High-Resolution Mapping of Sources Contributing to Urban Air Pollution Using Adjoint Sensitivity Analysis: Benzene and Diesel Black Carbon

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.

Network design for quantifying urban CO<sub>2</sub> emissions: assessing trade-offs between precision and network density

“…Numerous studies have performed top-down estimations of CO 2 emissions using observations from urban surface monitoring networks of various sizes (e.g., Gratani and Varone, 2005;McKain et al, 2012;Newman et al, 2013;Lauvaux et al, 2013;Bréon et al, 2015;Turnbull et al, 2015). However, it is not immediately clear how many sites are necessary to monitor the emissions from an urban area.…”

“…Yet most current urban monitoring networks have fewer than eight sites but operate for much longer than 8 weeks. For example, Gratani and Varone (2005) used a single site in Rome, Newman et al (2013) used a single site in Los Angeles, Lauvaux et al (2013) used two sites in Davos, Switzerland, McKain et al (2012) used a network of five sites in Salt Lake City, and Bréon et al (2015) used five sites in Paris. Recent work from Turnbull et al (2015) employed a denser network of 12 sites in Indianapolis.…”

“…A notable exception is the Open-Data Inventory for Anthropogenic Carbon dioxide (ODIAC) fossil fuel CO 2 (Oda and Maksyutov, 2011), which is based on satellite-observed nightlight data and available globally at 1 × 1 km 2 resolution. Highresolution fossil fuel CO 2 emissions are available for select cities and sectors such as Paris through the AirParif inventory (Bréon et al, 2015; http://www.airparif.asso.fr/en/index/ index) and Indianapolis, Los Angeles, Salt Lake City, and Phoenix through the Hestia project (Gurney et al, 2012;http: //hestia.project.asu.edu/); three recent studies (Gately et al, 2013;McDonald et al, 2014;Gately et al, 2015) developed high-resolution CO 2 emissions from vehicular traffic.…”

Premature deaths attributed to source-specific BC emissions in six urban US regions

“…Their analysis found that a maximum number of premature deaths (approximately 8 000) are attributed to road emissions in California, while road emissions in Georgia result in approximately 1 500 premature deaths. Bastien et al [38] used the CMAQ adjoint model at a 1 km horizontal resolution to perform source attribution of ambient BC and benzene concentrations in the San Francisco Bay area. They found that the largest sensitivities with respect to emissions occur in the winter, with contributions from local sources of BC contributing between 62% and 85%.…”

“…While the work of Dedoussi and Barrett [37], Bastien et al [38], Lee et al [44], and Turner et al [39] utilized adjoint models to perform source attribution studies at high spatial, temporal, and sectoral resolutions, each study focused on health impacts across a large domain, such as the entire globe or the entire US. The extent to which emissions from individual locations and sources cause adverse health effects in specific urban areas has thus yet to be addressed.…”