Constraining black carbon aerosol over Asia using OMI aerosol absorption optical depth and the adjoint of GEOS-Chem

Atmos. Chem. Phys., 15, 10281–10308, 2015 www.atmos-chem-phys.net/15/10281/2015/ doi:10.5194/acp-15-10281-2015 © Author(s) 2015. CC Attribution 3.0 License. Constraining black carbon aerosol over Asia using OMI aerosol absorption optical depth and the adjoint of GEOS-Chem L. Zhang1,2 , D. K. Henze1 , G. A. Grell2 , G. R. Carmichael3 , N. Bousserez1 , Q. Zhang4 , O. Torres5 , C. Ahn6 , Z. Lu7 , J. Cao8 , and Y. Mao9,10 1 Department of Mechanical Engineering, University of Colorado, Boulder, CO, USA 2 Global Systems Division, Earth System Research Laboratory, NOAA, Boulder, CO, USA 3 Department of Chemical and Biochemical Engineering, University of Iowa, Iowa City, IA, USA 4 Center for Earth System Science, Tsinghua University, Beijing, China 5 NASA Goddard Space Flight Center, Greenbelt, MD, USA 6 Science Systems and Applications, Inc., Lanham, MD, USA 7 Energy Systems Division, Argonne National Laboratory, Argonne, IL, USA 8 Key Lab of Aerosol Chemistry & Physics, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an, China 9 Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA, USA 10 State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China Correspondence to: D. K. Henze () Received: 19 August 2014 – Published in Atmos. Chem. Phys. Discuss.: 17 November 2014 Revised: 2 September 2015 – Accepted: 2 September 2015 – Published: 17 September 2015 Abstract. Accurate estimates of the emissions and distribution of black carbon (BC) in the region referred to here as Southeastern Asia (70–150◦ E, 11◦ S–55◦ N) are critical to studies of the atmospheric environment and climate change. Analysis of modeled BC concentrations compared to in situ observations indicates levels are underestimated over most of Southeast Asia when using any of four different emission inventories. We thus attempt to reduce uncertainties in BC emissions and improve BC model simulations by developing top-down, spatially resolved, estimates of BC emissions through assimilation of OMI (Ozone Monitoring Instrument) observations of aerosol absorption optical depth (AAOD) with the GEOS-Chem (Goddard Earth Observing System – chemistry) model and its adjoint for April and October 2006. Overwhelming enhancements, up to 500 %, in anthropogenic BC emissions are shown after optimization over broad areas of Southeast Asia in April. In October, the optimization of anthropogenic emissions yields a slight reduction (1–5 %) over India and parts of southern China, while emissions increase by 10–50 % over eastern China. Observational data from in situ measurements and AERONET (Aerosol Robotic Network) observations are used to evaluate the BC inversions and assess the bias between OMI and AERONET AAOD. Low biases in BC concentrations are improved or corrected in most eastern and central sites over China after optimization, while the constrained model still underestimates concentrations in Indian sites in both April and October, possibly as a consequence of low prior emissions. Model resolution errors may contribute up to a factor of 2.5 to the underestimation of surface BC concentrations over northern India. We also compare the optimized results using different anthropogenic emission inventories and discuss the sensitivity of top-down constraints on anthropogenic emissions with respect to biomass burning emissions. In addition, the impacts of brown carbon, the formulation of the observation operator, and different a priori constraints on the optimization are investigated. Overall, despite these limitations and uncertainties, using OMI AAOD to constrain BC sources improves model representation of BC distributions, particularly over China. Published by Copernicus Publications on behalf of the European Geosciences Union. 10282 1 Introduction Black carbon (BC) is a product of incomplete combustion of carbonaceous fuels, enhanced concentrations of which have led to a present-day overall positive radiative forcing and climate warming (Charlson and Pilat, 1969; Satheesh and Ramanathan, 2000; Bond et al., 2013). More than 10 years ago, Jacobson (2000) and Hansen et al. (2000) recognized that preindustrial to present increases in BC might warm the atmosphere about one-third as much as CO2 . Recently, an assessment by Bond et al. (2013) indicates that the global average preindustrial to present radiative forcing from BC is +1.1 W m−2 with 90 % uncertainty bounds of +0.17 to +2.1 W m−2 , which is more than two-thirds that of CO2 (+1.56 W m−2 ). Additionally, BC aerosols constitute up to 10–15 % of the mass concentration of fine particulate matter (PM2.5 ) over continental regions, exposure to which is known to adversely effect human health (e.g., Janssen et al., 2005, 2011; Schwartz et al., 2008). Given the magnitude of BC climate effects and health impacts, a number of studies have investigated its direct effect (Forster et al., 2007; Ramanathan and Carmichael, 2008), semi-direct effect (Ackeman et al., 2000; Johnson et al., 2004), indirect effect (Cozic et al., 2007; Liu et al., 2009; Oshima et al., 2009), and the albedo effect when deposited on snow (Hansen and Nazarenko, 2004; Hansen et al., 2005; Flanner et al., 2007; Qian et al., 2009) using various numerical models and observations. Central estimates of global annual emissions of BC are 8.0 Tg, of which 38 % comes from fossil fuel, 20 % from biofuel and 42 % from open burning (Bond et al., 2004). At the same time, estimates of BC emissions are recognized as having large uncertainties – 50 % at global scales and a factor of 2–5 at regional scales (Bond et al., 2004; Ramanathan and Carmichael, 2008). The Asian region referred to here as Southeast Asia (70–150◦ E, 11◦ S–55◦ N) is the major anthropogenic BC source region in the world, with growth in BC emissions of 21 % over China and 41 % over India from 1996 to 2010 associated with rapid economic and industrial development (Lu et al., 2011). BC emissions from both energy-related combustion and biomass burning that occur largely in Asia and Africa currently appear underestimated (Bond et al., 2013). A global top-down estimate of BC emissions using AERONET (Aerosol Robotic Network) observations by Cohen and Wang (2014) indicated that commonly used global BC emission data sets may be underestimated by a factor of 2 or more. Sixteen models from the AeroCom aerosol model intercomparison study underestimated the Southeast Asian BC surface concentrations by a factor of 2–3 (Koch et al., 2009). The GEOS-Chem (Goddard Earth Observing System – chemistry) model also underestimated monthly BC concentrations at almost all rural sites in China, particularly in January 2006, which indicated a regional underprediction of carbonaceous aerosol sources associated with anthropogenic activities (Fu et al., 2012; Wang Atmos. Chem. Phys., 15, 10281–10308, 2015 L. Zhang et al.: Constraining (...truncated)