•Isolation of coarse particles from fine particle sources.•Unique chemical composition of coarse particles.•Role of primary biological particles on aerosol loadings.The Desert Southwest Coarse Particulate Matter Study was undertaken to further our understanding of the spatial and temporal variability and sources of fine and coarse particulate matter (PM) in rural, arid, desert environments. Sampling was conducted between February 2009 and February 2010 in Pinal County, AZ near the town of Casa Grande where PM concentrations routinely exceed the U.S. National Ambient Air Quality Standards (NAAQS) for both PM10 and PM2.5. In this desert region, exceedances of the PM10 NAAQS are dominated by high coarse particle concentrations, a common occurrence in this region of the United States. This work expands on previously published measurements of PM mass and chemistry by examining the sources of fine and coarse particles and the relative contribution of each to ambient PM mass concentrations using the Positive Matrix Factorization receptor model (Clements et al., 2014).Coarse particles within the region were apportioned to nine sources including primary biological aerosol particles (PBAPs - 25%), crustal material (20%), re-entrained road dust (11%), feedlot (11% at the site closest to a cattle feedlot), secondary particles (10%), boron-rich crustal material (9%), and transported soil (6%), with minor contributions from ammonium nitrate, and salt (considered to be NaCl). Fine particles within the region were apportioned to six sources including motor vehicles (37%), road dust (29%), lead-rich (10%), with minor contributions from brake wear, crustal material, and salt. These results can help guide local air pollution improvement strategies designed to reduce levels of PM to below the NAAQS.

N-Nitrosodimethylamine (NDMA) is a chloramination disinfection by-product (DBP) with an uncertain regulatory future. While extensive literature exists on NDMA formation potentials (FP) for natural waters and for model compounds considered as NDMA precursors, less data exists on the kinetics of NDMA formation in surface and wastewaters. NDMA formation kinetics experiments were conducted in seven source waters at two monochloramine doses. NDMA formation was modeled by a simple, second-order model, using the measured NDMAmax and monitored monochloramine concentrations at selected reaction times. The model fits NDMA formation well (R2 > 0.88) in all source waters. While the extent of NDMA formation was highly variable, the rate constant (kapp) values from different waters fell in a narrow range (0.01–0.09 M−1 s−1). This suggests that a common precursor or rate limiting step for NDMA formation likely exists despite the differences in matrices. Although further studies are needed to validate the model over a wider range of water conditions such as pH and N:Cl2 ratios, the model could help water utilities to predict NDMA formation in distribution systems.

The occurrence, source, and sink processes of N-nitrosodimethylamine (NDMA) have been explored by means of combined laboratory, field, and model studies. Observations have shown the occurrence of NDMA in fogs and clouds at substantial concentrations (7.5−397 ng L−1). Laboratory studies were conducted to investigate the formation of NDMA from nitrous acid and dimethylamine in the homogeneous aqueous phase. While NDMA was produced in the aqueous phase, the low yields (<1%) observed could not explain observational concentrations. Therefore gaseous formation of NDMA with partitioning to droplets likely dominates aqueous NDMA formation. Box-model calculations confirmed the predominant contributions from gas phase formation followed by partitioning into the cloud droplets. Measurements and model calculations showed that while NDMA is eventually photolyzed, it might persist in the atmosphere for hours after sunrise and before sunset since the photolysis in the aqueous phase might be much less efficient than in the gas phase.

Cloudwater samples have been collected for the first time at a high-elevation site in the US interior Southwest. Cloud samples were collected at the summit of Mt. Elden near Flagstaff, Arizona. The samples were analyzed for pH, ionic composition, trace metals, organic carbon content, and volatile organic compounds. All of the samples showed high pH values (5.12–6.66), which appear to be the result of soil/crustal acid-neutralizing components. Ammonium and nitrate were the dominant ionic species. Organic carbon concentrations ranged from 3 to 18 mg/l. Volatile aromatic compounds (toluene, ethylbenzene, and xylenes) were detected, although they did not contribute significantly to the dissolved organic matter (<1% of dissolved organic carbon). Still, their aqueous-phase concentrations were substantially higher than equilibrium partitioning from the gas phase would suggest. Metal concentrations were high when compared to other cloud studies in remote areas. Overall, with the exception of pH, the cloud chemistry showed marked inter-event variability. The source of the variability was investigated using NOAA HYSPLIT dispersion calculations. Like the cloud composition, the air mass back trajectories differed widely from event to event, and consistently, air masses that passed over highly urbanized areas had higher trace metal, organic, and ion concentrations than more pristine air masses.

Organic compounds in ambient particulate matter (PM) samples are used as tracers for PM source apportionment. These PM samples are collected using high volume samplers; one such sampler is an impactor in which polyurethane foam (PUF) and polypropylene foam (PPF) are used as the substrates. The polymer substrates have the advantage of limiting particle bounce artifacts during sampling; however these substrates may contain background organic additives. A protocol of two extractions with isopropanol followed by three extractions with dichloromethane (DCM) was developed for both substrate precleaning and analyte extraction. Some residual organic contaminants were present after precleaning; expressed as concentrations in a 24-h ambient PM sample, the residual amounts were 1 μg m−3 for plasticizers and antioxidants, and 10 ng m−3 for n  -alkanes with carbon number lower than 26. The quantification limit for all other organic tracer compounds was ≈0.1≈0.1 ng m−3 in a 24-h ambient PM sample. Recovery experiments were done using NIST Standard Reference Material (SRM) Urban Dust (1649a); the average recoveries for polycyclic aromatic hydrocarbons (PAHs) from PPF and PUF substrates were 117±8%117±8% and 107±11%107±11%, respectively. Replicate extractions were also done using the ambient samples collected in Nogales, Arizona. The relative differences between repeat analyses were less than 10% for 47 organic tracer compounds quantified. After the first extraction of ambient samples, less than 7% of organic tracer compounds remained in the extracted substrates. This method can be used to quantify a suite of semi- and non-polar organic tracer compounds suitable for source apportionment studies in 24-h ambient PM samples.

•Oxy-polycyclic hydrocarbons (PAHs) can substantially partition to the aqueous phase.•Chemical lifetimes of PAHs can be significantly shortened in the presence of fog.•Temperature dependencies of partitioning ratios represent huge uncertainty.The importance of the atmospheric aqueous phase of fogs and clouds, for the processing and removal of polycyclic aromatic hydrocarbons (PAHs) is not well known. A multiphase model was developed to determine the fate and lifetime of PAHs in fogs and clouds for a limited set of daytime conditions. The model describes partitioning between three phases (aqueous, liquid organic, and gas), experimental and estimated (photo)oxidation rates. Using a limited set of microphysical and chemical input conditions, the loss rates of PAHs in the complex three-phase system are explored.At 25 °C, PAHs with two, three and four rings are predicted to be primarily in the gas phase (fraction in the gas phase xg > 90%) while five- and six-ring PAHs partition significantly into droplets with aqueous phase fractions of 1–6% and liquid organic phase fractions of 31–91%, respectively. The predicted atmospheric chemical lifetimes of PAHs in the presence of fog or cloud droplets (<8 h) are significantly shorter than literature predictions of PAH lifetimes due to wet and dry deposition (1–14 days and 5–15 months, respectively) and shorter than or equal to predicted lifetimes due to chemical reactions in the gas and organic particulate phases (1–300 h). Even though PAH solubilities are ≤4 × 10−2 g L−1, the results of the current study show that often the condensed phase of fog and cloud droplets cannot be neglected as a PAH sink.

N-Nitrosodimethylamine (NDMA) is a chloramination disinfection by-product (DBP) with an uncertain regulatory future. While extensive literature exists on NDMA formation potentials (FP) for natural waters and for model compounds considered as NDMA precursors, less data exists on the kinetics of NDMA formation in surface and wastewaters. NDMA formation kinetics experiments were conducted in seven source waters at two monochloramine doses. NDMA formation was modeled by a simple, second-order model, using the measured NDMAmax and monitored monochloramine concentrations at selected reaction times. The model fits NDMA formation well (R2 > 0.88) in all source waters. While the extent of NDMA formation was highly variable, the rate constant (kapp) values from different waters fell in a narrow range (0.01–0.09 M−1 s−1). This suggests that a common precursor or rate limiting step for NDMA formation likely exists despite the differences in matrices. Although further studies are needed to validate the model over a wider range of water conditions such as pH and N:Cl2 ratios, the model could help water utilities to predict NDMA formation in distribution systems.