Investigation of Multi-decadal Trends in Aerosol Direct Radiative Effect from Anthropogenic Emission Changes over North America and their effects on air quality by Using a Two-way Coupled Meteorology-chemistry Model

The complex feedback mechanisms among chemistry-aerosol-cloud-radiation-climate exist ubiquitously in the Earth system. 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 from past and current human activities will help us to build more confidence in the estimates of 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 short-lived aerosol-precursors over the past two decades (1990–2010) in the United States, their impacts on aerosol loading in North America, and subsequent impacts on regional radiation budgets. During this period, U.S. emissions of pollutants have had dramatic changes and significant decadal brightening of downwelling shortwave has been observed in the continental United States. In this study we test the hypothesis that changes in surface solar radiation over time are caused by the changing patterns of anthropogenic emissions of aerosols and aerosol precursors.

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. A newly developed 20-years emission inventory is used in order to accurately reflect the emission trends resulting from progressively more stringent air quality regulations as well as population trends, economic conditions, and technology changes in motor vehicles and electric power generation. The direct effects of aerosols on SW radiation are considered in this WRF/CMAQ model. New algorithms for the calculation of aerosol optical properties and radiation have been developed considering both computational efficiency and more realistic aerosol states. Preliminary model simulations for 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. Many important factors for the photochemical cycle in planetary boundary layer (PBL), such as temperature, PBL height, photolysis rate, etc., will be affected by the direct effects of aerosols. Thus, the influence of aerosol direct effects on air quality will be investigated in this study. Aerosol mixing state is a key factor for the calculations of aerosol optical properties and will be investigated in this study. A more realistic core-shell model will be tested to demonstrate the uncertainties in the treatment of aerosols in this study. How to constrain a regional climate model for a long-time-period simulation (e.g. decades) in order to get more reasonable results is a challenge. Different nudging strategies on atmosphere and soil will also be compared in this study.