Cort Anastasio

Aqueous phase photochemical formation of hydrogen peroxide in authentic cloud waters

Authentic cloud water samples absorb ultraviolet radiation and thereby initiate formation of peroxides (primarily hydrogen peroxide (H2O2)). This aqueous phase photoformation of H2O2 is a widespread phenomenon in the terrestrial troposphere; midday equinox-normalized H2O2 photoformation rates ranged up to 3.0 μM/h for 36 different cloud water samples collected from sites in New York, North Carolina, Ontario, Virginia, and Washington. By comparison, previously published field studies have shown that over the eastern United States and Canada, approximately 100% of winter cloud water samples, 58–100% of spring and fall samples, and 3–7% of summer samples had peroxide concentrations less than 5 μM.. Previously published model estimates of aqueous phase H2O2 formation rates in cloud drops from gas-to-drop transfer of H2O2 and HO2• range from 1–10 μM/h for all but the most pristine regions of the troposphere. Based on a comparison of these published field and modeling results with the measured aqueous phase H2O2 photoformation rates reported here, we conclude that aqueous phase photochemistry is a significant and in some cases probably dominant source of H2O2 to tropospheric cloud drops. Theory predicts and experiments confirm that the initial rate of aqueous phase H2O2 photoformation is linearly dependent on solar actinic flux. The chromophores responsible for aqueous phase peroxide photoformation have not been identified, but there is evidence that organic chromophores are responsible for the H2O2 photoformation reported here.

Chemistry of fog waters in California's Central Valley: 2. Photochemical transformations of amino acids and alkyl amines

Abstract Although amino compounds are seemingly ubiquitous in atmospheric particles and deposition, little is known of their fate in the troposphere. We report here on the fate of 21 amino acids and alkyl amines in fog waters from Davis, California, illuminated with simulated sunlight or monochromatic light. In all experiments four amino acids – histidine (His), methionine (Met), tryptophan (Trp), and tyrosine (Tyr) – consistently decayed, with half-lives that ranged from ∼1 h (Met) to ∼23 h (Tyr) in midday, winter-solstice sunlight at Davis, CA ( solar zenith angle =62°) . Half-lives for the remaining amino compounds examined were typically >45 h in our experiments. Reactions with photoformed hydroxyl radical ( · OH ) and singlet molecular oxygen (O 2 ( 1 Δ g ) or 1 O 2 ∗ ) accounted for essentially all of the loss of His and Tyr, the less reactive of the four amino acids that consistently decayed, but were minor sinks for the more reactive compounds (Met and Trp). Additional experiments revealed that methionine sulfoxide (MetSO) was formed with a yield of 58–88% during the oxidation of methionine, suggesting that the ratio of MetSO to Met might be a useful chemical marker for the age of atmospheric particles and drops. Other products expected from the transformation of amino compounds include ammonia, organic acids, and possibly mutagenic nitrosoaromatics. To complement our laboratory experiments, we also calculated rates of transformations of amino acids in near-neutral pH fog drops under ambient conditions. These calculations reveal that ozone should be a major sink for amino acids and that half-lives for many amino acids in ambient fog drops will be much shorter than those determined in our photochemistry experiments. Overall, our results indicate that reactions in atmospheric condensed phases will transform amino nitrogen compounds (including free amino acids as well as proteins and peptides) and, consequently, increase the bioavailability of nitrogen in atmospheric deposition.

A general scavenging rate constant for reaction of hydroxyl radical with organic carbon in atmospheric waters.

Hydroxyl radical (OH) is an important oxidant in atmospheric aqueous phases such as cloud and fog drops and water-containing aerosol particles. We find that numerical models nearly always overestimate aqueous hydroxyl radical concentrations because they overpredict its rate of formation and, more significantly, underpredict its sinks. To address this latter point, we examined OH sinks in atmospheric drops and aqueous particles using both new samples and an analysis of published data. Although the molecular composition of organic carbon, the dominant sink of OH, is extremely complex and poorly constrained, this sink behaves very similarly in different atmospheric waters and even in surface waters. Thus, the sink for aqueous OH can be estimated as the concentration of dissolved organic carbon multiplied by a general scavenging rate constant [kC,OH = (3.8 ± 1.9) × 10(8) L (mol C)(-1) s(-1)], a simple process that should significantly improve estimates of OH concentrations in atmospheric drops and aqueous particles.

Environmental Aging of Polycyclic Aromatic Hydrocarbons on Soot and its Effect on Source Identification

Soot-associated PAHs were exposed to simulated sunlight to investigate disappearance rates under environmental aging conditions and to examine the robustness of diagnostic ratios for PAH source apportionment. Naphthalene, acenaphthylene, acenaphthene, and fluorene showed an obvious two-phase disappearance in all experiments while phenanthrene and anthracene exhibited this behavior for all but the highest soot loading. The first phase loss is 5-40 times faster than the second phase loss and occurred within 3h for naphthalene, acenaphthylene, acenaphthene, and fluorene and within 10h for phenanthrene and anthracene. Two-phase disappearance was not observed for any of the higher molecular weight PAHs with 4-6 rings. Each PAH has a unique loss rate via photodegradation and volatilization and these rates of some PAHs were affected by soot loadings; phenanthrene and anthracene showed similar rates in the first phase and increased loss rates in the second phase as soot loading increased. In the absence of light, the loss of PAHs was related to both temperature and molecular characteristics. Due to differences in disappearance rates of individual PAHs under illumination over extended times, prolonged exposure to sunlight could change the interpretation of some diagnostic ratios used previously for PAH source identification. This result indicates that more consistent and accurate methods that take into consideration the longevity of particulate PAHs are needed for reliable source apportionment.

Secondary Organic Aerosol Production from Aqueous Reactions of Atmospheric Phenols with an Organic Triplet Excited State

Condensed-phase chemistry plays a significant role in the formation and evolution of atmospheric organic aerosols. Past studies of the aqueous photoformation of secondary organic aerosol (SOA) have largely focused on hydroxyl radical oxidation, but here we show that triplet excited states of organic compounds ((3)C*) can also be important aqueous oxidants. We studied the aqueous photoreactions of three phenols (phenol, guaiacol, and syringol) with the aromatic carbonyl 3,4-dimethoxybenzaldehyde (DMB); all of these species are emitted by biomass burning. Under simulated sunlight, DMB forms a triplet excited state that rapidly oxidizes phenols to form low-volatility SOA. Rate constants for these reactions are fast and increase with decreasing pH and increasing methoxy substitution of the phenols. Mass yields of aqueous SOA are near 100% for all three phenols. For typical ambient conditions in areas with biomass combustion, the aqueous oxidation of phenols by (3)C* is faster than by hydroxyl radical, although rates depend strongly on pH, oxidant concentrations, and the identity of the phenol. Our results suggest that (3)C* can be the dominant aqueous oxidant of phenols in areas impacted by biomass combustion and that this is a significant pathway for forming SOA.

Hydrogen peroxide formation in a surrogate lung fluid by transition metals and quinones present in particulate matter.

Inhaled ambient particulate matter (PM) causes adverse health effects, possibly by generating reactive oxygen species (ROS), including hydrogen peroxide (HOOH), in the lung lining fluid. There are conflicting reports in the literature as to which chemical components of PM can chemically generate HOOH in lung fluid mimics. It is also unclear which redox-active species are most important for HOOH formation at concentrations relevant to ambient PM. To address this, we use a cell-free, surrogate lung fluid (SLF) to quantify the initial rate of HOOH formation from 10 transition metals and 4 quinones commonly identified in PM. Copper, 1,2-naphthoquinone, 1,4-naphthoquinone, and phenanthrenequinone all form HOOH in a SLF, but only copper and 1,2-naphthoquinone are likely important at ambient concentrations. Iron suppresses HOOH formation in laboratory solutions, but has a smaller effect in ambient PM extracts, possibly because organic ligands in the particles reduce the reactivity of iron. Overall, copper produces the majority of HOOH chemically generated from typical ambient PM while 1,2-naphthoquinone generally makes a small contribution. However, measured rates of HOOH formation in ambient particle extracts are lower than rates calculated from soluble copper by an average (±1σ) of 44 ± 22%; this underestimate is likely due to either HOOH destruction by Fe or a reduction in Cu reactivity due to organic ligands from the PM.

The specific surface area and chemical composition of diamond dust near Barrow, Alaska

2006, when conditions were similar. The SSA of DD ranges from 79.9 to 223 m 2 kg � 1 . The calculated ice surface area in the atmosphere reaches 11000 (� 70%) mm 2 cm � 3 , much higher than the aerosol surface area. However, the impact of DD on the downwelling and upwelling light fluxes in the UV and visible is negligible. The composition of DD is markedly different from that of snow on the surface. Its concentrations in mineral ions are much lower, and its overall composition is acidic. Its concentrations in aldehydes, DOC, H2O2 and mercury are much higher than in surface snows. Our interpretation is that DOC from the oceanic surface microlayer, coming from open leads in the ice off of Barrow, is taken up by DD. Active chemistry in the atmosphere takes place on DD crystal surfaces, explaining its high concentrations in aldehydes and mercury. After deposition, active photochemistry modifies DD composition, as seen from the modifications in its absorption spectra and aldehyde and H2O2 content. This probably leads to the emissions of reactive species to the atmosphere.

Combustion-derived flame generated ultrafine soot generates reactive oxygen species and activates Nrf2 antioxidants differently in neonatal and adult rat lungs

BackgroundUrban particulate matter (PM) has been epidemiologically correlated with multiple cardiopulmonary morbidities and mortalities, in sensitive populations. Children exposed to PM are more likely to develop respiratory infections and asthma. Although PM originates from natural and anthropogenic sources, vehicle exhaust rich in polycyclic aromatic hydrocarbons (PAH) can be a dominant contributor to the PM2.5 and PM0.1 fractions and has been implicated in the generation of reactive oxygen species (ROS).ObjectivesCurrent studies of ambient PM are confounded by the variable nature of PM, so we utilized a previously characterized ethylene-combusted premixed flame particles (PFP) with consistent and reproducible physiochemical properties and 1) measured the oxidative potential of PFP compared to ambient PM, 2) determined the ability of PFPs to generate oxidative stress and activate the transcription factor using in vitro and ex vivo models, and 3) we correlated these responses with antioxidant enzyme expression in vivo.MethodsWe compared oxidative stress response (HMOX1) and antioxidant enzyme (SOD1, SOD2, CAT, and PRDX6) expression in vivo by performing a time-course study in 7-day old neonatal and young adult rats exposed to a single 6-hour exposure to 22.4 μg/m3 PFPs.ResultsWe showed that PFP is a potent ROS generator that induces oxidative stress and activates Nrf2. Induction of the oxidative stress responsive enzyme HMOX1 in vitro was mediated through Nrf2 activation and was variably upregulated in both ages. Furthermore, antioxidant enzyme expression had age and lung compartment variations post exposure. Of particular interest was SOD1, which had mRNA and protein upregulation in adult parenchyma, but lacked a similar response in neonates.ConclusionsWe conclude that PFPs are effective ROS generators, comparable to urban ambient PM2.5, that induce oxidative stress in neonatal and adult rat lungs. PFPs upregulate a select set of antioxidant enzymes in young adult animals, that are unaffected in neonates. We conclude that the inability of neonatal animals to upregulate the antioxidant response may, in part, explain enhanced their susceptibility to ultrafine particles, such as PFP.

Photodestruction of Dissolved Organic Nitrogen Species in Fog Waters

Despite the fact that significant quantities of organic nitrogen (ON) have been measured in rain drops and aerosol particles, relatively little is known about the chemical and photochemical reactivity of ON species in tropospheric condensed phases. The preliminary work presented here reports on the photode struction of a number of dissolved organic nitrogen (DON) species in illuminated fog waters collected in Davis, CA. Results from these experiments show that a number of DON compounds, including pyrrole, methionine, and tryptophan, are rapidly destroyed in illuminated fog waters. Calculated half-lives for these compounds in fog waters illuminated with midday, winter-solstice sunlight range from 0.6-2.3 h. The other DON compounds studied were destroyed more slowly; half-lives in winter-solstice sunlight ranged from ∼10 to over 48 h. Indirect photooxidation as a result of photoformed reactive species was the dominant mechanism for destruction of the DON species examined. Hydroxyl radical was a minor sink for the DON compounds that were most rapidly photodestroyed but was a significant sink for the amino acids which had longer lifetimes. The destruction of ON in these experiments indicates that tropospheric photoreactions might play a significant role in the biogeochemical cycling of nitrogen and likely increase the bioavailability of nitrogen in atmospheric deposition.

Determination of halogenated mono-alcohols and diols in water by gas chromatography with electron-capture detection

We have developed an analytical method for the detection of halogenated alcohols in water with particular focus on 3-chloro-1,2-propanediol and 3-bromo-1,2-propanediol. In this method the target analytes are extracted from water, derivatized with heptafluorobutyric anhydride, and then analyzed with gas chromatography with electron-capture detection. The effects of water, pH and seawater constituents on the method were investigated. Method detection limits for a 5 ml aqueous sample ranged from 0.14 microg l(-1) for 2-bromo-1,3-propanediol to 1.7 microg l(-1) for 1,3-dichloro-2-propanol (1,3DCP).