Abstract:California plans to reduce emissions of longâ€lived greenhouse gases (GHGs) through adoption of new energy systems that will also lower concentrations of shortâ€lived absorbing soot contained in airborne particulate matter (PM). Here we examine the direct and indirect effects of reduced PM concentrations under a lowâ€carbon energy (GHGâ€Step) scenario on radiative forcing in California. Simulations were carried out using the sourceâ€oriented WRF/Chem (SOWC) model with 12 km spatial resolution for the year 2054. The avoided aerosol emissions due to technology advances in the GHGâ€Step scenario reduce ground level PM concentrations by ~8.85% over land compared to the Businessâ€asâ€Usual (BAU) scenario, but changes to meteorological parameters are more modest. Topâ€ofâ€atmospheric forcing predicted by the SOWC model increased by 0.15 W m−2, surface temperature warmed by 0.001 K, and planetary boundary layer height (PBLH) increased by 2.20 cm in the GHGâ€Step scenario compared to the BAU scenario. PM climate feedbacks are small because the significant changes in ground level PM concentrations associated with the GHGâ€Step scenario are limited to the first few hundred meters of the atmosphere, with little change for the majority of the vertical column above that level. As an orderâ€ofâ€magnitude comparison, the longâ€term effects of global reductions in GHG emissions (RCP8.5–RCP4.5) lowered average surface temperature over the California study domain by approximately 0.76 K. The effects of longâ€lived climate pollutants such as CO2 are much stronger than the effects of shortâ€lived climate pollutants such as PM soot over California in the year 2054.

Keywords: greenhouse gas emissions, particulate matter, longâ€lived climate pollutants, short-lived climate pollutants, low-carbon energy