North American isoprene influence on intercontinental ozone pollution

Abstract

Changing land-use and climate may alter emissions of biogenic isoprene, a key ozone (O₃) precursor. Isoprene is also a precursor to peroxy acetyl nitrate (PAN) and thus affects partitioning among oxidized nitrogen (NO_y) species, shifting the balance towards PAN, which more efficiently contributes to long-range transport relative to nitric acid (HNO₃) which rapidly deposits. With a suite of sensitivity simulations in the MOZART-2 global tropospheric chemistry model, we gauge the relative importance of the intercontinental influence of a 20% increase in North American (NA) isoprene and a 20% decrease in NA anthropogenic emissions (nitrogen oxides (NO_x), non-methane volatile organic compounds (NMVOC) and NO_x + NMVOC + carbon monoxide + aerosols). The surface O₃ response to NA isoprene emissions (ΔO₃_ISOP) in surface air over NA is about one third of the response to all NA anthropogenic emissions (ΔO₃_ANTH; although with different signs). Over intercontinental distances, ΔO₃_ISOP is relatively larger; in summer and fall, ΔO₃_ISOP in surface air over Europe and North Africa (EU region) is more than half of ΔO₃_ANTH. Future increases in NA isoprene emissions could thus offset decreases in EU surface O₃ resulting from controls on NA anthropogenic emissions. Over the EU region, ΔPAN_ISOP at 700 hPa is roughly the same magnitude as ΔPAN_ANTH (oppositely signed). Outside of the continental source region, the percentage changes in PAN are at least twice as large as for surface O₃, implying that long-term PAN measurements at high altitude sites may help to detect O₃ precursor emission changes. We find that neither the baseline level of isoprene emissions nor the fate of isoprene nitrates contributes to the large diversity in model estimates of the anthropogenic emission influence on intercontinental surface O₃ or oxidized nitrogen deposition reported in the recent TF HTAP multi-model studies (TFHTAP, 2007).