Global polycyclic aromatic hydrocarbons (PAHs) and future climate and emissions, 2000-2050

We investigate effects of 2000-2050 emissions and climate
changes on the atmospheric transport of three polycyclic aromatic hydro-
carbons (PAHs): phenanthrene (PHE), pyrene (PYR), and benzo[a]pyrene
(BaP). We use the GEOS-Chem model coupled to meteorology from a general
circulation model and focus on impacts to northern hemisphere midlatitudes
and the Arctic. We project declines in anthropogenic emissions (up to 20%)
and concentrations (up to 37%), with particle-bound PAHs declining more,
and greater declines in midlatitudes versus the Arctic. Climate change causes
relatively minor increases in midlatitude concentrations for the more volatile
PHE and PYR (up to 4%) and decreases (3%) for particle-bound BaP. In the
Arctic, all PAHs decline slightly under future climate (up to 2%). Overall, we
observe a small 2050 "climate penalty" for volatile PAHs and "climate benefit"
for particle-bound PAHs. The degree of penalty or benefit depends on
competition between deposition and surface-to-air fluxes of previously
deposited PAHs. Particles and temperature have greater impacts on future transport than oxidants, with particle changes alone accounting for 15% of BaP decline under 2050 emissions. Higher temperatures drive increasing surface-to-air fluxes that cause PHE and PYR climate penalties. Simulations suggest ratios of more-to-less volatile species can be used to diagnose signals of climate versus emissions and that these signals are best observed in the Arctic.