Abstract. Surface ozone (O3) is an important air pollutant and
greenhouse gas. Land use and land cover is one of the critical factors
influencing ozone, in addition to anthropogenic emissions and climate. Land
use and land cover change (LULCC) can on the one hand affect ozone
“biogeochemically”, i.e., via dry deposition and biogenic emissions of
volatile organic compounds (VOCs). LULCC can on the other hand alter
regional- to large-scale climate through modifying albedo and
evapotranspiration, which can lead to changes in surface temperature,
hydrometeorology, and atmospheric circulation that can ultimately impact
ozone “biogeophysically”. Such biogeophysical
effects of LULCC on ozone are largely understudied. This study investigates
the individual and combined biogeophysical and biogeochemical effects of
LULCC on ozone and explicitly examines the critical pathway for how LULCC
impacts ozone pollution. A global coupled atmosphere–chemistry–land model is
driven by projected LULCC from the present day (2000) to the future (2050) under
RCP4.5 and RCP8.5 scenarios, focusing on the boreal summer. Results reveal
that when considering biogeochemical effects only, surface ozone is
predicted to have slight changes by up to 2 ppbv maximum in some areas due
to LULCC. It is primarily driven by changes in isoprene emission and dry
deposition counteracting each other in shaping ozone. In contrast, when
considering the combined effect of LULCC, ozone is more substantially
altered by up to 5 ppbv over several regions in North America and Europe
under RCP4.5, reflecting the importance of biogeophysical effects on ozone
changes. In boreal and temperate mixed forests with intensive reforestation,
enhanced net radiation and sensible heat induce a cascade of
hydrometeorological feedbacks that generate warmer and drier conditions
favorable for higher ozone levels. In contrast, reforestation in subtropical
broadleaf forests has minimal impacts on boundary-layer meteorology and
ozone air quality. Furthermore, significant ozone changes are also found in
regions with only modest LULCC, which can only be explained by “remote”
biogeophysical effects. A likely mechanism is that reforestation induces a
circulation response, leading to reduced moisture transport and ultimately
warmer and drier conditions in the surrounding regions with limited LULCC.
We conclude that the biogeophysical effects of LULCC are important pathways
through which LULCC influences ozone air quality both locally and in remote
regions even without significant LULCC. Overlooking the effects of
hydrometeorological changes on ozone air quality may cause underestimation
of the impacts of LULCC on ozone pollution.
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