North American isoprene influence on intercontinental ozone pollution
Changing land-use and climate may alter emis- sions of biogenic isoprene, a key ozone (O3) precursor. Iso- prene is also a precursor to peroxy acetyl nitrate (PAN) and thus affects partitioning among oxidized nitrogen (NOy) species, shifting the balance towards PAN, which more ef- ficiently contributes to long-range transport relative to nitric acid (HNO3) which rapidly deposits. With a suite of sen- sitivity simulations in the MOZART-2 global tropospheric chemistry model, we gauge the relative importance of the intercontinental influence of a 20% increase in North Ameri- can (NA) isoprene and a 20% decrease in NA anthropogenic emissions (nitrogen oxides (NOx), non-methane volatile or- ganic compounds (NMVOC) and NOx +NMVOC+carbon monoxide+aerosols). The surface O3 response to NA iso- prene emissions (O3 ISOP) in surface air over NA is about one third of the response to all NA anthropogenic emissions (O3 ANTH; although with different signs). Over intercon- tinental distances, O3 ISOP is relatively larger; in summer and fall, O3 ISOP in surface air over Europe and North Africa (EU region) is more than half of O3 ANTH. Fu- ture increases in NA isoprene emissions could thus offset decreases in EU surface O3 resulting from controls on NA anthropogenic emissions. Over the EU region, PAN ISOP at 700 hPa is roughly the same magnitude as PAN 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 emis- sion changes. We find that neither the baseline level of iso- prene emissions nor the fate of isoprene nitrates contributes to the large diversity in model estimates of the anthropogenic emission influence on intercontinental surfaceO3 or oxidized nitrogen deposition reported in the recent TF HTAP multi- model studies (TFHTAP, 2007).