Figure 10.

(a) Monthly box plot of the hourly signal to noise ratio (SNR) at all sites (afternoon hours only), calculated as the CO₂ enhancement (y_enh) above the background divided by the standard deviation of the backgrounds (Figure 8) for each hour. Red lines are medians; box edges indicate 25^th and 75^th percentiles; whiskers indicate the extent excluding outliers, which are shown in red + marks. The y-axis has been truncated for readability so some outliers, up to 30, are not shown. (b) Average SNR by month for each site. Black solid line in both panels indicates SNR=1.