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.

The mutual response between surface temperature and the mass concentration of regional black carbon (BC) aerosols has still remained far from understanding due to its complex nature. A detailed analysis presented in this study using long-term data indicates a significant pattern of mutual response between surface temperature and BC in restricted background weather conditions (water vapor, cloud cover and wind speeds). The analysis shows that a fall in surface temperature which naturally occurs daily after the sunrise, leads to the development of a stronger inversion in the near-surface level and this, in turn, contributes to the enhancement of BC fumigation peak. Further, the enhanced fumigation peak (especially during pre-monsoon) is found positively influencing the mid-day temperature rise possibly due to the immediate impact of the direct radiative forcing of BC aerosols. These observations lead us to consider a hypothesis that 'an extra fall in the morning hour surface temperature contributes to the enhancement of BC fumigation peak and can degrade the morning hour air quality which gives positive feedback to the mid-day temperature rise over a region'. A substantial in situ data [over Gadanki (13.5 degrees N, 79.2 degrees E)] along with MERRA-2 and ERA-5 data are used in this methodical analysis. Moreover, the validity of the hypothesis has been tested over other locations. Regional weather and seasonal cycle are found to have apparent interference with the feature of the observed mutual response pattern. The results from this study clearly indicate that the approach used, can be executed location independently. (C) 2020 Elsevier B.V. All rights reserved.