Understanding the organic matter/organic carbon (OM/OC) ratio in ambient particulate matter (PM) is critical to achieve mass closure in routine PM measurements, to assess the sources of and the degree of chemical processing organic aerosol particles have undergone, and to relate ambient pollutant concentrations to health effects. Of particular interest is how the OM/OC ratio varies in the urban environment, where strong spatial and temporal gradients in source emissions are common. We provide results of near-roadway high-time-resolution PM₁ OM concentration and OM/OC ratio observations during January 2008 at Fyfe Elementary School in Las Vegas, NV, 18 m from the U.S. 95 freeway soundwall, measured with an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-AMS). The average OM/OC ratio was 1.54 (± 0.20 standard deviation), typical of environments with a low amount of secondary aerosol formation. The 2-min average OM/OC ratios varied between 1.17 and 2.67, and daily average OM/OC ratios varied between 1.44 and 1.73. The ratios were highest during periods of low OM concentrations and generally low during periods of high OM concentrations. OM/OC ratios were low (1.52 ± 0.14, on average) during the morning rush hour (average OM = 2.4 µg/m³), when vehicular emissions dominate this near-road measurement site. The ratios were slightly lower (1.46 ± 0.10) in the evening (average OM = 6.3 µg/m³), when a combination of vehicular and fresh residential biomass burning emissions was typically present during times with temperature inversions. The hourly averaged OM/OC ratio peaked at 1.66 at midday. OM concentrations were similar, regardless of whether the monitoring site was downwind or upwind of the adjacent freeway throughout the day, though they were higher during stagnant conditions (wind speed < 0.5 m/sec). The OM/OC ratio generally varied more with time of day than with wind direction and speed. Implications: Day-to-day variability in the fine particle OM/OC ratio is quite large, suggesting that using a fixed OM/OC value in PM mass closure calculations, even one that changes seasonally, may be insufficient to achieve accurate mass closure on individual days. Health studies that rely on OC measurements may under- or overestimate exposure to OM, and converting OC to OM with a fixed OM/OC ratio represents a significant source of uncertainty; thus, air quality managers may not have sufficient information about the importance of OM contributions to PM_2.5 to make optimal regulatory decisions. Supplemental Materials: Supplemental materials are available for this paper. Go to the publisher's online edition of the Journal of the Air & Waste Management Association.

Air quality monitoring was conducted at a rural site with a tower in the middle of California’s San Joaquin Valley (SJV) and at elevated sites in the foothills and mountains surrounding the SJV for the California Regional PM₁₀/M_2.5 Air Quality Study. Measurements at the surface and on a tower at 90 m were collected in Angiola, CA, from ecember 2000 through February 2001 and included hourly black carbon (BC), particle counts from optical particle counters, nitric oxide, ozone, temperature, relative humidity, wind speed, and direction. Boundary site measurements were made primarily using 24-hr integrated particulate matter (PM) samples. These measurements were used to understand the vertical variations of PM and PM precursors, the effect of stratification in the winter on concentrations and chemistry aloft and at the surface, and the impact of aloft-versus-surface transport on PM concentrations. Vertical variations of concentrations differed among individual species. The stratification may be important to atmospheric chemistry processes, particularly nighttime nitrate formation aloft, because NO₂ appeared to be oxidized by ozone in the stratified aloft layer. Additionally, increases in accumulation-mode particle concentrations in the aloft layer during a fine PM (PM_2.5) episode corresponded with increases in aloft nitrate, demonstrating the likelihood of an aloft nighttime nitrate formation mechanism. Evidence of local transport at the surface and regional transport aloft was found; transport processes also varied among the species. The distribution of BC appeared to be regional, and BC was often uniformly mixed vertically. Overall, the combination of time-resolved tower and surface measurements provided important insight into PM stratification, formation, and transport.

Ambient non-refractory PM₁ aerosol particles were measured with an Aerodyne High Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-AMS) at an elementary school 18 m from the US 95 freeway soundwall in Las Vegas, Nevada, during January 2008. Additional collocated continuous measurements of black carbon (BC), carbon monoxide (CO), nitrogen oxides (NO_x), and meteorological data were collected. The US~Environmental Protection Agency's (EPA) positive matrix factorization (PMF) data analysis tool was used to apportion organic matter (OM) as measured by HR-AMS, and rotational tools in EPA PMF were used to better characterize the solution space and pull resolved factors toward known source profiles. Three- to six-factor solutions were resolved. The four-factor solution was the most interpretable, with the typical AMS PMF factors of hydrocarbon-like organic aerosol (HOA), low-volatility oxygenated organic aerosol (LV-OOA), biomass burning organic aerosol (BBOA), and semi-volatile oxygenated organic aerosol (SV-OOA). When the measurement site was downwind of the freeway, HOA composed about half the OM, with SV-OOA and LV-OOA accounting for the rest. Attempts to pull the PMF factor profiles toward source profiles were successful but did not qualitatively change the results, indicating that these factors are very stable. Oblique edges were present in G-space plots, suggesting that the obtained rotation may not be the most plausible one. Since solutions found by pulling the profiles or using F_peak retained these oblique edges, there appears to be little rotational freedom in the base solution. On average, HOA made up 26% of the OM, while LV-OOA was highest in the afternoon and accounted for 26% of the OM. BBOA occurred in the evening hours, was predominantly from the residential area to the north, and on average constituted 12% of the OM; SV-OOA accounted for the remaining third of the OM. Use of the pulling techniques available in EPA PMF and ME-2 suggested that the four-factor solution was very stable.

Speciated particulate matter (PM)2.5 data collected as part of the Interagency Monitoring of Protected Visual Environments (IMPROVE) program in Phoenix, AZ, from April 2001 through October 2003 wer...

Low-cost sensors can provide insight on the spatio-temporal variability of air pollution, provided that sufficient efforts are made to ensure data quality. Here, 19 AirBeam particulate matter (PM) sensors were deployed from December 2016 to January 2017 to determine the spatial variability of PM_2.5 in Sacramento, California. Prior to, and after, the study, the 19 sensors were deployed and collocated at a regulatory air monitoring site. The sensors demonstrated a high degree of precision during all collocated measurement periods (Pearson R² = 0.98 − 0.99 across all sensors), with little drift. A sensor-specific correction factor was developed such that each sensor reported a comparable value. Sensors had a moderate degree of correlation with regulatory monitors during the study (R² = 0.60 − 0.68 at two sites). In a multi-linear regression model, the deviation between sensor and reference measurements of PM_2.5 had the highest correlation with dew point and relative humidity. Sensor measurements were used to estimate the PM_2.5 spatial variability, finding an average pairwise coefficient of divergence of 0.22 and a range of 0.14 to 0.33, indicating mostly homogeneous distributions. No significant difference in the average sensor PM concentrations between environmental justice (EJ) and non-EJ communities (p value = 0.24) was observed.

To evaluate the feasibility of the Sunset semicontinuous organic and elemental carbon (OC/EC) monitor, the U.S. Environmental Protection Agency (EPA) sponsored the deployment of this monitor at Chemical Speciation Network (CSN) sites with OC and EC measurements via quartz fiber filter collection in Chicago, Illinois; Houston, Texas; Las Vegas, Nevada; St. Louis, Missouri; Rubidoux, California; and Washington, D.C. Houston, St. Louis, and Washington also had collocated Aethalometer black carbon (BC) measurements. Sunset OC generally compared well with the CSN OC (r² = 0.73 across five sites); the Sunset/CSN OC ratio was, on average, 1.06, with a range among sites of 0.96 to 1.12. Sunset thermal EC and CSN EC did not compare as well, with an overall r² of 0.22, in part because 26% of the hourly Sunset EC measurements were below the detection limit. Sunset optical EC had a much better correlation to CSN EC (r² = 0.67 across all sites), with an average Sunset/CSN ratio of 0.90 (range of 0.7 to 1.08). There was also a high correlation of Sunset optical EC with Aethalometer BC (r² = 0.77 across all sites), though with a larger bias (average Sunset/Aethalometer ratio of 0.56). When the Sunset instrument was working well, OC and OptEC data were comparable to CSN OC and EC.