Development of ASTM International D8405-Standard Test Method for Evaluating PM2.5 Sensors or Sensor Systems Used in Indoor Applications.

Sensors and sensor systems for monitoring fine particles with aerodynamic diameters smaller than 2.5 µm can provide real-time feedback on indoor air quality and thus can help guide actions to manage indoor air pollutant concentrations. Standardized verification of the performance and accuracy of sensors and sensor systems is crucial for predicting the efficacy of such monitoring. A new ASTM International standard test method (ASTM D8405) was created for this need and is the most exacting laboratory protocol published to date for evaluating indoor air quality sensors and sensor systems measuring particles smaller than 2.5 µm in diameter. ASTM D8405 subjects sensors and sensor systems to five test phases: (1) an initial particle concentration ramp; (2) exposure to various temperature and humidity conditions; (3) exposure to interfering particles; (4) temperature cycling; and (5) a final particle concentration ramp to assess drift. This paper discusses the development of the standard test method, key aspects of the testing process, example evaluation results, and a comparison of this standard test method against peer evaluation protocols.

A light-driven burst of hydroxyl radicals dominates oxidation chemistry in newly activated cloud droplets.

Aerosol particles and their interactions with clouds are one of the most uncertain aspects of the climate system. Aerosol processing by clouds contributes to this uncertainty, altering size distributions, chemical composition, and radiative properties. Many changes are limited by the availability of hydroxyl radicals in the droplets. We suggest an unrecognized potentially substantial source of OH formation in cloud droplets. During the first few minutes following cloud droplet formation, the material in aerosols produces a near-UV light-dependent burst of hydroxyl radicals, resulting in concentrations of 0.1 to 3.5 micromolar aqueous OH ([OH]aq). The source of this burst is previously unrecognized chemistry between iron(II) and peracids. The contribution of the "OH burst" to total OH in droplets varies widely, but it ranges up to a factor of 5 larger than previously known sources. Thus, this new process will substantially enhance the impact of clouds on aerosol properties.

Oxidative Potential by PM2.5 in the North China Plain: Generation of Hydroxyl Radical.

Adverse health effects of ambient PM2.5 ( dp < 2.5 µm) can be associated with the production of reactive oxygen species (ROS), among which hydroxyl radical (•OH) is the most reactive. However, â€¢OH generated by PM2.5 has not been quantified and studied in the North China Plain (NCP), which has suffered from heavy air pollution in recent years. In this study, PM2.5 samples were collected at an urban site (Beijing) and a suburban site (Wangdu), extracted in a cell-free surrogate lung fluid (SLF), and •OH generated in the extracts were quantified. The results show that more •OH is produced on heavily polluted days than that on clean days (2.0 and 1.6 times higher in Beijing and Wangdu, respectively). The production of •OH per unit mass (ng/µg·PM2.5) decreases with the increase of ambient PM2.5 concentration because SO42-, NO3-, and NH4+ dominate the increased PM2.5, while these secondary inorganic components do not contribute to the generation of •OH. Trace metals (e.g., Fe, Cu, Se) and carbonaceous species (organic carbon and elemental carbon) correlate well with the •OH production, indicating that particles from combustion sources including coal combustion, vehicle exhaust, and industry contribute more to •OH generation.

Rapid Aqueous-Phase Hydrolysis of Ester Hydroperoxides Arising from Criegee Intermediates and Organic Acids.

Stabilized Criegee intermediates react with organic acids in the gas phase and at the air-water interface to form a class of ester hydroperoxides, α-acyloxyalkyl hydroperoxides (αAAHPs). A number of recent studies have proposed the importance of αAAHPs to the formation and growth of secondary organic aerosol (SOA). The chemistry of αAAHPs has not been investigated due to a lack of commercially available chemical standards. In this work, the behavior of αAAHPs in condensed phases is investigated for the first time. Experiments were performed with two synthesized αAAHP species. αAAHPs decomposed rapidly in the aqueous phase, with the rate highly dependent on the solvent, temperature, solution pH, and other compounds present in the solution. The measured 1st-order decomposition rate coefficient varied between 10-3 and 10-5 s-1 under the conditions examined in this work. Elucidation of the reaction mechanism is complicated by byproducts arising from the synthetic procedure, but observations are consistent with a base-catalyzed hydrolysis of αAAHPs. The rapid hydrolysis of αAAHPs observed in this work implies their short lifetimes in ambient cloud and fog waters. Decomposition of αAAHPs likely gives rise to smaller peroxides, such as H2O2. The loss of αAAHPs is also relevant to filter extraction, which is commonly practiced in laboratory experiments, potentially explaining contradictory results reported in the existing literature regarding the importance of αAAHPs in SOA.

Influence of intramolecular hydrogen bonding on OH-stretching overtone intensities and band positions in peroxyacetic acid.

Vapor phase absorption spectra and integrated band intensities of the OH stretching fundamental as well as first and second overtones (2ν(OH) and 3ν(OH)) in peroxyacetic acid (PAA) have been measured using a combination of FT-IR and photoacoustic spectroscopy. In addition, ab initio calculations have been carried out to examine the low energy stable conformers of the molecule. Spectral assignment of the primary features appearing in the region of the 2ν(OH) and 3ν(OH) overtone bands are made with the aid of isotopic substitution and anharmonic vibrational frequency calculations carried out at the MP2/aug-cc-pVDZ level. Apart from features associated with the zeroth-order OH stretch, the overtone spectra are dominated by features assigned to combination bands composed of the respective OH stretching overtone and vibrations involving the collective motion of several atoms in the molecule resulting from excitation of the internal hydrogen bonding coordinate. Integrated absorption cross section measurements reveal that internal hydrogen bonding, the strength of which is estimated to be ∼20 kJ/mol in PAA, does not result in a enhanced oscillator strength for the OH stretching fundamental of the molecule, as is often expected for hydrogen bonded systems, but does cause a precipitous drop in the oscillator strength of its 2ν(OH) and 3ν(OH) overtone bands, reducing them, respectively, by a factor of 165 and 7020 relative to the OH stretching fundamental.