Megacity ozone air quality under four alternative future scenarios

Abstract

Abstract. The impact of the megacities of the world on global tropospheric ozone, and conversely, the extent to which megacities are influenced by emissions of ozone precursors from outside of the megacities is examined under the four alternative RCP ("Representative Concentration Pathway") emissions scenarios. Despite accounting for about 6 % of present-day anthropogenic emissions of ozone precursor species, the contribution of emissions from megacities to global tropospheric ozone is calculated to be 0.84 %. By 2100 this contribution falls to between 0.18 % and 0.62 % depending on the scenario, with the lower value being for the most-polluting of the four future emissions scenarios due to stringent controls on ozone precursor emissions from highly populated areas combined with a stronger tropospheric background ozone field. The higher end of this range is from the least-polluting of the four emissions scenarios, due to lower background tropospheric ozone combined with the use of a simpler downscaling methodology in the construction of the scenario, which results in higher emissions from megacities. Although the absolute impact of megacities on global ozone is small, an important result of this study is that under all future scenarios, future air quality in megacities is expected to be less influenced by local emissions within the cities, but instead more influenced by emission sources outside of the cities, with mixing ratios of background ozone projected to play an increasing role in megacity air quality throughout the 21st century. Assumptions made when downscaling the emissions scenarios onto the grids used in such modelling studies can have a large influence on these results; future generations of emissions scenarios should include spatially explicit representations or urban development suitable for air quality studies using global chemical transport models.

Tropospheric ozone and its precursors from the urban to the global scale from air quality to short-lived climate forcer

“…Previous work has shown that individual megacities are not well represented by global emission inventories compared to the detailed city-scale inventories (Butler et al, 2012), which can be an issue when trying to model ozone on a larger scale. Additional complexity is added by the nonlinearity of the atmospheric chemistry involved in ozone production processes.…”

“…In a model "annihilation" experiment where the emissions from grid cells containing megacities were removed, emissions from megacities contributed only 0.84 % to the global average tropospheric ozone column density, proportionally smaller than the 6 % of global anthropogenic ozone precursor emissions from megacities (Butler et al, 2012). This does not, however, represent human health exposure to ozone in megacities and is also a global average.…”

Modelling the impact of megacities on local, regional and global tropospheric ozone and the deposition of nitrogen species

“…Wild and Akimoto, 2001;Derwent et al, 2004;Fiore et al, 2009)). Changes to urban emissions therefore have the potential to influence air quality on much larger scales, as also investigated in the recent European collaborative project MEGAPOLI (Megacities: Emissions, urban, regional and Global Atmospheric POLlution and climate effects, and Integrated tools for assessment and mitigation) (Baklanov et al, 2010), and in a number of recent modelling studies (Mayer et al, 2000;Lawrence et al, 2007;Butler and Lawrence, 2009;Fiore et al, 2009;Butler et al, 2012). Many of these studies use pure modelling approaches to assess the impact of emission changes both in perturbation and future scenarios.…”

Future changes in surface ozone over the Mediterranean basin in the framework of the Chemistry-Aerosol Mediterranean Experiment (ChArMEx)

“…Several modeling studies and assessments have evaluated the future evolution of chemical and dynamical processes and have shown that future changes in ozone precursors have a significant impact on the evolution of tropospheric ozone (O 3 ) and particularly surface O 3 (Butler et al, 2012;West et al, 2007). Among the changes is the stratospheric influx increase due, on one hand, to the global warming resulting from the accentuation of residual atmospheric circulation forced by climate change 5 (Collins et al, 2003;Sudo et al, 2003;Butchart et al, 2006;Zeng et al, 2003) and, on the other hand, to the recovery of stratospheric ozone (Zeng et al, 2010;Kawase et al, 2011).…”

“…For the evaluation of the different models, we use a complete set of metrics (see Table 3): the coefficient of variation ratio (CvR), the correlation coefficient (R), the normalized mean biases (NMB), the Mean Bias (MnB), the Mean Absolute Gross Error (MAGE) and the Root Mean Square Error (RMSE). In addition 15 to these metrics, we use two unbiased symmetric metrics introduced by Yu et al (2006) that are found to be statistically robust and easier to interpret: the Normalized Mean Bias Factor (NMBF) and the normalized mean absolute error factor (NMAEF).…”