Providing top-down constraints on emissions is an important application of model-based analysis of remote-sensing observations of chemically reactive species, yet the degree to which column concentrations are governed by local vs distant sources within models themselves has yet to be fully characterized. We use a chemical transport model and its adjoint to quantify source contributions to weekly column concentrations of ammonia, nitrogen dioxide, sulfur dioxide, and formaldehyde. This efficient approach for quantifying the spatial extent of source influences is validated and then applied to an array of sites and seasons. Overall, we find instances for each species where emissions from at least 500km away must be taken into account to retrieve 90% of the total column influence. This demonstrates that the common practice of assuming a local relationship between satellite observations and emissions in top-down constraints could lead to considerable biases, an issue that will be exacerbated as the resolution of models and observations are refined in coming years.

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Journal Geophysical Research Letters
Turner, A.J., Henze, D.K., Martin, R.V., & Hakami, A. (2012). The spatial extent of source influences on modeled column concentrations of short-lived species. Geophysical Research Letters, 39(12). doi:10.1029/2012GL051832