Atmospheric Environment (v.36, #20)

Non-methane hydrocarbons in the Arctic boundary layer by J.R Hopkins; I.D Jones; A.C Lewis; J.B McQuaid; P.W Seakins (3217-3229).
C2–C7 non-methane hydrocarbons were measured in clean maritime boundary layer air at latitudes between 53°N and 81°N. Measurements were made as part of the Arctic ice and environmental variability cruise aboard RRS James Clark Ross using a high sensitivity automated gas chromatography system. The data were collected during summer 1999, a period of continuous Arctic sunlight. Hydrocarbons of anthropogenic origin were observed to decrease in concentration with increasing latitude, a combination of dispersion and extensive atmospheric chemical processing. At high latitudes, low boundary layer conditions were common and species of exclusively anthropogenic origin reached highly stable although non-zero values (e.g. acetylene 27.8±2.4 pptV). A number of hydrocarbons believed to be of oceanic origin showed wide variability in these regions of atmospheric boundary layer stability (average ethene=37.2±20.9 pptV), highlighting inhomogeneity in the ocean to atmosphere flux rates. Whilst substantial increases in biological activity and productivity in the marginal ice zone have been previously reported, no evidence for increased biogenic hydrocarbon emissions were observed during this cruise. Removal mechanisms for atmospheric hydrocarbons were generally dominated by OH chemistry. Under clean maritime conditions the observed iso-/n pentane and butane ratios were in good agreement with published sea-fluxes. Using the relative ratios of iso-/n-butane and iso-butane/propane, there was evidence of significant chlorine atom chemistry at these high latitudes.
Keywords: Non-methane hydrocarbons; Marine atmosphere; Long range transport; Biogenic emssions;

A PLP–LIF kinetic study of the atmospheric reactivity of a series of C4–C7 saturated and unsaturated aliphatic aldehydes with OH by José Albaladejo; Bernabé Ballesteros; Elena Jiménez; Pilar Martı́n; Ernesto Martı́nez (3231-3239).
Absolute rate coefficients for the reaction of OH radical with a series of saturated and unsaturated aliphatic aldehydes were measured with the pulsed laser photolysis/laser-induced fluorescence technique at room temperature and as a function of total pressure (p T=100–400 Torr). No pressure dependence of the rate coefficients was observed. The weighted average values obtained, k OH±2σ, in units of 10−11  cm3  molecule−1  s−1, were 2.88±0.26 for n-butanal, 2.48±0.24 for n-pentanal, 2.60±0.21 for n-hexanal, 2.96±0.23 for n-heptanal, 3.51±0.71 for crotonaldehyde, 2.35±0.32 for trans-2-pentenal, 2.95±0.45 for trans-2-hexenal and 2.45±0.30 for trans-2-heptenal, respectively. The results are compared with previous data when available and with the corresponding coefficients for the reactions with NO3 and O3. The dominant tropospheric chemical loss process for these aliphatic aldehydes is the daytime reaction with OH, except in the case of trans-2-heptenal where the estimated lifetime for the reaction with NO3 radical is smaller than the corresponding value for the OH reaction.
Keywords: Aliphatic aldehydes; Hydroxyl radical; Kinetic study; Gas-phase reactions; Laser photolysis;

Investigation of the light-enhanced emission of mercury from naturally enriched substrates by Mae Sexauer Gustin; Harald Biester; Christopher S Kim (3241-3254).
Incident radiation has been reported to facilitate mercury release from soils. In this study the influence of light on mercury emissions from substrates amended with pure synthetic mercury species, and from naturally and anthropogenically mercury-enriched substrates were investigated using laboratory experiments and in situ flux measurements. Light-enhanced emissions were found to occur from substrates amended with HgS, and from elemental mercury (Hg0) and HgCl2 amended iron oxide and organic containing substrates. The magnitude of light-enhanced emissions for natural substrates ranged from 1.5 to 116 times that occurring in the dark at the same substrate temperature. Substrates containing corderoite, metacinnabar and “matrix bound mercury” (that bound to organic or inorganic phases) exhibited a higher degree of light-enhanced emissions relative to that containing predominantly cinnabar. Calculated activation energies for both laboratory and field data indicate that photo-reduction is a process associated with the light-enhanced emissions. Activation energies, derived using in situ mercury fluxes and soil temperatures, indicated that photo-reduction was a dominant process facilitating release of Hg from substrates with sunrise. Activation energies, calculated using daytime data, were less than those calculated for sunrise. This is hypothesized to be due to a pool of Hg0 being developed with photo-reduction at first light that is released as soil temperatures and convective heat transfer increase during the day. This study demonstrated that light energy is the more dominant process controlling mercury emissions from naturally enriched substrates than soil temperature.

Spatial and temporal variability in outdoor, indoor, and personal PM2.5 exposure by J.L Adgate; G Ramachandran; G.C Pratt; L.A Waller; K Sexton (3255-3265).
Outdoor, indoor and personal PM2.5 measurements were made in a population of nonsmoking adults from three communities in the Minneapolis–St. Paul metropolitan area between April and November 1999. Thirty-two healthy adult subjects (23 females, 9 males; mean age 42±10, range: 24–64 yr) were monitored for 2–15 days during the spring, summer, and fall monitoring seasons. Twenty-four hour average gravimetric PM2.5 samples were collected using a federal reference monitor (Anderson RAAS2.5-300) located at outdoor (O) central sites in the Battle Creek (BCK), East St. Paul (ESP) and Phillips (PHI) communities. Concurrent 24-h average indoor (I) and personal (P), and a limited number of outdoor-at-home (O@H) samples were collected using inertial impactors (PEM™ Model 200, MSP, Inc). The O (geometric mean {GM}=8.6; n=271; range: 1.0–41 μg/m3) were lower than I concentrations (GM=10.7; n=294; range 1.3–131 μg/m3), which were lower than P concentrations (GM=19.0; n=332; range 2.2–298 μg/m3). Correlation coefficients between O concentrations in the three communities were high and measured GM O levels in BCK were significantly lower than ESP, most likely because of local sources, but GM concentrations in PHI were not significantly different from BCK or ESP. On days with paired samples (n=29), O concentrations were significantly lower (mean difference 2.9 μg/m3; p=0.026) than O@H measurements (GM=11.3; range: 3.5–33.8 μg/m3), likely due to local sources in communities. Observed I and P concentrations were more variable, probably because of residential central air conditioning and hours of household ventilation for I and P, and occupational and environmental tobacco smoke exposures outside the residence for P. Across all individuals and days the median PM2.5 “personal cloud” was 5.7 μg/m3, but the mean of the average for each participant was 15.7 μg/m3, with very low values in participants who did not work outside the home and much higher values in subjects with active lifestyles. Across all households and individuals the correlation between P and O concentrations was not significant, but the overall I–O correlation (0.27) and P–I correlation (0.51) were significant (p<0.05). Relatively little spatial variability was observed in O PM2.5 concentrations across the three communities compared to the variability associated with I and P samples, and the measured O levels were relatively low compared to other large metropolitan areas in the United States.
Keywords: Particulate matter; Urban; Monitoring; Season; Human exposure;

Dry deposition measured with a water surface sampler: a comparison to modeled results by Usama M. Shahin; Thomas M. Holsen; Mustafa Odabasi (3267-3276).
In this study, a water surface sampler (WSS) and a knife-leading-edge surrogate surface (KSS) covered with both a Nylasorb filter, and a greased strip were used to directly measure nitrate, sulfate, and ammonium dry fluxes in Chicago between May and October 1997. Concurrently, an annular denuder system was used to measure the ambient concentration of gaseous nitric acid, sulfur dioxide, and ammonia gases. Water evaporation and relative humidity were also simultaneously measured. The measured fluxes and ambient concentrations were used to calculate the mass transfer coefficients of all four species.The measured MTCs of water vapor, ammonia gas, nitric acid and sulfur dioxide were normalized to the square root of their diffusion coefficient and correlated to the wind speed at 10 m above the WSS. The best-fit model was k A =D A 0.5[(0.98±0.1)u10+(1.26±0.3)], where k A is the air side mass transfer coefficient (cm s−1), D A is the air side diffusion coefficient (cm2  s−1), u 10 is the wind speed 10 m above the WSS (m s−1), and the ± is the 95% confidence interval.Field measured MTCs of water vapor, ammonia gas, nitric acid, and sulfur dioxide were used to examine the applicability of two models, the resistance model and the water evaporation model. Results showed that the resistance model predictions were statistically the same as the measured MTCs to the surrogate surfaces while the predictions of the water evaporation model was statistically smaller than the measured values.
Keywords: Dry deposition; Nitric acid; Ammonia; Water surface sampler; Mass transfer coefficient;

Urban areas emit significant amounts of pollutants that impact forest ecosystems. One of the most important of these is nitric acid vapor (HNO3), a nitrogen-containing gas that deposits efficiently to forest canopies. Since measuring HNO3 fluxes directly is often impractical and costly in remote forest locales, inferential techniques are most often used to estimate HNO3 flux. Given the highly efficient deposition of HNO3, many of these inferential models assume that leaf surfaces are a ‘perfect sink’ for HNO3 (i.e., that resistance to HNO3 deposition is negligibly small or zero). This study tests the ‘perfect sink’ assumption in an open gas exchange system by exposing Abies magnifica, Abies concolor, and Pinus jeffreyi seedlings to concentrations of 1–13 ppb at 4–20% relative humidity. We find that, at these humidities and concentrations, cuticles are not perfect sinks for HNO3, with cuticular resistance values ranging from 20 to 184 s m−1. In addition, our results indicate that accumulating HNO3 on leaf cuticles at these concentrations leads to higher cuticular resistance over 8–12 h exposure periods. Based on this laboratory data, we then parameterized cuticular resistance using a single-layer inferential model for semi-arid forests in the Lake Tahoe Basin. Modeled fluxes using this modification were 33% lower during well-mixed daytime conditions than the fluxes from an identical model run using the perfect sink assumption. Since HNO3 can often account for more than half of atmospheric deposition, we conclude that inferential models that assume foliage to be perfect HNO3 sinks are inaccurate, especially in semi-arid forests where significant amounts of HNO3 can accumulate on leaf surfaces during dry periods.
Keywords: Dry deposition; Surface resistance; Numerical model; Conifer gas exchange; Lake Tahoe Basin;

Field performance of dichotomous sequential PM air samplers by N Poor; T Clark; L Nye; T Tamanini; K Tate; R Stevens; T Atkeson (3289-3298).
For over one year, the Environmental Protection Commission of Hillsborough County (EPCHC) in Tampa, Florida, operated two dichotomous sequential particulate matter air samplers collocated with a manual Federal Reference Method (FRM) air sampler at a waterfront site on Tampa Bay. The FRM was alternately configured as a PM2.5, then as a PM10 sampler. For the dichotomous sampler measurements, daily 24-h integrated PM2.5 and PM10–2.5 ambient air samples were collected at a total flow rate of 16.7 l min−1. A virtual impactor split the air into flow rates of 1.67 and 15.0 l min−1 onto PM10–2.5 and PM2.5 47-mm diameter PTFE® filters, respectively. Between the two dichotomous air samplers, the average concentration, relative bias and relative precision were 13.3 μg m−3, 0.02% and 5.2% for PM2.5 concentrations (n=282), and 12.3 μg m−3, 3.9% and 7.7% for PM10–2.5 concentrations (n=282). FRM measurements were alternate day 24-h integrated PM2.5 or PM10 ambient air samples collected onto 47-mm diameter PTFE® filters at a flow rate of 16.7 l min−1. Between a dichotomous and a PM2.5 FRM air sampler, the average concentration, relative bias and relative precision were 12.4 μg m−3, −5.6% and 8.2% (n=43); and between a dichotomous and a PM10 FRM air sampler, the average concentration, relative bias and relative precision were 25.7 μg m−3, −4.0% and 5.8% (n=102). The PM2.5 concentration measurement standard errors were 0.95, 0.79 and 1.02 μg m−3; for PM10 the standard errors were 1.06, 1.59, and 1.70 μg m−3 for two dichotomous and one FRM samplers, respectively, which indicate the dichotomous samplers have superior technical merit. These results reveal the potential for the dichotomous sequential air sampler to replace the combination of the PM2.5 and PM10 FRM air samplers, offering the capability of making simultaneous, self-consistent determinations of these particulate matter fractions in a routine ambient monitoring mode.
Keywords: Relative standard deviation; Relative precision; PM2.5; PM10; Technical merit;

A study of the gas-phase ozonolysis of terpenes: the impact of radicals formed during the reaction by Jerker Fick; Linda Pommer; Barbro Andersson; Calle Nilsson (3299-3308).
The gas-phase ozonolysis of α-pinene, Δ 3-carene and limonene was investigated at ppb levels and the impact of the ozone, relative air humidity (RH), and time was studied using experimental design. The amounts of terpene reacted varied in the different settings and were as high as 8.1% for α-pinene, 10.9% for Δ 3-carene and 23.4% for limonene. The designs were able to describe almost all the variation in the experimental data and were also successful in predicting omitted values. The results described the effects of time and ozone and also showed that RH did not have a statistically significant effect on the ozonolysis. The results also showed that all three terpenes were affected by an additional oxidation of OH radicals and/or other reactive species. The results from the designs states that this additional oxidation was responsible for 40% of the total amount of α-pinene reacted, 33% of the total amount of Δ 3-carene reacted and 41% of the total amount of limonene reacted at the settings 20 ppb terpene, 75 ppb ozone, 20% RH and a reaction time of 213 s. Additional experiments with 2-butanol as OH radical scavenger showed that the reaction with OH radicals was responsible for 37% of the total α-pinene reacted and 39% of the total Δ 3-carene reacted at the same settings. The scavenger experiments also showed that there were no significant amounts of OH radicals formed during the ozonolysis of limonene. The results from the designs were also compared to a mathematical model in order to evaluate further the data.
Keywords: Monoterpenes; OH radicals; Relative humidity; Criegee intermediate; Experimental design;

Ammonia volatilization from field-applied animal slurry—the ALFAM model by H.T Søgaard; S.G Sommer; N.J Hutchings; J.F.M Huijsmans; D.W Bussink; F Nicholson (3309-3319).
Keywords: Livestock slurry; Volatilization; Statistical analysis; European database; Agriculture;

Effects of temperature and land use on predictions of biogenic emissions in Eastern Texas, USA by Will Vizuete; Victoria Junquera; Elena McDonald-Buller; Gary McGaughey; Greg Yarwood; David Allen (3321-3337).
Accurate estimates of biogenic volatile organic compound emissions are critical for air quality planning in areas such as Eastern Texas where biogenic emissions comprise a significant fraction of the total volatile organic compound inventory. Uncertainties in biogenic volatile organic chemical emission estimates associated with different land use databases, surface temperature databases, and temperature interpolation methods were quantified and compared. The sensitivity of isoprene emissions to land use classification was investigated by comparing predictions based on land use data recently compiled for Eastern Texas to those based on the Biogenic Emissions Landcover Database version 3.1 (BELD3). Previous studies have only made these comparisons with the previous BELD version 2 database. Isoprene emission increased throughout much of Eastern Texas because areas classified as agricultural or savannah in BELD3 were more accurately classified as Post Oak, Live Oak, mesquite, and juniper in the new database. These results indicate the need for land use studies in areas poorly characterized in the BELD3. The sensitivity of isoprene emission estimates to uncertainties in surface temperatures were investigated by comparing predictions based on two different temperature databases and three different interpolation techniques. Spatial interpolations of surface temperatures collected at available Automated Surface Observing System (ASOS) stations in Houston, Austin, and Dallas were similar to the spatial interpolations of surface temperatures obtained from the ETA Data Assimilation System (EDAS). As a result, substantial variations in isoprene emissions were not observed over the majority of the modeling domain; however, differences of 4 F over localized regions produced a 35% difference in isoprene emissions. Comparisons between the isoprene emissions of the three interpolation methods sometimes revealed large variations, with maximum temperature differences of 4 F resulting in 60% differences in isoprene emissions in areas with the highest isoprene emissions. It was noted that the ASOS stations were clustered in urban areas and not in areas with the highest biogenic emissions. More ambient temperature monitors need to be located in rural locations to provide robust estimates of biogenic emissions and facilitate validation of interpolated temperature fields.
Keywords: Biogenic emissions; Ozone formation; GloBEIS; Isoprene; Land cover; EDAS; ASOS;

Following a request by environmental authorities in New Zealand, atmospheric modelling has been applied to delimitation of clean air zones for urban areas. This approach involved the integration of a kinematic trajectory model with an atmospheric mesoscale model to identify the spatial extent of the catchment of air affecting air pollution concentrations in Christchurch on nights of high air pollution. The Regional Atmospheric Modelling System (RAMS) was configured for the region and idealised simulations performed to obtain predicted wind fields for synoptic situations typical of winter smog events. The predicted surface wind fields on two grids, with horizontal resolutions of 1.5 and 0.5 km, respectively, were then used to derive back-trajectories from the late evening peak of pollution over the central city (around 2200 NZST) to the time at which people tend to first light their domestic fires (around 1800 NZST). The results indicate that although winds are often light, the air tends to travel from a significant distance outside the city boundary over this time period. In particular, cold air typically travels up to 20 km from the Canterbury Plains to the west into the city during these air pollution events, as well as from small valleys in the Port Hills to the south of the city. This research illustrates the significance of upstream sources of air for providing relatively clean air to the city, and acting as buffer zones. It is, therefore, possible to identify the area around the city to which urban air quality is particularly sensitive. This area could either be designated as a buffer zone, or included within the clean air zone of the city. This technique also provides a useful tool for identifying the role of different components of the local wind field responsible for air pollution dispersion and transport in different parts of the area.
Keywords: Clean air zone; Airflow modelling; Air pollution dispersion; Kinematic back-trajectory analysis; Air quality management;