Abstract
Given the importance of aerosol particles to radiative transfer via aerosol-radiation interactions, a methodology for tracking and diagnosing causes of temporal changes in regional-scale aerosol populations is illustrated. The aerosol optical properties tracked include estimates of total columnar burden (aerosol optical depth, AOD), dominant size mode (Ångström exponent, AE), and relative magnitude of radiation scattering versus absorption (single scattering albedo, SSA), along with metrics of the structure of the spatial field of these properties. Over well-defined regions of North America, there are generally negative temporal trends in mean and extreme AOD, and SSA. These are consistent with lower aerosol burdens and transition towards a relatively absorbing aerosol, driven primarily by declining sulfur dioxide emissions. Conversely, more remote regions are characterized by increasing mean and extreme AOD that is attributed to increased local wildfire emissions and long-range (transcontinental) transport. Regional and national reductions in anthropogenic emissions of aerosol precursors are leading to declining spatial autocorrelation in the aerosol fields and increased importance of local anthropogenic emissions in dictating aerosol burdens. However, synoptic types associated with high aerosol burdens are intensifying (becoming more warm and humid), and thus changes in synoptic meteorology may be offsetting aerosol burden reductions associated with emissions legislation.
Highlights
Atmospheric aerosol particles impact biogeochemical cycles, human health, and global and regional climate by scattering and absorbing radiation, acting as cloud condensation nuclei or ice nucleating particles and altering cloud lifetimes and albedo, and changing the atmospheric thermal structure and atmospheric stability
Aerosol properties in the MERRA-2 reanalysis product are derived in part based on assimilation of aerosol optical depth (AOD) at 550 nm derived from remotely sensed properties such as spectral reflectances, solar and instrument geometry, cloud cover, and surface features into the Goddard Earth Observing System, version 5 (GEOS-5) model[18]
MERRA-2 has been subject to extensive evaluation relative to independent observations, and only limited additional evaluation was undertaken as part of this study and is focused on evaluation of the joint probabilities of the key variables considered : AOD, and AE and Single scattering albedo (SSA) relative to those from ground-based measurements of columnar aerosol properties from AErosol RObotic NETwork (AERONET) stations[21]
Summary
Atmospheric aerosol particles (aerosols) impact biogeochemical cycles, human health, and global and regional climate by scattering and absorbing radiation, acting as cloud condensation nuclei or ice nucleating particles and altering cloud lifetimes and albedo, and changing the atmospheric thermal structure and atmospheric stability (ref.[1] and references therein). We propose a suite of aerosol-CIs, and illustrate how they are derived and applied using regions of the U.S National Climate Assessment (NCA) program (Fig. 1) We demonstrate how these aerosol-CIs can be used to quantify variability and temporal trends in aerosol populations, and attribute changes through time to specific drivers of aerosol variability: Gaseous precursor and primary aerosol emissions, and meteorological conditions at the synoptic scale. CIs must be predicated on high quality, uniform (gridded), and publically available data with well-defined provenance and an expectation that the variables on which they are based will continue to be measured into the future Observations, such as those from satellite- or ground-based remote sensing, are not suitable for deriving aerosol-CIs due to spatiotemporal discontinuities and a bias towards sampling cloud-free conditions[16]. MERRA-2 provides gridded global hourly output of observable aerosol optical properties, including in cloudy-sky scenes, with high fidelity when evaluated relative to independent (non-assimilated) observations[17]
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