Evaluation of TROPOMI operational standard NO₂ column retrievals (from version 1.3 to 2.4) with OMNO2 and QA4ECV OMI observations over China

Abstract. The TROPOMI satellite instrument plays a key role in nitrogen dioxide (NO2) monitoring on account of its unprecedented spatial resolution and stable quality of data. However, since 2019, TROPOMI operational NO2 retrieval has improved and updated in three versions (1.4, 2.2 and 2.4), with significant impact on retrieved NO2 column. Thus, studies including both TROPOMI NO2 data before and after the activation of these versions could show artificial jumps. Moreover, up to date evaluation result of TROPOMI NO2 data in current version 2.4 is not yet well documented in the literature. Therefore, in this work, we focus on evaluating TROPOMI's capability to detect NO2 under the different retrieval version conditions, by comparing with OMNO2 data and QA4ECV OMI data over China. We find a 38 % increase of tropospheric NO2 in version 1.4 due to improved FRESCO-wide cloud retrieval, and a 14 % increase in version 2.2 due to adjusted surface albedo for cloud-free scenes. We show that the upgrade to version 2.4 with new DLER surface albedo, led to an increase by 3 x 1014 molecules cm-2 of tropospheric NO2 over vegetation. Furthermore, we demonstrate that TROPOMI data shows strongest tropospheric NO2 seasonal variation compared to OMNO2 data and QA4ECV OMI data, and this seasonal effect was enhanced with the tropospheric NO2 retrieval version upgrades. Additionally, we examine for the first time the change of TROPOMI AMFs (air mass factors) in the different versions, and based on it, we arrive at a correction for the underestimation of TROPOMI NO2 column in previous versions. We also find a 33 % overestimation of NO2 reduction during the COVID-19 lockdown over China when using TROPOMI data before and after the activation of the NO2 version 1.4.