3 mm3 silicon cell with a built-in piezoresistive pressure sensor, have been designed. The models used take into account the viscous and thermal losses, which cannot be neglected for such small-sized resonators. The components have been fabricated either by additive manufacturing or microfabrication and characterized. Based on a compilation of experimental data, a similar sub-ppm limit of detection is demonstrated. All three versions of photoacoustic cells have their own domain of operation as each one has benefits and drawbacks, regarding fabrication, implementation, and ease of use. [ABSTRACT FROM AUTHOR]

Ambient concentrations of CO, as well as NOx and O3, were measured as a part of the PARADE campaign conducted at the Taunus Observatory on the summit of the Kleiner Feldberg between the 8th of August and 9th of September 2011. These measurements were made in an effort to provide insight into the characteristics of the effects of both biogenic and anthropogenic emissions on atmospheric chemistry in the rural south-western German environment. The overall average CO concentration was found to be 100.3±18.1 ppbv (within the range of 71 to 180 ppbv), determined from 10-min averages during the summer season. The background CO concentration was estimated to be ~90 ppbv. CO and NOx showed bimodal diurnal variations with peaks in the late morning (10:00-12:00 UTC) and in the late afternoon (17:00-20:00 UTC). Strong correlations between CO and NOx indicated that vehicular emission was the major contributor to the notable CO plumes observed at the sampling site. Both local meteorology and backward trajectory analyses suggest that CO plumes were associated with anthropogenically polluted air masses transferred by an advection to the site from densely populated city sites. Furthermore, a good linear correlation of R2 = 0.54 between CO and O3 (∆O3/∆CO=0.560±0.016 ppbv/ppbv) was observed, in good agreement with previous observations

Ambient concentrations of CO, as well as NOx and O3, were measured as a part of the PARADE campaign conducted at the Taunus Observatory on the summit of the Kleiner Feldberg between the 8th of August and 9th of September 2011. These measurements were made in an effort to provide insight into the characteristics of the effects of both biogenic and anthropogenic emissions on atmospheric chemistry in the rural south-western German environment. The overall average CO concentration was found to be 100.3±18.1 ppbv (within the range of 71 to 180 ppbv), determined from 10-min averages during the summer season. The background CO concentration was estimated to be ~90 ppbv. CO and NOx showed bimodal diurnal variations with peaks in the late morning (10:00-12:00 UTC) and in the late afternoon (17:00-20:00 UTC). Strong correlations between CO and NOx indicated that vehicular emission was the major contributor to the notable CO plumes observed at the sampling site. Both local meteorology and backward trajectory analyses suggest that CO plumes were associated with anthropogenically polluted air masses transferred by an advection to the site from densely populated city sites. Furthermore, a good linear correlation of R2 = 0.54 between CO and O3 (∆O3/∆CO=0.560±0.016 ppbv/ppbv) was observed, in good agreement with previous observations [ABSTRACT FROM AUTHOR]

Eddy-covariance (EC) scalar-flux measurements suffer from unavoidable biases introduced by high-frequency losses in the sampled scalar concentration fluctuations. This bias alone leads to an underestimation of scalar fluxes by as much as 20% in some cases, especially when a closed-path gas analyzer is used to sample concentration far from the inlet location. A novel method that directly corrects for these high-frequency losses using only the sampled scalar-concentration time series is proposed and tested. The sampled concentration fluctuation time series is adjusted, point-by-point, in the wavelet half-plane for each EC averaging interval (≈30 min). Similarity between scalars (and temperature) is not necessary and a pre-defined theoretical shape of the cospectrum is not required, making this method attractive at meteorologically non-ideal sites. When closed-path gas analyzers are used to measure HO concentration fluctuations, the method is shown to reproduce the dependence of the attenuation on air relative humidity. Nevertheless, the method is not able to account for excessively large spectral attenuation that occurs close to the spectral peak, as might be the case with long tubes and high relative humidity. Since the method corrects the original scalar concentration time series and not the cospectrum, other flow statistics-such as variances and integral time scales-are also adjusted. The proposed method can be used synergistically with conventional high-frequency cospectral correction methods given the differences in assumptions and approaches among these methods. When the conventional and the proposed methods agree, added confidence to the estimate of the high frequency correction is gained, and vice versa. [ABSTRACT FROM AUTHOR]

Abstract: The present paper describes an extremely sensitive apparatus based upon laser photoacoustic spectroscopy (LPAS) methods which can be used for the detection and measurement of trace gases at very low concentrations (parts per trillion by volume – pptV). Two experimental set-ups were designed and characterized with the photoacoustic (PA) cell in an external configuration: the first one with a low power CO2 laser where the saturation effects are negligible, and a second one with a high power CO2 laser where the saturation effects are important and have to be taken into consideration. In the first case, the minimum detectable concentration was 0.9ppbV (parts per billion by volume), while in the second case this parameter was improved to 0.29ppbV. Comparing with the best results published previously in the literature, our minimum detectable concentration is better by a factor of 4.2 in the first case and by a factor of 13.1 in the second case. All measurements were done in nitrogen and ethylene with the 10P(14) line of a continuous wave CO2 laser. This technology can dramatically impact detection in numerous areas. [Copyright &y& Elsevier]

The presence of water vapor in a sample of air reduces the concentration of a trace gas measured from the sample. We present a methodology to correct for this effect for those cases when the concentration of the trace gas has already been measured from a wet sample. The conversion or correction factor that takes the wet mole fraction to a dry mole fraction is determined by the mixing ratio of water vapor inside the sampling canister. For those samples where the water vapor is saturated inside the canister, the water vapor mixing ratio is largely determined by laboratory conditions; for the unsaturated samples, the mixing ratio is determined by station conditions. If the meteorology at the sampling station is known, the equations presented here can be used directly to calculate the appropriate correction factor. For convenience, we use climatological data to derive average monthly correction factors for seven common global sampling sites: Barrow, AK, US (71°N, 157°W); Cape Meares, OR, US (45°N, 124°W); Mauna Loa, HI, US (19°N, 155°W); Ragged Point, Barbados (13°N, 59°W); American Samoa (14°S, 171°W); Cape Grim, Tasmania, Australia (41°S, 145°E); South Pole (90°S). These factors adjust wet mole fractions upwards within a range of 0.002% for the South Pole to over 0.8% for saturated sites. We apply the correction factors to wet nitrous oxide (N2O) mole fractions. The corrected data are more consistent with our understanding of N2O sources. [Copyright &y& Elsevier]