Determination of primary and secondary sources of organic acids and carbonaceous aerosols using stable carbon isotopes

Stable carbon isotope ratio (δ¹³C) data can provide important information regarding the sources and the processing of atmospheric organic carbon species. Formic, acetic and oxalic acid were collected from Zurich city in August–September 2002 and March 2003 in the gas and aerosol phase, and the corresponding δ¹³C analysis was performed using a wet oxidation method followed by isotope ratio mass spectrometry. In August, the δ¹³C values of gas phase formic acid showed a significant correlation with ozone (coefficient of determination (r²) = 0.63) due to the kinetic isotope effect (KIE). This indicates the presence of secondary sources (i.e. production of organic acids in the atmosphere) in addition to direct emission. In March, both gaseous formic and acetic acid exhibited similar δ¹³C values and did not show any correlation with ozone, indicating a predominantly primary origin. Even though oxalic acid is mainly produced by secondary processes, the δ¹³C value of particulate oxalic acid was not depleted and did not show any correlation with ozone, which may be due to the enrichment of ¹³C during the gas - aerosol partitioning.The concentrations and δ¹³C values of the different aerosol fractions (water soluble organic carbon, water insoluble organic carbon, carbonate and black carbon) collected during the same period were also determined. Water soluble organic carbon (WSOC) contributed about 60% to the total carbon and was enriched in ¹³C compared to other fractions indicating a possible effect of gas - aerosol partitioning on δ¹³C of carbonaceous aerosols. The carbonate fraction in general was very low (3% of the total carbon).     

Impact of recent urbanization on formic and acetic acid concentrations in coastal North Carolina rainwater

Concentrations of formic and acetic acids in Wilmington, North Carolina, USA, rainwater collected between 1996 and 1998 have increased dramatically since an earlier study conducted at the same site between 1987 and 1989. The current volume-weighted concentrations of acetic acid are within the range of values reported for urban locations whereas values from the earlier study at this site were similar to those obtained for rural locations. The ratios of formic to acetic acids (F : A) in the current study (approximately 1 : 1) are considerably lower than those previously reported (approximately 2.7 : 1). Current F : A's are similar to F : A's from direct automobile emissions. Increases in formic and acetic acid concentrations and the shift in formic to acetic acid ratios likely reflect the impact of extensive population growth in the surrounding region. Assuming increases in formic and acetic acid concentrations result from increased anthropogenic sources, we estimate at least 1/2 of the formic and at least 2/3 of the acetic acid in Wilmington, NC growing season rainwater results from anthropogenic sources.     

Wet deposition of low molecular weight mono- and di-carboxylic acids,aldehydes and inorganic species in Los Angeles

About 60 rainwater samples were collected at west Los Angeles, California in 1981–1984 and were analyzed for C₁–C₉ monocarboxylic acids (0.33–79 μM, average (av.) 13±15 μM), C₂–C₁₀ dicarboxylic acids (2.9–51 μM, av. 7.5±14 μM) and C₁–C₄ aldehydes (0.85–28 μM, av. 9.2±11 μM). Distributions of monocarboxylic acids show a predominance of formic (average concentration: 6.5 μM) and acetic (av. 5.6 μM) acids followed by propionic acid (av. 0.44 μM). Oxalic acid is the dominant diacid (av. 3.9 μM) followed by succinic acid (av. 1.0 μM). Formaldehyde (av. 6.9 μM) is the dominant aldehyde, with the next most abundant, acetaldehyde, being minor (av. 0.65 μM). For select rain samples described in this paper, were found to comprise monocarboxylic acids 0.9–12.3% (av. 4.4±3.4%), diacids comprise 1.2–9.5% (av. 4.2±3.3%) and aldehydes comprise 0.2–6.2% (av. 2.1±2.2%) of total organic carbon (TOC, 2.0–18.6 mg C l⁻¹; av. 9.8±5.4 mg C l⁻¹). Annual rain fluxes of monocarboxylic acids and aldehydes during 1982–1983 were calculated to be 0.24 and 0.11 g m⁻² yr⁻¹, respectively, with an annual estimated wet deposition in the Los Angeles Basin of 3120 and 1430 tons, respectively. These fluxes are equivalent to 2500 times of the acids and 2.5 times of the aldehydes emitted from automobile exhausts in the Los Angeles air basin. This comparison suggests that major portions of the carboxylic acids detected in the rain are not directly emitted from auto-exhausts, but are most likely produced in the atmosphere by gaseous and/or aqueous phase photo-induced reactions.     

Short-chain oxygenated VOCs: Emission and uptake by plants and atmospheric sources,sinks, and concentrations

Emissions of volatile organic compounds (VOCs) have multiple atmospheric implications and play many roles in plant physiology and ecology. Among these VOCs, growing interest is being devoted to a group of short-chain oxygenated VOCs (oxVOCs). Technology improvements such as proton transfer reaction-mass spectrometry are facilitating the study of these hydrocarbons and new data regarding these compounds is continuously appearing. Here we review current knowledge of the emissions of these oxVOCs by plants and the factors that control them, and also provide an overview of sources, sinks, and concentrations found in the atmosphere.The oxVOCs reviewed here are formic and acetic acids, acetone, formaldehyde, acetaldehyde, methanol, and ethanol. In general, because of their water solubility (low gas–liquid partitioning coefficient), the plant-atmosphere exchange is stomatal-dependent, although it can also take place via the cuticle. This exchange is also determined by atmospheric mixing ratios. These compounds have relatively long atmospheric half-lives and reach considerable concentrations in the atmosphere in the range of ppbv. Likewise, under non-stressed conditions plants can emit all of these oxVOCs together at fluxes ranging from 0.2 up to 4.8 μg(C)g⁻¹(leaf dry weight)h⁻¹ and at rates that increase several-fold when under stress.Gaps in our knowledge regarding the processes involved in the synthesis, emission, uptake, and atmospheric reactivity of oxVOCs precludes the clarification of exactly what is conditioning plant-atmosphere exchange—and also when, how, and why this occurs—and these lacunae therefore warrant further research in this field.     

Emissions of volatile fatty acids from feed at dairy facilities

Recent studies suggest that dairy operations may be a major source of non-methane volatile organic compounds in dairy-intensive regions such as Central California, with short chain carboxylic acids (volatile fatty acids or VFAs) as the major components. Emissions of four VFAs (acetic acid, propanoic acid, butanoic acid and hexanoic acid) were measured from two feed sources (silage and total mixed rations (TMR)) at six Central California Dairies over a fifteen-month period. Measurements were made using a combination of flux chambers, solid phase micro-extraction fibers coupled to gas chromatography mass spectrometry (SPME/GC–MS) and infra-red photoaccoustic detection (IR-PAD for acetic acid only). The relationship between acetic acid emissions, source surface temperature and four sample composition factors (acetic acid content, ammonia-nitrogen content, water content and pH) was also investigated. As observed previously, acetic acid dominates the VFA emissions. Fluxes measured by IR-PAD were systematically lower than SPME/GC–MS measurements by a factor of two. High signals in field blanks prevented emissions from animal waste sources (flush lane, bedding, open lot) from being quantified. Acetic acid emissions from feed sources are positively correlated with surface temperature and acetic acid content. The measurements were used to derive a relationship between surface temperature, acetic acid content and the acetic acid flux. The equation derived from SPME/GC–MS measurements predicts estimated annual average acetic acid emissions of (0.7 + 1/?0.4) g m^?2 h^?1 from silage and (0.2 + 0.3/?0.1) g m^?2 h^?1 from TMR using annually averaged acetic acid content and meteorological data. However, during the summer months, fluxes may be several times higher than these values.     

The performance of a gas and aerosol monitoring system (GAMS) for the determination of acidic water soluble organic and inorganic gases and ammonia as well as related particles from the atmosphere

A new application of the quasi-simultaneous gas/particle phase sampling and analysis principle first proposed by Simon and Dasgupta (Anal. Chem. 34 (1995) 71) is described. For the first time, a gradient chromatograph is used in connection with such a sampling system to allow the simultaneous determination of major organic (formic, acetic, propionic, oxalic, malonic and succinic) and inorganic (SO₂, HNO₂, HNO₃, HCl and H₂F₂) acidic gases and related particles. Another addition to the previous systems is the analysis of cations other than ammonium from the particulate phase. The time resolution of the instrument still remains high, 1 h, during which gaseous water-soluble acidic compounds, ammonia, as well as related anionic particles and inorganic major cations are analysed. Sampling is based on diffusion in a wetted parallel plate denuder for gases and on growth in supersaturated water vapour for particles. The determination limits range from 2 ppt (acetate) to 0.4 ppb (ammonia) in the gas phase and 0.01 μg m⁻³ (citric acid) to 0.79 μg m⁻³ (calcium) for particulate matter. Collection efficiencies for gas and aerosol sampling were quantified and the supersaturation in the aerosol sampling apparatus investigated. The system has been used for field measurements at a background station; selected results of these measurements are presented.     

Low molecular weight carboxylic acids in an urban atmosphere: Winter measurements in São Paulo City,Brazil

Atmospheric gas and particle-phase carboxylic acids were measured during July 1996, Winter, in an urban area of São Paulo, a highly polluted Latin American city. Ion chromatography and capillary electrophoresis techniques were used to determine the species. As oxalic (36.2±21.4%), pyruvic (15.0±7.9%), β-hydroxy-butyric (9.15±9.00%) and glycolic (3.55±2.26%) acids were determined in aerosol particles, formic and acetic acids were determined both in the gaseous (4.36±2.70 and 3.66±2.63 ppbv, respectively) and particulate phases (17.8±12.4 and 18.2±9.8%, respectively). Approximately 98% of the total acetic and formic acids were in the gas-phase and the gas–aerosol equilibrium was influenced by high levels of relative humidity. Gaseous formic-to-acetic ratios fell in the 0.94–1.85 range. Photochemical production appeared to be a very likely source of the gaseous acetic and formic acid levels found in this investigation. Direct emissions, mainly motor exhaust of vehicles also contributed to their presence in air.The observed amounts of formic and acetic acids in the particle phase were comparable with those observed in other urban sites. Results from aerosol particles indicated lower concentrations of the carboxylic acids at night, but their diurnal and nocturnal variation were similar.Using a correlation matrix, it was possible to suggest some sources for the carboxylic acids in the particulate phase. During daytime, vehicular emission appeared to be the primary source of acetic acid, whereas formic and pyruvic acids should be formed photochemically. Moreover, emissions from biogenic primary sources appeared to be an important contribution to atmospheric concentrations of formic and glycolic acids. Presumably, the photooxidation of pyruvic and glycolic acids gave rise to the oxalic acid.No source for acetic and pyruvic acids at nighttime was possible to suggest. However, direct vehicular and biogenic emissions might be major sources of TOC in nocturnal measurements. Oxalic acid might result from vehicular emission, glycolic acid from biogenic emission and formic acid from both sources.     

Semivolatile behavior of dicarboxylic acids and other polar organic species at a rural background site (Nylsvley,RSA)

In this study aerosol samples from the South African savanna were analyzed for their polar organic constituents. Samples were collected with a front/back-up filter tandem system of quartz fiber filters (dual filter strategy). In all samples (n=15) dicarboxylic acids and a variety of phthalates, aldehydes and monocarboxylic acids were observed. Oxalic acid was the dominating compound with an average amount of 79.2 ng m⁻³ on the front filter and 11.3 ng m⁻³ on the back-up filter. The presence of significant concentrations of dicarboxylic acids on the back-up filter was rather unexpected. There are two possible sources to explain the presence of individual compounds on the back-up filter – particle penetration through the front filter or adsorption of compound parts from the gas phase. Interpretation of the data indicates that the dicarboxylic acid concentrations on the back-up filters appear to be caused by the adsorption of gaseous organic species. Dicarboxylic acids semivolatilic behavior is evident with this results. This conclusion refutes the commonly held view that dicarboxylic acids in the atmosphere were associated with the aerosol phase only. Additionally, it was found that the distribution of dicarboxylic acids between the gas and particle phase in the atmosphere is not only dependent on their vapor pressures. The actual gas phase concentration appears to be more determined by the chemical properties of the particles than by pure physical influences. Surprisingly, malonic acid exhibits an anomaly, as it does not show a semivolatile tendency.     

The chemistry of gaseous acids in medieval churches in Cyprus

Indoor and outdoor concentrations of HCl, HNO₃, HCOOH and CH₃COOH were determined in two medieval churches in Cyprus, during July 2003 and March 2004. The high air exchange rate through the open windows and doors led to lower indoor, compared to outdoor, acid concentrations in July 2003. Indoor pollutant emissions and a low air exchange rate resulted in higher indoor compared to outdoors acid concentrations in both churches during March 2004. Indoor to outdoor inorganic acid ratios were higher than the corresponding indoor to outdoor organic acid ratios during July 2003, whilst the opposite trend was observed during March 2004. Direct acid emission from candle burning appears to play a major role in the observed indoor acid concentrations. Emissions of volatile organic compounds from other sources, like humans, cleaning products and incense, led also to formation or depletion of the gaseous acids via homogeneous photochemical, heterogeneous and dark reaction sequences. Chemical reaction pathways were extensively investigated and appear to explain the observed results. The apparent indoor acid deposition velocities ranged between 0.05 and 0.15 cm s⁻¹.     

Investigations of emissions and heterogeneous formation of HONO in a road traffic tunnel

Simultaneous measurements of nitrous acid (HONO) and nitrogen dioxide (NO₂) using a differential optical absorption spectroscopy system, nitrogen oxide (NO) by an in situ chemiluminescence analyser and carbon dioxide (CO₂) by a gas chromatographic technique were carried out in the Wuppertal Kiesbergtunnel. At high traffic density HONO concentrations of up to 45 ppbV were observed. However, at low traffic density unexpectedly high HONO concentrations of up to 10 ppbV were measured caused by heterogeneous HONO formation on the tunnel walls. In addition to the tunnel campaigns, emission measurements of HONO, NO₂, NO and CO₂ from different single vehicles (a truck, a diesel and a gasoline passenger car) were also performed. For the correction of the HONO emission data, the heterogeneous HONO formation on the tunnel walls was quantified by two different approaches (a) in different NO₂ emission experiments in the tunnel without traffic and (b) on tunnel wall residue in the laboratory. The HONO concentration corrected for heterogeneous formation on the tunnel walls, in relation to the CO₂ concentration can be used to estimate the amount of HONO, which is directly emitted from the vehicle fleet. From the measured data, emission ratios (e.g. HONO/NO_x) and emission indices (e.g. mg HONO kg⁻¹ fuel) were calculated. The calculated emission index of 88±18 mg HONO kg⁻¹ fuel allows an estimation of the HONO emission rates from traffic into the atmosphere. Furthermore, the heterogeneous formation of HONO from NO₂ on freshly emitted exhaust particles is discussed.     

Temporal and spatial variation of sulfur-gas-transfer between coastal marine sediments and the atmosphere

The spatial and temporal variability of sulfur gas fluxes (H₂S, COS, CH₃SH, DMS, and CS₂) at the sediment–air interface were studied in the intertidal Wadden Sea area of Sylt-Rømø (Germany/Denmark) during eight measuring campaigns between June 1991 and September 1994. Measurements were performed mainly at four sites in a sheltered intertidal bay of approximately 6 km² (Königshafen) and discontinuously in a wider range of the 400 km² Sylt-Rømø tidal flat area. In situ fluxes of the S-gases were determined by a dynamic chamber technique focusing on dry sediment periods. Additional experiments were conducted in order to determine changes in S-gas concentrations in the sediment between the surface and 70 cm depth.In most cases H₂S was the dominant S-gas emitted from the sediment to the atmosphere, contributing up to 70% of the total S-emission at this interface. Mean H₂S emission rates ranged between 0.07 and 9.95 μg S m^-2 h^-1. Both emission rates and relative contribution of H₂S were lowest from fine sand and highest from muddy sites. Diurnal variation of H₂S emission was evident in summer and fall with up to 10-fold higher rates during night than during the day. Distinct seasonal variation of H₂S-transfer between the sediment and the atmosphere was observed with higher emission rates in the summer than in spring or fall. The emission of H₂S to the atmosphere was smaller by a factor of 1600–26 000 than the H₂S produced from sulfate reduction. Apparently, the efficiency by which H₂S produced in the sediment is retained and reoxidized by biogeochemical sediment processes is extremely high. Carbonyl sulfide (COS) was emitted with relatively constant rates in space and time with mean flux rates ranging between 0.24 and 2.0 μg S m^-2 h^-1. Carbon disulfide emission rates were comparable to those of COS and varied between 0.3 and 2.23 μg S m^-2 h^-1. DMS played a minor role in the S-gas transfer from uncovered sediment areas contributing between 3.1 and 23% to total S-emission from the sediment to the atmosphere.     

Acetic acid and formic acid concentrations in the museum environment measured by SPME-GC/MS

A method is described to sample and quantify acetic and formic acid vapours within 35 min. The detection limits are 5.3 and 28.9 μg/m³, respectively, and the technique used is manual SPME collection of the sample followed by analysis on a gas chromatography/mass spectrometry system. In both cases linearity between the standards prepared and the results measured are found. The technique can be used to analyse air in small confined areas, as the total volume of air sampled is <4 l.     

Characterization of gaseous pollutant and particulate matter emission rates from a commercial broiler operation part I: Observed trends in emissions

This paper characterizes the emission rates of size fractionated particulate matter, inorganic aerosols, acid gases, ammonia and methane measured over four flocks at a commercial broiler chicken facility. Mean emission rates of each pollutant, along with sampling notes, were reported in this paper, the first in a series of two. Sampling notes were needed because inherent gaps in data may bias the mean emission rates.The mean emission rates of PM₁₀ and PM_2.5 were 5.0 and 0.78 g day^?1 [Animal Unit, AU]^?1, respectively, while inorganic aerosols mean emission rates ranged from 0.15 to 0.46 g day^?1 AU^?1 depending on the season. The average total acid gas emission rate was 0.43 g day^?1 AU^?1 with the greatest contribution from nitrous and nitric acids and little contribution from sulfuric acid (as SO₂).Ammonia emissions were seasonally dependent, with a mean emission rate of 66.0 g day^?1 AU^?1 in the cooler seasons and 94.5 g day^?1 AU^?1 during the warmer seasons. Methane emissions were relatively consistent with a mean emission rate of 208 g day^?1 AU^?1.The diurnal pattern in each pollutant’s emission rate was relatively consistent after normalizing the hourly emissions according to each daily mean emission rate. Over the duration of a production cycle, all the measured pollutants’ emissions increased proportionally to the total live mass of birds in the house, with the exception of ammonia.Interrelationships between pollutants provide evidence of mutually dependent release mechanisms, which suggests that it may be possible to fill data gaps with minimal data requirements. In the second paper (Roumeliotis, T.S., Dixon, B.J., Van Heyst, B.J. Characterization of gaseous pollutants and particulate matter emission rates from a commercial broiler operation part II: correlated emission rates. Atmospheric Environment, 2010.), regression correlations are developed to estimate daily mean emission rates for data gaps and, using the normalized hourly diurnal patterns from this paper, emission factors were generated for each pollutant.     

Seasonal soil and leaf CO2 exchange rates in a Mediterranean holm oak forest and their responses to drought conditions

We measured the soil and leaf CO₂ exchange in Quercus ilex and Phillyrea latifolia seasonally throughout the year in a representative site of the Mediterranean region, a natural holm oak forest growing in the Prades Mountains in southeastern Catalonia. In the wet seasons (spring and autumn), we experimentally decreased soil moisture by 30%, by excluding rainfall and water runoff in 12 plots, 1×10 m, and left 12 further plots as controls. Our aim was to predict the response of these gas exchanges to the drought forecasted for the next decades for this region by GCM and ecophysiological models.Annual average soil CO₂ exchange rate was 2.27±0.27 μmol CO₂ m⁻² s⁻¹. Annual average leaf CO₂ exchange rates were 8±1 and 5±1 μmol m⁻² s⁻¹ in Q. ilex and P. latifolia, respectively. Soil respiration rates in control treatments followed a seasonal pattern similar to photosynthetic activity. They reached maximum values in spring and autumn (2.5–3.8 μmol m⁻² s⁻¹ soil CO₂ emission rates and 7–15 μmol m⁻² s⁻¹ net photosynthetic rates) and minimum values (almost 0 for both variables) in summer, showing that soil moisture was the most important factor driving the soil microbial activity and the photosynthetic activity of plants. In autumn, drought treatment strongly decreased net photosynthesis rates and stomatal conductance of Q. ilex by 44% and 53%, respectively. Soil respiration was also reduced by 43% under drought treatment in the wet seasons. In summer there were larger soil CO₂ emissions in drought plots than in control plots, probably driven by autotrophic (roots) metabolism. The results indicate that leaf and soil CO₂ exchange may be strongly reduced (by ca. 44%) by the predicted decreases of soil water availability in the next decades. Long-term studies are needed to confirm these predictions or to find out possible acclimation of those processes.     

Shouldn’t snowpacks be sources of monocarboxylic acids?

We report the first measurements of the mixing ratios of acetic (CH₃COOH) and formic (HCOOH) acids in the air filling the pore spaces of the snowpacks (firn air) at Summit, Greenland and South Pole. Both monocarboxylic acids were present at levels well above 1 ppbv throughout the upper 35 cm of the snowpack at Summit. Maximum mixing ratios in Summit firn air reached nearly 8 ppbv CH₃COOH and 6 ppbv HCOOH. At South Pole the mixing ratios of these acids in the top 35 cm of firn air were also generally >1 ppbv, though their maximums barely exceeded 2.5 ppbv of CH₃COOH and 2.0 ppbv of HCOOH. Mixing ratios of the monocarboxylic acids in firn air did not consistently respond to diel and experimental (fast) variations in light intensity, unlike the case for N oxides in the same experiments. Air-to-snow fluxes of CH₃COOH and HCOOH apparently support high mixing ratios (means of (CH₃COOH/HCOOH) 445/460 and 310/159 pptv at Summit and South Pole, respectively) in air just above the snow during the summer sampling seasons at these sites. We hypothesize that oxidation of carbonyls and alkenes (that are produced by photo- and OH-oxidation of ubiquitous organic compounds) within the snowpack is the source of the monocarboxylic acids.     

Characteristics of mercury exchange flux between soil and air in the heavily air-polluted area,eastern Guizhou,China

To investigate the characteristics of mercury exchange between soil and air in the heavily air-polluted area, total gaseous mercury (TGM) concentration in air and Hg exchange flux were measured in Wanshan Hg mining area (WMMA) in November, 2002 and July–August, 2004. The results showed that the average TGM concentrations in the ambient air (17.8–1101.8 ng m⁻³), average Hg emission flux (162–27827 ng m⁻² h⁻¹) and average Hg dry deposition flux (0–9434 ng m⁻² h⁻¹) in WMMA were 1–4 orders of magnitude higher than those in the background area. It is said that mercury-enriched soil is a significant Hg source of the atmosphere in WMMA. It was also found that widely distributed roasted cinnabar banks are net Hg sources of the atmosphere in WMMA. Relationships between mercury exchange flux and environmental parameters were investigated. The results indicated that the rate of mercury emission from soil could be accelerated by high total soil mercury concentration and solar irradiation. Whereas, highly elevated TGM concentrations in the ambient air can restrain Hg emission from soil and even lead to strongly atmospheric Hg deposition to soil surface. A great amount of gaseous mercury in the heavily polluted atmosphere may cycle between soil and air quickly and locally. Vegetation can inhibit mercury emission from soil and are important sinks of atmospheric mercury in heavily air-polluted area.     

Aircraft-based volatile organic compounds flux measurements with relaxed eddy accumulation

Regional estimates of fluxes of volatile organic compounds (VOCs) are required to improve our understanding of their role in the chemistry of the atmosphere. Flux measurements on such a scale can best be obtained using aircraft-based systems. These systems usually rely on the eddy covariance technique, which requires fast response gas sensors for flux measurement, but such sensors are not available for most organic compounds, therefore, the relaxed eddy-accumulation (REA) technique was selected. An aircraft-based REA sampling system was developed and used to measure isoprene emission over the boreal forest during the 1996 summer. Over a short period in July at the Boreal Ecosystem/Atmosphere Study (BOREAS) southern study area (SSA), the isoprene fluxes ranged from −0.06 to 1.79 μg m^-2 s^-1, with a mean of 0.59±0.34 μg m^-2 s^-1, while in August at the BOREAS northern study area (NSA) the isoprene fluxes ranged from 0.00 to 0.26 μg m^-2 s^-1, with a mean of 0.14±0.09 μg m^-2 s^-1. In the SSA, the isoprene fluxes over aspen ranged from 0.44 to 1.79 μg m^-2 s^-1, with a mean of 0.92±0.33 μg m^-2 s^-1, whereas over black spruce, isoprene fluxes ranged from −0.06 to 0.54 μg m^-2 s^-1, with a mean of 0.36±0.21 μg m^-2 s^-1. The isoprene fluxes were exponentially correlated with solar radiation and radiative surface temperature. High correlations between isoprene fluxes and the fluxes of CO₂ and latent heat were also observed. Carbon lost through isoprene emissions was about 0.7 and 0.8% of the CO₂ assimilation rate for aspen and black spruce, respectively. The results demonstrate that the aircraft-based relaxed eddy-accumulation technique is a promising approach for quantifying the atmosphere–surface exchange of VOCs on a regional scale.     

Origin of low-molecular-weight dicarboxylic acids and their concentration and size distribution variation in suburban aerosol

The concentrations and size distributions of low molecular weight dicarboxylic acids in suburban particulate matter collected in early and mid-autumn 2002 and early and mid-summer 2003 in Tainan, Taiwan, were analyzed. PM_2.5 contained, on average, 449.3 ng m⁻³ oxalic acid, 53.0 ng m⁻³ malic acid, 45.5 ng m⁻³ maleic acid, 29.6 ng m⁻³ succinic acid, 20.8 ng m⁻³ malonic acid, and 11.6 ng m⁻³ tartaric acid. Bar tartaric acid, concentrations were higher during the day, indicating that these acids are photochemical products. Furthermore, the malonic acid–succinic acid ratio of 0.79 during daytime and 0.60 during nighttime demonstrates that more succinic acid is converted to malonic acid during daytime, and that aerosol dicarboxylic acids predominantly originate from photochemical oxidation during daytime. The concentration peak of oxalic acid occurred in the condensation and droplet modes (0.32–1.0 μm), as did that of sulfate. In early summer, succinic acid, malonic acid, and oxalic acid major concentration peaks occurred at 0.32–0.54 μm, indicative of the relationship created by photochemical decomposition of succinc acid into malonic acid into oxalic acid. This photochemical decomposition accelerated in mid-summer such that most concentration peaks for succinic and malonic acids also occurred at 0.32–1.0 μm. Mid-summer is also the wettest period of the four in Tainan, with 85% RH. As a result of hygroscopic reactions in mid-summer, malonic acid and oxalic acid major concentration peaks shifted from 0.32–0.54 μm or 0.54–1.0 μm to 1.0–1.8 μm, thus extending the range in which these species were found to larger particle sizes, and this shift was highly correlated with a shift in succinic acid size distribution. This latter observation offers additional evidence that succinic acid is photochemically decomposed into malonic acid and oxalic acid and that the presence of malonic and oxalic acids in the wet mid-summer atmosphere is made more obvious via hygroscopic growth. Close correlation between succinic acid and Na⁺ and succinic acid and NO₃⁻ in the coarse mode is related to sea spray.     

Isoprene emission capacity for US tree species

Isoprene emission capacity measurements are presented from 18 North American oak (Quercus) species and species from six other genera previously found to emit significant quantities of isoprene. Sampling was conducted at physiographically diverse locations in North Carolina, Central California, and Northern Oregon. Emissions from several sun leaves of each species were measured at or near standard conditions (leaf temperature of 30°C and photosynthetically active radiation of 1000 μmol m⁻² s⁻¹) using environmentally controlled cuvette systems and gas chromatography with reduction gas detectors. Species mean emission capacity ranged from 39 to 158 μg C g⁻¹ h⁻¹ (mean of 86), or 22 to 79 nmol m⁻² s⁻¹ (mean of 44). These rates are 2–28 times higher than those previously reported from the same species, which were summarized in a recent study where isoprene emission rates were assigned based on published data and taxonomy. These discrepancies were attributed to differences in leaf environment during development, measurement technique (branch or plant enclosure versus leaf enclosure), and lack of environmental measurements associated with some of the earlier branch enclosure measurements. Mass-based emission capacities for 15 of 18 oak species, sweetgum (Liquidambar styraciflua), and poplars (Populus trichocarpa and P. deltoides) were within ranges used in current biogenic volatile organic compound (BVOC) emission models, while measured rates for the remaining three oak species, Nyssa sylvatica, Platanus occidentalis, Robinia pseudoacacia, Salix nigra, and Populus hybrids (Populus trichocarpa × P. deltoides) were considerably higher. In addition, mean specific leaf mass of the oak species was 30% higher than assumed in current emission models. Emission rates reported here and in other recent studies support recent conclusions that isoprene emission capacities for sun leaves of high emitting species may be better represented by a value of 100±50 μg C g⁻¹ h⁻¹ during hot summer conditions. We also find that intermediate isoprene emission rates previously suggested for some tree species may not represent their true emission capacities, and that broadleaf plant species may have either low (<1.0 μg C g⁻¹ h⁻¹) or very high (∼100 μg C g⁻¹ h⁻¹) genetic capacity to emit isoprene when mature foliage is exposed to a high ambient temperature and light environment.     

Isoprene and monoterpene emission from the coniferous species Abies Borisii-regis—implications for regional air chemistry in Greece

The emission of isoprene has been studied from a forest of Abies Borisii-regis, a Mediterranean fir species previously thought to emit only monoterpenes. Emission studies from two independent enclosure experiments indicated a standardised isoprene emission rate of (18.4±3.8) μg g_dry-weight⁻¹ h⁻¹, similar in magnitude to species such as eucalyptus and oak which are considered to be strong isoprene emitters. Isoprene emission depended strongly on both leaf temperature (2°C–34°C) and photosynthetically active radiation (PAR) below 250 μmol m⁻² s⁻¹, becoming saturated with respect to PAR above this value. The annual isoprene emission rate was estimated to be (132±29) kT yr⁻¹ for those trees growing within Greece, comparable to current estimates of the total isoprene budget of Greece as a whole, and contributing significantly to regional ozone and carbon monoxide budgets. Monoterpene emission exhibited exponential temperature dependence, with 1,8-cineole, α-pinene, β-pinene and limonene forming the primary emissions. A standardised total monoterpene emission rate of (2.7±1.1) μg g_dry-weight⁻¹ h⁻¹ was calculated, corresponding to an annual monoterpene emission rate of (24±12) kT yr⁻¹. Research was conducted as part of the AEROBIC’97 (AEROsol formation from BIogenic organic Carbon) series of field campaigns.