High-spatial-resolution mapping and source apportionment of aerosol composition in Oakland, California, using mobile aerosol mass spectrometry
Authors: Shah Rishabh U. , Robinson Ellis S. , Gu Peishi , Robinson Allen L. , Apte Joshua S. , Presto Albert A. 
Published: November 2018

Abstract. We investigated spatial and temporal patterns in the concentration and
composition of submicron particulate matter (PM1) in Oakland,
California, in the summer of 2017 using an aerosol mass spectrometer mounted
in a mobile laboratory. We performed ∼160 h of mobile sampling in the
city over a 20-day period. Measurements are compared for three adjacent
neighborhoods with distinct land uses: a central business district
(“downtown”), a residential district (“West Oakland”), and a major
shipping port (“port”). The average organic aerosol (OA) concentration is
5.3 µg m−3 and contributes ∼50 % of the PM1
mass. OA concentrations in downtown are, on average,
1.5 µg m−3 higher than in West Oakland and port. We
decomposed OA into three factors using positive matrix factorization:
hydrocarbon-like OA (HOA; 20 % average contribution), cooking OA (COA;
25 %), and less-oxidized oxygenated OA (LO-OOA; 55 %). The collective
45 % contribution from primary OA (HOA + COA) emphasizes the
importance of primary emissions in Oakland. The dominant source of primary OA
shifts from HOA-rich in the morning to COA-rich after lunchtime. COA in
downtown is consistently higher than West Oakland and port due to a large
number of restaurants. HOA exhibits variability in space and time. The
morning-time HOA concentration in downtown is twice that in port, but port
HOA increases more than two-fold during midday, likely because trucking
activity at the port peaks at that time. While it is challenging to
mathematically apportion traffic-emitted OA between drayage trucks and cars,
combining measurements of OA with black carbon and CO suggests that while
trucks have an important effect on OA and BC at the port, gasoline-engine
cars are the dominant source of traffic emissions in the rest of Oakland.
Despite the expectation of being spatially uniform, LO-OOA also exhibits
spatial differences. Morning-time LO-OOA in downtown is roughly 25 %
(∼0.6 µg m−3) higher than the rest of Oakland. Even as
the entire domain approaches a more uniform photochemical state in the
afternoon, downtown LO-OOA remains statistically higher than West Oakland and
port, suggesting that downtown is a microenvironment with higher
photochemical activity. Higher concentrations of particulate sulfate (also of
secondary origin) with no direct sources in Oakland further reflect higher
photochemical activity in downtown. A combination of several factors (poor
ventilation of air masses in street canyons, higher concentrations of
precursor gases, higher concentrations of the hydroxyl radical) likely
results in the proposed high photochemical activity in downtown. Lastly,
through Van Krevelen analysis of the elemental ratios (H ∕ C, O ∕ C)
of the OA, we show that OA in Oakland is more chemically reduced than several
other urban areas. This underscores the importance of primary emissions in
Oakland. We also show that mixing of oceanic air masses with these primary
emissions in Oakland is an important processing mechanism that governs the
overall OA composition in Oakland.