Comment on amt-2023-12

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Atmospheric ammonia (NH₃) is a reactive nitrogen compound that pollutes our environment and threatens public health. Monitoring the spatial and temporal variations is important for quantifying its emissions and depositions and evaluating the strategies for managing anthropogenic sources of NH₃. In this study, we present an NH₃ retrieval algorithm based on the optimal estimation method for the Geostationary Interferometric Infrared Sounder (GIIRS) onboard China&rsquo;s FengYun-4B satellite (FY-4B/GIIRS). In particular, we examine the information content based on the degree of freedom for signal (DOFS) in retrieving the diurnal NH₃ in East Asia, with a focus on two source regions including North China Plain and North India. Different from previous studies, our retrievals are implemented using two different absorption micro-windows (920&ndash;940 cm^&minus;1 and 955&ndash;975 cm^&minus;1), and their difference in retrievals is used to imply the impact of spectral noise on the retrieval uncertainty. Retrieval results using FY-4B/GIIRS spectra from July to December 2022 show that the two retrieval sets using different micro-windows are highly consistent, with a correlation coefficient of 0.81 and a root-mean-square-error of 3.2 &times; 10¹⁵ molec/cm² when DOFS &gt; 0.5. no large systematic bias is observed. The DOFS for the majority ranges from 0 to 1.0, mainly depending on the thermal contrast (TC) defined as the temperature difference between the surface and the lower atmosphere. Consistent with retrievals from low-earth-orbit (LEO) infrared sounders, the detection sensitivity, as quantified by the averaging kernel (AK) matrix, peaks in the lowest 2 km atmospheric layers. The DOFS and TC are highly correlated, resulting in a typical "butterfly" shape. That is, the DOFS increases when TC becomes either more positive or more negative. The NH₃ columns from FY-4B/GIIRS show a significant diurnal cycle in summer (July&ndash;August) in North China Plain, in good agreement with the day-night gradient from the collocating IASI retrievals. The weak and moderate diurnal cycles in September&ndash;October and November&ndash;December, respectively, are also presented from both FY-4B/GIIRS and IASI retrievals. This study demonstrates the capability of FY-4B/GIIRS in capturing the diurnal NH₃ changes in East Asia, which will have the potential to improve regional and global air quality and climate research.