Black carbon radiative forcing derived from AERONET measurements and models over an urban location in the southeastern Iberian Peninsula
Introduction
Black carbon (BC) is the dominant absorbing aerosol in the solar radiation spectrum, playing a key role in the estimation of the direct radiative forcing although it accounts for < 5% of the mass of atmospheric aerosol in most areas of the world (Haywood and Shine, 1997, Zhang et al., 2012). Bond et al. (2013) provided an inventory of the major BC sources. They are listed according to the importance of emission: The bottom-up estimates predicts that open burning contributes about 40% of total BC emissions. Residential Solid Fuels (Wood, agricultural waste, dung, and coal) provide another 25% of BC emissions. The diesel-engine category (on-road and off-road engines) is estimated that contributed about 20% of global BC emissions of the total. Finally, industrial coal combustion is estimated to provide about 9% of global emissions. BC is directly emitted into the air from these sources, not formed in the atmosphere from precursor substances. Estimates of BC global annual emissions were 8.0 Tg. The uncertainties were about a factor of 2, with uncertainty range of 4.3–22 Tg/yr. BC particles are generally produced from the incomplete combustion of fossil fuels and biomass burning (Zhao et al. 2015). BC can raise the amount of solar radiation absorbed in the visible and infrared spectral ranges within the Earth's climate system and, consequently, heat the atmosphere and surface (Hansen et al., 2000, Ramanathan and Carmichael, 2008). Ramanathan and Carmichael (2008) performed a comparison of radiative forcing caused by greenhouse gases and BC. They found that the direct radiative forcing due to BC was larger than that due to any other greenhouse gas except CO2. BC causes large atmospheric warming constituting about 55% of CO2 forcing on the global scale (Ramanathan and Carmichael, 2008, Keil et al., 2001, Babu et al., 2002, Bond et al., 2013). Bond et al. (2013) estimated a climate forcing due to BC of + 1.1 W/m2 with 90% uncertainty limits of + 0.17 to + 2.1 W/m2. The process through which BC suspended in the atmosphere scatters and absorbs incoming solar radiation is termed “direct effect”. The absorption by BC suspended warms the air, but the extinction of radiation results a negative forcing at the Earth's surface (Ramanathan and Carmichael 2008). The “semi-direct effect” assumes that BC particles reside interstitially between cloud droplets (Johnson et al., 2004, Chung and Seinfeld, 2005, Jacobson, 2006, Jones et al., 2007). BC also has significant effects on clouds by changing atmospheric stability, affecting cloud formation (Ackerman et al. 2000). Therefore, BC is considered as a potential cause of global warming (Hansen et al., 2000, Bond et al., 2013). BC may also play a relevant role for the aerosol cloud (“indirect effect”) since it is injected into the atmosphere as primary aerosol particle, affecting the number of particles available in cloud condensation (Oshima et al. 2009).
There are a large number of papers focused on the analysis of aerosol measurements over the Iberian Peninsula (e.g., Silva et al., 2002, Olmo et al., 2006, Estellés et al., 2007, Cachorro et al., 2008, Prats et al., 2008, Toledano et al., 2009, Pereira et al., 2011, Valenzuela et al., 2012a). However, none of them were focused on studying the BC aerosol in this region. To our knowledge, only Lyamani et al. (2011) performed a detailed analysis about BC concentration at the surface level. They measured significant amount of BC over surface in Granada with mean value of 3.0 ± 1.5 μg/m3. These authors also reported that BC exhibited a well-defined seasonal variation with the highest concentration during winter likely due to increased emissions from domestic heating and a lower planetary layer height (Granados-Muñoz et al., 2012). The disadvantage of in situ measurements is that require considerable effort and this technique does not provide large spatial and temporal coverage (Derimian et al. 2008). Recently, some studies have paid attention to retrieve aerosol composition from AERONET retrievals, since it provides worthy information on aerosol optical and physical properties such as column-averaged aerosol refractive indices and size distributions (Schuster et al., 2005, Dey et al., 2006, Arola et al., 2011). They derived information of BC concentration from AERONET imaginary refractive indices assumes that absorption is due to three components: Black Carbon (BC), Brown Carbon (BrC) and mineral dust (MD). In addition, it can also provide a long term view and extensive spatial coverage, as it comprises > 300 sun photometers placed throughout the world (Holben et al. 1998).
Long-range transport of BC has a great significance on the climate change and air quality. However, large uncertainties remain between simulated and observed global transport of BC. Uncertainties in models result from many factors, including BC emissions inventories, the parameterizations of BC aging, wet removal, and dry deposition processes (Liu et al., 2011, Shen et al., 2014). BC aging process occurring during long-range transport is a key factor in simulated concentrations of BC. The aging process refers to a transformation from hydrophobic to hydrophilic aerosols, where aged BC particles can act as CCN and, thus, they can be removed by wet scavenging when BC is trapped in cloud droplets or ice crystals. From all pathways for the hydrophobic-to-hydrophilic conversion, chemical aging is the least understood, but potentially could affect hydrophobic-to-hydrophilic conversion on time scales shorter than days or weeks (Kanakidou et al. 2005). Therefore, the rate of aging significantly affects the atmospheric lifetime of BC, being one of the key factors controlling long-range transport of these particles and, consequently, affecting their global distribution (Liu et al. 2011). However, the aging of BC is highly simplified in global models and, thus, errors related to the BC wet scavenging should be taken into account.
Backward trajectory analysis is a well-known technique to link air mass-origin with aerosol optical properties at the measurements area (e.g. Kokkalis et al., 2017, Kumar et al., 2017, Zdun et al., 2016, Valenzuela et al., 2015). The analysis of backward trajectories provides objective interpretations related to source regions, residence times over each region and different circulation patterns (curvature and length) of air masses. The accuracy of back-trajectories showed position errors with the travel's distance (Stohl 1998). These errors produce divergence in the back-trajectories and they are associated with five causes (Harris et al. 2005): differences in computational methodology, 3–4%; time interpolation, 9–25%; vertical movement method, 18–34%; meteorological input data, 30–40%; and combined two-way differences in vertical transport method and meteorological input data, 39–47%. This sensitivity test was performed for 96 h of flight time. In other studies, seven-day back trajectory calculations were done taking into account the aerosols residence time of around one week in the lower atmosphere in the northern midlatitudes (Kumar Bharath and Verma, 2016).
Some studies have focused on the characterization of the aerosol radiative forcing during desert dust events over southeastern Iberian Peninsula (e.g., Antón et al., 2012, Valenzuela et al., 2012b). However, to our knowledge, no study has addressed the characterization of the BC content in the entire atmospheric column and their radiative effects over the southeastern Iberian Peninsula. Hence, it is a challenge to carry out the first study of BC content retrieved from sun-photometer measurements over southeastern Iberian Peninsula, especially considering that is has relevance not only from the local point of view, but also from a regional perspective. Thus we develop a detailed analysis of BC aerosol in Granada urban atmosphere for the period 2005–2012. Furthermore, the BC radiative effects in the shortwave spectral range will be determined and, compared with the radiative effects of the total aerosol (composite aerosol), although restricted to fine mode-dominated cases.
Section snippets
Experimental site, instrumentation and data
Ground-based data were retrieved at the radiometric station located on the rooftop of the Andalusian Institute for Earth System Research (IISTA-CEAMA, 37.168 N, 3.608 W) in Granada (South-Eastern Spain) which is found at 680 m a.s.l. (Fig. 1). Granada is a medium-sized city with little emissions associated with large-scale industrial activities and with a population of 300,000 inhabitants. However anthropogenic fine aerosols load are expected from domestic heating and intense road traffic in the
BC content
The technique used by Arola et al. (2011) has been utilized in this work to retrieve information about BC aerosol concentration. This method is based on the approach employed previously by Schuster et al. (2005). Further information regarding the method can be found in the mentioned works. Thus, AERONET measurements of refractive index and the above mentioned approach were used to retrieve information about BC aerosol fraction. For a mixture of BC, BrC and Ammonium Sulfate (NH4)2(SO4) embedded
Temporal variation of BC content
BC aerosol concentrations were retrieved from 734 sun-photometer observations corresponding to 265 days from 2005 to 2012 over Granada. BC concentrations exhibited clear seasonal pattern, evident in the monthly BC concentrations observed during the entire period (Fig. 2). The monthly mean BC concentrations were high in winter (December to February) and autumn (September to November) and low in summer (June to August) and spring (March to May), with the highest concentrations in winter and the
Conclusions
The BC concentration values presented in this study were retrieved for the period 2005–2012. BC concentrations showed high average values in January (4.0 ± 2.6 mg/m2) and December (4.2 ± 3.3 mg/m2) and low average values in July (1.6 ± 1.2 mg/m2) and August (2.0 ± 0.6 mg/m2). The winter average BC concentration (3.8 ± 0.7 mg/m2) was 30% higher than the eight-year average (2.9 ± 0.9 mg/m2) whereas the summer average BC concentration (1.9 ± 0.3 mg/m2) was 35% lower than the overall mean. The reduction in the use of
Acknowledgments
This work was supported by the Andalusia Regional Government through project P12-RNM-2409, by the Spanish Ministry of Economy and Competitiveness through projects CGL2013-45410-R and CGL2016-81092-R and by the European Union's Horizon 2020 research and innovation programme through project ACTRIS-2 (grant agreement No. 654109). The authors thankfully acknowledge the FEDER program for the instrumentation used in this work. Antonio Valenzuela thanks Universidad de Granada for the award of a
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