Nitrogen oxide fluxes between corn (Zea mays L.) leaves and the atmosphere

In the United States, fertilized corn fields, which make up approximately 5% of the total land area, account for approximately 45% of total soil NO_x emissions. Leaf chamber measurements were conducted of NO and NO₂ fluxes between individual corn leaves and the atmosphere in (1) field-grown plants near Champaign, IL (USA) in order to assess the potential role of corn canopies in mitigating soil–NO_x emissions to the atmosphere, and (2) greenhouse-grown plants in order to study the influence of various environmental variables and physiological factors on the dynamics of NO₂ flux. In field-grown plants, fluxes of NO were small and inconsistent from plant to plant. At ambient NO concentrations between 0.1 and 0.3 ppbv, average fluxes were zero. At ambient NO concentrations above 1 ppbv, NO uptake occurred, but fluxes were so small (14.3±0.0 pmol m⁻² s⁻¹) as to be insignificant in the NO_x inventory for this site. In field-grown plants, NO₂ was emitted to the atmosphere at ambient NO₂ concentrations below 0.9 ppbv (the NO₂ compensation point), with the highest rate of emission being 50 pmol m⁻² s⁻¹ at 0.2 ppbv. NO₂ was assimilated by corn leaves at ambient NO₂ concentrations above 0.9 ppbv, with the maximum observed uptake rate being 643 pmol m⁻² s⁻¹ at 6 ppbv. When fluxes above 0.9 ppbv are standardized for ambient NO₂ concentration, the resultant deposition velocity was 1.2±0.1 mm s⁻¹. When scaled to the entire corn canopy, NO₂ uptake rates can be estimated to be as much as 27% of the soil-emitted NO_x. In greenhouse-grown and field-grown leaves, NO₂ deposition velocity was dependent on incident photosynthetic photon flux density (PPFD; 400–700 nm), whether measured above or below the NO₂ compensation point. The shape of the PPFD dependence, and its response to ambient humidity in an experiment with greenhouse-grown plants, led to the conclusion that stomatal conductance is a primary determinant of the PPFD response. However, in field-grown leaves, measured NO₂ deposition velocities were always lower than those predicted by a model solely dependent on stomatal conductance. It is concluded that NO₂ uptake rate is highest when N availability is highest, not when the leaf deficit for N is highest. It is also concluded that the primary limitations to leaf-level NO₂ uptake concern both stomatal and mesophyll components.     

FT-IR product study of the OH-initiated oxidation of DMS in the presence of NOx

The influence of NO_x (NO+NO₂) concentrations on the product distribution of the OH-initiated oxidation of DMS has been studied at room temperature using total NO_x concentrations varying from 0 to ∼1800 ppbv (30–600 ppbv NO₂ and 140–1760 ppbv NO). Clear trends in the formation yields of the products SO₂, COS, MSA, MTF (methyl thiolformate), MSPN (methanesulphonyl peroxynitrate), DMSO and DMSO₂ have been observed with variation in NO_x. The presence of low levels of NO reduces the yields of both MTF and COS to zero. The formation yields of MSA and DMSO₂ increase with increasing NO_x concentration, whereas the yields of DMSO and SO₂ decrease. The following approximate changes in the yield, not corrected for possible loss processes, have been measured for variation of NO_x between 0 and ∼1800 ppbv: DMSO decreases from 20 to 3%S; DMSO₂ increases from 3 to 15%S, SO₂ decreases from 70 to 30%S and MSA increases from 4 to 17%S. Under the experiments conditions NO_x levels of several tens of ppbv are required before a perceptible change is observed in the MSA yield. If applicable to the atmosphere such a situation is only likely to be observed near coastal areas affected by pollution. MSPN (CH₃SO₂O₂NO₂) is observed as an oxidation product in the presence of NO₂ even at low levels (e.g. 60 ppbv). Its possible role as a NO_x reservoir in the troposphere is considered.     

Formation of particulate sulfur species (sulfate and methanesulfonate) during summer over the Eastern Mediterranean: A modelling approach

To improve our understanding of the mechanisms of particulate sulfur formation (non sea-salt sulfate, nss-SO₄²⁻) and methanesulfonate (MS_x used here to represent the sum of gaseous methanesulfonic acid, MSA, and particulate methanesulfonate, MS⁻) in the eastern Mediterranean and to evaluate the relative contribution of biogenic and anthropogenic sources to the S budget, a chemical box model coupled offline with an aerosol–cloud model has been used.Based on the measurements of gaseous dimethyl sulfide (DMS) and methanesulfonic acid (MSA) and the MSA sticking coefficient determined during the Mediterranean Intensive Oxidant Study (MINOS) experiment, the yield of gaseous MSA from the OH-initiated oxidation of DMS was calculated to be about 0.3%. Consequently, MSA production from gas-phase oxidation of DMS is too small to explain the observed levels of MS⁻. On the other hand, heterogeneous reactions of dimethyl sulfoxide (DMSO) and its gas-phase oxidation product methanesulfinic acid (MSIA) can account for most of the observed MS⁻ levels. The modelling results indicate that about 80% of the production of MS⁻ can be attributed to heterogeneous reactions.Observed submicron nss-SO₄²⁻ levels can be fully explained by homogeneous (photochemical) gas-phase oxidation of sulfur dioxide (SO₂) to sulfuric acid (H₂SO₄), which is subsequently scavenged by (mainly submicron) aerosol particles. The predominant oxidant during daytime is hydroxyl radical (OH) showing very high peak levels in the area during summer mostly under cloudless conditions. Therefore, during summer in the east Mediterranean, heterogeneous sulfate production appears to be negligible. This result is of particular interest for sulfur abatement strategy. On the other hand only about 10% of the supermicron nss-SO₄²⁻ can be explained by condensation of gas-phase H₂SO₄, the rest must be formed via heterogeneous pathways.Marine biogenic sulfur emissions contribute up to 20% to the total oxidized sulfur production (SO₂ and H₂SO₄) in good agreement with earlier estimates for the area.     

Examination of the impact of photoexcited NO2 chemistry on regional air quality

Impact of the excited nitrogen dioxide (NO₂¹) chemistry on air quality in the U.S. is examined using the Community Multiscale Air Quality (CMAQ) model for a summer month. Model simulations were conducted with and without the NO₂¹ chemistry. The largest impact of the NO₂¹ chemistry in the eastern U.S. occurred in the northeast and in the western U.S. occurred in Los Angeles. While the single largest daily maximum 8-h ozone (O₃) increased by 9 ppbv in eastern U.S. and 6 ppbv in western U.S., increases on most days were much lower. No appreciable change in model performance statistics for surface-level O₃ predictions relative to measurements is noted between simulations with and without the NO₂¹ chemistry. Based on model calculations using current estimates of tropospheric emission burden, the NO₂¹ chemistry can increase the monthly mean daytime hydroxyl radicals (OH) and nitrous acid (HONO) by a maximum of 28% and 100 pptv, respectively.     

Solubility of ammonia in liquid water and generation of trace levels of standard gaseous ammonia

Equilibrium gas phase concentration of ammonia in dilute solution has been measured as a function of total ammonia + ammonium concentration (0.002–0.10 M), pH (6–10) and temperature (278.8−290.6 K). Henry's Law is obeyed under these conditions and may be expressed as In K_H(M atm⁻¹) = 4092/T −9.70 with a relative standard error of less than 5 %, in good agreement with NBS thermodynamic data. Convenient generation of trace levels of ammonia (1.33 × 10⁻⁸–7.77 × 10⁻⁴ atm) using a porous membrane tube is described.     

Measurements of the aerosol concentrations of methanesulphonic acid,dimethyl sulphoxide and dimethyl sulphone in the marine atmosphere of the British Isles

Aerosol concentrations of methanesulphonic acid (MSA), dimethyl sulphoxide (DMSO) and dimethyl sulphone (DMSO₂) have been measured from landbased stations at Plymouth (Devon, U.K.), Galway (EIRE), and from various shipboard stations in the North Sea and the North Atlantic Ocean. MSA, DMSO and DMSO₂ all show seasonal cycles with spring/summer maxima and winter minima. The summer concentrations of MSA are approximately an order of magnitude higher than in winter. The general levels of MSA (July 1985 mean = 9.27 × 10⁻⁹ mol m⁻³, December 1986 mean = 1.14 × 10⁻⁹ mol m⁻³) are comparable to those reported from Cape Grim, Tasmania. Modelling indicates that neither MSA nor DMSO₂ are present in sufficient quantity to represent major oxidation pathways for dimethyl sulphide (DMS). Rate constant ratios for both the reactions of DMS and DMSO with OH and IO have been estimated. Hydroxyl radical does not appear to be reactive enough for it to be the major sink of atmospheric DMS. It is also shown that the rate constants for the destruction of DMSO (the main reaction product of the DMS/IO system) with either IO or OH are likely to be slow. Thus low tropospheric concentrations of DMSO tend to indicate that it also is not a major product of DMS oxidation.     

Nitric acid-air diffusion coefficient: Experimental determination

Trace gaseous HNO₃ in air is removed in a laminar flow nylon tube. The HNO₃ deposition pattern was obtained by sectioning the tube, extracting with an aqueous solution, and measuring the concentration by ion chromatography. Mass transport analysis of the deposition pattern demonstrated the HNO₃ was removed from the air stream at a rate controlled by gaseous diffusion. The HNO₃-air diffusion coefficient = 0.118 ± 0.003 cm² s⁻¹ (n = 7) for T = 298 K and P = 1 atm. It exhibited no dependence on relative humidity over the range 5–95 %.     

C2–C8 NMHCs over the Eastern Mediterranean: Seasonal variation and impact on regional oxidation chemistry

More than 2500 measurements of C₂–C₈ non-methane hydrocarbons (NMHCs) have been conducted at Finokalia sampling station on the island of Crete over a thirty-month period (September 2003–February 2006), to investigate the factors controlling NMHC levels and estimate their role in the oxidizing capacity of the Eastern Mediterranean atmosphere. Atmospheric concentrations of NMHCs range from below the detection limit (5 pptv) to a few ppbv and present a hydroxyl radical (OH) driven seasonal pattern with lower values during summer. The diel variability was also influenced by the reaction of the NMHC with the OH radical, exhibiting a nighttime maximum and a midday or early afternoon minimum. Long-lived compounds demonstrate higher concentrations under the influence of the northern sector (European continent), indicating that besides chemistry, transport significantly contributes to NMHCs levels in the area. Based on the observed NMHCs diurnal cycles, mean OH radical levels of 3.5 × 10⁶ molecules cm⁻³ have been derived for May–October period.     

A synergism between ultraviolet and gamma radiation in producing aerosol particles from SO2H2SO4 laden atmospheres

Experiments were performed to compare the capability of u.v. light, γ-radiation and simultaneous u.v. and γ-radiation to produce aerosol particles in a mixture of SO₂, NO₂ and synthetic air at different humidities. In the presence of u.v. radiation, γ-radiation was found to enhance the particle production at dose rates as low as 10⁻⁴ μGy s⁻¹ at 0% r.h., and 2 × 10⁻² μGy s⁻¹ at 75% r.h. γ-radiation alone did not produce aerosol particles at the same dose rates. The possible processes underlying this synergistic effect are discussed.     

Organic trace gas composition of the marine boundary layer over the northwest Indian Ocean in April 2000

In April 2000 atmospheric trace gas measurements were performed on the western Indian Ocean on a cruise of the Dutch research vessel Pelagia from the Seychelles (5°S, 55°E) to Djibouti (12°N, 43°E). The measurements included analysis of dimethyl sulfide (DMS), acetone and acetonitrile every 40 s using PTR-MS (proton-transfer-reaction mass spectrometry) and gas chromatographic analyses of C₂–C₇ hydrocarbons in air samples taken during the cruise. The measurements took place at the end of the winter monsoon season and the sampled air masses came predominantly from the Southern Hemisphere, resulting in low concentrations of some long-lived hydrocarbons, halocarbons, acetone (350 pptv) and acetonitrile (120 pptv). On three consecutive days a diurnal cycle in DMS concentration was observed, which was used to estimate the emission of DMS (1.5±0.7×10¹³ molecules m⁻² s⁻¹) and the 24 h averaged concentration of hydroxyl (OH) radicals (1.4±0.7×10⁶ molecules cm⁻³). A strongly increased DMS concentration was found at a location where upwelling of deeper ocean waters took place, coinciding with a marked decrease in acetone and acetonitrile. In the northwestern Indian Ocean a slight increase of some trace gases was noticed showing a small influence of pollution from Asia and from northeast Africa as indicated with back trajectory calculations. The air masses from Asia had elevated acetonitrile concentrations showing some influence of biomass burning as was also found during the 1999 Indian Ocean Experiment, whereas the air masses from northeast Africa seemed to have other sources of pollution.     

Isoprene above the Eastern Mediterranean: Seasonal variation and contribution to the oxidation capacity of the atmosphere

Isoprene is one of the most important biogenic volatile organic compounds with large terrestrial emissions and comparatively a small oceanic source on a global scale. This marine source seems to strongly depend on environmental parameters such as phytoplankton abundance, light, temperature, wind speed, and thus, to be highly variable. However, this source can consequently affect the chemistry of the marine boundary layer on a local or mesoscale. The present study investigates the factors that control isoprene levels and estimates the marine source of isoprene and its role in the oxidizing capacity of the atmosphere at a coastal site in the East Mediterranean. More than 2000 measurements of isoprene have been conducted at Finokalia sampling station on the island of Crete over an 8-month period from February to October 2004. Isoprene varies between 5 and 1200 pptv with the highest values observed in summer. The origin of the air masses determines the atmospheric abundance and the prevailing source of isoprene. According to chemical box model calculations, during daytime the isoprene observed under marine conditions is reducing hydroxyl (OH) and hydroperoxy (HO₂) radicals by up to 26% and 13%, respectively, whereas, it can increase the sum of peroxy radicals by a factor of 4. At night, isoprene of marine origin is depressing nitrate radicals by up to 25% and increases the low nighttime levels of OH and HO₂ radicals by up to 25% and 30%, respectively. The seawater emissions of isoprene in the area are estimated between 10⁸ and 6×10⁹ molecules cm⁻² s⁻¹ with a strong seasonal variability.     

A calibrated source of N2O5

A calibrated source of N₂O₅ was developed for use in evaluating instruments designed to measure reactive odd-nitrogen species in the atmosphere. The desirable features of the source are the output of a very low mixing ratio of N₂O₅, the measurement of total reactive nitrogen NO_y in the source output, and the use of commercially-available detection and calibration equipment. N₂O₅ was derived from a solid crystalline sample at 192 K. Mixing ratios of N₂O₅ less than 2 ppmv (parts per million by volume) were produced in a small flow ( < 50 STPcc min⁻¹) of He carrier gas. This small flow was then diluted to obtain mixing ratios less than 50 ppbv (parts per billion by volume) in flows greater than 1 STPℓ min⁻¹. The N₂O₅ mixing ratio was determined with an accuracy of ± 15 % by using a thermal dissociator, calibrated NO/N₂ mixtures, and an NO chemiluminescence detector. The level of other reactive nitrogen species in the source output was determined with an Au catalytic converter and the NO detector.     

C1–C8 volatile organic compounds in the atmosphere of Hong Kong: Overview of atmospheric processing and source apportionment

We present measurements of C₁–C₈ volatile organic compounds (VOCs) at four sites ranging from urban to rural areas in Hong Kong from September 2002 to August 2003. A total of 248 ambient VOC samples were collected. As expected, the urban and sub-urban sites generally gave relatively high VOC levels. In contrast, the average VOC levels were the lowest in the rural area. In general, higher mixing ratios were observed during winter/spring and lower levels during summer/fall because of seasonal variations of meteorological conditions. A variation of the air mass composition from urban to rural sites was observed. High ratios of ethyne/CO (5.6 pptv/ppbv) and propane/ethane (0.50 pptv/pptv) at the rural site suggested that the air masses over the territory were relatively fresh as compared to other remote regions. The principal component analysis (PCA) with absolute principal component scores (APCS) technique was applied to the VOC data in order to identify and quantify pollution sources at different sites. These results indicated that vehicular emissions made a significant contribution to ambient non-methane VOCs (NMVOCs) levels in urban areas (65±36%) and in sub-urban areas (50±28% and 53±41%). Other sources such as petrol evaporation, industrial emissions and solvent usage also played important roles in the VOC emissions. At the rural site, almost half of the measured total NMVOCs were due to combustion sources (vehicular and/or biomass/biofuel burning). Petrol evaporation, solvent usage, industrial and biogenic emissions also contributed to the atmospheric NMVOCs. The source apportionment results revealed a strong impact of anthropogenic VOCs to the atmosphere of Hong Kong in both urban/sub-urban and rural areas.     

Atmospheric gas and particle measurements at a rural northeastern U.S. site

Ammonia, nitric acid, sulfur dioxide and particles in two size ranges were collected at a rural site in northeastern U.S. in January–March 1984. Ammonia was collected with an oxalic acid coated denuder, all other components were collected on filters. The concentrations of ammonia ranged between 0.0 and 0.5 ppbv, nitric acid: 0.1 and 2.3 ppbv and sulfur dioxide: 1 and 52 ppbv. Ammonium and sulfate in the fine particles were highly correlated, the regression line indicated that the most abundant compound was ammonium sulphate. The content of free hydronium ions in the fine particles was well below the ammonium content. No correlation between NH₃ and NH⁺₄, HNO₃ and NO₃ SO₃ and SO⁻²₄ could be observed.     

Influence of ship emissions on air quality and input of contaminants in southern Alaska National Parks and Wilderness Areas during the 2006 tourist season

The impact of ship emissions on air quality in Alaska National Parks and Wilderness Areas was investigated using the Weather Research and Forecasting model inline coupled with chemistry (WRF/Chem). The visibility and deposition of atmospheric contaminants was analyzed for the length of the 2006 tourist season. WRF/Chem reproduced the meteorological situation well. It seems to have captured the temporal behavior of aerosol concentrations when compared with the few data available. Air quality follows certain predetermined patterns associated with local meteorological conditions and ship emissions. Ship emissions have maximum impacts in Prince William Sound where topography and decaying lows trap pollutants. Along sea-lanes and adjacent coastal areas, NO_x, SO₂, O₃, PAN, HNO₃, and PM_2.5 increase up to 650 pptv, 325 pptv, 900 pptv, 18 pptv, 10 pptv, and 100 ng m^?3. Some of these increases are significant (95% confidence). Enhanced particulate matter concentrations from ship emissions reduce visibility up to 30% in Prince William Sound and 5–25% along sea-lanes.     

Laboratory study of cluster ions formation in H2SO4–H2O system: Implications for threshold concentration of gaseous H2SO4 and ion-induced nucleation kinetics

Ion-induced binary H₂SO₄–H₂O nucleation is an important mechanism of aerosol formation in the atmosphere. Ions are created in the atmosphere mainly by galactic cosmic rays. The importance of ion-induced nucleation is recognized in some of the observed nucleation events in the background atmosphere. However, the predictions of current ion–aerosol models are highly uncertain mostly due to the lack of detailed experimental information concerning the thermodynamics and kinetics of ion clustering reactions. Here we continue the report of results of our laboratory experiments on the formation and growth of positive and negative cluster ions in H₂SO₄–H₂O vapours in the flow reactor started in Wilhelm et al. [2004. Ion-induced aerosol formation: new insights from laboratory measurements of mixed cluster ions HSO₄⁻(H₂SO₄)_a(H₂O)_w and H⁺ (H₂SO₄)_a(H₂O)_w. Atmospheric Environment 38, 1735–1744] and Sorokin et al. [2006. Formation and growth of sulphuric acid–water cluster ions: experiments, modelling, and implications for ion-induced aerosol formation. Atmospheric Environment 40, 2030–2045]. The main attention is given to the definition of the concentration of gaseous sulphuric acid in experiment and also to some aspects of the kinetics of small cluster ions formation. The performed analysis has indicated a threshold concentration of gaseous sulphuric acid for binary homogeneous nucleation of at least about 10¹⁰ cm⁻³ at room temperature and low relative humidity.     

A dynamic dilution system for the production of sub-ppb concentrations of reactive and labile species

A three-stage dynamic dilution system has been developed capable of generating calibration standards below 100 pptv using gravimetrically calibrated permeation tubes as a trace species source. Multistage dilution permits the use of equivalent one-step diluent flows approaching 1000 std 1 min ⁻¹. An allfluorocarbon flow path facilitates the handling of reactive and labile species. Accuracies of about ± 3 % have been demonstrated for several sulfur compounds including H₂S and SO₂ at mixing ratios below 200 ppt.     

Vehicle emissions of radical precursors and related species observed in the 2009 SHARP campaign

The 2009 Study of Houston Atmospheric Radical Precursors (SHARP) field campaign had several components that yielded information on the primary vehicular emissions of formaldehyde (HCHO) and nitrous acid (HONO), in addition to many other species. Analysis of HONO measurements at the Moody Tower site in Houston, TX, yielded emission ratios of HONO to the vehicle exhaust tracer species NO_x and CO of 14 pptv/ppbv and 2.3 pptv/ppbv, somewhat smaller than recently published results from the Galleria site, although evidence is presented that the Moody Tower values should be upper limits to the true ratios of directly emitted HONO, and are consistent with ratios used in current standard emissions models. Several other Moody Tower emission ratios are presented, in particular a value for HCHO/CO of 2.4 pptv/ppbv. Considering only estimates of random errors, this would be significantly lower than a previous value, though the small sample size and possible systematic differences should be taken into account. Emission factors for CO, NO_x, and HCHO, as well as various volatile organic compounds (VOCs), were derived from mobile laboratory measurements both in the Washburn Tunnel and in on-road exhaust plume observations. These two sets of results and others reported in the literature all agree well, and are substantially larger than the CO, NO_x, and HCHO emission factors derived from the emission ratios reported from the Galleria site.

Implications: Emission factors for the species measured in the various components of the 2009 SHARP campaign in Houston, TX, including HCHO, HONO, CO, CO₂, nitrogen oxides, and VOCs, are needed to support regional air quality monitoring. Components of the SHARP campaign measured these species in several different ways, each with their own potential for systematic errors and differences in vehicle fleets sampled. Comparisons between data sets suggest that differences in sampling place and time may result in quite different emission factors, while also showing that different vehicle mixes can yield surprisingly similar emission factors.     

Diffusion scrubber technique used for measurements of atmospheric ammonia

A diffusion scrubber (DS) was developed to measure trace levels of gaseous ammonia in ambient air. The sampling resolution time for this method is 10 min and the detection limit is estimated to be 0.01 ppbv. The response to the NI-I₃ concentrations is found to be dependent on the relative humidity in the ambient air and the temperature. The method is calibrated by using a diluted NH₃ cylinder gas, and the concentrations of the calibration gas were in the range 0.02–2 ppbv during the test. Sampling performed with the DS-method is compared to sampling performed by a filter pack and a continuous flow denuder (AMANDA). The DS-method shows good agreement with the continuous flow denuder and the filter pack.     

Carbon monoxide photoproduction from rice and maize leaves

We investigated CO photoproduction from intact leaves of rice (Oryza sativa L.) and maize (Zea mays L.) by laboratory experiments. CO photoproduction showed positive correlation with light intensity and was positively dependent on oxygen concentration. The average CO photoproduction was 2.6±0.3×10¹⁰ molecules cm⁻² s⁻¹ from rice leaves and 2.2±0.1×10¹⁰ molecules cm⁻² s⁻¹ from maize leaves (n=5) at a radiation intensity of 49 mW cm⁻². CO photoproduction from senescent rice leaves was 9 times greater (25.7±1.5×10¹⁰ molecules cm⁻² s⁻¹, n=2) at the same radiation intensity than from live leaves, and responded slowly to changes in oxygen concentration and light intensity. CO photoproduction showed no correlation with CO₂ concentration or humidity. This indicates that CO photoproduction in leaves is not directly controlled by carbon metabolism or stomatal conductance. The lack of dependence on stomatal conductance leads to the conclusion that the diffusion of CO from inside the leaves to the atmosphere is not a controlling factor for CO photoproduction from rice and maize leaves.