Top-Down vs Bottom-Up Emission Calculations

Top-down and bottom-up emission calculations are two complementary approaches used to estimate greenhouse gas and pollutant emissions. Top-down methods start with large-scale atmospheric measurements or inventories and work backward to infer emissions. They use data such as satellite observations, atmospheric monitoring stations, and inverse modeling to estimate total emissions over a region. This approach captures real-world emissions including those from unknown or unreported sources, providing a broad and integrated view. However, it may lack detailed source attribution and can be influenced by atmospheric transport uncertainties. Bottom-up methods involve aggregating emissions from individual sources based on activity data and emission factors. For example, calculating emissions from vehicles by multiplying miles traveled by emission rates per mile. This approach offers detailed, source-specific information and is useful for regulatory compliance and targeted mitigation. Yet, it can underestimate emissions if sources are missing or emission factors are inaccurate. In practice, combining both approaches improves accuracy. Bottom-up inventories provide granular emission data, while top-down assessments validate and adjust these estimates against real atmospheric observations. This integrated approach enhances emission inventories’ reliability, informing effective environmental policies and climate action.