Light Non-methane Hydrocarbons Research Articles

Online observation of light non-methane hydrocarbons (C2–C5) over the central Himalayas: Influence of the Indo-Gangetic Plain region

Studies pertaining to non-methane hydrocarbons (NMHCs) are very limited in South Asia, particularly at high-mountain sites. Therefore, online in-situ observations of light NMHCs (C2–C5) were conducted in the Central Himalayas (Nainital, 1958 m) from January 2017 to December 2020. Additionally, air samples were collected from an Indo-Gangetic Plain (IGP) site (Haldwani, 554 m), located about 18 km and the nearest source regions for the mountain site. The diurnal variations show daytime higher values at the mountain site, which is typical for a pristine site. Correlation analysis and percentage contributions confirm the major roles of biomass burning at the mountain site and Liquefied Petroleum Gas (LPG) emission, automobile and domestic combustion at the IGP site. Nevertheless, the anthropogenic influences also reaches the Himalayan site, mainly in spring, which is confirmed by the concentrated weighted trajectory (CWT) analysis. Such influences are limited to the IGP site in winter, primarily due to the shallower boundary layer. The estimated photochemical age varies from 9 days in winter to 16 days in summer-monsoon. It is shown that the hydroxyl radical reactivity, ozone formation potential (OFP) and secondary organic aerosol formation potential (SOAFP) are 4–6 times higher at the IGP site compared to the Himalayan site. Propylene, ethylene and n-butane are major contributor in these at both sites. Ethane is found to be most abundant (40%) at the Himalayan site, while propane (27%) at the IGP site. An assessment of percentage contribution of different NMHCs in emission inventories for the Himalayan region is suggested.

Decrease of the recovery rate of light non-methane hydrocarbons by interference of air oxygen in a measurement of non-methane hydrocarbon concentration under air

Decrease of the recovery rate of light non-methane hydrocarbons by interference of air oxygen in a measurement of non-methane hydrocarbon concentration under air

Variations in surface ozone and carbon monoxide in the Kathmandu Valley and surrounding broader regions during SusKat-ABC field campaign: role of local and regional sources

Abstract. Air pollution resulting from rapid urbanization and associated human activities in the Kathmandu Valley of Nepal has been leading to serious public health concerns over the past 2 decades. These concerns led to a multinational field campaign SusKat-ABC (Sustainable atmosphere for the Kathmandu Valley – Atmospheric Brown Clouds) that measured different trace gases, aerosols and meteorological parameters in the Kathmandu Valley and surrounding regions during December 2012 to June 2013 to understand local- to regional-scale processes influencing air quality of the Kathmandu Valley. This study provides information about the regional distribution of ozone and some precursor gases using simultaneous in situ measurements from a SusKat-ABC supersite at Bode, Nepal, and two Indian sites: a high-altitude site, Nainital, located in the central Himalayan region and a low-altitude site, Pantnagar, located in the Indo-Gangetic Plain (IGP). The diurnal variations at Bode showed a daytime buildup in O3 while CO shows morning and evening peaks. Similar variations (with lower levels) were also observed at Pantnagar but not at Nainital. Several events of hourly ozone levels exceeding 80 ppbv were also observed at Bode. The CO levels showed a decrease from their peak level of about 2000 ppbv in January to about 680 ppbv in June at Bode. The hourly mean ozone and CO levels showed a strong negative correlation during winter (r2 = 0.82 in January and r2 = 0.71 in February), but this negative correlation gradually becomes weaker, with the lowest value in May (r2 = 0.12). The background O3 and CO mixing ratios at Bode were estimated to be about 14 and 325 ppbv, respectively. The rate of change of ozone at Bode showed a more rapid increase ( ∼ 17 ppbv h−1) during morning than the decrease in the evening (5–6 ppbv h−1), suggesting the prevalence of a semi-urban environ. The lower CO levels during spring suggest that regional transport also contributes appreciably to springtime ozone enhancement in the Kathmandu Valley on top of the local in situ ozone production. We show that regional pollution resulting from agricultural crop residue burning in northwestern IGP led to simultaneous increases in O3 and CO levels at Bode and Nainital during the first week of May 2013. A biomass-burning-induced increase in ozone and related gases was also confirmed by a global model and balloon-borne observations over Nainital. A comparison of surface ozone variations and composition of light non-methane hydrocarbons among different sites indicated the differences in emission sources of the Kathmandu Valley and the IGP. These results highlight that it is important to consider regional sources in air quality management of the Kathmandu Valley.

First observations of light non-methane hydrocarbons (C2–C5) over a high altitude site in the central Himalayas

Abstract This study presents observations of methane (CH 4 ) and light non-methane hydrocarbons (NMHCs) for the first time from a high altitude site Nainital (29.4°N, 79.5°E, 1958 m amsl) in the central Himalayas. The whole air samples collected with a frequency of 3 samples per week during April 2009–December 2011 are analyzed using a Gas Chromatograph equipped with Flame Ionization Detector (GC-FID). Additionally, samples were collected from two semi-urban sites (Haldwani and Pantnagar) in the adjoining Indo Gangetic plain region. CH 4 and NMHCs show a distinct seasonal cycle over this region with more frequent observations of higher levels during winter (DJF) and late autumn (SON) and lower levels during the summer–monsoon (JJA). Different NMHCs exhibit better correlations during autumn/winter as compared to the summer–monsoon season. The annual mean mixing ratios of methane, ethane, ethene, propane, propene, i-butane, n-butane, acetylene, and i-pentane at Nainital are measured to be 1.9 ± 0.1 ppmv, 1.8 ± 1.0, 0.7 ± 0.9, 0.6 ± 0.8, 0.6 ± 0.7, 0.6 ± 0.7, 0.5 ± 0.6, 1.0 ± 0.8, and 0.5 ± 0.6, respectively (all in ppbv). The seasonal cycle of CH 4 at Nainital is found to be similar to other global high altitude sites (Jungfraujoch and Mauna Loa) but somewhat different than a high altitude site Mt. Abu in India. NMHCs, other than ethane and propane, are found to be higher over this central Himalayan region than other sites. Additionally, composition of NMHCs is shown to be different over the study region when compared with other sites in the IGP region. A correlation study between ln((n-butane)/(ethane)) and ln((i-butane)/(ethane)) showed that oxidation by the OH radical is the main removal mechanism of these species over the central Himalaya and dilution maintains the ratios of these species. The lowest slope of propane and acetylene with CO during summer and spring are indicating absence of fresh air mass over this region. This study fills a major gap in observational data for light NMHCs in the Himalayas and has implications for better understanding of tropospheric chemistry over this region.

Study of C2–C5 Non-methane Hydrocarbons and Their Ozone Formation Potential at Bhubaneswar, an Eastern Coastal Site in India

A preliminary study has been taken up to evaluate the NHHCs concentration and their effect on atmospheric chemistry at Bhubaneswar for the very first time during winter months when pollution load is prevalent. For this purpose ambient air samples were collected during a period of 3 months (Dec 2013–Feb 2014). The samples were analyzed for C2–C5 light non-methane hydrocarbons (NMHCs) using a gas chromatograph with a thermal desorption system. It was observed that level of NMHCs over the measurement site was lower in comparison to other urban locations within India but higher in comparison to Bay of Bengal. Statistical interpretation suggests a non-significant variation of NMHCs concentration between the observation months. Diurnal observations revealed a higher concentration of both n-pentane and i-pentane which was mostly attributed to solvent evaporation. Liquefied petroleum gas usage is believed to be a major contributor to the mixing ratios of propane (1.5 ppbv) and butane (0.027 ppbv) while ethane and ethylene emissions were attributed to traffic volume and vehicular exhausts. The propylene-equivalent and ozone formation potential of NMHCs have also been calculated in order to find out their OH reactivity and contribution to the photochemical ozone formation. Relative humidity was also observed to have a significant correlation with NMHCs concentration. Variation of total non-methane hydrocarbons (TNMHCs) with ozone and CO suggest the role of TNMHC as precursor for ozone formation.

Emissions of Volatile Organic Compounds from Biomass Burning Sources and their Ozone Formation Potential over India

Thousands of volatile organic compounds (VOCs) in the Earth's atmosphere exist which play an important role in various photochemical processes. However, the global model simulations of tropospheric chemistry deal with limited data of speciated VOCs. In the present study, we have used the Global Fire Emissions Database inventory of VOCs emitted from biomass burning in India during the period from 1997 to 2009. We have also analysed data of some VOCs measured in the upper troposphere over India for the year 2008. In this study, the major species analysed are C 2 H 4 , C 2 H 4 O, C 2 H 6 , C 2 H 6 S, C 3 H 6 , C 3 H 6 S, C 3 H 8 , C 5 H 8 , CH 3 OH, higher alkanes, higher alkenes, terpenes and toluene lumps. The biomass burning emissions of VOCs show large inter-annual variation. For example, the annual emission estimates of non-methane hydrocarbons (NMHCs) and CH3OH varied in the range 100-470 and 46-211 Gg yr -1 respectively. The major biomass sources were broadly categorized as deforestation, fuel-wood, forest and agricultural residues. The agricultural residue burning is the most dominant among the several biomass burning sources contributing to the emissions of CH 3 OH (59%), isoprene (80%) and toluene (72%). On the other hand, the major sources for NMHCs emission were agricultural residues and deforestation during all the years. The fire count data detected using the satellite-based Along Track Scanning Radiometer have been used to directly refer to the seasonal and inter-annual variations of biomass burning activities. We have estimated the propylene-equivalent concentrations of different light NMHCs measured in the upper troposphere over India. Role of stratospheric intrusion in the distribution of NMHCs has been analysed using the potential vorticity data.

Variability in ozone and its precursors over the Bay of Bengal during post monsoon: Transport and emission effects

AbstractSimultaneous measurements of O3, CO, NOx, CH4, and light nonmethane hydrocarbons were made over the Bay of Bengal (BoB) during 28 October to 17 November 2010 to study the role of chemistry and dynamics. The measurements revealed large variability in O3 (11 to 60 ppbv) and CO (45 to 260 ppbv). Estimated south to north latitudinal gradients in O3 (3.95 ppbv/°) and CO (16.56 ppbv/°) were significantly higher than those observed during earlier campaigns. Hybrid Single‐Particle Lagrangian Integrated Trajectory simulated back air trajectories were used to classify these measurements into pollution plumes from nearby sources (India‐Bangladesh region and Southeast Asia), influenced by long‐range transport and pristine marine air from the Indian Ocean. Interspecies correlations were used to identify emission signatures in these air masses, e.g., chemical proxies suggested influence of biofuel/biomass burning in NE‐BoB and E‐BoB air masses. Principle component analysis indicated contributions of ship emissions to NOx levels over the BoB. Influences of fire from the Myanmar and Thailand regions are shown to be the potential contributor to enhanced CO levels (>250 ppbv) over the BoB during 14–15 November. Diurnal variations in surface O3 revealed effects of advection, entrainment, and photochemistry. A chemical box model simulated the photochemical buildup in O3 in polluted air masses and daytime destruction in pristine oceanic air masses.

油ガス田地域(新潟)と非油ガス田地域(茨城・群馬)における軽質非メタン炭化水素濃度の比較II : 1994年〜1999年冬季

油ガス田地域(新潟)と非油ガス田地域(茨城・群馬)における軽質非メタン炭化水素濃度の比較II : 1994年〜1999年冬季

Light non-methane hydrocarbons at two sites in the Indo-Gangetic Plain

Measurements of light (C(2)-C(5)) non-methane hydrocarbons (NMHCs) were made along with ozone (O(3)), oxides of nitrogen (NO(x)), carbon monoxide (CO) and methane (CH(4)) at Hissar and Kanpur in the Indo-Gangetic Plain (IGP) in India during the month of December, 2004. Air samplings during noon and evening hours provided an opportunity to study the emission characteristics and changes during this period at these sites. The mixing ratio of O(3) was higher during noon hours due to photochemical formation, while the levels of precursor gases showed elevated values during the evening hours on a clear sky day. On foggy days there is no such variation. The lower mixing ratios of O(3) observed on foggy days could be due to the slower rate of photochemical formation caused by a reduction in solar flux and surface deposition caused by the presence of a stable planetary boundary layer. Propene and ethene show the highest evening to noon ratio due to their faster reactivities with OH radicals. Correlations among different species of the measured gases indicate contributions of emissions from biomass and biofuel burning as well as fossil fuel combustion. Although qualitatively in relation to O(3), the propylene (propene) equivalents of NMHCs have been calculated to investigate their roles in O(3) photochemistry and compared with the data from Ahmedabad, an urban site in western India. The important result, which has emerged from the analysis of the observed data, is that while the total amount of these NMHCs is least at Hissar and highest at Ahmedabad, the total propylene-equivalent is highest at Hissar and lowest at Ahmedabad. Further, these two sites in the IGP show significant contributions, almost 72-77%, by propene and ethene while the contribution by these two gases at Ahmedabad is only about 47%. The surface level mixing ratios of O(3) could be treated as representative for the chemical characterization of air mass at a regional scale over the IGP as the month long trends of O(3) show significant similarity compared to the trends in precursors at the two sites.

Seasonality in the latitudinal distributions of NMHCs over Bay of Bengal

Abstract Surface level measurements of light non-methane hydrocarbons (C 2 –C 4 NMHCs) were conducted during summer and winter monsoon campaigns over Bay of Bengal (BOB). The mixing ratios of major NMHCs during winter monsoon were significantly higher compared to summer monsoon. During both seasons, the distributions of NMHCs show clear north–south decreasing gradients over open oceanic region of BOB. During the summer (winter) monsoons, the latitudinal gradients of ethane, propane and acetylene were 29 ± 3 (136 ± 4) pptv deg −1 , 14 ± 2 (46 ± 8) pptv deg −1 and 14 ± 4 (148 ± 10) pptv deg −1 , respectively over open oceanic region of BOB. For measurements during summer monsoon, the latitudinal gradients in the mixing ratios of NMHCs were caused by the transport of pollutants from peninsular India to the northern BOB and flow of cleaner marine air over southern BOB. On the other hand, the gradients in the mixing ratios of NMHCs during winter monsoon are due to the dilution and photochemical loss towards the southern BOB. In both seasons, no such clear latitudinal trends were observed along the east coast of India. The measurements along the coastal tracks were influenced by stagnant flow and photochemically aged air which also lacks clear north–south directionality. Alkenes show opposite seasonality to that of major NMHCs as the mixing ratios of both ethene and propene during summer monsoon were higher than the winter values. In addition, the distributions of alkenes also show significant heterogeneity, however, no clear latitudinal trends can be seen during both seasons. The investigations of wind and ocean color parameters suggest substantial emissions of alkenes from surface sea waters which also support the seasonality of these species over BOB.

Sampling, storage, and analysis of C 2–C 7 non-methane hydrocarbons from the US National Oceanic and Atmospheric Administration Cooperative Air Sampling Network glass flasks

An analytical technique was developed to analyze light non-methane hydrocarbons (NMHC), including ethane, propane, iso-butane, n-butane, iso-pentane, n-pentane, n-hexane, isoprene, benzene and toluene from whole air samples collected in 2.5 l-glass flasks used by the National Oceanic and Atmospheric Administration, Earth System Research Laboratory, Global Monitoring Division (NOAA ESRL GMD, Boulder, CO, USA) Cooperative Air Sampling Network. This method relies on utilizing the remaining air in these flasks (which is at below-ambient pressure at this stage) after the completion of all routine greenhouse gas measurements from these samples. NMHC in sample aliquots extracted from the flasks were preconcentrated with a custom-made, cryogen-free inlet system and analyzed by gas chromatography (GC) with flame ionization detection (FID). C 2–C 7 NMHC, depending on their ambient air mixing ratios, could be measured with accuracy and repeatability errors of generally ≤10–20%. Larger deviations were found for ethene and propene. Hexane was systematically overestimated due to a chromatographic co-elution problem. Saturated NMHC showed less than 5% changes in their mixing ratios in glass flask samples that were stored for up to 1 year. In the same experiment ethene and propene increased at ∼30% yr −1. A series of blank experiments showed negligible contamination from the sampling process and from storage (<10 pptv yr −1) of samples in these glass flasks. Results from flask NMHC analyses were compared to in-situ NMHC measurements at the Global Atmospheric Watch station in Hohenpeissenberg, Germany. This 9-months side-by-side comparison showed good agreement between both methods. More than 94% of all data comparisons for C 2–C 5 alkanes, isoprene, benzene and toluene fell within the combined accuracy and precision objectives of the World Meteorological Organization Global Atmosphere Watch (WMO-GAW) for NMHC measurements.

Transport of nonmethane hydrocarbons to Jupiter's troposphere by descent of smog particles

Ethane and other C 2 and C 3 nonmethane hydrocarbons were observed by the Galileo Probe Mass Spectrometer to have a deep minimum in mixing ratio near and below the tropopause, rising to much larger values at and below the 15 bar level. This minimum requires that these hydrocarbons condense on smog particles at low temperatures. The large amounts at deeper levels were produced in the pumping system of the instrument and any contribution from the ambient atmosphere cannot be resolved, although evaporation from the smog particles is proposed and incorporated in a model. In the stratosphere some of the ethane is released by photon-stimulated desorption. The other light nonmethane hydrocarbons are not modeled, but can be expected to show the same behavior as ethane.

Impact of transport from the surrounding continental regions on the distributions of ozone and related trace gases over the Bay of Bengal during February 2003

Simultaneous surface level measurements of O3, CO, methane, and light nonmethane hydrocarbons (NMHCs) were made over the Bay of Bengal during a cruise campaign between 19 February and 28 February 2003. The mixing ratios of O3, CO, methane, ethane and acetylene were observed in the ranges of 20–52 ppbv, 126–293 ppbv, 1.65–1.85 ppmv, 622–2088 pptv and 134–1388 pptv, respectively, during the campaign period. Ratios of some measured trace gases have been used to estimate the level of photochemical processes and transport times of air parcels. Three types of air masses have been identified on the basis of source regions and transport pathways. The study region is frequently affected by transport of pollutants from the nearby continental emission sources, but the strongest pollution event during this period was due to long‐range transport from extratropical Northern Hemisphere. These higher mixing ratios of trace gases are due to faster transport of air parcels from the source regions via free troposphere. On the basis of C2H2/CO ratio, it is also observed that this pollution plume was photochemically fresh. Latitudinal distributions of all the measured trace gases show significant north‐south decreasing trends. In particular, gradients in the mixing ratios of NMHCs over the Bay of Bengal compare 2–3 times higher than those observed over the Arabian Sea, the Indian Ocean and the Pacific Ocean. Comparisons with previous measurements over the Arabian Sea and the Indian Ocean show significantly higher levels of these trace gases over the Bay of Bengal.

Measurements of the 12C/13C Kinetic Isotope Effects in the Gas-Phase Reactions of Light Alkanes with Chlorine Atoms

The carbon kinetic isotope effects (KIEs) of the reactions of several light non-methane hydrocarbons (NMHC) with Cl atoms were determined at room temperature and ambient pressure. All measured KIEs, defined as the ratio of the Cl reaction rate constants of the light isotopologue over that of the heavy isotopologue (Clk12/Clk13) are greater than unity or normal KIEs. For simplicity, measured KIEs are reported in per mil according to Clepsilon=(Clk12/Clk13 -1)x1000 per thousand unless noted otherwise. The following average KIEs were obtained (all in per thousand): 10.73+/-0.20 (ethane), 6.44+/-0.14 (propane), 6.18+/-0.18 (methylpropane), 3.94+/-0.01 (n-butane), 1.79+/-0.42 (methylbutane), 3.22+/-0.17 (n-pentane), 2.02+/-0.40 (n-hexane), 2.06+/-0.19 (n-heptane), 1.54+/-0.15 (n-octane), 3.04+/-0.09 (cyclopentane), 2.30+/-0.09 (cyclohexane), and 2.56+/-0.25 (methylcyclopentane). Measurements of the 12C/13C KIEs for the Cl atom reactions of the C2-C8 n-alkanes were also made at 348 K, and no significant temperature dependence was observed. To our knowledge, these 12C/13C KIE measurements for alkanes+Cl reactions are the first of their kind. Simultaneous to the KIE measurement, the rate constant for the reaction of each alkane with Cl atoms was measured using a relative rate method. Our measurements agree with published values within+/-20%. The measured rate constant for methylcyclopentane, for which no literature value is available, is (2.83+/-0.11)x10-10 cm3 molecule-1 s-1, 1sigma standard error. The Clepsilon values presented here for the C2-C8 alkanes are an order of magnitude smaller than reported methane Clepsilon values (Geophys. Res. Lett., 2000, 27, 1715), in contrast to reported OHepsilon values for methane (J. Geophys. Res. (Atmos.), 2001, 106, 23, 127) and C2-C8 alkanes (J. Phys. Chem. A, 2004, 108, 11537), which are all smaller than 10 per thousand. This has important implications for atmospheric modeling of saturated NMHC stable carbon isotope ratios. 13C-structure reactivity relationship values (13C-SRR) for alkane-Cl reactions have been determined and are similar to previously reported values for alkane-OH reactions.

Two high-speed, portable GC systems designed for the measurement of non-methane hydrocarbons and PAN: results from the Jungfraujoch High Altitude Observatory.

Near real-time measurements of light non-methane hydrocarbons (NMHCs) and peroxyacetyl nitrate (PAN) have been performed in the free troposphere using two fast gas chromatography (GC) instruments designed for use on aircraft. A GC-helium ionisation detector (HID) system measured 15 C(2)-C(5) hydrocarbons with 5 min time resolution and a dual channel GC-Electron Capture Detector (ECD) measured PAN with 90 s resolution. Both instruments had low parts per trillion by volume (pptV) detection limits and ran continuously at the remote Jungfraujoch (JFJ) research station in the Swiss Alps (46.55[degree]N, 7.98[degree]E), 3580 m above mean sea level (AMSL), during February/March 2003. Carbon monoxide, ozone, nitrogen oxide and nitrogen dioxide and all odd nitrogen species (NO(y)) were also measured continuously. Hydrocarbons and CO were strongly correlated in all air-masses whilst PAN exhibited both positive and negative correlations with respect to O(3), dependent on age and origin of the air-mass sampled. PAN was found to contribute [similar]20% to the NO(y) sampled on average. The experiment, as well as providing interesting datasets from this remote location, also demonstrated that when optimised, GC techniques have the potential to measure at a time resolution significantly greater than is traditionally considered, with high sensitivity and low uncertainty.

Method for Measuring Carbon Kinetic Isotope Effects of Gas-Phase Reactions of Light Hydrocarbons with the Hydroxyl Radical

A technique for the accurate measurement of the carbon kinetic isotope effects (KIEs) of the reactions of light non-methane hydrocarbons (NMHC) with OH radicals in the gas phase is presented. The method uses a variant of established relative rate techniques, which has been optimized for measurements of KIEs. The instrumentation consists of three basic components: (1) a gas-phase reaction chamber, (2) a gas chromatograph (GC) for separating the components of the reaction mixture, and (3) an isotope ratio mass spectrometer coupled on-line to the GC. Using this technique, measurements of hydroxyl radical reaction KIEs with C2−C9 saturated and unsaturated hydrocarbons having rate constants between 10-11 and 10-13 cm3 molecule-1 s-1 are achievable. This method has been used successfully to measure the carbon KIE for the reactions of OH with a wide range of hydrocarbons in the gas phase.

Isoprene production by Prochlorococcus, a marine cyanobacterium, and other phytoplankton

The oceans are a small source of light (C 2–C 6) non-methane hydrocarbons (NMHC), which influence the photo-oxidant chemistry of the remote marine atmosphere. Previous work has shown that water column sources of alkenes include photochemical processes, and that various phytoplankton species can produce isoprene. However, only a few phytoplankton species have been studied, and no assessment has been performed of the effects of other pelagic microorganisms on NMHC cycling. The dependence of phytoplanktonic isoprene production on light, temperature, and organism size has also not been investigated. In this work, laboratory cultures of five different marine phytoplankton species ( Prochlorococcus, Synechococcus, Micromonas pusilla, Pelagomonas calceolata, and Emiliania huxleyi) were examined for NMHC production capabilities. All species were found to produce isoprene at constant rates during the balanced exponential growth phase; rates ranged from 1×10 −21 to 4×10 −19 mol cell −1 day −1 over all cell species and growth conditions tested. No other NMHC was consistently produced or consumed by these cells. The presence of heterotrophic bacteria in phytoplankton cultures had no effect on isoprene production rates per phytoplankton cell. A positive allometric relationship was observed between isoprene production rate and cell volume; highest production rates were found for the largest cell, E. huxleyi, and lowest rates for Prochlorococcus, the smallest cell. Isoprene production was a function of light intensity and temperature in Prochlorococcus, with patterns that were similar to those between growth rate and these environmental variables. The maximum production with light intensity occurred in the photoinhibited regime, and the maximum with temperature was at the maximum of growth rate for this species, near 23 °C. Nanoflagellate grazing by Cafeteria roenbergensis on, and phage infection of, Prochlorococcus controlled total isoprene produced in the flask by controlling cell abundances. Phage infection also decreased the isoprene production rate per cell during latent period of infection as compared to healthy cells. With certain assumptions, combining the measured laboratory isoprene production rates with observed water column phytoplankton abundances resulted in a maximum estimated sea-to-air flux of isoprene that was on the same order of magnitude as previously reported values determined using in situ measured seawater and atmospheric measurements.

Seasonal variations of C2–C4 nonmethane hydrocarbons and C1–C4 alkyl nitrates at the Summit research station in Greenland

We report measurements of light (C2–C4) nonmethane hydrocarbons (NMHCs) and C1–C4 alkyl nitrates made at Summit, Greenland, over a full annual cycle (June 1997–1998). The remoteness of the Summit camp from industrial source regions resulted in trends of these trace gases that showed a clear seasonal variation with low variability. Variability (calculated as the percentage 1‐sigma deviation of a running 10‐point mean) averaged 7–9% for ethane over the entire study. The shorter‐lived species (ethyne, propane, and the butanes) exhibited variability from 12–20% in winter to 20–100% in summer. The best fit curve to the annual cycle of ethane is a sinusoidal oscillation, but each of the shorter‐lived NMHCs exhibited flat periods of low concentrations during the summer months. The C2–C4 NMHCs peaked between 8 and 24 February, with the longer‐lived NMHCs maximizing latest. These data were broadly consistent with literature values, confirming that high‐latitude Northern Hemisphere (NH) emissions are similar year to year. Employing their linear fall accumulation rates, we calculated average NMHC ratios versus ethane of 0.59(±0.10):0.33(±0.05):0.26(±0.08):0.14(±0.04) for propane, ethyne, n‐butane, and i‐butane, respectively. We suggest that these ratios represent useful quantities with which to compare averaged mid‐ and high‐latitude NH emission ratios. We also report the first year‐round observations of the seasonal cycle of light C1–C4 alkyl nitrates. Similar to the NMHCs, the seasonal trend of these gases shows primary dependence on transport to Summit during winter, and photochemical removal during summer. Unlike their parent NMHCs, alkyl nitrate concentrations did not asymptote to low levels during summer and they exhibited winter maxima later with decreasing photochemical lifetime.

The stable carbon isotope fractionation for reactions of selected hydrocarbons with OH‐radicals and its relevance for atmospheric chemistry

Measurements of the kinetic isotope effect (KIE) for the reactions of light n‐alkanes as well as for several unsaturated hydrocarbons, including alkenes, dienes, benzene, and ethyne with OH‐radicals are presented. All measured KIEs are positive; that is, molecules containing only C‐12 react faster than the C‐13 labeled molecules. However, the KIEs for n‐alkanes are quite small; between one and four permil. They can be explained mainly by the mass dependence of the collision frequency between the n‐alkanes and OH‐radicals. KIEs for the reaction of alkenes with OH‐radicals are considerably higher. They can be explained by a fractionation of 24.5±1.1‰ for the addition of an OH‐radical to a double bond. Inverse dependence on number of carbon atoms and mass dependence of the collision frequencies explain our observations. For benzene the KIE is slightly higher; for ethyne it is somewhat lower than expected from this simple model. For the reaction of many light nonmethane hydrocarbons (NMHC), especially of unsaturated hydrocarbons, with OH‐radicals the KIEs are sufficiently large to have significant impact on the isotopic composition of atmospheric NMHC. A small series of stable carbon isotope ratio measurements of atmospheric NMHC were made in the greater Toronto area. Traffic related NMHC emissions were also studied for their stable carbon isotope ratios. From these data it is possible to quantitatively determine the extent of photochemical processing due to OH‐radical reactions that the individual NMHC has experienced. Thus such measurements allow quantitative evaluation of the extent of chemical processing the different NMHC have gone through. This also includes the possibility to differentiate between the impact of local sources and regional or large scale transport. It is shown that in combination with concentration measurements isotope ratio measurements are extremely valuable to study the complex interaction between chemical removal mechanisms, mixing, and dilution processes.

Source determination of light non-methane hydrocarbons by simultaneous multi-site sampling in a metropolitan area

Concentration profiles of C2–C6 non-methane hydrocarbons (NMHCs) were obtained in metropolitan Taipei, Taiwan based on 50 air samples collected simultaneously during an evening traffic rush hour. Although motor vehicle exhaust contributes most of the measured hydrocarbons in the city, levels of propane and butane are significant, and do not correlate well with typical NMHCs emitted from vehicle exhaust, suggesting that the leakage from liquefied petroleum gas (LPG) is the primary source. Natural gas (NG) leakage was also investigated using ethane as a tracer, and a similar profile to propane was observed, suggesting that the use of LPG and NG in metropolitan Taipei is not exclusive but rather complementary. The reactivity of ethane, propane, iso-and n-butanes released from NG and LPG was estimated to be about 5–6% of the total reactivity summed from C2–C6 NMHCs, compared to their mass fraction of about 27%. The profiles computed from simultaneous sampling provide a direct view of NMHC distributions in Taipei metropolitan area, and are useful to indicate source strength with respect to geographic locations. The principle component analysis, however, can reveal relative importance of various sources in a particular location or area.