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Measured off-axis ICOS transmission spectra of an uncontaminated water standard, a 100 ppmv methanol-in-water mixture (blue), and a 1% ethanol-in-water mixture (green). Insets show the non-linear, least-squares fits to the measured water standard and methanol mixture in red with residuals shown in grey. The methanol adds discrete, narrowband absorptions that can be clearly identified in the marked regions of interest. Ethanol (and larger organics) acts as a broadband absorber, which shifts the baseline offset coefficient, b0.
Δδ 18O scales linearly with m BB , whereas Δδ 2H follows a 3rd order polynomial. Standard #1 and Standard #2 were measured twice for each ethanol concentration (total of four points at each doping level). Data points are an average of 4 injections and error bars show the standard error of the average. Note that the data are plotted versus (m BB − 1) such that isotope measurement error is zero at m BB = 1. Fits are forced through (0,0). The approximate ethanol concentration is shown on the upper x axis. Data plotted are from instrument #4.
Δδ 18O and Δδ 2H scale linearly with log(m NB ). Standard #1 and Standard #2 were measured twice for each methanol concentration (total of four points at each doping level). Data points are an average of 4 injections and error bars show the standard error of the average. Note that the x axis zero is defined by the average log(mNB) value of uncontaminated water standards. Fits are forced through (0,0). For larger values of m NB , a small deviation from linear behavior is visible; other groups have used a piecewise function to describe this relationship but observed a similar logarithmic trend.10 Approximate methanol concentration is shown on the upper x axis. Data plotted are from instrument #7.
Isotope error vs. metric fits for all 14 instruments: (a) 3rd order polynomial fits to Δδ 2H vs. m BB -1 showing a wide variety of responses to contamination with ethanol. Poor fits (i.e., low R2) typically have a small total deviation, indicating minimal error dependence on m BB and thus ethanol contamination. (b) Linear fits to Δδ 18O vs. m BB −1. The terminal markers in (a) and (b) correspond to 2% ethanol. (c) Linear fits to Δδ 2H vs. log(m NB ). (d) Linear fits to Δδ 18O vs. log(m NB ). The terminal markers in (c) and (d) correspond to 100 ppmv methanol. R2 values for each fit are shown in the legend.
Average absolute deviation after correction using the metrics described in this note. Black points were contaminated with a maximum of 100 ppmv methanol, red points with a maximum 2% ethanol.
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