Assessment of the interactions among tropospheric aerosol loading, radiative balance and clouds through examination of their multi-decadal trends

Description

While aerosol radiative effects have been recognized as some of the largest sources of uncertainty among the forcers of climate change, the verification of the spatial and temporal variability of aerosol radiative forcing has remained challenging. Anthropogenic emissions of primary aerosol and gaseous precursors have witnessed dramatic changes over the past two decades across the northern hemisphere. During the period 1990-2010, SO2 and NOx emissions across the US have reduced by about 66% and 50%, respectively, while emissions have increased dramatically in many developing regions. These changing emissions have resulted in contrasting trends in regional aerosol burden across the northern hemisphere and provide constraints for model calculations of aerosol radiative effects. A systematic investigation of aerosol direct effects is conducted in this study comprising of three major components: (1) development of a consistent emission inventory for the U.S. based on trends in population, economic conditions, and technology changes for 1990-2010; (2) a comprehensive analysis of multi-decadal trends in observations of PM2.5, chemical composition, optical properties, and surface solar radiation, and (3) hemispheric to regional scale multi-decadal simulations with WRF-CMAQ to examine the model’s ability to capture trends in aerosol direct radiative effects through comparisons with in-situ, columnar, and remote sensing observations. The model successfully captured increasing AOD trends along with correspondingly increased TOA-SWR (upwelling) and decreased surface-SWR (downwelling) in eastern China and northern Pacific with opposite trends for these metrics in eastern U.S., Europe and northern Atlantic (2000-2010). Nearly linear relationships between the responses in meteorological variables and AOD suggest super-linear responses of air quality are expected, leading to a greater enhancement of pollution in more polluted areas. Model calculations over the U.S. using finer resolution (36km and 12km) also show radiation brightening associated with declining PM burden during 1990-2010, with the association more pronounced in the eastern U.S. Multi-decadal trends in the aerosol optical depth (AOD) are captured by the model, even though the magnitude of AOD is underestimated. Detailed closure experiments with observations from the CARES campaign suggest that the underestimation of AOD primarily results from underestimation of PM constituents.