LA Megacity: An Integrated Land-Atmosphere System for Urban CO2 Emissions

Abstract:

About 10% of the global population lives in the word’s 20 megacities (cities with urban populations greater than 10 million people). Megacities account for approximately 20% of the global anthropogenic fossil fuel CO₂ (FFCO₂) emissions, and their proportion of emissions increases monotonically with the world population and urbanization. Megacities range in spatial extent from ~1000 – 10,000 km² with complex topography and variable landscapes. We present here the first attempt at building an integrated land-atmosphere modeling system for megacity environments, developed and evaluated for urban CO₂ emissions over the Los Angeles (LA) Megacity area. The Weather Research and Forecasting (WRF) – Chem model was coupled to a ~1.3-km FFCO₂ emission product, “Hestia-LA”, to simulate the transport of CO₂ across the LA magacity.\

We define the optimal model resolution to represent both the spatial variability of the atmospheric dynamics and the spatial patterns from the CO₂ emission distribution. In parallel, we evaluate multiple configurations of WRF with various physical schemes, using meteorological observations from the CalNex-LA campaign of May-June 2010. Our results suggest that there is no remarkable difference between the medium- (4-km) and high- (1.3-km) resolution simulations in terms of atmospheric model performance. However, the high-resolution modeled CO₂ mixing ratios clearly outperform the results at medium resolution for capturing both the spatial distribution and the temporal variability of the urban CO₂ signals.

We compare the impact of physical representation errors and emission aggregation errors on the modeled CO₂ mixing ratios across the LA megacity. Finally, we present a novel approach to evaluate the design of the current surface network over the LA megacity using the modeled spatial correlations. These results reinforce the importance of using high-resolution emission products over megacities to represent correctly the large spatial gradients in atmospheric CO₂ mixing ratios.