In this study, we investigated the aerosol radiative forcing (ARF) using ground-based measurements of PM2.5 and black carbon aerosols at a semi-arid, rain shadow location, Solapur in peninsular India. It is observed that aerosols caused a net cooling effect at top of the atmosphere (TOP) indicating that the aerosols reflect more solar radiation back to space than they absorb. At the surface, the aerosols caused a net cooling effect indicating more presence of scattering type aerosols. The resulting ARF of the aerosols was found to be ranging from +38 Wm-2 in monsoon to +53 Wm-2 in pre-monsoon indicating trapping of energy which resulted in a warming of the atmosphere. However, BC -only forcing indicated a significant warming effect at TOP as well as in the atmosphere which showed the potential of the absorbing carbonaceous aerosols. Overall, BC was responsible for 44% and 32% of the composite ARF, even though it formed only 7% and 2% of composite aerosol in the dry and wet periods, respectively. The warming impact of BC aerosols was also manifested in terms of their contribution to aerosol radiative forcing efficiency (ARFE) which was about four times more for BC-only than that for composite aerosols. More atmospheric heating rates were observed during dry periods for composite and BC-only aerosols than during wet period. These findings have important implications for aerosol-cloud-precipitation studies as well as the atmospheric thermodynamics and hydrological cycle over this semi-arid region where the total aerosol load is not significant and rainfall amount is scarce.

Debris flows can develop into mega catastrophes in semi-arid regions when the source materials come from landslides, and both snowmelt and precipitation are involved in increasing water discharge. In such environments, the formation of large-scale debris flows exhibits a distinguishable pattern, in which a multi-fold lower triggering rainfall threshold holds compared to humid regions. Previous research mainly focuses on mechanisms in humid environments or neglects variations across aridity classes. In this study, the formation and evolutionary mechanism of a debris flow occurring in a semi-arid context is investigated via field surveys, granularity measurement, terrain and climate analyses, and snow cover change detection. By examining the July 22, 2021, Xiao Dongsuo debris flow at Amidongsuo Park in the Qilian Ranges on the northeastern margin of the Tibetan Plateau, the mechanism of debris flows in semi-arid regions is revealed. The research finds that the large debris flow, whose course erosion scales up the disaster by 0.12 million m3, is primarily supplied by landslide deposits of 1.16 million m3. The debris flow is empowered by the integrated flow of extreme precipitation and extreme heat-stimulated snowmelt. However, the precipitation required to trigger the debris flow is much lower than that of precipitation-dominated ones and those in humid regions. In semi-arid mountains, prolonged extreme heat tends to increase soil moisture in areas covered by snow or permafrost. This reduces slope stability and induces slope failures, amplifying the disaster magnitude and raising disaster risks through extended deterioration. Hence, this study inspects the failure mechanism associated with debris flows in semi-arid regions for a more comprehensive understanding to constitute viable control plans for analogous disasters.

South Asian pollutants can be transported and deposited via wet/dry deposition to the remote areas of the Himalayas and could pose a serious threat to the mountain ecosystems. Therefore, in order to understand the concentrations, fluxes, seasonal variation and origin of the mercury (Hg), major ions and trace elements, precipitation samples were collected during 2012-2013 from a data gap region, Jomsom, the high elevation semiarid mountain valley in the central Himalayas. The volume-weighted mean (VWM) concentrations of ions followed the order of Ca2+ > Mg2+ > Na+ > NH4+ > SO42- > Cl- > NO3- > K+. The concentration of Cd was lowest (0.07 mu g L-1) whereas that of Fe was the highest (1073.59 mu g L-1) in the precipitation samples. Wet deposition level of all the measured inorganic species was comparable to urban Lhasa but higher than those in remote alpine sites of the Tibetan Plateau (TP). This study shows that Hg and other inorganic constituents were higher in the non-monsoon season compared to monsoon due to enhanced washout of aerosols. Enrichment factor (EF), sea salt fraction, crustal and anthropogenic fractions, principal component analysis (PCA) and correlation coefficient analysis suggested that crustal dust and anthropogenic activities as the major sources of measured chemical species whereas the influence of sea-salt was minimal. In addition, local anthropogenic emissions were low suggesting that the majority of the pollutants could have been transported from the South Asian region to the high elevation mountains. Meanwhile, low precipitation and dry environment could have enhanced the concentrations of inorganic species in the arid region than other sites over the central Himalayas. This work adds new dataset of inorganic pollutants in wet precipitation and provides baseline information for an arid region environmental protection. However, there is a need for further long-term monitoring to understand the precipitation chemistry of the arid regions.

A continuing increase in droughts/floods in Asian monsoon regions and worsening air quality due to aerosols are the two biggest threats to the health and well being of over 60% of the world's population. This study focuses on in-situ observations of atmospheric aerosols and their impact on shortwave direct aerosol radiative forcing (SDARF) during the southwest monsoon season (June-September) from 2015 to 2020 over a semi-arid station in Southern India. The Standardized precipitation index (SPI) is used to identify the droughts and normal monsoon years. Based on the SPI index, 2015, 2016, and 2018 were considered the drought monsoon years, while 2017, 2019, and 2020 were chosen as the normal monsoon years. During the drought monsoon years (normal monsoon years), the monthly mean black carbon (BC) was 1.17 +/- 0.25 (0.72 +/- 0.18), 1.02 +/- 0.31 (0.64 +/- 0.17), 1.02 +/- 0.38 (0.74 +/- 0.28), and 1.28 +/- 0.35 mu g/m(3) (0.88 +/- 0.21 mu g/m(3)), for June, July, August and September respectively. The lower BC concentration during the normal monsoon years is mainly due to the enhanced wet-removal rates by high rainfall over the measurement location. In July, there was a high ventilation coefficient (VC) and low concentration of BC, while in September, low VC, and a high concentration of BC was observed in both the drought and the normal monsoon years. In addition, a plane-parallel radiative transfer model was used to estimate shortwave direct aerosol radiative forcing for composite and without BC at various surfaces, including the surface (SUF), atmosphere (ATM), and top of the atmosphere (TOA). During the drought monsoon years (normal monsoon years), the estimated monthly ATM forcing was 17.6 +/- 2.4 (13.9 +/- 2.1), 17.5 +/- 7.5 (12.7 +/- 4.4), 17.2 +/- 4.0 (13.5 +/- 1.9), and 17.4 +/- 2.8 Wm(-2) (14.6 +/- 0.7 Wm(-2)) for June, July, August, and September, respectively. During the drought monsoon years, the estimated BC forcing was substantially larger (8.8 +/- 2.6 Wm(-2)) than that of normal monsoon years (6.0 +/- 1.5 Wm(-2)). It indicates the important role of absorbing BC aerosols during the drought monsoon years in introducing additional heat to the lower atmosphere, particularly over peninsular India.

To achieve an in-depth understanding of radiative forcing due to aerosols is a crucial challenge for climate change studies. The first-ever long-term measurement of direct shortwave composite and black carbon aerosol radiative properties over a semi-arid region, Anantapur, in southern India is presented. Long-term variations in Aerosol Optical Depth (AOD) and Black Carbon (BC) mass concentration from December 2007 to November 2017 are discussed with specific emphasis on intra-seasonal variation in aerosol optical properties, meteorology, transport pathways, and their implications for direct short wave radiative forcing over Anantapur. The intraseasonal mean AOD showed strong seasonal dependence with the highest (0.47 +/- 0.03) during summer and lowest (0.28 +/- 0.03) during the monsoon. Meanwhile, the intra-seasonal mean (+/- sigma) BC mass concentration was about 3.57 +/- 0.45, 2.60 +/- 0.58, 1.22 +/- 0.18 and 2.24 +/- 0.28 mu g m(-3) during winter, summer, monsoon and postmonsoon respectively. Furthermore, there is an obvious temporal variation in intra-seasonal BC mass concentration during the dry season (winter and summer). To be more specific, the intra-seasonal mean (+/- sigma) BC mass concentration before 2012 (after 2012) during the dry season was about 3.37 +/- 0.7 mu g m(-3 )(2.80 +/- 0.58 mu g m(-3)), respectively. Concentration weighted trajectory analyses (CWT) revealed that the air masses originated from the continental and polluted environments located in the central and northern parts of India (except monsoon), in regulating BC mass concentration over measurement location. Further, Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) derived aerosol vertical extinction profiles (532 nm) showed that majority aerosols (>250 Mm(-1)) are confined within 2 km from the surface during winter while in summer particles are distributed throughout the profile (similar to 6 km) with extinction coefficient varying between 200 and 250 Mm(-1). The Santa Barbara Discrete Ordinate Radiative Transfer (SBDART) model estimated intra-seasonal mean direct shortwave composite aerosol radiative forcing (DARE) in the atmosphere (ATM) was about 31.13 +/- 3.36, 34.82 +/- 3.89, 17.10 +/- 1.15, and 17.44 +/- 1.81 Wm(-2) during winter, summer, monsoon and post-monsoon, respectively. The positive signs of ATM forcing in all seasons indicate a warming of the atmosphere, and the corresponding heating rate was around a factor of two higher during the dry season (0.92 +/- 0.12 Kday(-1)) than the wet season (monsoon and post-monsoon) (0.49 +/- 0.04 Kday(-1)). The intra-seasonal mean BC forcing in ATM before 2012 (After 2012) during the dry season was about 24.14 +/- 2.85Wm(-2) (20.09 +/- 2.59Wm(-2)), respectively. The contribution of BC alone to the composite forcing during the study period over the station was similar to 68%. These findings would be helpful for regional climate studies and making air pollution control policy over the region.

The effects of the depth of the active layer of permafrost on aboveground vegetation in semi-arid and semi-humid regions of the Qinghai-Tibetan Plateau were studied. The depth of active permafrost was measured and aboveground vegetation recorded. Differences in correspondence between permafrost depth and aboveground vegetation in semi-arid and semi-humid regions were analyzed. Vegetation cover and biomass were well correlated with permafrost depth in both semi-arid and semi-humid regions, but the correlation coefficient in the semi-arid region was larger than in the semi-humid region. With the increase in permafrost depth, vegetation cover and biomass decreased in both regions. Species richness and diversity decreased with increasing depth of permafrost in the semi-arid region. In the semi-humid region, these at first increased and then decreased as permafrost depth increased. It seems likely that vegetation on the Qinghai-Tibetan Plateau will degenerate to different degrees due to permafrost depth increasing as a result of climatic warming. The influence would be especially remarkable in the semi-arid region.