## 2 Data Sources  
### RSS: MSU Temperature Data  
The Remote Sensing Systems (RSS) provide a gridded monthly dataset of atmospheric temperature (lower and mid-tropospheric) since late 1978, derived from the satellite-based MSU and AMSU. The present study uses the latest versions of monthly mean tropospheric layer temperature (TMT: temperature middle troposphere; TLT: temperature lower troposphere) records from the MSU and AMSU available from the RSS. The complete dataset (1978-2022) is accessible online at the RSS website [[http://www.ssmi.com](http://www.ssmi.com)]([http://www.ssmi.com](http://www.ssmi.com)) (latest v4.0; last accessed online on 18 January 2023). From 1978 to 2005, a combined total of nine MSU instruments were used, followed by a sequence of AMSU instruments that commenced in the middle of 1998 and are still operational today. The Microwave Sounding Unit (MSU) operates by utilizing microwave radiation emitted by oxygen molecules in the Earth's atmosphere to derive temperature profiles at different atmospheric layers. Molecular oxygen exhibits strong absorption lines around 60 GHz, allowing for the selection of specific frequencies to measure radiation from different atmospheric layers. The MSU instruments, which are cross-track scanners mounted on polar-orbiting weather satellites, gather microwave radiance data across four channels within the range of 50.3 to 57.95 GHz. These channels are positioned on the lower edge of the Oxygen absorption band. The four channels enable the measurement of atmospheric temperature across four layers, starting from the lower troposphere in channel 1 and extending to the lower stratosphere in channel 4. Similar to the MSU, the AMSU component is a temperature sounder consisting of 15 channels. Among these channels, 11 (channels 4 through 14) are situated within the 60 GHz absorption complex, which makes them highly relevant for determining atmospheric temperatures at different altitudes above the Earth's surface. The specific MSU and AMSU channels that are combined to form the new TLT and TMT datasets that are used in this study are explained in [72; 73]. The TLT and TMT records were extended beyond 2004 by merging data from channel 5 (AMSU5) in the newer AMSU instruments with data from channel 2 (MSU2) in the earlier MSU instruments. This long-term single-quality climate record suffers from several calibration issues, time variations, and diurnal variations that were addressed by several groups at Remote Sensing  
Figure 1: The sub-division of the study region into high-altitude regions (Tibet, western and eastern Himalayas) and plains (western and eastern Indo-Gangetic plains (IG plains)). The major dust source regions of Asia-Africa affecting the IG plains and Himalayas are marked as Afghanistan-Pakistan, Iran-Pakistan-Thar desert, Middle East, Arabian region, and Sahara Desert. Systems (RSS), the University of Alabama, Huntsville (UAH), the University of Washington (UAW), the University of Maryland (UMD), and the NOAA National Environmental Satellite, Data, and Information Service (NOAA/NESDIS) [72; 74; 75; 76], resulting in several different long-term Climate Data Record (CDR) versions for the different atmospheric layers (lower troposphere, middle troposphere, near the tropopause, and lower stratosphere). Determining trends from satellite data faces challenges such as calibration drift, temporal drift, and the need for inter-calibration among various satellites to ensure accurate and consistent measurements over time. The stratospheric weight contributes significantly (5-15%) to the total, partially masking tropospheric warming [77]. Their analysis suggests that this effect led to a cooling influence of 0.04 \({}^{\circ}\)K/decade on global TMT trends from 1979 to 2005. Several lower tropospheric datasets were developed to minimize the impact of stratospheric cooling using different methods. The MSU instrument was used to quantify the weighted difference between near-limb and near-nadir images.