Abstract
Abstract. Tropical tropospheric ozone affects Earth's radiative forcing and the oxidative capacity of the atmosphere. Considerable work has been devoted to the study of the processes controlling its budget. Yet, large discrepancies between simulated and observed tropical tropospheric ozone remain. Here, we characterize some of the mechanisms by which the photochemistry of isoprene impacts the budget of tropical ozone. At the regional scale, we use forward sensitivity simulation to explore the sensitivity to the representation of isoprene nitrates. We find that isoprene nitrates can account for up to 70% of the local NOx = NO+NO2 sink. The resulting modulation of ozone can be well characterized by their net modulation of NOx. We use adjoint sensitivity simulations to demonstrate that the oxidation of isoprene can affect ozone outside of continental regions through the transport of NOx over near-shore regions (e.g., South Atlantic) and the oxidation of isoprene outside of the boundary layer far from its emissions regions. The latter mechanism is promoted by the simulated low boundary-layer oxidative conditions. In our simulation, ~20% of the isoprene is oxidized above the boundary layer in the tropics. Changes in the interplay between regional and global effect are discussed in light of the forecasted increase in anthropogenic emissions in tropical regions.
Highlights
The impact of tropical ozone on Earth’s radiative forcing (Forster et al, 2007) and oxidative capacity have motivated considerable work to unravel the complex interplay between dynamics, surface emissions and chemistry that controls its distribution
In order to rationalize the simulated sensitivity of ozone to changes in the representation of isoprene nitrates, we define the local net removal of NOx by isoprene nitrate chemistry as: DING = PING0 + PING1 − LING1 − LING2 − α × LING0 where PING0 and PING1 are the photochemical sources of ING0 and ING1; LING0, LING1 and LING2 are the photochemical losses of ING0, ING1 and ING2
The rapid economic development of the tropical regions calls for a better assessment of how anthropogenic perturbations may influence the concentration of ozone and assess the consequences of this change on food security (Van Dingenen et al, 2009; Hewitt et al, 2009), human health (World Health Organization, 2005) and radiative forcing (directly or indirectly (e.g., Sitch et al, 2007)
Summary
The impact of tropical ozone on Earth’s radiative forcing (Forster et al, 2007) and oxidative capacity have motivated considerable work to unravel the complex interplay between dynamics, surface emissions and chemistry that controls its distribution. We focus on the regions located between 15◦S and 7◦N (Fig. 1) where more than 50 % of the global terrestrial isoprene emissions are located These estimates remain uncertain (Guenther et al, 2006) as (a) bottom-up estimates are derived from ground studies that are too sparse to capture the diversity of plants in the rain forest, and (b) topdown estimates using satellite measurements of formaldehyde (Palmer et al, 2003; Barkley et al, 2008) rely on chemical mechanisms that are known to poorly represent the photochemistry under high biogenics and low NOx conditions (e.g., Lelieveld et al, 2008; Stone et al, 2010). We conclude by presenting some avenues that may help tease out these different processes and improve their representations in chemical transport models (Sect. 4.4)
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