Long-term observations of black carbon aerosol over a rural location in southern peninsular India: Role of dynamics and meteorology

Highlights

• 10 years of BC observations over a rural location are reported for the first time.

• Significant decreasing trend is found in the highest and lowest values of BC.

• Strong association of BC with dynamics and meteorology is found.

• Angstrom exponent values suggest fossil fuel combustion as a dominant source.

Abstract

Ten years (2008–2017) of Black Carbon (BC) observations obtained using Aethalometer (AE-31) are analyzed to investigate the seasonal trends and temporal variabilities over a tropical site Gadanki (13.5° N, 79.2° E) located in south-east India. Diurnal variations of BC have two peak structures one in the morning (∼08 IST) in all seasons and second in the evening (∼20 IST) only during the pre-monsoon (March–May). Intra-annual variation in BC indicated February and March months as the bio-mass burning with highest BC mass concentration (3000–5000 ng/m³). About 46% of air parcel back trajectories found passing across the in-land regions of southern peninsular India brining transported aerosol to the source location during pre-monsoon. The lowest BC (∼1500 ng/m³) is noticed during the monsoon months (June–September). The average BC (2200 ng/m³) represents observational site as a typical rural site. The inter-annual variability of BC did not show any significant trend. However, trends in the maximum (March) and minimum (July) BC values show statistically significant decreasing trend suggesting reduction in bio-mass burning sources during March supported by the decrease in the fire counts. Diurnal variation in the absorption angstrom exponent indicates that the morning and evening peaks are contributed by the bio-mass combustion with values above threshold of 1. However, angstrom exponent values are found below 1 during noon time of monsoon season suggesting fossil fuel contribution. Strong coupling is found between aerosol concentration and tropospheric dynamics, meteorology in addition to the sources. The present study is expected to provide valuable input to the modelers and observational physicists as BC is climate sensitive variable.

Introduction

Black Carbon (BC) is a primary anthropogenic aerosol originated from biomass burning and incomplete combustion of fossil fuel. BC formed out of pure graphitic carbon is an important component of carbonaceous aerosol. Due to its property of light absorption BC is also known as ‘soot’ particle with appearance of black in color (Hansen et al., 1984, 1982). Climate effects of BC aerosol are multi fold. They do absorb the incoming solar radiation in short wavelengths leading to warming within the atmosphere and cooling at surface (Bond et al., 2013a; Bond and Bergstrom, 2006; Menon et al., 2002; Ramanathan and Carmichael, 2008). The warming produced by the direct effect (absorption) of BC is found to be next to the carbon dioxide (Jacobson, 2001). From the latest IPCC reports it has been found that the total warming by BC is 1 W/m² ( IPCC, 2013; Le Treut et al., 2007). The presence of BC inside a cloud actually can make the cloud system to disappear known as semi-direct effect or cloud inhibition effect (Ackerman et al., 2000; Ghan et al., 2008; Koren et al., 2004; Lohmann, 2006; Lohmann and Feichter, 2004; Ming et al., 2010; Ramana and Devi, 2016; Ramanathan et al., 2001; Sreekanth et al., 2007). On the other hand, climate and weather prediction models are not accurate both in simulation of the optical properties of the BC and their climate effects. This is mainly due to two aspects, firstly the improper parameterization of phenomena of BC. Secondly, the lack of representation of emission sources in emission inventories especially of open bio-mass burning (Bond et al., 2013b; Gadhavi et al., 2015; Koch et al., 2009; Myhre, 2009). BC being a fine mode particle can be inhaled directly by the human and pose serious health issues related to cardiovascular (Janssen et al., 2011). Human exposure to the BC is seriously pursued in the recent times as seen in many international projects (Tonne et al., 2017).

In general, the factors that influence the BC concentrations over a location are outlined to be various sources (local and long-range transport), meteorology (rainfall, solar insolation etc.) and dynamics (winds and boundary layer height). Long-term observations of BC concentrations at different geographical locations are essential to estimate their climatic effects and to predict the future scenarios. Statistically significant temporal variabilities established using long-term observations are useful in the validation of the skill of weather and climate models in the representation of diurnal, intra and inter-annual variabilities. Being an air pollutant, long-term trends in BC concentrations is very much required to propose mitigation strategies to control their population in the environment. Sensitivity studies using chemical composition of aerosol assume different values for BC; however, observations would fill this gap as BC is a major anthropogenic component of particulate matter.

Globally there are a few studies focused on long-term dataset (Ahmed et al., 2014; Hirdman et al., 2010; Kanaya et al., 2016; Sciare et al., 2009; Sharma et al., 2004). To mention a few, thirteen years of aerosol absorption data from Canadian Arctic station ‘Alert’ using Aethalometer (AE-6) reported 55% decrease in BC concentrations over the time and attributed to the emission source strengths and long-range transport (Sharma et al., 2004). In continuation to this, using long-term observations from three Arctic locations (Alert, Barrow and Zeppelin), decreasing trends found in BC were attributed to emission strengths as dominant factor rather than long-range transport (Hirdman et al., 2010). From a remote oceanic location in the Austral Ocean (Amsterdam Island, Southern Indian Ocean), five years of Aethalometer observations revealed the location as the pristine marine (annual mean BC of 5.8 ± 2.8 ng/m³) influenced by long-range transport of bio-mass burning aerosols (Sciare et al., 2009). Twenty seven years of thermal optical method based observations of BC over north eastern United States showed 32% decrease in BC with negative trend of −5.5 ng/m³ per year (Ahmed et al., 2014). Over different geographical locations in India, BC mass concentrations have been already reported but not using the long-term dataset. They are mostly either campaign data or less than 3–5 years of data. These observational sites are clearly depicted in a map shown in Fig. 1a. We have systematically listed the magnitude of BC (based on reported values from these sites), details on diurnal and seasonal patterns in the subsequent sections of the manuscript.

For the first time, ten years (2008–2017) of long-term observations of ambient BC concentrations carried out using Aethalometer (AE-31) are reported here over Gadanki, an in land rural location from Southern Peninsular India. The strong association with meteorological and dynamical parameters in determining the seasonal and intra-annual variation of BC is also established. Observational site, instrument, analytical approach (followed in data analysis), data reduction methods and available datasets are described in Section 2. The long-term variations in meteorological parameters including MODIS fire counts, atmospheric boundary layer height, rainfall are provided in Section 3. In Section 4, results are discussed with respect to diurnal, intra and inter-annual variations in BC and also explained in terms of their association with the rainfall, long-range transport, fire counts and boundary layer height. Using the angstrom parameter first approximation of sources of BC (among fossil fuel and bio-mass burning) is also discussed. Temporal variabilities over different locations (about 25) in southern Peninsular India are reported to distinguish the current observational site among others. Finally, the results are summarized in Section 5.