Planetary Boundary Layer and aerosol interactions over the Indian sub-continent M.N. Patil n , S.D. Patil, R.T. Waghmare, T. Dharmaraj Indian Institute of Tropical Meteorology, Pune 411008, India article info Article history: Received 19 August 2013 Received in revised form 29 January 2014 Accepted 25 February 2014 Available online 12 March 2014 Keywords: Planetary Boundary Layer Indian summer monsoon Aerosols Green house gases abstract Aerosols, both natural as well as anthropogenic, affect the radiative forcing of Earth's climate and reduce surface albedo. The Planetary Boundary Layer (PBL) height, which depends upon surface heat budget, is analyzed considering the increase in green house gases (GHGs) from pre-industrial to post-industrial era. The PBL climatology shows deeper PBL during pre-monsoon and summer monsoon seasons as compared to post-monsoon and winter. The PBL height has decreased in post-industrial decade compared to pre-industrial decade. The PBL height reduction is due to increasing aerosol and GHGs' concentrations in the recent decades, which causes surface warming and upper tropospheric cooling. Similarly, due to higher loading of the volcanic aerosol injected from the low latitude eruptions, the atmospheric circulation has been affected. & 2014 Elsevier Ltd. All rights reserved. 1. Introduction The turbulent nature of the Planetary Boundary Layer (PBL) is one of its most conspicuous and important features and is considered to be a circulatory system of the biosphere in many aspects. The PBL, as a consequence of the interactions between the atmosphere and underlying surface (Arya, 1988, Garratt, 1992, Stull, 1988), transfers heat, moisture and pollutants to disperse horizontally and vertically, effectively air-conditioning the bio- sphere providing a conduit for energy to power the weather system on all scales. Aerosols and clouds directly affect the energy balance of the earth's surface by interacting with the shortwave and long-wave radiations (Ardanuy et al., 1989; Charlson et al., 1992; Satheesh and Ramanathan, 2000; Latham et al., 2008; Panicker et al., 2008). Dust and black carbon are characterized by their ability to heat the atmosphere by absorbing solar radia- tion (Lau and Kim, 2006) whereas pure sulphate aerosols primarily scatter solar radiation and cause cooling (Charlson et al., 1991). Increase in aerosols causes an increase in the droplet concentra- tion which leads to an increase in the reflection of solar radiation to space from clouds, leading to climate cooling (Haywood and Boucher, 2000). If the condensed moisture inside the cloud is not altered by the increase in aerosols, the radius of the droplet will decrease because of the increase in its concentration resulting in a decrease in the precipitation efficiency (Haywood and Boucher, 2000). The aerosols produced by humans enhance the scattering and absorption of solar radiation producing brighter clouds that are less efficient at releasing precipitation (Ramanathan et al., 2001). The surface-emitted pollutants can be diluted greatly in PBL due to boundary layer turbulence (Stull, 1988). On micro- meteorological scale, the aerosol concentrations could be strong in shallow PBL and sparse in the convective one. Feingold et al. (2005) found that if smoke aerosols exist close enough to the ground, the heating of the absorbing aerosols could make PBL unstable and stimulate convection. It is evident that, in the recent decades, the green house gases (GHGs) and aerosols loading have been increased enormously in the atmosphere due to substantial increase in industries, fossil fuel and biomass burning, and could be responsible for the global warming (IPCC (Intergovernmental Panel on Climate Change) Climate Change 2007). Therefore, in recent years, more attention has been given to measurement and modeling the aerosols–radiation interactions (Wendisch et al., 2002; Rosenfeld, 2006; Liang and Zhang, 2013). The measured aerosol black carbon (BC) concentration over an urban site, Hyderabad, India, rapidly decreased with height in the PBL region and weakly above up to few kilometers, and the BC mass concentration was nearly 3.35 μgm 3 at ground, which reduced to nearly 1.05 μgm 3 at the top of the PBL (Moorthy et al., 2004). Recent observations (Tiwari et al., 2013) over New Delhi showed varied mass concentrations of BC from 0.9 to 25.5 μgm 3 , with an annual mean of 6.7 75.7 μgm 3 , with seasonal changes (minimum in monsoon and maximum in win- ter). The long term BC mass concentration measurements over the Indian subcontinent also varied from one station to another (NCAP, 2011). The diurnal variability of BC and PM 2.5 mass concentrations was highly associated with the combined effect of variations in Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jastp Journal of Atmospheric and Solar-Terrestrial Physics http://dx.doi.org/10.1016/j.jastp.2014.02.007 1364-6826/& 2014 Elsevier Ltd. All rights reserved. n Corresponding author. E-mail address: patil@tropmet.res.in (M.N. Patil). Journal of Atmospheric and Solar-Terrestrial Physics 112 (2014) 38–42