High-Resolution
Mapping of Sources Contributing
to Urban Air Pollution
Using Adjoint Sensitivity Analysis: Benzene and Diesel Black Carbon
- Publication date
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Abstract
The adjoint of the Community Multiscale
Air Quality (CMAQ) model
at 1 km horizontal resolution is used to map emissions that contribute
to ambient concentrations of benzene and diesel black carbon (BC)
in the San Francisco Bay area. Model responses of interest include
population-weighted average concentrations for three highly polluted
receptor areas and the entire air basin. We consider both summer (July)
and winter (December) conditions. We introduce a novel approach to
evaluate adjoint sensitivity calculations that complements existing
methods. Adjoint sensitivities to emissions are found to be accurate
to within a few percent, except at some locations associated with
large sensitivities to emissions. Sensitivity of model responses to
emissions is larger in winter, reflecting weaker atmospheric transport
and mixing. The contribution of sources located within each receptor
area to the same receptor’s air pollution burden increases
from 38–74% in summer to 56–85% in winter. The contribution
of local sources is higher for diesel BC (62–85%) than for
benzene (38–71%), reflecting the difference in these pollutants’
atmospheric lifetimes. Morning (6–9am) and afternoon (4–7
pm) commuting-related emissions dominate region-wide benzene levels
in winter (14 and 25% of the total response, respectively). In contrast,
afternoon rush hour emissions do not contribute significantly in summer.
Similar morning and afternoon peaks in sensitivity to emissions are
observed for the BC response; these peaks are shifted toward midday
because most diesel truck traffic occurs during off-peak hours