IMPLICATIONSSemi-volatile organic and inorganic compounds can represent a substantial fraction of ambient particulate mass. Loss of semi-volatile compounds during filter collection can occur due to changes in the composition of the air sample. Heating of the air sample to a temperature 10-20 °C above ambient to remove particle-bound water can further enhance loss of the semi-volatile constituents of ambient particles. These losses depend on particle composition, which varies with location and time. Thus, it is not possible to determine the true concentration by using empirical factors to correct the observed concentrations. Therefore, to minimize losses of semi-volatile compounds, we recommend that particle mass measurements be made at temperatures comparable to ambient. In urban areas, a substantial fraction of ambient PM can be semi-volatile material. A larger fraction of this component of PM 10 may be lost from the TEOM-heated filter than the Federal Reference Method (FRM). The observed relationship between TEOM and FRM methods varied widely among sites and seasons. In East Coast urban areas during the summer, the methods were highly correlated with good agreement. In the winter, correlation was somewhat lower, with TEOM PM concentrations generally lower than the FRM. Rubidoux, CA, and two Mexican sites (Tlalnepantla and Merced) had the highest levels of PM 10 and the largest difference between TEOM and manual methods.PM 2.5 data from collocation of 24-hour manual samples with the TEOM are also presented. As most of the semi-volatile PM is in the fine fraction, differences between these methods are larger for PM 2.5 than for PM 10 .

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The use of low-cost air quality sensors has proliferated among non-profits and citizen scientists, due to their portability, affordability, and ease of use. Researchers are examining the sensors for their potential use in a wide range of applications, including the examination of the spatial and temporal variability of particulate matter (PM). However, few studies have quantified the performance (e.g., accuracy, precision, and reliability) of the sensors under real-world conditions. This study examined the performance of two models of PM sensors, the AirBeam and the Alphasense Optical Particle Counter (OPC-N2), over a 12-week period in the Cuyama Valley of California, where PM concentrations are impacted by wind-blown dust events and regional transport. The sensor measurements were compared with observations from two well-characterized instruments: the GRIMM 11-R optical particle counter, and the Met One beta attenuation monitor (BAM). Both sensor models demonstrated a high degree of collocated precision (R2 = 0.8–0.99), and a moderate degree of correlation against the reference instruments (R2 = 0.6–0.76). Sensor measurements were influenced by the meteorological environment and the aerosol size distribution. Quantifying the performance of sensors in real-world conditions is a requisite step to ensuring that sensors will be used in ways commensurate with their data quality.

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Ambient O 3 concentrations in California's South Coast Air Basin (SoCAB) can be as much as 55% higher on weekends than on weekdays under comparable meteorological conditions. This is paradoxical because emissions of O 3 precursors (hydrocarbons, CO, and nitrogen oxides [NO x ]) are lower on weekends. Day-of-week emissions activity data were collected and analyzed to investigate the hypothesized causes of the "weekend O 3 effect." Emission activity data were collected for various mobile, area, and point sources throughout the SoCAB, including onroad vehicles, lawn and garden equipment, barbecues, fireplaces, solvent use, and point sources with continuous emission monitoring data. The results of this study indicate significant differences between weekday and weekend emission activity patterns and emissions. Their combined effect results in a 12-18% decrease in reactive organic gases (ROGs) and a 35-41% decrease in NO x emissions on Saturdays and Sundays, respectively, relative to weekdays in summer 2000. These changes in emissions result in an increase of more than 30% in the ROG/NO x ratio on weekends compared with weekdays, which, along with lower NO x emissions, leads to increased O 3 production on weekends.