Investigation of multi-decadal trends in aerosol direct radiative effect using a continental-scale coupled meteorology-chemistry model

Description

While aerosol radiative effects have been recognized as some of the largest sources of uncertainty among the forcers of climate change, there has been little effort devoted to verification of the spatial and temporal variability of the magnitude and directionality of aerosol radiative forcing. A comprehensive investigation of the processes regulating aerosol distributions, their optical properties, and their radiative effects is needed in order to build confidence in the projected impacts from changes in anthropogenic forcing and climate change. This study addresses this issue through a systematic investigation of changes in anthropogenic emissions of SO2 and NOx over the past two decades in the United States, their impacts on aerosol loading in North America, and subsequent impacts on regional radiation budgets.

A newly developed two-way coupled meteorology and air quality model composed of the Weather Research and Forecasting (WRF) model and the Community Multiscale Air Quality (CMAQ) model is being run for 20 years (1990–2010) on a 12-km resolution grid that covers most of North America. During this period, U.S. emissions of SO2 and NOx have been reduced by about 66% and 50%, respectively, mainly due to Title IV of the U.S. Clean Air Act Amendments that aimed to reduce emissions that contribute to acid deposition. Thus, by simulating this period we can assess model performance for reproducing observed trends in air pollutants, such as sulfate and nitrate aerosols, and their consequences on trends in surface radiation.

The WRF/CMAQ model includes direct effects of aerosols on SW radiation and the direct effects of tropospheric ozone on LW. A new Mie scattering algorithm has been developed for a wider range of wavelengths, which will enable consideration of aerosol direct effects on LW radiation. The two-way WRF-CMAQ also includes an experimental implementation of indirect effects where aerosols from CMAQ are activated as CCN that determine the droplet number concentration for the cloud microphysics model. The resulting effective droplet radius is used in the radiation model to compute cloud optical properties. Indirect effects are being tested by comparing cloud radiative forcing to satellite and aircraft measurements.

Preliminary model simulations for the summer seasons of 1990 and 2006 are being evaluated both for their performance in comparison to observed concentrations and simulation of observed trends in concentrations and surface radiation.