33 research outputs found
Deviations from ozone photostationary state during the International Consortium for Atmospheric Research on Transport and Transformation 2004 campaign: Use of measurements and photochemical modeling to assess potential causes
Nitric oxide (NO) and nitrogen dioxide (NO2) were monitored at the University of New Hampshire Atmospheric Observing Station at Thompson Farm (TF) during the ICARTT campaign of summer 2004. Simultaneous measurement of ozone (O3), temperature, and the photolysis rate of NO2 (jNO2) allow for assessment of the O3 photostationary state (Leighton ratio, Ί). Leighton ratios that are significantly greater than unity indicate that peroxy radicals (PO2), halogen monoxides, nitrate radicals, or some unidentified species convert NO to NO2 in excess of the reaction between NO and O3. Deviations from photostationary state occurred regularly at TF (1.0 †Ί †5.9), particularly during times of low NOx (NOx = NO + NO2). Such deviations were not controlled by dynamics, as indicated by regressions between Ί and several meteorological parameters. Correlation with jNO2 was moderate, indicating that sunlight probably controls nonlinear processes that affect Ί values. Formation of PO2 likely is dominated by oxidation of biogenic hydrocarbons, particularly isoprene, the emission of which is driven by photosynthetically active radiation. Halogen atoms are believed to form via photolysis of halogenated methane compounds. Nitrate radicals are believed to be insignificant. Higher Ί values are associated with lower mixing ratios of isoprene and chloroiodomethane and lower ratios of NOx to total active nitrogen, indicating that photochemical aging may very well lead to increased Ί values. PO2 levels calculated using a zeroâdimensional model constrained by measurements from TF can account for 71% of the observed deviations on average. The remainder is assumed to be associated with halogen atoms, most likely iodine, with necessary mixing ratios up to 0.6 or 1.2 pptv, for chlorine and iodine, respectively
Estimates of Cl atom concentrations and hydrocarbon kinetic reactivity in surface air at Appledore Island, Maine (USA), during International Consortium for Atmospheric Research on Transport and Transformation/Chemistry of Halogens at the Isles of Shoals
Average hydroxyl radical (OH) to chlorine atom (Cl·) ratios ranging from 45 to 119 were determined from variabilityâlifetime relationships for selected nonmethane hydrocarbons (NMHC) in surface air from six different transport sectors arriving at Appledore Island, Maine, during July 2004. Multiplying these ratios by an assumed average OH concentration of 2.5 Ă 106 cmâ3 yielded estimates of Cl· concentrations of 2.2 to 5.6 Ă 104 cmâ3. Summed reaction rates of methane and more than 30 abundant NMHCs with OH and Cl· suggest that Cl· reactions increased the kinetic reactivity of hydrocarbons by 16% to 30% over that due to OH alone in air associated with the various transport sectors. Isoprene and other abundant biogenic alkenes were the most important hydrocarbon contributors after methane to overall kinetic reactivity
Carbonyl sulfide exchange in a temperate loblolly pine forest grown under ambient and elevated CO2
Vegetation, soil and ecosystem level carbonyl sulfide (COS) exchange was observed at Duke Forest, a temperate loblolly pine forest, grown under ambient (Ring 1, R1) and elevated (Ring 2, R2) CO2. During calm meteorological conditions, ambient COS mixing ratios at the top of the forest canopy followed a distinct diurnal pattern in both CO2 growth regimes, with maximum COS mixing ratios during the day (R1=380±4 pptv and R2=373±3 pptv, daytime mean ± standard error) and minimums at night (R1=340±6 pptv and R2=346±5 pptv, nighttime mean ± standard error) reflecting a significant nighttime sink. Nocturnal vegetative uptake (â11 to â21 pmol mâ2sâ1, negative values indicate uptake from the atmosphere) dominated nighttime net ecosystem COS flux estimates (â10 to â30 pmol mâ2sâ1) in both CO2 regimes. In comparison, soil uptake (â0.8 to â1.7 pmol mâ2 sâ1) was a minor component of net ecosystem COS flux. In both CO2 regimes, loblolly pine trees exhibited substantial COS consumption overnight (50% of daytime rates) that was independent of CO2 assimilation. This suggests current estimates of the global vegetative COS sink, which assume that COS and CO2 are consumed simultaneously, may need to be reevaluated. Ambient COS mixing ratios, species specific diurnal patterns of stomatal conductance, temperature and canopy position were the major factors influencing the vegetative COS flux at the branch level. While variability in branch level vegetative COS consumption measurements in ambient and enhanced CO2 environments could not be attributed to CO2 enrichment effects, estimates of net ecosystem COS flux based on ambient canopy mixing ratio measurements suggest less nighttime uptake of COS in R2, the CO2 enriched environment
Coastal water source of shortâlived halocarbons in New England
Shortâlived halocarbon tracers were used to investigate marine influences on air quality in a coastal region of New England. Atmospheric measurements made at the University of New Hampshire\u27s Observing Station at Thompson Farm (TF) in Durham, New Hampshire, indicate that relatively large amounts of halocarbons are emitted from local estuarine and coastal oceanic regions. Bromineâcontaining halocarbons of interest in this work include bromoform (CHBr3) and dibromomethane (CH2Br2). The mean mixing ratios of CHBr3 and CH2Br2 from 11 January to 5 March 2002 were 2.6 pptv and 1.6 pptv, and from 1 June to 31 August 2002 mean mixing ratios were 5.9 pptv and 1.4 pptv, respectively. The mean mixing ratio of CHBr3 was not only highest during summer, but both CHBr3 and CH2Br2 exhibited large variability in their atmospheric mixing ratios during this season. We attribute the greater variability to increased production combined with faster atmospheric removal rates. Other seasonal characteristics of CHBr3 and CH2Br2 in the atmosphere, as well as the impact of local meteorology on their distributions at this coastal site, are discussed. Tetrachloroethene (C2Cl4) and trichloroethene (C2HCl3) were used to identify time periods influenced by urban emissions. Additionally, measurements of CHBr3, CH2Br2, C2Cl4, methyl iodide (CH3I), and ethyl iodide (C2H5I) were made at TF and five sites throughout the nearby Great Bay estuarine area between 18 and 19 August 2003. These measurements were used to elucidate the effect of the tidal cycle on the distributions of these gases. The mean mixing ratios of CHBr3, CH2Br2, CH3I, and C2H5I were âŒ82%, 46%, 14%, and 17% higher, respectively, near the coast compared to inland sites, providing evidence for a marine source of shortâlived halocarbons at TF. Correlation between the tidal cycle and atmospheric concentrations of marine tracers on the night of 18 August 2003 showed that the highest values for the brominated species occurred âŒ2â3 hours after high tide. Emission fluxes of CHBr3, CH2Br2, CH3I, and C2H5I on this night were estimated to be 26 ± 57, 4.7 ± 5.4, 5.9 ± 4.6, and 0.065 ± 0.20 nmol mâ2 hâ1, respectively. Finally, the anthropogenic source strength of CHBr3 was calculated to determine its impact on atmospheric levels observed in this region. Although our results indicate that anthropogenic contributions could potentially range from 15 to 60% of the total dissolved CHBr3 in the Great Bay, based on the observed ratio of CH2Br2/CHBr3 and surface seawater measurements in the Gulf of Maine, it appears unlikely that anthropogenic activities are a significant source of CHBr3 in the region
Nanoparticle growth following photochemical αâ and ÎČâpinene oxidation at Appledore Island during International Consortium for Research on Transport and Transformation/Chemistry of Halogens at the Isles of Shoals 2004
Nanoparticle events were observed 48 times in particle size distributions at Appledore Island during the International Consortium for Atmospheric Research on Transport and Transformation/Chemistry of Halogens on the Isles of Shoals (ICARTT/CHAiOS) field campaign from 2 July to 12 August of 2004. Eighteen of the nanoparticle events showed particle growth and occurred during mornings when peaks in mixing ratios of αâ and ÎČâpinene and ozone made production of condensable products from photochemical oxidation probable. Many pollutants and other potential precursors for aerosol formation were also at elevated mixing ratios during these events, including NO, HNO3, NH3, HCl, propane, and several other volatile organic carbon compounds. There were no consistent changes in particle composition, although both submicron and supermicron particles included high maximum concentrations of methane sulfonate, sulfate, iodide, nitrate, and ammonium during these events. Nanoparticle growth continued over several hours with a nearly linear rate of increase of diameter with time. The observed nanoparticle growth rates varied from 3 to 13 nm hâ1. Apparent nanoparticle aerosol mass fractions (yields) were estimated to range from less than 0.0005 to almost 1 using αâ and ÎČâpinene as the presumed particle source. These apparent high aerosol mass fractions (yields) at low changes in aerosol mass are up to two orders of magnitude greater than predictions from extrapolated laboratory parameterizations and may provide a more accurate assessment of secondary organic aerosol formation for estimating the growth of nanoparticles in global models
Controls on atmospheric chloroiodomethane (CH2ClI) in marine environments
Mixing ratios of chloroiodomethane (CH2ClI) in ambient air were quantified in the coastal North Atlantic region (Thompson Farm, Durham, New Hampshire, and Appledore Island, Maine) and two remote Pacific areas (Christmas Island, Kiribati, and Oahu, Hawaii). Average mixing ratios were 0.15 ± 0.18 and 0.68 ± 0.66 parts per trillion by volume (pptv) at Thompson Farm and Appledore Island, respectively, compared to 0.10 ± 0.05 pptv at Christmas Island and 0.04 ± 0.02 pptv in Hawaii. Photolysis constrained the daytime mixing ratios of CH2ClI at all locations with the minimum occurring at 1600 local time. Daily average fluxes to the atmosphere were estimated from mixing ratios and loss due to photolysis at Appledore Island, Christmas Island and Hawaii, and were 58 ± 9, 19 ± 3, and 5.8 ± 1.0 nmol CH2ClI mâ2 dâ1, respectively. The measured seaâtoâair flux from seawater equilibrator samples obtained near Appledore Island was 6.4 ± 2.9 nmol CH2ClI mâ2 dâ1. Mixing ratios of CH2ClI at Appledore Island increased with increasing wind speed. The maximum mixing ratios observed at Thompson Farm (1.6 pptv) and Appledore Island (3.4 pptv) are the highest reported values to date, and coincided with high winds associated with the passage of Tropical Storm Bonnie. We estimate that high winds during the 2004 hurricane season increased the flux of CH2ClI from the North Atlantic Ocean by 8 ± 2%
Characterization of aerosol associated with enhanced small particle number concentrations in a suburban forested environment
Two elevated particle number/mass growth events associated with Aitkenâmode particles were observed during a sampling campaign (13â29 September 2004) at the Duke University FreeâAir CO2 Enrichment facility, a forested field site located in suburban central North Carolina. Aerosol growth rates between 1.2 and 4.9 nm hrâ1 were observed, resulting in net increases in geometric mean diameter of 21 and 37 nm during events. Growth was dominated by addition of oxidized organic compounds. Campaignâaverage aerosol mass concentrations measured by an Aerodyne quadrupole aerosol mass spectrometer (QâAMS) were 1.9 ± 1.6 (Ï), 1.6 ± 1.9, 0.1 ± 0.1, and 0.4 ± 0.4 ÎŒg mâ3 for organic mass (OM), sulfate, nitrate, and ammonium, respectively. These values represent 47%, 40%, 3%, and 10%, respectively, of the measured submicron aerosol mass. Based on QâAMS spectra, OM was apportioned to hydrocarbonâlike organic aerosol (HOA, likely representing primary organic aerosol) and two types of oxidized organic aerosol (OOAâ1 and OOAâ2), which constituted on average 6%, 58%, and 36%, respectively, of the apportioned OM. OOAâ1 probably represents aged, regional secondary organic aerosol (SOA), while OOAâ2 likely reflects less aged SOA. Organic aerosol characteristics associated with the events are compared to the campaign averages. Particularly in one event, the contribution of OOAâ2 to overall OM levels was enhanced, indicating the likelihood of less aged SOA formation. Statistical analyses investigate the relationships between HOA, OOAâ1, OOAâ2, other aerosol components, gasâphase species, and meteorological data during the campaign and individual events. No single variable clearly controls the occurrence of a particle growth event
Bromoform and dibromomethane measurements in the seacoast region of New Hampshire, 2002â2004
Atmospheric measurements of bromoform (CHBr3) and dibromomethane (CH2Br2) were conducted at two sites, Thompson Farm (TF) in Durham, New Hampshire (summer 2002â2004), and Appledore Island (AI), Maine (summer 2004). Elevated mixing ratios of CHBr3 were frequently observed at both sites, with maxima of 37.9 parts per trillion by volume (pptv) and 47.4 pptv for TF and AI, respectively. Average mixing ratios of CHBr3 and CH2Br2 at TF for all three summers ranged from 5.3â6.3 and 1.3â2.3 pptv, respectively. The average mixing ratios of both gases were higher at AI during 2004, consistent with AI\u27s proximity to sources of these bromocarbons. Strong negative vertical gradients in the atmosphere corroborated local sources of these gases at the surface. At AI, CHBr3 and CH2Br2 mixing ratios increased with wind speed via seaâtoâair transfer from supersaturated coastal waters. Large enhancements of CHBr3 and CH2Br2 were observed at both sites from 10 to 14 August 2004, coinciding with the passage of Tropical Storm Bonnie. During this period, fluxes of CHBr3 and CH2Br2 were 52.4 ± 21.0 and 9.1 ± 3.1 nmol mâ2 hâ1, respectively. The average fluxes of CHBr3 and CH2Br2 during nonevent periods were 18.9 ± 12.3 and 2.6 ± 1.9 nmol mâ2 hâ1, respectively. Additionally, CHBr3 and CH2Br2 were used as marine tracers in case studies to (1) evaluate the impact of tropical storms on emissions and distributions of marineâderived gases in the coastal region and (2) characterize the transport of air masses during pollution episodes in the northeastern United States
BIBLE A whole-air sampling as a window on Asian biogeochemistry
Asian trace gas and aerosol emissions into carbon, nitrogen, and other elemental cycles will figure prominently in near term Earth system evolution. Atmospheric hydrocarbon measurements resolve numerous chemical species and can be used to investigate sourcing for key geocarriers. A recent aircraft study of biomass burning and lightning (BIBLE A) explored the East Asian atmosphere and was unique in centering on the Indonesian archipelago. Samples of volatile organics taken over/between the islands of Japan, Saipan, Java, and Borneo are here examined as a guide to whole-air-based studies of future Asian biogeochemistry. The midlatitude onshore/offshore pulse and tropical convection strongly influence concentration distributions. As species of increasing molecular weight are considered, rural, combustion, and industrial source regimes emerge. Methane-rich inputs such as waste treatment and rice cultivation are evidenced in the geostrophic outflow. The Indonesian atmosphere is rich in biomass burning markers and also those of vehicular activity. Complexity of air chemistry in the archipelago is a direct reflection of diverse topography, land use, and local economies in a rapidly developing nation. Conspicuous in its absence is the fingerprint for liquefied petroleum gas leakage, but it can be expected to appear as demand for clean fossil fuels rises along with per capita incomes. Combustion tracers indicate high nitrogen mobilization rates, linking regional terrestrial geocycles with open marine ecosystems. Sea to air fluxes are superimposed on continental and marine backgrounds for the methyl halides. However, ocean hot spots are not coordinated and suggest an intricate subsurface kinetics. Levels of long-lived anthropogenic halocarbons attest to the success of international environmental treaties while reactive chlorine containing species track industrial air masses. The dozens of hydrocarbons resolvable by gas chromatographic methods will enable monitoring of upcoming Asian modernization. Crucial uncertainties are underscored. Signatures for Asian combustion processes and megacities have been obtained only indirectly or at a distance. Detailed fingerprinting must be combined with regular aircraft and ground station measurements to maximize utility of the database
Are biogenic emissions a significant source of summertime atmospheric toluene in the rural Northeastern United States?
Summertime atmospheric toluene enhancements at Thompson Farm in the rural northeastern United States were unexpected and resulted in a toluene/benzene seasonal pattern that was distinctly different from that of other anthropogenic volatile organic compounds. Consequently, three hydrocarbon sources were investigated for potential contributions to the enhancements during 2004â2006. These included: (1) increased warm season fuel evaporation coupled with changes in reformulated gasoline (RFG) content to meet US EPA summertime volatility standards, (2) local industrial emissions and (3) local vegetative emissions. The contribution of fuel evaporation emission to summer toluene mixing ratios was estimated to range from 16 to 30 pptv dâ1, and did not fully account for the observed enhancements (20â50 pptv) in 2004â2006. Static chamber measurements of alfalfa, a crop at Thompson Farm, and dynamic branch enclosure measurements of loblolly pine trees in North Carolina suggested vegetative emissions of 5 and 12 pptv dâ1 for crops and coniferous trees, respectively. Toluene emission rates from alfalfa are potentially much larger as these plants were only sampled at the end of the growing season. Measured biogenic fluxes were on the same order of magnitude as the influence from gasoline evaporation and industrial sources (regional industrial emissions estimated at 7 pptv dâ1 and indicated that local vegetative emissions make a significant contribution to summertime toluene enhancements. Additional studies are needed to characterize the variability and factors controlling toluene emissions from alfalfa and other vegetation types throughout the growing season