Real-Time, Single-Particle Volatility, Size, and Chemical Composition Measurements of Aged Urban Aerosols

Aerosol particles undergo significant amounts of atmospheric processing within the Los Angeles basin. To assess the major sources and degree of aging, ambient particle volatility, size, and chemical composition were measured concurrently in real-time during the Study of Organic Aerosols conducted in Riverside, CA in November 2005. A thermodenuder (TD) was coupled to an aerosol time-of-flight mass spectrometer (ATOFMS) to characterize the chemistry of the individual submicrometer particles remaining after heating. Aged organic carbon (OC) particles contained >50% by volume secondary species, primarily ammonium nitrate, ammonium sulfate, and amines. At 230 °C, the chemistry of the 100−150 nm residues were elemental carbon (29% by number), OC (27%), and biomass burning (15%). Sea salt (47%) and dust (15%) were the major contributors at the larger sizes (750−800 nm). Many particles at 230 °C possessed signatures similar to those of fresh vehicle emissions, biomass burning, sea salt, and dust particles, showing that the TD-ATOFMS method can be used to apportion particles in highly aged environments to their original sources, while providing insight into the relative contributions of primary and secondary species

Investigations of the Diurnal Cycle and Mixing State of Oxalic Acid in Individual Particles in Asian Aerosol Outflow

The mixing state of oxalic acid was measured in Asian outflow during ACE-Asia by direct shipboard measurements using an ATOFMS single-particle mass spectrometer. Oxalic and malonic acids were found to be predominantly internally mixed with mineral dust and aged sea salt particles. A persistent diurnal cycle of oxalic acid in mineral dust occurred for over 25 days in marine, polluted marine, and dust storm air masses. The preferential enrichment of diacids in mineral dust over carbonaceous particles and their diurnal behavior indicate a photochemical source of the diacids. Oxalate was only detected simultaneously with elevated aged dust particle counts. This suggests that the diurnal production of diacids most likely results from episodic atmospheric processing of the polluted dust aerosol. We propose a mechanism to explain these observations in which the photochemical oxidation of volatile organic compounds is followed by partitioning of the diacids and precursors to the alkaline Asian dust, with subsequent heterogeneous and aqueous oxidation. Our data indicate that the particulate diacids were produced over just a few hours close to the source; no significant production or destruction appears to have occurred during long-range transport to the ship. No evidence of extensive cloud processing of the sampled aerosol was found. This mixing state of diacids has important implications for the solubility and cloud nucleation properties of the dominant fraction of water-soluble organics and the bioavailability of iron in dust

Determination of Single Particle Mass Spectral Signatures from Light-Duty Vehicle Emissions

In this study, 28 light-duty gasoline vehicles (LDV) were operated on a chassis dynamometer at the California Air Resources Board Haagen-Smit Facility in El Monte, CA. The mass spectra of individual particles emitted from these vehicles were measured using aerosol time-of-flight mass spectrometry (ATOFMS). A primary goal of this study involves determining representative size-resolved single particle mass spectral signatures that can be used in future ambient particulate matter source apportionment studies. Different cycles were used to simulate urban driving conditions including the federal testing procedure (FTP), unified cycle (UC), and the correction cycle (CC). The vehicles were selected to span a range of catalytic converter (three-way, oxidation, and no catalysts) and engine technologies (vehicles models from 1953 to 2003). Exhaust particles were sampled directly from a dilution and residence chamber system using particle sizing instruments and an ATOFMS equipped with an aerodynamic lens (UF-ATOFMS) analyzing particles between 50 and 300 nm. On the basis of chemical composition, 10 unique chemical types describe the majority of the particles with distinct size and temporal characteristics. In the ultrafine size range (between 50 and 100 nm), three elemental carbon (EC) particle types dominated, all showing distinct EC signatures combined with Ca, phosphate, sulfate, and a lower abundance of organic carbon (OC). The relative fraction of EC particle types decreased as particle size increased with OC particles becoming more prevalent above 100 nm. Depending on the vehicle and cycle, several distinct OC particle types produced distinct ion patterns, including substituted aromatic compounds and polycyclic aromatic hydrocarbons (PAH), coupled with other chemical species including ammonium, EC, nitrate, sulfate, phosphate, V, and Ca. The most likely source of the Ca and phosphate in the particles is attributed to the lubricating oil. Significant variability was observed in the chemical composition of particles emitted within the different car categories as well as for the same car operating under different driving conditions. Two-minute temporal resolution measurements provide information on the chemical classes as they evolved during the FTP cycle. The first two minutes of the cold start produced more than 5 times the number of particles than any other portion of the cycle, with one class of ultrafine particles (EC coupled with Ca, OC, and phosphate) preferentially produced. By number, the three EC with Ca classes (which also contained OC, phosphate, and sulfate) were the most abundant classes produced by the nonsmoking vehicles. The smoker category produced the highest number of particles, with the dominant classes being OC comprised of substituted monoaromatic compounds and PAHs, coupled with Ca and phosphate, thus suggesting used lubricating oil was associated with many of these particles. These studies show, by number, EC particles dominate gasoline emissions in the ultrafine size range particularly for the lowest emitting newer vehicles, suggesting the EC signature alone cannot be used as a unique tracer for diesels. This represents the first report of high time- and size-resolved chemical composition data showing the mixing state of nonrefractory elements in particles such as EC for vehicle emissions during dynamometer source testing

Single Particle Characterization of Ultrafine and Accumulation Mode Particles from Heavy Duty Diesel Vehicles Using Aerosol Time-of-Flight Mass Spectrometry

The aerodynamic size and chemical composition of individual ultrafine and accumulation mode particle emissions (Da = 50−300 nm) were characterized to determine mass spectral signatures for heavy duty diesel vehicle (HDDV) emissions that can be used for atmospheric source apportionment. As part of this study, six in-use HDDVs were operated on a chassis dynamometer using the heavy heavy-duty diesel truck (HHDDT) five-cycle driving schedule under different simulated weight loads. The exhaust emissions were passed through a dilution/residence system to simulate atmospheric dilution conditions, after which an ultrafine aerosol time-of-flight mass spectrometer (UF-ATOFMS) was used to sample and characterize the HDDV exhaust particles in real-time. This represents the first study where refractory species including elemental carbon and metals are characterized directly in HDDV emissions using on-line mass spectrometry. The top three particle classes observed with the UF-ATOFMS comprise 91% of the total particles sampled and show signatures indicative of a combination of elemental carbon (EC) and engine lubricating oil. In addition to the vehicle make/year, the effects of driving cycle and simulated weight load on exhaust particle size and composition were investigated

Seasonal Volatility Dependence of Ambient Particle Phase Amines

During the summer and fall of 2005 in Riverside, California, the seasonal volatility behavior of submicrometer aerosol particles was investigated by coupling an automated thermodenuder system to an online single-particle mass spectrometer. A strong seasonal dependence was observed for the gas/particle partitioning of alkylamines within individual ambient submicrometer aged organic carbon particles internally mixed with ammonium, nitrate, and sulfate. In the summer, the amines were strongly correlated with nitrate and sulfate, suggesting the presence of aminium nitrate and sulfate salts which were nonvolatile and comprised ∼6−9% of the average particle mass at 230 °C. In the fall, 86 ± 1% of the amines volatilized below 113 °C with aminium nitrate and sulfate salts representing less than 1% of the particle mass at 230 °C. In the summer, a more acidic particle core led to protonation of the amines and subsequent formation of aminium sulfate and nitrate salts; whereas, in the fall, the particles contained more ammonium and thus were less acidic, causing fewer aminium salts to form. Therefore, the acidity of individual particles can greatly affect gas/particle partitioning of organic species in the atmosphere, and the concentrations of amines, as strong bases, should be included in estimations of aerosol pH

Effect of Structural Heterogeneity in Chemical Composition on Online Single-Particle Mass Spectrometry Analysis of Sea Spray Aerosol Particles

Knowledge of the surface composition of sea spray aerosols (SSA) is critical for understanding and predicting climate-relevant impacts. Offline microscopy and spectroscopy studies have shown that dry supermicron SSA tend to be spatially heterogeneous particles with sodium- and chloride-rich cores surrounded by organic enriched surface layers containing minor inorganic seawater components such as magnesium and calcium. At the same time, single-particle mass spectrometry reveals several different mass spectral ion patterns, suggesting that there may be a number of chemically distinct particle types. This study investigates factors controlling single particle mass spectra of nascent supermicron SSA. Depth profiling experiments conducted on SSA generated by a fritted bubbler and total ion intensity analysis of SSA generated by a marine aerosol reference tank were compared with observations of ambient SSA observed at two coastal locations. Analysis of SSA produced by utilizing controlled laboratory methods reveals that single-particle mass spectra with weak sodium ion signals can be produced by the desorption of the surface of typical dry SSA particles composed of salt cores and organic-rich coatings. Thus, this lab-based study for the first time unifies findings from offline and online measurements as well as lab and field studies of the SSA particle-mixing state

Real-Time Detection and Mixing State of Methanesulfonate in Single Particles at an Inland Urban Location during a Phytoplankton Bloom

Dimethyl sulfide (DMS), produced by oceanic phytoplankton, is oxidized to form methanesulfonic acid (MSA) and sulfate, which influence particle chemistry and hygroscopicity. Unlike sulfate, MSA has no known anthropogenic source making it a useful tracer for ocean-derived biogenic sulfur. Despite numerous observations of MSA, predominately in marine environments, the production pathways of MSA have remained elusive highlighting the need for additional measurements, particularly at inland locations. During the Study of Organic Aerosols in Riverside, CA from July-August 2005, MSA was detected in submicrometer and supermicrometer particles using real-time, single-particle mass spectrometry. MSA was detected due to blooms of DMS-producing organisms along the California coast. The detection of MSA depended on both the origin of the sampled air mass as well as the concentration of oceanic chlorophyll present. MSA was mainly mixed with coastally emitted particle types implying that partitioning of MSA occurred before transport to Riverside. Importantly, particles containing vanadium had elevated levels of MSA compared to particles not containing vanadium, suggesting a possible catalytic role of vanadium in MSA formation. This study demonstrates how anthropogenic, metal-containing aerosols can enhance the atmospheric processing of biogenic emissions, which needs to be considered when modeling coastal as well as urban locations

Impact of Emissions from the Los Angeles Port Region on San Diego Air Quality during Regional Transport Events

Oceangoing ships emit an estimated 1.2−1.6 million metric tons (Tg) of PM10 per year and represent a significant source of air pollution to coastal communities. As shown herein, ship and other emissions near the Los Angeles and Long Beach Port region strongly influence air pollution levels in the San Diego area. During time periods with regional transport, atmospheric aerosol measurements in La Jolla, California show an increase in 0.5−1 μm sized single particles with unique signatures including soot, metals (i.e., vanadium, iron, and nickel), sulfate, and nitrate. These particles are attributed to primary emissions from residual oil sources such as ships and refineries, as well as traffic in the port region, and secondary processing during transport. During regional transport events, particulate matter concentrations were 2−4 times higher than typical average concentrations from local sources, indicating the health, environmental, and climate impacts from these emission sources must be taken into consideration in the San Diego region. Unless significant regulations are imposed on shipping-related activities, these emission sources will become even more important to California air quality as cars and truck emissions undergo further regulations and residual oil sources such as shipping continue to expand

Real-Time, Single-Particle Measurements of Oligomers in Aged Ambient Aerosol Particles

Unique high mass negative ions in the −200 to −400 mass/charge range with repetitive spacings of 12, 14, and 16 units, representative of oligomeric species, have been detected in single ambient submicrometer aerosol particles using real-time single-particle mass spectrometry during the Study of Organic Aerosols field campaign conducted in Riverside, CA (SOAR) in August and November 2005. These oligomer-containing particles represented 33−40% of the total detected particles and contained other indicators of aging including oxidized organic carbon, amine, nitrate, and sulfate ion markers. Overall, the highest mass oligomeric patterns were observed in small acidic 140−200 nm particles in the summer. Also during the summer, increased oligomer intensities were observed when the particles were heated with a thermodenuder. We hypothesize that heat removed semivolatile species, thereby increasing particle acidity, while concentrating the oligomeric precursors and accelerating oligomer formation. Differences in oligomer behavior with respect to particle size and heating can be attributed to seasonal differences in photochemical oxidation, the relative amount of ammonium, and particle acidity

Impacts of Aerosol Aging on Laser Desorption/Ionization in Single-Particle Mass Spectrometers

<div><p>Single-particle mass spectrometry (SPMS) has been widely used for characterizing the chemical mixing state of ambient aerosol particles. However, processes occurring during particle ablation and ionization can influence the mass spectra produced by these instruments. These effects remain poorly characterized for complex atmospheric particles. During the 2005 Study of Organic Aerosols in Riverside (SOAR), a thermodenuder was used to evaporate the more volatile aerosol species in sequential temperature steps up to 230°C; the residual aerosol particles were sampled by an aerosol mass spectrometer (AMS) and a single-particle aerosol time-of-flight mass spectrometer (ATOFMS). Removal of the secondary species (e.g., ammonium nitrate/sulfate) through heating permitted assessment of the change in ionization patterns as the composition changed for a given particle type. It was observed that a coating of secondary species can reduce the ionization efficiency by changing the degree of laser absorption or particle ablation, which significantly impacted the measured ion peak areas. Nonvolatile aerosol components were used as pseudo-internal standards (or “reference components”) to correct for this LDI effect. Such corrected ATOFMS ion peak areas correlated well with the AMS measurements of the same species up to 142°C. This work demonstrates the potential to accurately relate SPMS peak areas to the mass of specific aerosol components.</p><p>Copyright 2014 American Association for Aerosol Research</p></div