Comparison between Spatially Resolved Airborne Flux Measurements and Emission Inventories of Volatile Organic Compounds in Los Angeles

Los Angeles is a
major hotspot for ozone and particulate matter
air pollution in the United States. Ozone and PM2.5 in
this region have not improved substantially for the past decade, despite
a reduction in vehicular emissions of their precursors, NOx and volatile organic compounds (VOCs). This reduction
in “traditional” sources has made the current emission
mixture of air pollutant precursors more uncertain. To map and quantify
emissions of a wide range of VOCs in this urban area, we performed
airborne eddy covariance measurements with wavelet analysis. VOC fluxes
measured include tracers for source categories, such as traffic, vegetation,
and volatile chemical products (VCPs). Mass fluxes were dominated
by oxygenated VOCs, with ethanol contributing ∼29% of the total.
In terms of OH reactivity and aerosol formation potential, terpenoids
contributed more than half. Observed fluxes were compared with two
commonly used emission inventories: the California Air Resources Board
inventory and the combination of the Biogenic Emission Inventory System
with the Fuel-based Inventory of Vehicle Emissions combined with Volatile
Chemical Products (FIVE-VCP). The comparison shows mismatches regarding
the amount, spatial distribution, and weekend effects of observed
VOC emissions with the inventories. The agreement was best for typical
transportation related VOCs, while discrepancies were larger for biogenic
and VCP-related VOCs.