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Discussion papers | Copyright
https://doi.org/10.5194/acp-2018-148
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 13 Feb 2018

Research article | 13 Feb 2018

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This discussion paper is a preprint. A revision of the manuscript is under review for the journal Atmospheric Chemistry and Physics (ACP).

Changes in the aerosol direct radiative forcing from 2001 to 2015: observational constraints and regional mechanisms

Fabien Paulot1,2, David Paynter1, Paul Ginoux1, Vaishali Naik1, and Larry Horowitz1 Fabien Paulot et al.
  • 1Geophysical Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration, Princeton, New Jersey, USA
  • 2Program in Atmospheric and Oceanic Sciences, Princeton University, New Jersey, USA

Abstract. We present observation and model-based estimates of the changes in the direct shortwave effect of aerosols under clear-sky (SDRECS) from 2001 to 2015. Observation-based estimates are obtained from changes in the outgoing shortwave clear-sky radiation (Rsutcs) measured by the Clouds and the Earth's Radiant Energy System (CERES), accounting for the effect of variability in surface albedo, water vapor, and ozone. We find increases in SDRECS (i.e., less radiation scattered to space by aerosols) over Western Europe (0.7–1Wm−2dec−1) and the Eastern US (0.9–1.8Wm−2dec−1), decreases over India (−0.5–−1.9Wm−2dec−1) and no significant change over Eastern China. Comparisons with the GFDL chemistry climate model AM3, driven by CMIP6 historical emissions, show that changes in SDRECS over Western Europe and the Eastern US are well captured, which largely reflects the mature understanding of the sulfate budget in these regions. In contrast, the model overestimates the trends in SDRECS over India and Eastern China. Over China, this bias can be partly attributed to the decline of SO2 emissions after 2007, which is not captured by the CMIP6 emissions. In both India and Eastern China, we find much larger contributions of nitrate and black carbon to changes in SDRECS than in the US and Europe, which highlights the need to better constrain their precursors and chemistry. Globally, our model shows that changes in the all-sky aerosol direct forcing between 2001 and 2015 (+0.03Wm−2) are dominated by black carbon (+0.12Wm−2) with significant offsets from nitrate (−0.03Wm−2) and sulfate (−0.03Wm−2). Changes in the sulfate (+7%) and nitrate (+60%) all-sky direct forcing between 2001 and 2015 are only weakly related to changes in the emissions of their precursors (−12.5% and 19% for SO2 and NH3, respectively), due mostly to chemical non linearities.

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Observations show that the sunlight reflected to space by particles has decreased over the US and Europe, increased over India, and not changed over China from 2001 to 2015. These changes are attributed to different types of particules, namely sulfate over the US and Europe, and black carbon, sulfate, and nitrate over China and India. Our results suggest that the recent shift in human emissions from the US and Europe to Asia has altered their impact on the Earth's outgoing energy.
Observations show that the sunlight reflected to space by particles has decreased over the US...
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