The heterogeneous replacement of chloride by nitrate in individual sea-salt particles was monitored continuously over time in the troposphere with the use of aerosol time-of-flight mass spectrometry. Modeling calculations show that the observed chloride displacement process is consistent with a heterogeneous chemical reaction between sea-salt particles and gas-phase nitric acid, leading to sodium nitrate production in the particle phase accompanied by liberation of gaseous HCl from the particles. Such single-particle measurements, combined with a single-particle model, make it possible to monitor and explain heterogeneous gas/particle chemistry as it occurs in the atmosphere.

Size-segregated atmospheric aerosols were collected from urban and rural locations in Massachusetts using a micro-orifice impactor. The samples were analyzed for polycyclic aromatic hydrocarbons (PAH) with molecular weights between 178 and 302, using gas chromatography/mass spectrometry. Fifteen PAH were quantified in the urban samples and nine in the rural samples. The quantification results are in good agreement with available ambient monitoring data. In the urban samples, PAH were distributed among aerosol size fractions based on molecular weight. PAH with molecular weights between 178 and 202 were approximately evenly distributed between the fine (aerodynamic diameter <2 μm) and coarse (aerodymanic diameter >2 μm) aerosols. PAH with molecular weights greater than 228 were associated primarily with the fine aerosol fraction. In the rural samples, low and high molecular weight PAH were associated with both the fine and coarse aerosols. Slow mass transfer by vaporization and condensation is proposed to explain the observed PAH partitioning among aerosol size fractions.

[1] Submicrometer sea salt aerosol (SSA) particles are routinely observed in the remote marine boundary layer (MBL); these aerosols include cloud condensation nuclei and so affect the earth's radiative balance. Here foams designed to mimic oceanic whitecaps were generated in the laboratory using a range of bubbling flow rates and aqueous media: unfiltered seawater, filtered seawater, artificial seawater, and mixtures of filtered and artificial seawater. The number and sizes of dried foam droplets in the particle diameter, D p , range 15-673 nm were measured. Particle size distributions for natural and artificial seawaters were unimodal with a dN/d logD p mode at D p % 100 nm (%200 nm at 80% RH). The foam droplet mode falls within the range of reported mode diameters (D p = 40-200 nm) for submicrometer SSA particles observed in the remote MBL. The present laboratory results were scaled up to estimate submicrometer SSA particle fluxes; this extrapolation supports the hypothesis that foam droplets are the most important source of SSA particles by number. The foam droplet flux from the oceans was estimated to be 980 cm À2 s À1 for a fractional white cap coverage, W, of 0.2%. These results compared well with foam droplet fluxes reported elsewhere. The origins of variability in foam droplet fluxes were also evaluated. Natural organic matter affected foam droplet flux by a factor of 1.5; this was less than (1) the effect of bubbling flow rate on foam droplet flux (factor of 5) and (2) the uncertainty in W (factor of 3-7).

Size-segregated atmospheric particles were collected in Boston, MA, using a micro-orifice impactor. The samples were analyzed for oxygenated polycyclic aromatic hydrocarbons (OPAH) using gas chromatography/mass spectrometry. Seven PAH ketones (1-acenaphthenone, 9-fluorenone, 11H-benzo[a]fluoren-11-one, 7H-benzo[c]fluoren-7-one, 11H-benzo[b]fluoren-11-one, benzanthrone, and 6H-benzo[cd]pyrene-6-one), four PAH diones (1,4-naphthoquinone, phenanthrenequinone, 5,12-naphthacenequinone, and benzo[a]pyrene-6,12-dione), and one PAH dicarboxylic acid anhydride (naphthalic anhydride) were identified. Seven additional compounds with mass spectra typical of OPAH were tentatively identified. OPAH were generally distributed among aerosol size fractions based on molecular weight. Compounds with molecular weights between 168 and 208 were ap proximately evenly distributed between the fine (aerodynamic diameter, D p, < 2 μm) and coarse (D p > 2 μm) particles. OPAH with molecular weights of 248 and greater were associated primarily with the fine aerosol fraction. Most OPAH were distributed with particle size in a broad, unimodal hump similar to the the distributions observed for PAH in the same samples. These results suggest that OPAH are initially associated with fine particles after formation by either combustion or gas phase photooxidation and then partition to larger particles by vaporization and sorption. Two OPAH were distributed in bimodal distributions with peaks at D p ≈ 2 μm and D p ≈ 2 μm. These bimodal distributions may be indicative of sorption behavior different from PAH and other OPAH.

Aerosol time-of-flight mass spectrometers (ATOFMS) measure the size and chemical composition of single aerosol particles. To date, these instruments have provided qualitative descriptions of aerosols, in part because the fraction of particles actually present in the atmosphere that is detected by these instruments has not been known. In this work, the particle detection efficiencies of three ATOFMS instruments are determined under ambient sampling conditions from the results of colocated sampling with more conventional reference samplers at three locations in southern California. ATOFMS particle detection efficiencies display a power law dependence on particle aerodynamic diameter (D a ) over a calibration range of 0.32 < D a < 1.8 microns. Detection efficiencies are determined by comparison of ATOFMS data with inertial impactor data and are compared to detection efficiencies determined independently by the use of laser optical particle counters. Detection efficiencies are highest for the largest particles and decline by approximately 2 orders of magnitude for the smallest particles, depending on the ATOFMS design. Calibration functions are developed here and applied to scale ATOFMS data to yield continuous aerosol mass concentrations as a function of particle size over an extended period of time.

An air quality model that follows the evolution of single particles in the atmosphere has been combined with new emissions measurements and then used to predict the size distribution and chemical composition of the airborne fine particle mixture observed at Long Beach, Fullerton, and Riverside, CA, during September 1996. Model predictions show good agreement with ambient measurements of particle size and chemical composition at all three air monitoring sites. The air quality model is used to separately track individual particles released from different sources as they evolve over time. Four major classes of particles are observed: (1) large mineral dust and road dust particles that accumulate only small amounts of secondary aerosol products; (2) primary combustion particles (released initially from diesel vehicles, noncatalyst gasoline-powered vehicles, and food processing) that grow by accumulation of secondary reaction products; (3) sea salt particles that are almost completely transformed by conversion from NaCl to NaNO 3 during transport across the air basin; and (4) sulfate-containing nonsea salt background particles advected into the air basin from upwind over the ocean. The sulfate-containing nonsea salt background particles have an initial PM2.5 concentration of only 8 µg m -3 , but they accumulate significant secondary aerosol reaction products to produce a largely nitrate-containing aerosol having a PM2.5 concentration of 40 µg m -3 by the time that the air masses studied here reach Riverside, CA.

Trajectory analysis shows that the air masses arriving at Riverside, CA, on the afternoons of September 24 and 25, 1996, previously passed near air monitoring sites at Santa Catalina Island, Long Beach, and Fullerton, CA, in succession. At those sites, electrical aerosol analyzers and optical particle counters acquired continuous particle size distribution data, inertial impactor and bulk filter samples were taken with 4-h time resolution for determination of particle size and chemical composition during intensive sampling periods once per day at each site, and aerosol time-of-flight mass spectrometers acquired continuous data on particle size and composition at the single-particle level. These data permit particle evolution to be studied within single air masses as they sequentially pass several monitoring sites over a 2-day period. Air parcels associated with both of the trajectories studied show mineral dust, organic carbon, particulate nitrate and ammonium, and total suspended particulate matter concentrations that increase as transport occurs across the air basin. Large increases in particulate ammonium and nitrate concentrations occur between Fullerton and Riverside due to overnight air stagnation in an area with high gaseous ammonia emissions. The aerosol time-of-flight mass spectrometers show how the externally mixed population of individual particles is modified chemically during transport from Long Beach to Riverside, CA. The coastal aerosol at Long Beach containing sea-salt particles and primary carbon particles is changed substantially as these particles individually accumulate secondary ammonium nitrate and organics during travel across the air basin.

Sea‐salt aerosol (SSA) particles affect the Earth's radiative balance and moderate heterogeneous chemistry in the marine boundary layer. Using conventional and environmental transmission electron microscopes (ETEM), we investigated the hygroscopic growth and liquid‐layer compositions of particles generated from three types of aqueous salt solutions: sodium chloride, laboratory‐synthesized seawater (S‐SSA particles), and natural seawater (N‐SSA particles). Three levels of morphological change were observed with the ETEM as the laboratory‐generated particles were exposed to increasing relative humidity (RH). The first level, onset of observable morphological changes, occurred on average at 70, 48, and 35% RH for the NaCl, S‐SSA, and N‐SSA particles, respectively. The second level, rounding, occurred at 74, 66, and 57% RH for NaCl, S‐SSA, and N‐SSA particles, respectively. The third level, complete deliquescence, occurred at 75% RH for all particles. Collected ambient SSA particles were also examined. With the exception of deliquescence, they did not exhibit the same hygroscopic characteristics as the NaCl particles. The ambient particles, however, behaved most similarly to the synthesized and natural SSA particles, although the onset of morphological change was slightly higher for the S‐SSA particles. We used energy‐dispersive X‐ray spectrometry to study the composition of the liquid layer formed on the S‐SSA and N‐SSA particles. The layer was enriched in Mg, S, and O relative to the solid particle core. An important implication of these results is that MgSO4‐enriched solutions on the surface of SSA particles may be the solvents of many heterogeneous reactions.