Abstract

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.

Highlights

  • Organic aerosol (OA) contributes a significant fraction of the total ambient particulate matter (PM) mass (Zhang et al, 2007), which is of utmost concern for its detrimental effects on human health (Apte et al, 2015) and the Earth’s radiative budget (Myhre et al, 2013)

  • The results shown in this figure are reinforced with statistical confidence by using bootstrap resampling (Fig. S5)

  • Port measurements have more positive skewness than West Oakland; port has a larger fraction of magnets with high OA concentrations than West Oakland

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Summary

Introduction

Organic aerosol (OA) contributes a significant fraction of the total ambient particulate matter (PM) mass (Zhang et al, 2007), which is of utmost concern for its detrimental effects on human health (Apte et al, 2015) and the Earth’s radiative budget (Myhre et al, 2013). Concentrations of ultrafine particles, NO, CO, and particulate black carbon (BC) are enhanced near highways by a factor of 2–3 relative to areas > 100 m from roadways (Choi et al, 2012; Apte et al, 2017; Saha et al, 2018a). In the near-source region fresh emissions of BC and primary OA rapidly mix with background air, reducing concentrations through both dilution and OA partitioning. This rapid mixing occurs over tens to hundreds of meters downwind of the source (Canagaratna et al, 2010; Saha et al, 2018a)

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