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In this work, we deployed a single particle aerosol mass spectrometer (SPAMS) at a suburban coastal site in Hong Kong from February 04 to April 17, 2013 to study individual oxalate particles and a monitor for aerosols and gases in ambient air (MARGA) to track the bulk oxalate concentrations in particle matter smaller than 2.5 μm in diameter (PM

Surgical masks have been worn by the public worldwide during the COVID-19 pandemic, yet hazardous chemicals in the petroleum-derived polymer layer of masks are currently ignored and unregulated. These organic compounds pose potential health risks to the mask wearer through dermal contact or inhalation. Here, we show that surgical masks from around the world are loaded with semivolatile and volatile organic compounds (VOCs), including alkanes, polycyclic aromatic hydrocarbons (PAHs), phthalate esters, and reactive carbonyls at ng to μg/mask levels. Naphthalene was the most abundant mask-borne PAH, accounting for over 80% of total PAH levels; acrolein, a mutagenic carbonyl, was detected in most of the mask samples, and di(2-ethylhexyl) phthalate, an androgen antagonist, was detected in one-third of the samples. Furthermore, there is large mask-to-mask variability of the residue VOCs, revealing the uneven quality of masks. We confirm that masks containing more residue VOCs lead to significantly higher exposure levels and associated disease risks to the wearer, which should warrant the attention of the general public and regulatory agencies. We find that heating the masks at 50 °C for as short as 60 min lowers the total VOC content by up to 80%, providing a simple method to limit our exposure to mask-borne VOCs.

International audience

In this study, we assessed the feasibility of using ordinary face masks as a sampling means to collect airborne polycyclic aromatic hydrocarbons (PAHs). Nonwoven fabric masks can trap three-ring or larger PAHs at a high efficiency (>70%) and naphthalene at ∼17%. The sampling method is quantitative as confirmed by comparison with the standard method of the National Institute for Occupational Safety and Health. In conjunction with sensitive fluorescence detection, the method was applied to quantify nine airborne PAHs in a range of indoor and outdoor environments. Wearing the mask for 2 h allowed quantification of individual PAHs as low as 0.07 ng/m3. The demonstration shows applicability of the method in monitoring PAHs down to ∼30-80 ng/m3 in university office and laboratory settings and up to ∼900 ng/m3 in an incense-burning temple. Compared with traditional filter-/sorbent tube-based approaches, which require a sampling pump, our new method is simple, convenient, and inexpensive. More importantly, it closely tracks human exposure down to the individual level, thus having great potential to facilitate routine occupational exposure monitoring and large-scale surveillance of PAH concentrations in indoor and outdoor environments.

International audience; Nitrous acid (HONO) is an important radical precursor that can impact secondary pollutant levels, especially in urban environments. Due to uncertainties in its heterogeneous formation mechanisms, models often under predict HONO concentrations. A number of heterogeneous sources at the ground have been proposed but there is no consensus about which play a significant role in the urban boundary layer. We present a new one-dimensional chemistry and transport model which performs surface chemistry based on molecular collisions and chemical conversion, allowing us to add detailed HONO formation chemistry at the ground. We conducted model runs for the 2010 CalNex campaign, finding good agreement with observations for key species such as O3, NOx, and HOx. With the ground sources implemented, the model captures the diurnal and vertical profile of the HONO observations. Primary HOx production from HONO photolysis is 2–3 times more important than O3 or HCHO photolysis at mid-day, below 10 m. The HONO concentration, and its contribution to HOx, decreases quickly with altitude. Heterogeneous chemistry at the ground provided a HONO source of 2.5 × 1011 molecules cm−2 s−1 during the day and 5 × 1010 molecules cm−2 s−1 at night. The night time source was dominated by NO2 hydrolysis. During the day, photolysis of surface HNO3/nitrate contributed 45%–60% and photo-enhanced conversion of NO2 contributed 20%–45%. Sensitivity studies addressing the uncertainties in both photolytic mechanisms show that, while the relative contribution of either source can vary, HNO3/nitrate is required to produce a surface HONO source that is strong enough to explain observations.

In this study, we investigated the influences of stratospheric intrusion (SI) on the tropospheric ozone (O3) at Lulin Atmospheric Background Station (LABS, 2862 m MSL), a representative high-altitude site in East Asia. Thresholds for surface O3, carbon monoxide (CO), and relative humidity (RH) were set to identify SI events at LABS. Accordingly, 3.0% of the observation period was impacted by SI leading to a net O3 enhancement of 14.6 ± 9.6 ppb at LABS. Factors influencing the two most common SI event types affecting LABS, i.e. tropopause folds (major) and tropical cyclones (minor), were discussed. We utilized ERA5 reanalysis products (meteorology and O3) to trace and analyze the pathway and mechanism of these SI types over sub-tropical East Asia. With a conceptual model, we illustrate two driving circulations for SI air at ∼200 hPa that affect subtropical East Asia. SI air generated due to Rossby wave breaks are transported equatorward and downward into the troposphere. The southern westerlies passing south of the Tibetan Plateau can further push the SI air eastward. A SI event in 2007 with a significant O3 enhancement of 43.4 ppb caused by typhoon Nari was detected and discussed to depict the second SI type.

Many aromatic compounds (e.g., polycyclic aromatic hydrocarbons (PAHs)) found in atmospheric aerosols are toxic and exist in both unsubstituted and substituted forms. Previous studies have mainly concentrated on investigating unsubstituted PAHs, leaving the substituted compounds largely uncharacterized. This study focuses on detection of both unsubstituted and substituted aromatics in ambient aerosol samples using ultrahigh-resolution mass spectrometry. Aerosol samples collected from roadside, urban, and suburban sites in Hong Kong were characterized by Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) coupled with atmospheric pressure photoionization (APPI) or electrospray ionization (ESI). In the APPI+ mode, 166 aromatic CH formulas (i.e., formulas containing C and H only and with a double bond equivalent (DBE) of 4 or higher) were determined through molecular formula calculations based on an accurate m/z determination. Among the determined aromatic CH formulas, 141 are possible alkylated monocyclic aromatic hydrocarbon (MAH) or PAH formulas, and account for ≥ 45% of the total intensity by aromatic CH+ formulas. Both APPI+ and ESI+ are effective in detecting nitroaromatics (i.e., CHO2N1 formulas and DBE ≥ 5). The two ionization modes provide complementary formula coverage, with formulas determined by APPI+ extending to higher DBE and those by ESI+ covering higher carbon numbers. Alkylated nitrobenzene compounds are the most abundant among nitroaromatics, and they, together with alkylated nitro-PAHs, account for > 80% of the total intensity of aromatic CHO2N+ formulas, indicating the importance of these compounds in real aerosol samples. Aromatic CHN+ and CHO+ formulas are also determined, confirming the atmospheric presence of some previously reported O- and N-containing aromatic compounds and revealing new possible formulas. The determination of aromatic organic formulas in this study provides useful guidance for future quantitative analysis of hazardous aromatic compounds. Future work is needed to determine the abundance and to study the toxicity of alkylated MAHs and PAHs outside the 16 US EPA priority PAH compounds.

Knowledge of the relative abundance of primary organic aerosol (POA) and secondary organic aerosol (SOA) forms an important scientific basis for formulating particulate matter (PM) control policies. Taking advantage of a comprehensive chemical composition data set of PM2.5 including both POA and SOA tracers (most notably, SOA tracers of a few biogenic voltaic organic compound precursors), we investigate the impact of inclusion of SOA tracers on the source apportionment of organic carbon (OC) and PM2.5 in the Pearl River Delta region of China using positive matrix factorization (PMF). In PMF runs incorporating SOA tracers (PMFw), ten PMF factors were resolved including four secondary factors: (1) SOA I (α-pinene, β-caryophyllene, and naphthalene-derived SOA), (2) SOA II (isoprene-derived SOA), (3) a secondary sulfate factor, and (4) a secondary nitrate factor. In PMF tests without SOA tracers (PMFwo), the SOA I and SOA II factors could not be extracted, but the remaining eight source factors were resolved....

Gas-phase biogenic volatile organic compounds (BVOCs) are oxidized in the troposphere to produce secondary pollutants such as ozone (O3), organic nitrates (RONO2), and secondary organic aerosol (SOA). Two coupled zero-dimensional models have been used to investigate differences in oxidation and SOA production from isoprene and α-pinene, especially with respect to the nitrate radical (NO3), above and below a forest canopy in rural Michigan. In both modeled environments (above and below the canopy), NO3 mixing ratios are relatively small (3 contributes 12 % of total daytime α-pinene oxidation below the canopy while only contributing 4 % above. Increasing background pollutant levels to simulate a more polluted suburban or peri-urban forest environment increases the average contribution of NO3 to daytime below-canopy α-pinene oxidation to 32 %. Gas-phase RONO2 produced through NO3 oxidation undergoes net transport upward from the below-canopy environment during the day, and this transport contributes up to 30 % of total NO3-derived RONO2 production above the canopy in the morning (∼ 07:00). Modeled SOA mass loadings above and below the canopy ultimately differ by less than 0.5 µg m−3, and extremely low-volatility organic compounds dominate SOA composition. Lower temperatures below the canopy cause increased partitioning of semi-volatile gas-phase products to the particle phase and up to 35 % larger SOA mass loadings of these products relative to above the canopy in the model. Including transport between above- and below-canopy environments increases above-canopy NO3-derived α-pinene RONO2 SOA mass by as much as 45 %, suggesting that below-canopy chemical processes substantially influence above-canopy SOA mass loadings, especially with regard to monoterpene-derived RONO2.

Fine particulate matter (PM) is capable of scattering and absorbing light and is the main culprit of visibility degradation. Major PM chemical components have been characterized for their light absorption and scattering efficiency, but separating the organic components has yet to be fully parameterized with light extinction coefficients. In this study, light extinction data and PM2.5 chemical composition data were monitored at a suburban site in Hong Kong over a two-year period (2013-2014). Using the IMPROVE formula to reconstruct the light scattering coefficient under-estimates the measured scattering coefficient (slope=0.85), but explains the data variability well (R2: 0.92). A multilinear regression analysis using the 'local' PM2.5 composition data and measured extinction coefficients was performed to empirically establish mass scattering and absorption efficiencies (i.e., MSE and MAE) for the different PM2.5 components. During this process, the stepwise separation of organic matter (OM) was performed according to water solubility: water soluble organic carbon (WSOC) and water insoluble organic carbon (WISOC); then according to water affinity: hydrophilic carbon (HPI) and hydrophobic carbon (HPO), the latter being the sum of humic-like substance carbon (HULISc) and WISOC. The localized formulas predict the measured extinction coefficients (i.e., σsp and σap) very well (slope=0.99 for both). The results showed that the dry MSE of ammonium sulfate and ammonium nitrate were comparable with those used in the IMPROVE equation while MSE for OC is noticeably larger in the localized formula (13.1 vs. 7.2m2g-1). Splitting the OM into different fractions revealed the MSE for hydrophilic carbon (16.1m2g-1) was distinctly higher than for hydrophobic carbon, including HULIS (11.0m2g-1) and WISOC (12.8m2g-1). Regression analysis of light absorption against EC and OC indicates that absorption is not fully accounted for considering only EC. OC also contributes to light absorption.

The formation of sulfate (SO42−) in the atmosphere is linked chemically to its direct precursor, sulfur dioxide (SO2), through several key oxidation paths for which nitrogen oxides or NOx (NO and NO2) play essential roles. Here we present a coherent description of the dependence of SO42– formation on SO2 and NOx under haze-fog conditions, in which fog events are accompanied by high aerosol loadings and fog-water pH in the range of 4.7–6.9. Three SO42– formation regimes emerge as defined by the role played by NOx. In the low-NOx regime, NOx act as catalyst for HOx, which is a major oxidant for SO2, whereas in the high-NOx regime, NO2 is a sink for HOx. Moreover, at highly elevated NOx levels, a so-called NO2-oxidant regime exists in which aqueous NO2 serves as the dominant oxidant of SO2. This regime also exists under clean fog conditions but is less prominent. Sensitivity calculations using an emission-driven box model show that the reduction of SO42– is comparably sensitive to the reduction of SO2 and NOx emissions in the NO2-oxidant regime, suggesting a co-reduction strategy. Formation of SO42− is relatively insensitive to NOx reduction in the low-NOx regime, whereas reduction of NOx actually leads to increased SO42– production in the intermediate high-NOx regime. Distinct dependence of atmospheric SO42– formation on NOx levels in haze-fog events is revealed by SO42– production isopleths that are obtained through simulations of atmospheric chemistry with a box model.

International audience; Measurements of hydroxyl (OH) and hydroperoxy (HO2*) radical concentrations were made at the Pasadena ground site during the CalNex-LA 2010 campaign using the laser-induced fluorescence-fluorescence assay by gas expansion technique. The measured concentrations of OH and HO2* exhibited a distinct weekend effect, with higher radical concentrations observed on the weekends corresponding to lower levels of nitrogen oxides (NOx). The radical measurements were compared to results from a zero-dimensional model using the Regional Atmospheric Chemical Mechanism-2 constrained by NOx and other measured trace gases. The chemical model overpredicted measured OH concentrations during the weekends by a factor of approximately 1.4 ± 0.3 (1σ), but the agreement was better during the weekdays (ratio of 1.0 ± 0.2). Model predicted HO2* concentrations underpredicted by a factor of 1.3 ± 0.2 on the weekends, while measured weekday concentrations were underpredicted by a factor of 3.0 ± 0.5. However, increasing the modeled OH reactivity to match the measured total OH reactivity improved the overall agreement for both OH and HO2* on all days. A radical budget analysis suggests that photolysis of carbonyls and formaldehyde together accounted for approximately 40% of radical initiation with photolysis of nitrous acid accounting for 30% at the measurement height and ozone photolysis contributing less than 20%. An analysis of the ozone production sensitivity reveals that during the week, ozone production was limited by volatile organic compounds throughout the day during the campaign but NOx limited during the afternoon on the weekends.

In this study, we investigated the seasonal, diurnal, and episodic characteristics of aerosol nitrate concentrations in PM2.5 at a suburban receptor site in Hong Kong using an hourly MARGA sampled dataset. At the site, large spikes in the NO3− concentration have been observed in all seasons, and are easily overlooked in datasets examining 24 h average concentrations. As a key component to PM2.5, nitrate constituted between 5 and 12% of the mass concentration on average per month, but contributed up to 25% during some episodic cases spanning only a few hours. Seasonal variations of PM2.5 nitrate concentrations at the site were driven by temperature and excess [NH4+] in the aerosol, defined as the amount of ammonium in excess of that required for satisfying [NH4+]/[SO42−] = 1.5. The vast majority of winter nitrate data was associated with ammonium-rich aerosols ([NH4+]/[SO42−] > 1.5), with the diurnal variation tracking the availability of excess [NH4+]. Distinctly different than winter conditions, the summer nitrate data was in ammonium-poor regime and tracked nitric acid concentrations, a photochemical tracer. A regression analysis of measured nitrate with the excess [NH4+] shows good correlation in spring, summer and winter (R2: 0.72–0.81), with slopes greater than 0.7 indicating the majority of excess NH4+ is associated with PM2.5 nitrate. It was found that measured nitrate exceeded excess [NH4+] in samples of low excess [NH4+] availability, leading to our finding that nitric acid attaching to sea salt and crustal particles in the fine mode is a non-negligible route (constituting up to ∼20% of the PM2.5 nitrate in this study) to assimilate nitrate into the PM2.5 aerosol. Accounting for both this minor route and the ammonia + nitric acid route may prove useful in modeling efforts to capture PM2.5 nitrate measurement fluctuations, particularly during events of a large influx of alkali particles, such as dust storms.

Recent observations suggest a large and unknown daytime source of nitrous acid (HONO) to the atmosphere. Multiple mechanisms have been proposed, many of which involve chemistry that reduces nitrogen dioxide (NO2) on some time scale. To examine the NO2 dependence of the daytime HONO source, we compare weekday and weekend measurements of NO2 and HONO in two U.S. cities. We find that daytime HONO does not increase proportionally to increases in same-day NO2, i.e., the local NO2 concentration at that time and several hours earlier. We discuss various published HONO formation pathways in the context of this constraint.

Liquid water content (LWC) is the amount of liquid water on aerosols. It contributes to visibility degradation, provides a surface for gas condensation, and acts as a medium for heterogeneous gas/particle reactions. In this study, 520 half-hourly measurements of ionic chemical composition in PM2.5 at a receptor site in Hong Kong are used to investigate the dependence of LWC on ionic chemical composition, particularly on the relative abundance of sulfate and nitrate. LWC was estimated using a thermodynamic model (AIM-III). Within this data set of PM2.5 ionic compositions, LWC was highly correlated with the multivariate combination of sulfate and nitrate concentrations and RH (R2 = 0.90). The empirical linear regression result indicates that LWC is more sensitive to nitrate mass than sulfate. During a nitrate episode, the highest LWC (80.6 ± 17.9 μg m−3) was observed and the level was 70% higher than that during a sulfate episode despite a similar ionic PM2.5 mass concentration. A series of sensitivity tests were conducted to study LWC change as a function of the relative nitrate and sulfate abundance, the trend of which is expected to shift to more nitrate in China as a result of SO2 reduction and increase in NOx emission. Starting from a base case that uses the average of measured PM2.5 ionic chemical composition (63% SO42−, 11% NO3−, 19% NH4+, and 7% other ions) and an ionic equivalence ratio, [NH4+]/(2[SO42−] + [NO3−]), set constant to 0.72, the results show LWC would increase by 204% at RH = 40% when 50% of the SO42− is replaced by NO3− mass concentration. This is largely due to inhibition of (NH4)3H(SO4)2 crystallization while PM2.5 ionic species persist in the aqueous phase. At RH = 90%, LWC would increase by 12% when 50% of the SO42− is replaced by NO3− mass concentration. The results of this study highlight the important implications to aerosol chemistry and visibility degradation associated with LWC as a result of a shift in PM2.5 ionic chemical composition to more nitrate in atmospheric environments as is expected in many Chinese cities.

Total hydroxyl radical (OH) reactivity was measured at the PROPHET (Program for Research on Oxidants: PHotochemistry, Emissions, and Transport) forested field site in northern Michigan during the 2009 Community Atmosphere–Biosphere INteraction EXperiment (CABINEX). OH reactivity measurements were made with a turbulent-flow reactor instrument at three heights from the forest floor above (21 and 31 m) and below (6 m) the canopy at three different time periods during the CABINEX campaign. In addition to total OH reactivity measurements, collocated measurements of volatile organic compounds (VOCs), inorganic species, and ambient temperature were made at the different heights. These ancillary measurements were used to calculate the total OH reactivity, which was then compared to the measured values. Discrepancies between the measured and calculated OH reactivity, on the order of 1–24 s−1, were observed during the daytime above the canopy at the 21 and 31 m heights, as previously reported for this site. The measured OH reactivity below the canopy during the daytime was generally lower than that observed above the canopy. Closer analysis of the measurements of OH reactivity and trace gases suggests that the missing OH reactivity could come from oxidation products of VOCs. These results suggest that additional unmeasured trace gases, likely oxidation products, are needed to fully account for the OH reactivity measured during CABINEX.

Research on relationships between dissolved nutrients and land-use at the watershed scale is a high priority for protecting surface water quality. We measured dissolved nitrogen (DN) and ortho-phosphorus (P) along 130 km of the Calapooia River (Oregon, USA) and 44 of its sub-basins for 3 years to test for associations with land-use. Nutrient concentrations were analyzed for spatial and seasonal patterns and for relationships with land-use and stream discharge. Ortho-P and DN were higher in lower-elevation sub-basins dominated by poorly drained soils and agricultural production compared with higher-elevation sub-basins dominated by well-drained soils and forests. Eight lower basins had at least one sample period with nitrate-N 10 mg L(-1). The Calapooia River had lower concentrations of dissolved nutrients compared with lower sub-basins, often by an order of magnitude. Dissolved organic N represented a greater proportion of DN in the upper forested sub-basins. Seasonal nutrient concentrations had strong positive correlations to the percent of a sub-basin that was managed for agriculture in all seasons (p values ≤ 0.019) except summer. Results suggest that agricultural lands are contributing to stream nutrient concentrations. However, poorly drained soils in agricultural areas may also contribute to the strong relationships that we found between dissolved nutrients and agriculture.

Hydroxyl (OH) and hydroperoxyl (HO2) radicals are key species driving the oxidation of volatile organic compounds that can lead to the production of ozone and secondary organic aerosols. Previous measurements of these radicals in forest environments with high isoprene, low NOx conditions have shown serious discrepancies with modeled concentrations, bringing into question the current understanding of isoprene oxidation chemistry in these environments. During the summers of 2008 and 2009, OH and peroxy radical concentrations were measured using a laser-induced fluorescence instrument as part of the PROPHET (Program for Research on Oxidants: PHotochemistry, Emissions, and Transport) and CABINEX (Community Atmosphere-Biosphere INteractions EXperiment) campaigns at a forested site in northern Michigan. Supporting measurements of photolysis rates, volatile organic compounds, NOx (NO + NO2 and other inorganic species were used to constrain a zero-dimensional box model based on the Regional Atmospheric Chemistry Mechanism, modified to include the Mainz Isoprene Mechanism (RACM-MIM). The CABINEX model OH predictions were in good agreement with the measured OH concentrations, with an observed-to-modeled ratio near one (0.70 ± 0.31) for isoprene mixing ratios between 1–2 ppb on average. The measured peroxy radical concentrations, reflecting the sum of HO2 and isoprene-based peroxy radicals, were generally lower than predicted by the box model in both years.

In recent years in a few Chinese megacities, fog events lasting one to a few days have been frequently associated with high levels of aerosol loading characterized by high sulfate (as high as 30 μg m(-3)), therefore termed as haze-fog events. The concomitant pollution characteristics include high gas-phase mixing ratios of SO2 (up to 71 ppbv) and NO2 (up to 69 ppbv), high aqueous phase pH (5-6), and smaller fog droplets (as low as 2 μm), resulting from intense emissions from fossil fuel combustion and construction activities supplying abundant Ca(2+). In this work, we use an observation-based model for secondary inorganic aerosols (OBM-SIA) to simulate sulfate formation pathways under conditions of haze-fog events encountered in Chinese megacities. The OBM analysis has identified, at a typical haze-fogwater pH of 5.6, the most important pathway to be oxidation of S(IV) by dissolved NO2, followed by the heterogeneous reaction of SO2 on the aerosol surface. The aqueous phase oxidation of S(IV) by H2O2 is a very minor formation pathway as a result of the high NOx conditions suppressing H2O2 formation. The model results indicate that the unique cocktail of high fogwater pH, high concentrations of NO2, SO2, and PM, and small fog droplets are capable of greatly enhancing sulfate formation. Such haze-fog conditions could lead to rapid sulfate production at night and subsequently high PM2.5 in the morning when the fog evaporates. Sulfate formation is simulated to be highly sensitive to fogwater pH, PM, and precursor gases NO2 and SO2. Such insights on major contributing factors imply that reduction of road dust and NOx emissions could lessen PM2.5 loadings in Chinese megacities during fog events.

Slugs are common pests of grass seed fields in western Oregon and the current focus of repeated, and often unsuccessful, efforts by growers to control them using molluscicides. Here we document rapid loss of molluscicidal bait pellets to earthworms and the resulting adverse effects on slug control. Three years of field studies were conducted at 17 locations with contrasting crops, soil types, residue levels, and tillage management programs. Baits were isolated in covered, sunken, open bottom arenas in the field to exclude removal by slugs, rodents, or birds. Forty hours of nighttime field observations and photographic documentation were collected to support the hypothesis that earthworms were removing slug bait before it had the opportunity to kill slugs. Greenhouse studies were conducted on the gray field slug (Derocerus reticulatum Mueller), in screen-topped arenas to determine the effects on mortality, seedling survival, and egg fecundity in a bait-depleting environment. Field data showed that an average of 17% of all bait pellet types were removed nightly by earthworms, with a range of 5.1e6.4 days until 100% disappearance. Individual earthworms in the field were observed removing up to three bait pellets per hour. Earthworms removed a 5% metaldehyde formulation significantly faster than either 4% metaldehyde or 1% iron phosphate pellet baits, possibly because of the smaller physical pellet size. Seedling survival in the greenhouse � 65% was achieved by the highest rate of 4% metaldehyde bait pellets, the two highest rates of 7.5% metaldehyde granules, and the two highest rates of 25% metaldehyde liquid formulation. Earthworms showed no behavioral interest in the granular or liquid formulations, providing growers with attractive alternatives to the ineffectual pelleted baits currently in widespread use. Published by Elsevier Ltd.

Vegetation emits large quantities of biogenic volatile organic compounds (BVOC). At remote sites, these compounds are the dominant precursors to ozone and secondary organic aerosol (SOA) production, yet current field studies show that atmospheric models have difficulty in capturing the observed HOx cycle and concentrations of BVOC oxidation products. In this manuscript, we simulate BVOC chemistry within a forest canopy using a one-dimensional canopy-chemistry model (Canopy Atmospheric CHemistry Emission model; CACHE) for a mixed deciduous forest in northern Michigan during the CABINEX 2009 campaign. We find that the base-case model, using fully-parameterized mixing and the simplified biogenic chemistry of the Regional Atmospheric Chemistry Model (RACM), underestimates daytime in-canopy vertical mixing by 50–70% and by an order of magnitude at night, leading to discrepancies in the diurnal evolution of HOx, BVOC, and BVOC oxidation products. Implementing observed micrometeorological data from above and within the canopy substantially improves the diurnal cycle of modeled BVOC, particularly at the end of the day, and also improves the observation-model agreement for some BVOC oxidation products and OH reactivity. We compare the RACM mechanism to a version that includes the Mainz isoprene mechanism (RACM-MIM) to test the model sensitivity to enhanced isoprene degradation. RACM-MIM simulates higher concentrations of both primary BVOC (isoprene and monoterpenes) and oxidation products (HCHO, MACR+MVK) compared with RACM simulations. Additionally, the revised mechanism alters the OH concentrations and increases HO2. These changes generally improve agreement with HOx observations yet overestimate BVOC oxidation products, indicating that this isoprene mechanism does not improve the representation of local chemistry at the site. Overall, the revised mechanism yields smaller changes in BVOC and BVOC oxidation product concentrations and gradients than improving the parameterization of vertical mixing with observations, suggesting that uncertainties in vertical mixing parameterizations are an important component in understanding observed BVOC chemistry.

We reanalyze a data set of hydrocarbons in ambient air obtained by gas chromatography-mass spectrometry at a surface site in Pasadena in the Los Angeles basin during the NOAA California Nexus study in 2010. The number of hydrocarbon compounds quantified from the chromatograms is expanded through the use of new peak-fitting data analysis software. We also reexamine hydrocarbon removal processes. For alkanes, small alkenes, and aromatics, the removal is determined by the reaction with hydroxyl (OH) radicals. For several highly reactive alkenes, the nighttime removal by ozone and nitrate (NO3) radicals is also significant. We discuss how this nighttime removal affects the determination of emission ratios versus carbon monoxide (CO) and show that previous estimates based on nighttime correlations with CO were too low. We analyze model output from the Weather Research and Forecasting-Chemistry model for hydrocarbons and radicals at the Pasadena location to evaluate our methods for determining emission ratios from the measurements. We find that our methods agree with the modeled emission ratios for the domain centered on Pasadena and that the modeled emission ratios vary by 23% across the wider South Coast basin. We compare the alkene emission ratios with published results from ambient measurements and from tunnel and dynamometer studies of motor vehicle emissions. We find that with few exceptions the composition of alkene emissions determined from the measurements in Pasadena closely resembles that of motor vehicle emissions.

Photolysis of nitrous acid generates hydroxyl radicals — a key atmospheric oxidant. Measurements at a forest in Michigan suggest that sunlight-induced breakdown of nitric acid on the canopy surface serves as a significant source of nitrous acid to the overlying atmosphere.

Riparian ecosystems have the capacity to lower NO 3 - concentrations in groundwater entering from nonpoint agricultural sources. The processes responsible for decreases in riparian groundwater NO," concentrations in the Willamette Valley of Oregon are not well understood. Our objective was to determine if denitrification and/or dissimilatory NO 3 - reduction to NH 4 + (DNRA) could explain decreases in groundwater NO 3 - moving from a perennial ryegrass cropping system into a mixed-herbaceous riparian area. In situ denitrification rates (DN) were not different between the riparian area (near-stream or near-cropping system) and cropping system the first year. In the second year, during the transition to a clover planting, DN was highest just inside of the riparian/cropping system border. Median denitrification enzyme activity (DEA) rates ranged from 29.5 to 44.6 mg N 2 O-N kg -1 d -1 for surface soils (0-15 cm) and 0.7 to 1.7 μg N 2 O-N kg -1 d -1 in the subsoil (135-150 cm). Denitrification enzyme activity rates were not different among the zones and were most often correlated to soil moisture and NH 4 + . Nitrate additions to surface soils increased DEA rates, indicating a potential to denitrify additional NO 3 - . Based on groundwater velocity estimates, NO 3 - (3.8 mg NO 3 - -N L -1 ) entering the riparian surface soil could have been consumed in 0.2 to 7 m by denitrification and 0.03 to 1.0 m by DNRA. Denitrification rates measured in the subsoil could not explain the spatial decrease in NΟ 3 - . However, with the potentially slow movement of water in the subsoil, denitrification and DNRA (0 to 264 pg N kg -1 d -1 ) together could have completely consumed NO 3 - within 0.5 m of entering the riparian zone.

Efforts to improve land-use practices to prevent contamination of surface waters with soil are limited by an inability to identify the primary sources of soil present in these waters. We evaluated the utility of fatty acid methyl ester (FAME) profiles of dry reference soils for multivariate statistical classification of soils collected from surface waters adjacent to agricultural production fields and a wooded riparian zone. Trials that compared approaches to concentrate soil from surface water showed that aluminum sulfate precipitation provided comparable yields to that obtained by vacuum filtration and was more suitable for handling large numbers of samples. Fatty acid methyl ester profiles were developed from reference soils collected from contrasting land uses in different seasons to determine whether specific fatty acids would consistently serve as variables in multivariate statistical analyses to permit reliable classification of soils. We used a Bayesian method and an independent iterative process to select appropriate fatty acids and found that variable selection was strongly impacted by the season during which soil was collected. The apparent seasonal variation in the occurrence of marker fatty acids in FAME profiles from reference soils prevented preparation of a standardized set of variables. Nevertheless, accurate classification of soil in surface water was achieved utilizing fatty acid variables identified in seasonally matched reference soils. Correlation analysis of entire chromatograms and subsequent discriminant analyses utilizing a restricted number of fatty acid variables showed that FAME profiles of soils exposed to the aquatic environment still had utility for classification at least 1 wk after submersion.

Fatty acid methyl ester (FAME) and length heterogeneity-polymerase chain reaction (LH-PCR) analyses were used to generate ‘fingerprints’ of FAMEs and eubacterial 16S rDNA sequences characteristic of agricultural soil communities. We hypothesized that pooling data from two methods that characterized different components of soil biological communities would improve the resolution of fingerprints characterizing the effects of contrasting tillage and ground cover practices. By using supervised classifications of FAME and LH-PCR, a discriminant analysis procedure distinguished soils from contrasting tillage and ground cover management and predicted the origin of soil samples. Used independently, FAME provided higher resolution of tillage, ground cover, and field location than LH-PCR, but LH-PCR was effective at identifying field location. Pooling data from both methods did not enhance the predictive power. A comparison of linear discriminant analysis, quadratic discriminant analysis, and nonparametric density estimation demonstrated that minimizing assumptions about data distribution improved the capacity of FAME analysis to resolve differences in soil types. Use of a purely statistical Bayesian method to select a subset of fatty acids (FA’s) as variables in discriminant analyses identified FA’s that represented signature FA’s for specific groups of organisms. Published by Elsevier Science Ltd.

The contribution of potentially intense denitrification in the saturated zone of hydromorphic soils to atmospheric N2O levels is poorly understood because few data exist on shallow ground water N2O production, consumption and transport to the atmosphere. The objective of the present study was to investigate the contribution of the saturated zone to surface N2O emission for two fen soils and a Gleyic Luvisol with ground water tables at the surface during the experimental period. Total denitrification, denitrifier N2O production, ground water dissolved N2 and/or N2O, and surface N2O emissions were measured in situ. Concentrations of dissolved gases and surface emissions were also simulated with a simple process-based model assuming measured rates of N2and N2O production or constant profiles of dissolved N2O concentration. NO3 − and N2O were abundant in all samples of the three sites. Substantial N2O surface emission originating from the saturated zone was measured at both fen sites. Total denitrification ranged from 0.9 to 1.58 mg N l−1 day−1 in the 15 – 35-cm layer of the fen soils and from 0.005 to 0.13 mg N l−1 day−1 in the 50 – 100-cm layer of the Gleyic Luvisol. The ratio of N2O production to total denitrification ranged from 0.07 to 0.32 in the fen soils and from 0.06 to 0.08 in the Gleyic Luvisol. The ratio of N2O production to total denitrification ranged from 0.07 to 0.32, and from 0.06 to 0.08, respectively. Concentrations of dissolved N2O measured at the fen sites were 1 to 2 orders of magnitude lower than simulated concentrations. In contrast, measured N2O emissions were within the order of magnitude of emissions simulated assuming measured N2 and N2O production rates. The agreement between measured and simulated N2O concentrations and surface emissions was satisfactory when gas diffusivity was multiplied by 10 and N2O reduction rate was multiplied by 20 in the simulation. Simulation of diffusive N2O emission assuming constant values of measured N2O concentration profiles resulted in emissions approximately one order of magnitude lower than the measured values. At the Lake Creek site, the measured peak concentration of dissolved N2 and the concentration simulated assuming measured values of N2 production were relatively close with values of 2.4 and 3.5 times the atmospheric equilibrium concentration, respectively. It was concluded that the disagreements between measured and simulated values of N2O concentrations and emissions resulted from the underestimation of model parameters for gas transfer and/or for N2O reduction to N2,which were not measured. Future modeling attempts should include use of measured values for all model parameters to obtain a more realistic description of the dynamics of N2O emission from the saturated zone.

Prescription based nitrogen (N) fertilization of crops in a given local environment begins by understanding factors affecting crop N use and relating these factors to a time scale most directly related to crop growth. For these reasons the following objectives were sought for an economically important grass seed crop in western Oregon, Italian ryegrass (Lolium multiflorum Lam.). One, to determine the influence of N source (NH4‐N and NO3‐N) on crop growth and seed yield. Second, to determine the relationship between accumulated growing degree days (GDD) and plant ontogeny and relate these findings to N‐source effects on growth and seed yield parameters. Field plots were established in 1991 and again in 1992. Five N‐source treatments were applied: calcium nitrate (CN), ammonium nitrate (AN), ammonium sulfate (AS), ammonium chloride (AC), and urea‐dicyandiamide (DCD). Nitrogen‐source treatments had no effect on root and shoot dry mass accumulation in either year. Ammonium‐N fertilization increased t...

To develop optimum nitrogen (N) fertilization practices with the least impact on environmental quality and with the greatest economic return, it is imperative that a greater understanding of crop and soil N dynamics be sought. This paper reports on research conducted with these objectives: (i) to determine the relationship between plant N and dry matter accumulation and soil N status as affected by N‐source fertilization as a function of accumulated growing degree days (GDD), and (ii) to determine if western Oregon soil conditions favor ammonium (NH4) over nitrate (NO3) nutrition during the period of grass seed crop growth. In a companion paper, plant growth and seed yield component data were discussed in relation to N‐source treatments and climatic year effects. Western Oregon field plots of Italian ryegrass (Lolium multiflorum Lam.) were fertilized with calcium nitrate, ammonium nitrate, ammonium sulfate, ammonium chloride, and urea‐dicyandiamide (DCD) to manipulate soil NH4 and NO3 ratios. Ita...

Biogenic volatile organic compounds (BVOCs) can react in the atmosphere to form organic nitrates, which serve as NOx (NO + NO2) reservoirs, impacting ozone and secondary organic aerosol production, the oxidative capacity of the atmosphere, and nitrogen availability to ecosystems. To examine the contributions of biogenic emissions and the formation and fate of organic nitrates in a forest environment, we simulated the oxidation of 57 individual BVOCs emitted from a rural mixed forest in northern Michigan. Key BVOC-oxidant reactions were identified for future laboratory and field investigations into reaction rate constants, yields, and speciation of oxidation products. Of the total simulated organic nitrates, monoterpenes contributed ~70% in the early morning at ~12 m above the forest canopy when isoprene emissions were low. In the afternoon, when vertical mixing and isoprene nitrate production were highest, the simulated contribution of isoprene-derived organic nitrates was greater than 90% at all altitudes, with the concentration of secondary isoprene nitrates increasing with altitude. Notably, reaction of isoprene with NO3 leading to isoprene nitrate formation was found to be significant (~8% of primary organic nitrate production) during the daytime, and monoterpene reactions with NO3 were simulated to comprise up to ~83% of primary organic nitrate production at night. Lastly, forest succession, wherein aspen trees are being replaced by pine and maple trees, was predicted to lead to increased afternoon concentrations of monoterpene-derived organic nitrates. This further underscores the need to understand the formation and fate of these species, which have different chemical pathways and oxidation products compared to isoprene-derived organic nitrates and can lead to secondary organic aerosol formation.

Nitrogen (N) is one of the most critical elements affecting grass seed yield. In soil and cropping conditions of Western Oregon, ammonium‐N may play an important role in the growth and development of ryegrass grown for seed. Our objectives were to determine the physiological and biochemical effects of ammonium and nitrate nutrition on ryegrass vegetative growth and subsequent expression of floral tillers, and changes in plant reduced‐N and carbohydrate composition. Plants were grown in hydroponics (active pH‐controlled) and fed nitrate and ammonium alone or in combination at 10 mM total N. The experiment was terminated at anthesis, which occurred at 68 days after planting (DAP). Changes in plant tiller number, height, dry weight, reduced‐N, and water‐soluble carbohydrates (WSC) were recorded. Vegetative growth rate of plants receiving lower ratios of nitrate/ammonium was up to twice those receiving total nitrate or 75/25 nitrate/ammonium. Total primary tiller number at 68 DAP (anthesis) was 30% g...

[1] Recent laboratory and field studies have indicated that glyoxal is a potentially large contributor to secondary organic aerosol mass. We present in situ glyoxal measurements acquired with a recently developed, high sensitivity spectroscopic instrument during the CalNex 2010 field campaign in Pasadena, California. We use three methods to quantify the production and loss of glyoxal in Los Angeles and its contribution to organic aerosol. First, we calculate the difference between steady state sources and sinks of glyoxal at the Pasadena site, assuming that the remainder is available for aerosol uptake. Second, we use the Master Chemical Mechanism to construct a two-dimensional model for gas-phase glyoxal chemistry in Los Angeles, assuming that the difference between the modeled and measured glyoxal concentration is available for aerosol uptake. Third, we examine the nighttime loss of glyoxal in the absence of its photochemical sources and sinks. Using these methods we constrain the glyoxal loss to aerosol to be 0–5 × 10−5 s−1 during clear days and (1 ± 0.3) × 10−5 s−1 at night. Between 07:00–15:00 local time, the diurnally averaged secondary organic aerosol mass increases from 3.2 μg m−3 to a maximum of 8.8 μg m−3. The constraints on the glyoxal budget from this analysis indicate that it contributes 0–0.2 μg m−3 or 0–4% of the secondary organic aerosol mass.

Aspartate aminotransferase (AAT), a key enzyme in the biosynthesis of aspartate and asparagine, occurs as two forms in alfalfa (Medicago sativa L.), AAT-1 and AAT-2. Both forms were purified to near homogeneity, and high titer polyclonal antibodies produced to the native proteins. Alfalfa AAT-1 was purified from root suspension culture cells, while AAT-2 was purified from effective root nodules. Antibodies prepared to AAT-1 and used as probes for western blots readily recognized native and SDS forms of AAT-1 but did not recognize either native or SDS forms of AAT-2. Conversely, antibodies to AAT-2 readily recognized native and SDS forms of AAT-2 but did not recognize AAT-1. Immunotitrations further confirmed the immunological distinction between AAT-1 and AAT-2. AAT-1 antibodies immunotitrated 100% of the in vitro activity of purified AAT-1 but had no effect on AAT-2 in vitro activity. Likewise, AAT-2 antibodies removed 100% of the in vitro activity of purified AAT-2 but did not affect AAT-1 in vitro activity. Sequential titration of total AAT activity from roots and nodules showed that AAT-1 comprised the major form (62%) of AAT in roots, while AAT-2 was the predominant form (90%) in nodules. Last, SDS-PAGE western blots showed that the molecular masses of AAT-1 and AAT-2 were 42 and 40 kilodaltons, respectively. These data indicate that AAT is under the control of at least two distinct genes in alfalfa.

Oxalic acid is one of the most abundant dicarboxylic acids in the atmosphere, receiving a great deal of attention due to its potential influence on cloud condensation nucleus activities. In this work, we report 10 months of hourly oxalate measurements in particulate matter of less than 2.5 µm in aerodynamic diameter (PM2.5) by a Monitor for Aerosols and Gases in ambient Air at a suburban coastal site in Hong Kong from April 2012 to February 2013. A total of more than 6000 sets of oxalate and inorganic ion data were obtained. The mean (±SD) oxalate concentration was 0.34 (±0.18) µg m−3, accounting for 2.8% of the total ion mass and 1.5% of the PM2.5 mass. Seasonal variation showed higher concentrations in fall and winter (0.54 and 0.36 µg m−3, respectively) and lower concentrations in spring and summer (~0.26 µg m−3). Different from the inorganic ions, a shallow dip in the oxalate concentration consistently occurred in the morning after sunrise (around 9:00 A.M.) throughout all seasons. Our analysis suggests that this was likely due to photolysis of oxalate-Fe (III) complex under sunlight. In summer, a small daytime peak was discernable for oxalate and nitrate. This characteristic, together with a more evident diurnal variation of O3, indicates comparatively more active photochemical oxidation in summer than other seasons. High correlations were observed between oxalate and non-sea-salt SO42− (NSS) (R2 = 0.63) and Ox (O3 + NO2) (R2 = 0.48), indicating significant commonality in their secondary formation. Positive matrix factorization analysis of oxalate and other real-time gas and particle-phase component data estimates that secondary formation processes, including secondary gas or aqueous oxidation processes (49%), oxidation processes of biomass burning emissions (37%), accounted for the majority of PM2.5 oxalate. A backward trajectories cluster analysis found that higher oxalate/NSS ratios were associated with low pollution samples under the influence of marine air masses while the ratios were lower in high pollution samples that were typically associated with continental air masses passing through areas of high anthropogenic emissions. Isolating the “low pollution marine” aerosols across the entire data set indicates that oxalate production increased in the summer compared to other seasons, suggesting either more active marine emissions of oxalate precursors or stronger photochemical processes in the summer.

Carbon metabolism in Bradyrhizobium japonicum bacteroids is reviewed. Additionally, the bacteroid tricarboxylic acid (TCA) cycle and its regulation under oxygen-limited conditions is considered, with emphasis on possible sites of TCA cycle rate-limiting reactions. Furthermore, we consider other adaptive pathways that may be employed by these organisms while in symbiosis. These pathways include: (1) a poly-β-hydroxy-butyrate shunt, (2) a malate-aspartate shuttle, (3) an α-ketoglutarate-glutamate shunt, (4) a modified dicarboxylic acid cycle, and (5) fermentation pathways leading to lactate, acetaldehyde and ethanol. The effects of oxygen limitation on host carbon metabolism are also considered briefly.

Stems of Vicia faba plants were used to study phloem unloading because they are hollow and have a simple anatomical structure that facilitates access to the unloading site. After pulse labeling of a source leaf with (14)CO(2), stem sections were cut and the efflux characteristics of (14)C-labeled sugars into various buffered solutions were determined. Radiolabeled sucrose was shown to remain localized in the phloem and adjacent phloem parenchyma tissues after a 2-hour chase. Therefore, sucrose leakage from stem segments prepared following a 75-minute chase period was assumed to be characteristic of phloem unloading. The efflux of (14)C assimilates from the phloem was enhanced by 1 millimolar p-chloromercuribenzene sulfonic acid (PCMBS) and by 5 micromolar carbonyl cyanide m-chlorophenly hydrazone (CCCP). However, PCMBS inhibited and CCCP enhanced general leakage of nonradioactive sugars from the stem segments. Sucrose at concentrations of 50 millimolar in the free space increased efflux of [(14)C]sucrose, presumably through an exchange mechanism. This exchange was inhibited by PCMBS and abolished by 0.2 molar mannitol. Increasing the osmotic concentration of the efflux medium with mannitol reduced [(14)C]sucrose efflux. However, this inhibition seems not to be specific to sucrose unloading since leakage of total sugars, nonlabeled sucrose, glucose, and amino acids from the bulk of the tissue was reduced in a similar manner. The data suggest that phloem unloading in cut stem segments is consistent with passive efflux of sucrose from the phloem to the apoplast and that sucrose exchange via a membrane carrier may be involved. This is consistent with the known conductive function of the stem tissues, and contrasts with the apparent nature and function of unloading in developing seeds.

In vitro sugar transport into developing isolated maize embryos was studied. Embryo fresh and dry weight increased concomitantly with endogenous sucrose concentration and glucose uptake throughout development. However, endogenous glucose and fructose concentration and sucrose uptake remained constant. The uptake kinetics of radiolabeled sucrose, glucose, and fructose showed a biphasic dependence on exogenous substrate concentration. Hexose uptake was four to six times greater than sucrose uptake throughout development. Carbonylcyanide-m-chlorophenylhydrazone and dinitrophenol inhibited sucrose and glucose uptake significantly, but 3-O-methyl glucose uptake was less affected. The uptake of 1 millimolar sucrose was strongly pH dependent while glucose was not. Glucose and fructose were readily converted to sucrose and insoluble products soon after absorption into the embryo. Thus, sucrose accumulated, while glucose pools remained low. Based on the findings of this and other studies a model for sugar transport in the developing maize kernel is presented.

Short-term transport studies were conducted using excised whole Zea mays kernels incubated in buffered solutions containing radiolabeled sugars. Following incubation, endosperms were removed and rates of net 14C-sugar uptake were determined. Endogenous sugar gradients of the kernel were estimated by measuring sugar concentrations in cell sap collected from the pedicel and endosperm. A sugar concentration gradient from the pedicel to the endosperm was found. Uptake rates of 14C-labeled glucose, fructose, and sucrose were linear over the concentration range of 2 to 200 millimolar. At sugar concentrations greater than 50 millimolar, hexose uptake exceeded sucrose uptake. Metabolic inhibitor studies using carbonylcyanide-m-chlorophenylhydrazone, sodium cyanide, and dinitrophenol and estimates of Q10 suggest that the transport of sugars into the developing maize endosperm is a passive process. Sucrose was hydrolyzed to glucose and fructose during uptake and in the endosperm was either reconverted to sucrose or incorporated into insoluble matter. These data suggest that the conversion of sucrose to glucose and fructose may play a role in sugar absorption by endosperm. Our data do not indicate that sugars are absorbed actively. Sugar uptake by the endosperm may be regulated by the capacity for sugar utilization (i.e. starch synthesis).