Non-reversible aging can increase solar absorption in African biomass burning aerosol plumes of intermediate age

Recent studies highlight that biomass-burning aerosol over the remote southeast Atlantic is some of the most sunlight-absorbing aerosol on the planet. In-situ measurements of single-scattering albedo at the 530 nm wavelength (SSA530nm) range from 0.83 to 0.89 within six flights (five in September, 2016 and one in late August, 2017) of the ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) aircraft campaign, increasing with the organic aerosol to black carbon (OA : BC) mass ratio. OA : BC mass ratios of 10 to 14 are lower than some model values and consistent with BC-enriched source emissions, based on indirect inferences of fuel type (savannah grasslands) and dry, flame-efficient combustion conditions. These primarily explain the low single-scattering albedos. We investigate whether continued chemical aging of aerosol plumes of intermediate age (4–7 days after emission, as determined from model tracers) within the free troposphere can further lower the SSA530nm. A mean OA to organic carbon mass ratio of 2.2 indicates highly oxygenated aerosol with the chemical marker f44 indicating the free-tropospheric aerosol continues to oxidize after advecting offshore of continental Africa. Two flights, for which BC to carbon monoxide (CO) ratios remain constant with increasing chemical age, are analyzed further. In both flights, the OA : BC mass ratio decreases over the same time span, indicating continuing net aerosol loss. One flight sampled younger (∼ 4 days) aerosol within the strong zonal outflow of the 4–6 km altitude African Easterly Jet-South. This possessed the highest OA : BC mass ratio of the 2016 campaign and overlaid slightly older aerosol with proportionately less OA, although the age difference of one day is not enough to attribute to a large-scale recirculation and subsidence pattern. The other flight sampled aerosol constrained closer to the coast by a mid-latitude disturbance and found older aerosol aloft overlying younger aerosol. Its vertical increase in OA : BC and nitrate to BC was less pronounced than when younger aerosol overlaid older aerosol, consistent with compensation between a net aerosol loss through aging and a thermodynamical partitioning. Organic nitrate provided 68 % on average of the total nitrate for the 6 flights, in contrast to measurements made at Ascension Island that only found inorganic nitrate. Some evidence for the thermodynamical partitioning to the particle phase at higher altitudes with higher relative humidities for nitrate is still found. The 470–660 nm absorption Angstrom exponent is slightly higher near the African coast than further offshore (approximately 1.2 versus 1.0–1.1), indicating some brown carbon may be present near the coast. The data support the following parameterization: SSA530nm = 0.80+0056*(OA : BC). This indicates a 20 % decrease in SSA can be attributed to chemical aging, or the net 25 % reduction in OA : BC documented for constant BC : CO ratios.

Headache provocation by nitric oxide in men who have never experienced a headache

Introduction In the general population 4% have never experienced a headache. Freedom from headache could be due to distinctive protective mechanisms or a lack of environmental risk factors for headache. Isosorbide-5-mononitrate is an organic nitrate which in the body is metabolised to nitric oxide. The nitric oxide pathway plays a crucial role in the primary headaches. We hypothesized that people who are free from headache are protected by distinctive mechanisms in the nitric oxide pathway. Methods We performed an observer blinded case-control study using nitric oxide to provoke a headache. 32 headache free male participants and 26 randomly selected male controls received 60 mg Isosorbide-5-mononitrate orally on the study day. Participants fill out a headache diary with headache intensity and characteristics until 12 hours after administration of Isosorbide-5-mononitrate. Primary endpoint were areas under the curve of headache intensity score. Results All 58 participants completed the study. There was no significant difference in headache incidence, headache intensity score or migraine-like attack between headache free participants and controls. Conclusion We show that men who have never experienced a headache develop a headache when provoked with Isosorbide-5-mononitrate. This indicates that freedom from headache in men is not related to the nitric oxide pathway which is involved in the primary headache disorders.

Photochemical Evolution of the 2013 California Rim Fire: Synergistic Impacts of Reactive Hydrocarbons and Enhanced Oxidants

Large wildfires markedly alter regional atmospheric composition, but chemical complexity challenges model predictions of downwind impacts. Here, we elucidate key facets of gas-phase photochemistry and assess novel chemical processes via a case study of the 2013 California Rim Fire plume. Airborne in situ observations, acquired during the NASA Studies of Emissions, Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) mission, illustrate the evolution of volatile organic compounds (VOC), oxidants, and reactive nitrogen over 12 hours of atmospheric aging. Measurements show rapid formation of ozone and peroxyacyl nitrates (PNs), sustained peroxide production, and prolonged enhancements in oxygenated VOC and nitrogen oxides (NOx). Measurements and Lagrangian trajectories constrain a 0-D puff model that approximates plume photochemical history and provides a framework for evaluating key processes. Simulations examine the effects of 1) previously-unmeasured reactive VOC identified in recent laboratory studies, and 2) emissions and secondary production of nitrous acid (HONO). Inclusion of estimated unmeasured VOC leads to a 250 % increase in OH reactivity and a 70 % increase in radical production via oxygenated VOC photolysis. HONO amplifies radical cycling and serves as a downwind NOx source, although two different HONO production mechanisms (particulate nitrate photolysis and heterogeneous NO2 conversion) exhibit markedly different effects on ozone, NOx, and PNs. Analysis of radical initiation rates suggests that oxygenated VOC photolysis is a major radical source, exceeding HONO photolysis when averaged over the first 2 hours of aging. Ozone production chemistry transitions from VOC-sensitive to NOx-sensitive within the first hour of plume aging, with both peroxide and organic nitrate formation contributing significantly to radical termination. To simulate smoke plume chemistry accurately, models should simultaneously account for the full reactive VOC pool and all relevant oxidant sources.

Glyceryl Trinitrate: History, Mystery, and Alcohol Intolerance

Glyceryl trinitrate (GTN) is one of the earliest known treatments for angina with a fascinating history that bridges three centuries. However, despite its central role in the nitric oxide (NO) story as a NO-donating compound, establishing the precise mechanism of how GTN exerts its medicinal benefit has proven to be far more difficult. This review brings together the explosive and vasodilatory nature of this three-carbon molecule while providing an update on the likely in vivo pathways through which GTN, and the rest of the organic nitrate family, release NO, nitrite, or a combination of both, while also trying to explain nitrate tolerance. Over the last 20 years the alcohol detoxification enzyme, aldehyde dehydrogenase (ALDH), has undoubtedly emerged as the front runner to explaining GTN’s bioactivation. This is best illustrated by reduced GTN efficacy in subjects carrying the single point mutation (Glu504Lys) in ALDH, which is also responsible for alcohol intolerance, as characterized by flushing. While these findings are significant for anyone following the GTN story, they appear particularly relevant for healthcare professionals, and especially so, if administering GTN to patients as an emergency treatment. In short, although the GTN puzzle has not been fully solved, clinical study data continue to cement the importance of ALDH, as uncovered in 2002, as a key GTN activator.

Nighttime chemistry of biomass burning emissions in urban areas: A dual mobile chamber study

Residential biomass burning for heating purposes is an important source of air pollutants during winter. Here we test the hypothesis that significant secondary organic aerosol production can take place even during winter nights through oxidation of the emitted organic vapors by the nitrate (NO3) radical produced during the reaction of ozone and nitrogen oxides. We use a mobile dual smog chamber system which allows the study of chemical aging of ambient air against a control reference. Ambient urban air sampled during a wintertime campaign during nighttime periods with high concentrations of biomass burning emissions was used as the starting point for the aging experiments. Biomass burning organic aerosol (OA) was, on average, 70 % of the total OA at the beginning of our experiments. Ozone was added in the perturbed chamber to simulate mixing with background air (and subsequent NO3 radical production and aging), while the second chamber was used as a reference. Following the injection of ozone, rapid OA formation was observed in all experiments, leading to increases in the OA concentration by 20 %–70 %. The oxygen-to-carbon ratio of the OA increased on average by 50 %, and the mass spectra of the produced OA was quite similar to the oxidized OA mass spectra reported during winter in urban areas. Furthermore, good correlation was found for the OA mass spectra between the ambient-derived emissions in this study and the nocturnal aged laboratory-derived biomass burning emissions from previous work. Concentrations of NO3 radicals as high as 25 ppt (parts per trillion) were measured in the perturbed chamber, with an accompanying production of 0.1–3.2 µg m−3 of organic nitrate in the aerosol phase. Organic nitrate represented approximately 10 % of the mass of the secondary OA formed. These results strongly indicate that the OA in biomass burning plumes can chemically evolve rapidly even during wintertime periods with low photochemical activity.

A Systematic Re-evaluation of Methods for Quantification of Bulk Particle-phase Organic Nitrates Using Real-time Aerosol Mass Spectrometry

Organic nitrate (RONO2) formation in the atmosphere represents a sink of NOx (NOx = NO + NO2) and termination of the NOx/HOx (HOx = HO2 + OH) ozone formation and radical propagation cycles, can act as a NOx reservoir transporting reactive nitrogen, and contributes to secondary organic aerosol formation. While some fraction of RONO2 is thought to reside in the particle phase, particle-phase organic nitrates (pRONO2) are infrequently measured and thus poorly understood. There is an increasing prevalence of aerosol mass spectrometer (AMS) instruments, which have shown promise for determining quantitative total organic nitrate functional group contribution to aerosols. A simple approach that relies on the relative intensities of NO+ and NO2+ ions in the AMS spectrum, the calibrated NOx+ ratio for NH4NO3, and the inferred ratio for pRONO2 has been proposed as a way to apportion the total nitrate signal to NH4NO3 and pRONO2. This method is increasingly being applied to field and laboratory data. However, the methods applied have been largely inconsistent and poorly characterized, and therefore, a detailed evaluation is timely. Here, we compile an extensive survey of NOx+ ratios measured for various pRONO2 compounds and mixtures from multiple AMS instruments, groups, and laboratory and field measurements. We show that, in the absence of pRONO2 standards, the pRONO2 NOx+ ratio can be estimated using a ratio referenced to the calibrated NH4NO3 ratio, a so-called Ratio-of-Ratios method (RoR = 2.75 ± 0.41). We systematically explore the basis for quantifying pRONO2 (and NH4NO3) with the RoR method using ground and aircraft field measurements conducted over a large range of conditions. The method is compared to another AMS method (positive matrix factorization, PMF) and other pRONO2 and related (e.g., total gas + particle RONO2) measurements, generally showing good agreement/correlation. A broad survey of ground and aircraft AMS measurements shows a pervasive trend of higher fractional contribution of pRONO2 to total nitrate with lower total nitrate concentrations, which generally corresponds to shifts from urban-influenced to rural/remote regions. Compared to ground campaigns, observations from all aircraft campaigns showed substantially lower pRONO2 contributions at mid ranges of total nitrate (0.01–0.1 up to 2–5 μg m−3), suggesting that the balance of effects controlling NH4NO3 and pRONO2 formation and lifetimes — such as higher humidity, lower temperatures, greater dilution, different sources, higher particle acidity, and pRONO2 hydrolysis (possibly accelerated by particle acidity) — favors lower pRONO2 contributions for those environments and altitudes sampled.

Atmospheric photooxidation and ozonolysis of Δ³-carene and 3-caronaldehyde: rate constants and product yields

The oxidation of Δ3-carene and one of its main oxidation products, caronaldehyde, by the OH radical and O3 was investigated in the atmospheric simulation chamber SAPHIR under atmospheric conditions for NOx mixing ratios below 2 ppbv. Within this study, the rate constants of the reaction of Δ3-carene with OH and O3 and of the reaction of caronaldehyde with OH were determined to be (8.0±0.5)×10-11 cm3 s−1 at 304 K, (4.4±0.2)×10-17 cm3 s−1 at 300 K and (4.6±1.6)×10-11 cm3 s−1 at 300 K, in agreement with previously published values. The yields of caronaldehyde from the reaction of OH and ozone with Δ3-carene were determined to be 0.30±0.05 and 0.06±0.02, respectively. Both values are in reasonably good agreement with reported literature values. An organic nitrate (RONO2) yield from the reaction of NO with RO2 derived from Δ3-carene of 0.25±0.04 was determined from the analysis of the reactive nitrogen species (NOy) in the SAPHIR chamber. The RONO2 yield of the reaction of NO with RO2 derived from the reaction of caronaldehyde with OH was found to be 0.10±0.02. The organic nitrate yields of Δ3-carene and caronaldehyde oxidation with OH are reported here for the first time in the gas phase. An OH yield of 0.65±0.10 was determined from the ozonolysis of Δ3-carene. Calculations of production and destruction rates of the sum of hydroxyl and peroxy radicals (ROx=OH+HO2+RO2) demonstrated that there were no unaccounted production or loss processes of radicals in the oxidation of Δ3-carene for conditions of the chamber experiments. In an OH-free experiment with added OH scavenger, the photolysis frequency of caronaldehyde was obtained from its photolytical decay. The experimental photolysis frequency was a factor of 7 higher than the value calculated from the measured solar actinic flux density, an absorption cross section from the literature and an assumed effective quantum yield of unity for photodissociation.

Nitrate-functionalized patch confers cardioprotection and improves heart repair after myocardial infarction via local nitric oxide delivery

Nitric oxide (NO) is a short-lived signaling molecule that plays a pivotal role in cardiovascular system. Organic nitrates represent a class of NO-donating drugs for treating coronary artery diseases, acting through the vasodilation of systemic vasculature that often leads to adverse effects. Herein, we design a nitrate-functionalized patch, wherein the nitrate pharmacological functional groups are covalently bound to biodegradable polymers, thus transforming small-molecule drugs into therapeutic biomaterials. When implanted onto the myocardium, the patch releases NO locally through a stepwise biotransformation, and NO generation is remarkably enhanced in infarcted myocardium because of the ischemic microenvironment, which gives rise to mitochondrial-targeted cardioprotection as well as enhanced cardiac repair. The therapeutic efficacy is further confirmed in a clinically relevant porcine model of myocardial infarction. All these results support the translational potential of this functional patch for treating ischemic heart disease by therapeutic mechanisms different from conventional organic nitrate drugs.

Formation of oxidized gases and secondary organic aerosol from a commercial oxidant-generating electronic air cleaner

Airborne virus transmission during the COVID-19 pandemic increased the demand for indoor air cleaners. While some commercial electronic air cleaners could be effective in reducing primary pollutants and inactivating bioaerosol, studies on the formation of secondary products from oxidation chemistry during their use are limited. Here, we measured oxygenated volatile organic compounds (OVOCs) and the chemical composition of particles generated from a hydroxyl radical generator in an office. During operation, enhancements in OVOCs, especially low-molecular-weight organic and inorganic acids, were detected. Rapid increases in particle number and volume concentrations were observed, corresponding to the formation of highly-oxidized secondary organic aerosol (SOA) (O:C ~1.3). The organic mass spectra showed an enhanced signal at m/z 44 (CO2+) and the aerosol evolved with a slope of ~ -1 in the Van Krevelen diagram. These results suggest that organic acids generated during VOC oxidation contributed to particle nucleation and SOA formation. Nitrate, sulfate, and chloride also increased during the oxidation without a corresponding increase in ammonium, suggesting organic nitrate, organic sulfate, and organic chloride formation. As secondary species are reported to have detrimental health effects, further studies are needed to evaluate potential OVOCs and SOA formation from electronic air cleaners in different indoor environments.