The influence of instrumental line shape degradation on NDACC gas retrievals
Atmospheric Measurement Techniques Discussions
Instrumental line shape (ILS) degradation can cause significant biases between global FTIR (Fourier transform infrared) networks if not properly treated. Currently, how ILS degradation influences the global NDACC (Network for Detection of Atmospheric Composition Change) gases retrieval and how much ILS deviation is acceptable for each NDACC gas are still not fully quantified. We simulated ILS degradations with respect to typical types of misalignment, and compared their influence on each NDACC
... ence on each NDACC gas. The sensitivities of total column, root mean square of fitting residual (RMS), total random uncertainty, total systematic uncertainty, total uncertainty, degrees of freedom for signal (DOFs), and profile with respect to different levels of ILS degradation for all current NDACC gases, i.e., O<sub>3</sub>, HNO<sub>3</sub>, HCl, HF, ClONO<sub>2</sub>, CH<sub>4</sub>, CO, N<sub>2</sub>O, C<sub>2</sub>H<sub>6</sub>, and HCN, were investigated. The influence of an imperfect ILS on NDACC gases retrieval were assessed, and the consistency under different meteorological conditions and solar zenith angles (SZA) were examined. The study concluded that the influence of ILS degradation can be approximated by the linear sum of individual modulation efficiency (ME) amplitude influence and phase error (PE) influence. The PE influence is of secondary importance compared with the ME amplitude influence. For total column retrieval, the stratospheric gases are more sensitive to ILS degradation than the tropospheric gases. For profile retrieval, the positive ME has more influence on tropospheric gases than the stratospheric gases. In contrast, the negative ME has more influence on stratospheric gases than the tropospheric gases. In order to suppress the influence on total column for ClONO<sub>2</sub> and other NDACC gases within 10&thinsp;% and 1&thinsp;%, respectively, the permitted maximum ILS degradation for each NDACC gas was deduced (summarized in Table 5).