5 research outputs found
Photochemical Aging of α‑pinene and β‑pinene Secondary Organic Aerosol formed from Nitrate Radical Oxidation
The
nitrate radical (NO<sub>3</sub>) is the dominant nighttime
oxidant in most urban and rural environments and reacts rapidly with
biogenic volatile organic compounds to form secondary organic aerosol
(SOA) and organic nitrates (ON). Here, we study the formation of SOA
and ON from the NO<sub>3</sub> oxidation of two monoterpenes (α-pinene
and β-pinene) and investigate how they evolve during photochemical
aging. High SOA mass loadings are produced in the NO<sub>3</sub>+β-pinene
reaction, during which we detected 41 highly oxygenated gas- and particle-phase
ON possessing 4 to 9 oxygen atoms. The fraction of particle-phase
ON in the β-pinene SOA remains fairly constant during photochemical
aging. In contrast to the NO<sub>3</sub>+β-pinene reaction,
low SOA mass loadings are produced during the NO<sub>3</sub>+α-pinene
reaction, during which only 5 highly oxygenated gas- and particle-phase
ON are detected. The majority of the particle-phase ON evaporates
from the α-pinene SOA during photochemical aging, thus exhibiting
a drastically different behavior from that of β-pinene SOA.
Our results indicate that nighttime ON formed by NO<sub>3</sub>+monoterpene
chemistry can serve as either permanent or temporary NO<sub><i>x</i></sub> sinks depending on the monoterpene precursor
Secondary Organic Aerosol (SOA) from Nitrate Radical Oxidation of Monoterpenes: Effects of Temperature, Dilution, and Humidity on Aerosol Formation, Mixing, and Evaporation
Nitrate
radical (NO<sub>3</sub>) oxidation of biogenic volatile
organic compounds (BVOC) is important for nighttime secondary organic
aerosol (SOA) formation. SOA produced at night may evaporate the following
morning due to increasing temperatures or dilution of semivolatile
compounds. We isothermally dilute the oxidation products from the
limonene+NO<sub>3</sub> reaction at 25 °C and observe negligible
evaporation of organic aerosol via dilution. The SOA yields from limonene+NO<sub>3</sub> are approximately constant (∼174%) at 25 °C and
range from 81 to 148% at 40 °C. Based on the difference in yields
between the two temperatures, we calculated an effective enthalpy
of vaporization of 117–237 kJ mol<sup>–1</sup>. The
aerosol yields at 40 °C can be as much as 50% lower compared
to 25 °C. However, when aerosol formed at 25 °C is heated
to 40 °C, only about 20% of the aerosol evaporates, which could
indicate a resistance to aerosol evaporation. To better understand
this, we probe the possibility that SOA from limonene+NO<sub>3</sub> and β-pinene+NO<sub>3</sub> reactions is highly viscous. We
demonstrate that particle morphology and evaporation is dependent
on whether SOA from limonene is formed before or during the formation
of SOA from β-pinene. This difference in particle morphology
is present even at high relative humidity (∼70%)
Designing better cloth masks: The effect of fabric and attachment-style on discomfort
Cloth masks are a tool for controlling community transmission during pandemics, as well as during other outbreak situations. However, cloth masks vary in their designs, and the consequences of this variability for their effectiveness as source control have received little attention, particularly in terms of user discomfort and problematic mask-wearing behaviors. In the present studies, common design parameters of cloth masks were systematically varied to ascertain their effect(s) on the subjective discomfort and frequency of problematic mask-wearing behaviors, which detract from the effectiveness of cloth masks as source control. The type of fabric comprising a mask (flannel or twill made of 100% cotton) and the attachment-style of a mask (i.e., ear loops or fabric ties) were varied in adults (18 to 65 years) and children (ages 6 to 11 years). For adults, ear loops were less comfortable than ties (p = .035) and were associated with greater face- (p = .005) and mask-touching (p = .001). Children, however, found flannel masks to be more breathable than twill masks (p = .007) but touched their masks more frequently when wearing a mask made of flannel than twill (p = .033). Common design parameters of cloth masks not only affect user discomfort and behavior but do so differently in adults and children. To improve the effectiveness of cloth masks as source control, the present studies highlight the importance of measuring the effect(s) of design decisions on user discomfort and behavior in different populations.</p
Organic Aerosols Associated with the Generation of Reactive Oxygen Species (ROS) by Water-Soluble PM<sub>2.5</sub>
We
compare the relative toxicity of various organic aerosol (OA)
components identified by an aerosol mass spectrometer (AMS) based
on their ability to generate reactive oxygen species (ROS). Ambient
fine aerosols were collected from urban (three in Atlanta, GA and
one in Birmingham, AL) and rural (Yorkville, GA and Centerville, AL)
sites in the Southeastern United States. The ROS generating capability
of the water-soluble fraction of the particles was measured by the
dithiothreitol (DTT) assay. Water-soluble PM extracts were further
separated into the hydrophobic and hydrophilic fractions using a C-18
column, and both fractions were analyzed for DTT activity and water-soluble
metals. Organic aerosol composition was measured at selected sites
using a high-resolution time-of-flight AMS. Positive matrix factorization
of the AMS spectra resolved the organic aerosol into isoprene-derived
OA (Isop_OA), hydrocarbon-like OA (HOA), less-oxidized oxygenated
OA, (LO-OOA), more-oxidized OOA (MO-OOA), cooking OA (COA), and biomass
burning OA (BBOA). The association of the DTT activity of water-soluble
PM<sub>2.5</sub> (WS_DTT) with these factors was investigated by linear
regression techniques. BBOA and MO-OOA were most consistently linked
with WS_DTT, with intrinsic water-soluble activities of 151 ±
20 and 36 ± 22 pmol/min/μg, respectively. Although less
toxic, MO-OOA was most widespread, contributing to WS_DTT activity
at all sites and during all seasons. WS_DTT activity was least associated
with biogenic secondary organic aerosol. The OA components contributing
to WS_DTT were humic-like substances (HULIS), which are abundantly
emitted in biomass burning (BBOA) and include highly oxidized OA from
multiple sources (MO-OOA). Overall, OA contributed approximately 60%
to the WS_DTT activity, with the remaining probably from water-soluble
metals, which were mostly associated with the hydrophilic WS_DTT fraction
Collection of Nitrogen Dioxide for Nitrogen and Oxygen Isotope DeterminationLaboratory and Environmental Chamber Evaluation
The family of atmospheric oxides of nitrogen, NOy (e.g., nitrogen oxides (NOx) +
nitric acid (HNO3) + nitrous acid (HONO) + peroxyacetyl
nitrate (PAN) + particulate nitrate (pNO3–) + other), have an influential role in atmospheric chemistry, climate,
and the environment. The nitrogen (δ15N) and oxygen
(δ18O and Δ17O) stable isotopes
of NOy are novel tools for potentially
tracking emission sources and quantifying oxidation chemistry. However,
there is a lack of well-established methods, particularly for speciated
gas-phase components of NOy, to accurately
quantify δ15N, δ18O, and Δ17O. This work presents controlled laboratory experiments and
complex chamber α-pinene/NOx oxidation
experiments of a sampling apparatus constructed for the simultaneous
capture of multiple NOy species for isotope
analysis using a series of coated denuders, with a focus on nitrogen
dioxide (NO2•). The laboratory tests
indicate complete NO2• capture for the
targeted concentration of 15 ppbv for at least 24 h collections
at 10 liters per minute, with δ15N and δ18O precisions of ±1.3‰ and 1.0‰, respectively,
and minimal (2.2% ± 0.1%) NO2• collection
on upstream denuders utilized for the capture of HNO3 and
other acidic gases. The multispecies NOy collection system showed excellent concentration correlations with
online instrumentation for both HNO3 and NO2• and isotope reproducibility of ±1.7‰,
±1.8‰, and ±0.7‰ for δ15N,
δ18O, and Δ17O, respectively, for
replicate experiments and highly time-resolved collections. This work
demonstrates a new method that can enable the simultaneous collection
of HNO3 and NO2• for accurate
quantification of concentration and isotopic composition