Mixing Ratios Of Non-methane Hydrocarbons Research Articles

Ozone sensitivity factor: NOX or NMHCs?: A case study over an urban site in Delhi, India

The paper reports relations between surface ozone (O3) and its main precursors, nitrogen oxides (NOx) and non-methane hydrocarbons (NMHCs) due to its importance from scientific perspective, besides implementation of policies concerning O3 pollution mitigation. Measurements and analysis of abovementioned trace gases revealed relationships among them in an urban location in Delhi, India during April to December 2014. Seasonal average mixing ratio of O3 during the study period was 37 ± 13 ppb (pre-monsoon), 26 ± 8 ppb (monsoon) and 25 ± 16 ppb (post-monsoon). Monthly average NMHCs to NOx ratio for the study period was 18.3 ± 7.2. Ozone isopleths plotted to study ozone production as a function of NOx and NMHC mixing ratios confirmed that the most effective ratio to produce high O3 in the study area was ~0.1. Results inclined towards NOX sensitive O3 as it increased with high NOX and was low at low to medium NOX. High NMHC/NOX ratios also specified that NOX played a more critical role in controlling ozone concentration at the study site. Ozone production conditions were more favourable during pre-monsoon and post-monsoon seasons, as they were comparatively nearer to optimum ratio. The study also suggests positive, far-reaching implications of controlling NOx emissions, which in current scenario could be a solution for decreasing ground-level ozone.

Evaluating oil and gas contributions to ambient nonmethane hydrocarbon mixing ratios and ozone-related metrics in the Colorado Front Range

Recently, oil and natural gas (O&NG) production activities in the Denver-Julesburg Basin have expanded rapidly. Associated nonmethane hydrocarbon (NMHC) emissions contribute to photochemical formation of ground-level ozone and include benzene as well as other hazardous air pollutants. Using positive matrix factorization (PMF) and chemical mass balance (CMB) methods, we estimate how much O&NG activities and other sources contribute to morning NMHC mixing ratios measured from 2013 to mid-2016 at a site in Platteville, CO, in the Denver-Julesburg Basin, and at a contrasting site in downtown Denver. A novel adjoint sensitivity analysis method is then used to estimate corresponding contributions to ozone and ozone-linked mortality in the Denver region. Average 6–9 am NMHC mixing ratios in Platteville were seven times higher than those in Denver in 2013 but four times higher in 2016. CMB estimates that O&NG activities contributed to the Platteville (Denver) site an average of 96% (56%) of NMHC on a carbon basis while PMF indicated 92% (33%). Average vehicle-related contributions of NMHC are estimated as 41% by CMB and 53% by PMF in Denver. Estimates of the fractional contribution to potential ozone and ozone-linked mortality from O&NG activities are smaller while those from vehicles are larger than the NMHC contributions. CMB (PMF) indicate that greater than 78% (40%) of annual average benzene in Denver is attributable to vehicle emissions while greater than 75% (67%) of benzene in Platteville is attributable to O&NG activities.

Volatile organic compounds at a rural site in Beijing: influence of temporary emission control and wintertime heating

Abstract. While residential coal/biomass burning might be a major and underappreciated emission source for PM2.5, especially during winter, it is not well constrained whether burning solid fuels contributes substantially to ambient volatile organic compounds (VOCs), which are precursors to secondary organic aerosols (SOAs) that typically have a higher contribution to particulate matter during winter haze events. In this study, ambient air samples were collected in 2014 from 25 October to 31 December at a rural site on the campus of the University of Chinese Academy of Sciences (UCAS) in northeastern Beijing for the analysis of VOCs. Since temporary intervention measures were implemented on 3–12 November to improve the air quality for the Asian-Pacific Economic Cooperation (APEC) summit held on 5–11 November in Beijing, and wintertime central heating started on 15 November in Beijing after the APEC summit, this sample collection period provided a good opportunity to study the influence of temporary control measures and wintertime heating on ambient VOCs. As a result of the temporary intervention measures implemented during 3–12 November (period II), the total mixing ratios of non-methane hydrocarbons averaged 11.25 ppb, approximately 50 % lower than the values of 23.41 ppb in period I (25 October–2 November) and 21.71 ppb in period III (13 November–31 December). The ozone and SOA formation potentials decreased by ∼50 % and ∼70 %, respectively, during period II relative to period I, with the larger decrease in SOA formation potentials attributed to more effective control over aromatic hydrocarbons mainly from solvent use. Back trajectory analysis revealed that the average mixing ratios of VOCs in southerly air masses were 2.3, 2.3 and 2.9 times those in northerly air masses during periods I, II and III, respectively; all VOC episodes occurred under the influence of southerly winds, suggesting much stronger emissions in the southern urbanized regions than in the northern rural areas. Based on a positive matrix factorization (PMF) receptor model, the altered contributions from traffic emissions and solvent use could explain 47.9 % and 37.6 % of the reduction in ambient VOCs, respectively, during period II relative to period I, indicating that the temporary control measures on vehicle emissions and solvent use were effective at lowering the ambient levels of VOCs. Coal/biomass burning, gasoline exhaust and industrial emissions were among the major sources, and they altogether contributed 60.3 %, 78.6 % and 78.7 % of the VOCs during periods I, II and III, respectively. Coal/biomass burning, mostly residential coal burning, became the dominant source, accounting for 45.1 % of the VOCs during the wintertime heating period, with a specifically lower average contribution percentage in southerly air masses (38.2 %) than in northerly air masses (48.8 %). The results suggest that emission control in the industry and traffic sectors is more effective in lowering ambient reactive VOCs in non-heating seasons; however, during the winter heating season reducing emissions from residential burning of solid fuels would be of greater importance and would have health co-benefits from lowering both indoor and outdoor air pollution.

Effect of traffic restriction on reducing ambient volatile organic compounds (VOCs): Observation-based evaluation during a traffic restriction drill in Guangzhou, China

Traffic restriction (TR) is a widely adopted control measure in case of heavy air pollution particularly in urban areas, yet it is hard to evaluate the effect of TR on reducing VOC emissions based on monitoring data since ambient VOC mixing ratios are influenced not only by source emissions but also by meteorological conditions and atmospheric degradation. Here we collected air samples for analysis of VOCs before, during and after a TR drill carried out in Guangzhou in September 2010 at both a roadside and a rooftop (∼50 m above the ground) site. TR measures mainly included the “odd-even license” rule and banning high-emitting “yellow label” vehicles. The mixing ratios of non-methane hydrocarbons (NMHCs) did not show significant changes at the roadside site with total NMHCs of 39.0 ± 11.8 ppbv during non-TR period and 39.1 ± 14.8 ppbv during TR period, whereas total NMHCs decreased from 30.4 ± 14.3 ppbv during the non-TR period to 22.1 ± 10.6 ppbv during the TR period at rooftop site. However, the ratios of methyl tert-butyl ether (MTBE), benzene and toluene against carbon monoxide (MTBE/CO, T/CO and B/CO) at the both sampling sites dropped significantly. The ratios of toluene to benzene (T/B) instead increased significantly. Changes in these ratios all consistently indicated reduced input from traffic emissions particularly gasoline vehicles. Source attribution by positive matrix factorization (PMF) confirmed that during the TR period gasoline vehicles contributed less VOCs in percentages while industrial sources, biomass burning and LPG shared larger percentages. Assuming that emissions from industrial sources remained unchanged during the TR and non-TR periods, we further used the PMF-retrieved contribution percentages to deduce the reduction rate of traffic-related VOC emissions, and obtained a reduction rate of 31% based on monitoring data at the roadside site and of 34% based on the monitoring data at the rooftop site. Considering VOC emissions from all sources in Guangzhou city, the TR control measures adopted could reduce VOC up to 15%.

Sources of C2–C4 alkenes, the most important ozone nonmethane hydrocarbon precursors in the Pearl River Delta region

Surface ozone is becoming an increasing concern in China's megacities such as the urban centers located in the highly industrialized and densely populated Pearl River Delta (PRD) region, where previous studies suggested that ozone production is sensitive to VOC emissions with alkenes being important precursors. However, little was known about sources of alkenes. Here we present our monitoring of ambient volatile organic compounds at four representative urban, suburban and rural sites in the PRD region during November–December 2009, which experienced frequent ozone episodes. C2–C4 alkenes, whose total mixing ratios were 11–20% of non-methane hydrocarbons (NMHCs) quantified, accounted for 38–64% of ozone formation potentials (OFPs) and 30–50% of the total hydroxyl radical (OH) reactivity by NMHCs. Ethylene was the most abundant alkene, accounting for 8–15% in total mixing ratios of NMHCs and contributed 25–46% of OFPs. Correlations between C2–C4 alkenes and typical source tracers suggested that ethylene might be largely related to vehicle exhausts and industry activities, while propene and butenes were much more LPG-related. Positive Matrix Factorization (PMF) confirmed that vehicle exhaust and liquefied petroleum gas (LPG) were two major sources that altogether accounted for 52–62%, 58–77%, 73–83%, 68–79% and 73–84% for ethylene, propene, 1-butene, trans-2-butene and cis-2-butene, respectively. Vehicle exhausts alone contributed 32–49% ethylene and 35–41% propene. Industry activities contributed 13–23% ethylene and 7–20% propene. LPG instead contributed the most to butenes (38–65%) and substantially to propene (23–36%). Extensive tests confirmed high fractions of propene and butenes in LPG then used in Guangzhou and in LPG combustion plumes; therefore, limiting alkene contents in LPG would benefit regional ozone control.

Sources and variability of non-methane hydrocarbons in the Eastern Mediterranean

The volume mixing ratios of non-methane hydrocarbons including saturated, unsaturated and aromatic ones (C2 to C8 as non methane hydrocarbons, NMHCS) were measured at three distinct sites (natural, rural, urban) in the lower troposphere of the Eastern Mediterranean from February 2006 to March 2007. Average concentrations for most of the NMHC show clear seasonal variations along the year mainly attributed to photochemistry. Significant correlations found among various hydrocarbons indicate contribution from mobile and stationary sources (exhaust and combustion). Leakages from natural gas (NG) or liquefied petroleum gas (LPG) probably from the continental platforms may account for important sources for the background concentration of ethane and propane in the area. Isoprene has been found to have mostly a biogenic source but its dual anthropogenic–biogenic origin was also evident at the rural site.

The influence of sampling protocol on nonmethane hydrocarbon mixing ratios

The effect of sampling protocol on ambient air hydrocarbon mixing ratios was examined on eight sampling days in Los Angeles during 2007 and 2008. Four protocols, which were based on previously published multi-city urban hydrocarbon studies in the United States, were compared and differences were quantified. Whole air canister samples were collected and analyzed for nonmethane hydrocarbons (NMHCs). Differing sampling protocols resulted in large differences in mixing ratios, up to an order of magnitude, for certain NMHCs on the same sampling day. However, the magnitude of the variability between NMHC levels obtained by the four protocols was not consistent throughout the eight sampling days. It was found that sampling time, followed by sampling location, had the greatest influence on the magnitude of the mixing ratio. Ratios between hydrocarbons, often used in urban studies to gain information on emission sources, also varied depending on the protocol used. Comparison of absolute NMHC mixing ratios collected in urban environments using differing sampling protocols should be made with care.

Ambient mixing ratios of nonmethane hydrocarbons (NMHCs) in two major urban centers of the Pearl River Delta (PRD) region: Guangzhou and Dongguan

The Pearl River Delta (PRD) region can be considered one of the most economically developed areas of mainland China. In September 2005, a total of 96 whole air samples were collected in Guangzhou and Dongguan, two important urban centers of the PRD region. Guangzhou is considered the economic center of Guangdong province, and Dongguan is a rapidly expanding industrial city. Here, we report mixing ratios of 50 nonmethane hydrocarbons (NMHCs) that were quantified in the ambient air of these PRD centers. The discussion focuses on understanding the main sources responsible for NMHC emissions, and evaluating the role of the identified sources towards ozone formation. Propane was the most abundant species in Guangzhou, with an average mixing ratio of 6.8ppbv (±0.7ppbv S.E.), compared to 2.5±0.2ppbv in Dongguan. Toluene was the most abundant hydrocarbon in Dongguan (6.1±0.8ppbv, compared to 5.9±0.7ppbv in Guangzhou). Based on an analysis of the correlation between vehicular-emitted compounds and the measured NMHCs, together with the benzene-to-toluene (B/T) ratio, vehicular emission appears to be the dominant source of NMHCs measured in Guangzhou. By contrast, selected species (including toluene) in many of the Dongguan samples were influenced by an additional source, most likely related to industrial activities. A specific B/T ratio (<0.20) is proposed here and used as indicator of samples strongly affected by industrial emissions. The ozone formation potential (OFP) is calculated, and the role of the different NMHCs associated with industrial and combustion sources is evaluated.

Measurements of nonmethane hydrocarbons in 28 United States cities

Between 1999 and 2005 a sampling campaign was conducted to identify and quantify the major species of atmospheric nonmethane hydrocarbons (NMHCs) in United States cities. Whole air canister samples were collected in 28 cities and analyzed for methane, carbon monoxide (CO) and NMHCs. Ambient mixing ratios exhibited high inter- and intra-city variability, often having standard deviations in excess of 50% of the mean value. For this reason, ratios of individual NMHC to CO, a combustion tracer, were examined to facilitate comparison between cities. Ratios were taken from correlation plots between the species of interest and CO, and most NMHCs were found to have correlation coefficients ( r 2) greater than 0.6, particularly ethene, ethyne and benzene, highlighting the influence of vehicular emissions on NMHC mixing ratios. Notable exceptions were the short-chain alkanes, which generally had poor correlations with CO. Ratios of NMHC vs. CO were also used to identify those cities with unique NMHC sources.

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.

Nonmethane hydrocarbon mixing ratios in continental outflow air from eastern North America: Export of ozone precursors to Bermuda

High surface ozone levels at Bermuda during springtime are associated with the transport of large scale frontal systems from the U.S. east coast over the North Atlantic. An objective of the AEROCE III study was to adopt a meteorologically informed sampling strategy of the chemical characteristics of air masses both in advance and behind eastward progressing cold fronts in order to differentiate between vertical sources of these elevated tropospheric ozone signals. Concentrations of hydrocarbons in air sampled over the eastern US and in offshore flights varied in a complex manner with sample altitude and prevailing meteorological situation. In several aircraft flights, the presence at altitude of distinct layers of air of elevated concentrations of NMHC's attested to the dynamic mixing of lower and upper air masses associated with springtime frontal activity. Layers of mid‐tropospheric air of high O3 (140 ppbv) and low background NMHC mixing ratios (1.44 ppbv ethane, 0.034 ppbv propene, 0.247 ppbv propane, 0.034 ppbv isobutene, 0.041 ppbv n‐butane, 0.063 ppbv benzene, 0.038 ppbv toluene) mixing ratios were indicative of descending, stratospherically influenced air on a flight to the east of Norfolk, VA on April 24 (alt 4600 m). Layers of high O3 (60–70 ppbv) and elevated NMHC concentrations (1.88 ppbv ethane, 0.092 ppbv propene, 0.398 ppbv propane, 0.063 ppbv isobutene, 0.075 ppbv n‐butane, 0.106 ppbv benzene, 0.102 ppbv toluene) observed on a flight to the west of Bermuda on April 28 (alt. 4100 m) indicated upwardly lifting convective activity of continentally sourced air masses. A meteorologically informed sampling strategy employed in the study was valuable in providing a framework with which to both optimize NMHC sampling decisions and to provide a context with which to interpret their observed mixing ratios.

Nonmethane hydrocarbon measurements in the North Atlantic Flight Corridor during the Subsonic Assessment Ozone and Nitrogen Oxide Experiment

Mixing ratios of nonmethane hydrocarbons (NMHCs) were not enhanced in whole air samples collected within the North Atlantic Flight Corridor (NAFC) during the fall of 1997. The investigation was conducted aboard NASA's DC‐8 research aircraft, as part of the Subsonic Assessment (SASS) Ozone and Nitrogen Oxide Experiment (SONEX). NMHC enhancements were not detected within the general organized tracking system of the NAFC, nor during two tail chases of the DC‐8's own exhaust. Because positive evidence of aircraft emissions was demonstrated by enhancements in both nitrogen oxides and condensation nuclei during SONEX, the NMHC results suggest that the commercial air traffic fleet operating in the North Atlantic region does not contribute at all or contributes negligibly to NMHCs in the NAFC.

Aircraft measurements of the latitudinal, vertical, and seasonal variations of NMHCs, methyl nitrate, methyl halides, and DMS during the First Aerosol Characterization Experiment (ACE 1)

Canister sampling for the determination of atmospheric mixing ratios of nonmethane hydrocarbons (NMHCs), selected halocarbons, and methyl nitrate was conducted aboard the National Center for Atmospheric Research (NCAR) C‐130 aircraft over the Pacific and Southern Oceans as part of the First Aerosol Characterization Experiment (ACE 1) during November and December 1995. A latitudinal profile, flown from 76°N to 60°S, revealed latitudinal gradients for most trace gases. NMHC and halocarbon gases with predominantly anthropogenic sources, including ethane, ethyne, and tetrachloroethene, exhibited significantly higher mixing ratios in the northern hemisphere at all altitudes. Methyl chloride exhibited its lowest mixing ratios at the highest northern hemisphere latitudes, and the distributions of methyl nitrate and methyl iodide were consistent with tropical and subtropical oceanic sources. Layers containing continental air characteristic of aged biomass burning emissions were observed above about 3 km over the remote southern Pacific and near New Zealand between approximately 19°S and 43°S. These plumes originated from the west, possibly from fires in southern Africa. The month‐long intensive investigation of the clean marine southern midlatitude troposphere south of Australia revealed decreases in the mixing ratios of ethane, ethyne, propane, and tetrachloroethene, consistent with their seasonal mixing ratio cycle. By contrast, increases in the average marine boundary layer concentrations of methyl iodide, methyl nitrate, and dimethyl sulfide (DMS) were observed as the season progressed to summer conditions. These increases were most appreciable in the region south of 44°S over Southern Ocean waters characterized as subantarctic and polar, indicating a seasonal increase in oceanic productivity for these gases.

Seasonal trends and local influences on nonmethane hydrocarbon concentrations in the Canadian boreal forest

Mixing ratios of nonmethane hydrocarbons, both biogenic and anthropogenic, have been monitored year‐round at several sites in the boreal forests of Saskatchewan and Ontario. Seasonal patterns in hydrocarbon mixing ratios are explained by the wintertime effect of the Arctic air mass and the summertime increase in photochemical processing at all sites. Summer‐time differences in hydrocarbon distributions between sites are indicative of the density of anthropogenic sources and the oxidative capacity of the atmosphere in the region. Consideration of hydrocarbon mixing ratios and OH reaction rate constants suggests that the biogenic hydrocarbon isoprene may limit the oxidative capacity of the atmosphere in boreal regions. The observed temperature dependence of the isoprene mixing ratio suggests that the ambient isoprene concentration is strongly influenced by the emission rate.

In situ nonmethane hydrocarbon measurements on SAGA 3

During the third Soviet‐American Gas and Aerosol (SAGA 3) expedition to the central Pacific in February to March 1990 we observed C1‐C5 nonmethane hydrocarbons (NMHCs) both in the atmosphere and in the ocean. Atmospheric NMHCs project strongly on two factors: those NMHCs with lifetimes over 1 week (ethane, ethyne, propane, cyclopropane) display north‐south latitudinal gradients whose magnitudes are proportional to their photochemical loss rates, which is consistent with a northern hemispheric, continental source, while those NMHCs with lifetimes under 1 week (all alkenes and pentane) do not display latitudinal gradients but do vary in common in a more complicated manner, consistent with a heterogeneous, marine source. Data taken from a sea water equilibrator shows that alkene concentrations vary strongly with wind speed, supporting previous conclusions that sea‐air evasion is a major loss term for mixed layer alkenes. A reasonable balance exists between sea‐air fluxes and atmospheric column removal for some alkenes (ethene, propene, 1‐butene, and 1‐pentene), while the remainder are out of balance, with partial pressures insufficient to support atmospheric observations. The total NMHC sea‐air flux is about 0.8 μmole m−2d−1. Both our air and water concentrations fall near the low end of previously observed ranges. It is argued that some previous higher atmospheric observations may reflect systematic problems with canister sampling. During SAGA 3 the role of NMHCs in the remote atmospheric OH budget was secondary though important, accounting (in conjunction with NMHC oxidation products) for 10 to 20% of all OH removal during baseline periods and a substantially larger fraction during episodes of increased NMHC mixing ratios.

Model studies of the oxidation of light hydrocarbons in the troposphere and stratosphere

Abstract Our model studies of light non‐methane hydrocarbons (NMHCs) in the troposphere and lower stratosphere for seasonally varying insolation and transport conditions indicate that NMHC mixing ratios (MRs) are generally highest in the winter. These results reflect our assumptions of a constant surface source for each season. Comparisons of these calculated NMHC MRs with measurements suggest that the sources of the light NMHC may vary seasonally. There is a serious discrepancy between calculated and observed values of formic and acetic acids. It appears that the direct emissions or the aqueous phase chemistry of these acids, or their precursors, are of importance. An injection of NMHCs into the upper troposphere to stimulate transport of boundary‐layer air by rapid vertical convection in clouds, has a significant impact on the local chemistry on a time‐scale of a few days. The most important effects besides the increase in NMHCs and their products are the generation of peroxyacetyl nitrate precursors an...

Two‐dimensional distribution of light hydrocarbons: Results from the STRATOZ III experiment

During the STRATOZ III flights in June 1984, about 120 whole‐air samples were collected and analyzed later on in the laboratory for several atmospheric trace components, including light hydro‐carbons. The STRATOZ III mission consisted of 21 flight segments and covered a latitude range from 70°N to 60°S and altitudes up to 12 km. The results of these measurements were used to construct latitude‐altitude profiles in the form of isolines for ethane, ethene, acetylene, propane, propene, n‐butane, isobutane, n‐pentane, and isopentane. These results are compared with the latitudinal cross sections obtained during a previous, very similar flight mission (STRATOZ II) in May–June 1980. Also, the few published latitudinal or vertical profiles for these nonmethane hydrocarbons (NMHCs) are used for a comparison. The two‐dimensional distributions for the longer‐lived NMHCs, especially ethane and to some extent also propane and acetylene, are reasonably representative, even on a global scale. The high variability of the short‐lived NMHCs, the C4 and C5 alkanes, and the light alkenes, prevents the determination of representative two‐dimensional distributions for these species. The distributions of these short‐lived compounds give at best an extremely rough idea on the distributions and should in general be considered as descriptions of a given momentary situation. These latitude‐altitude profiles indicate the existence of fast mechanisms for vertical mixing in the troposphere. The observation of high mixing ratios of short‐lived hydrocarbons in the middle and upper troposphere proves the existence of vertical mixing processes with a time scale comparable to, or even shorter than, the atmospheric lifetime of these reactive NMHCs. As a consequence, there exist several regions, even above the boundary layer, with NMHC mixing ratios high enough to make them important participants in the atmospheric photochemical reaction cycles.

Hydrocarbons and carbon monoxide in African savannah air

Recent measurements of tropospheric mixing ratios of methane, non‐methane hydrocarbons (NMHC), and carbon monoxide (CO) from the Kenyan savannah are reported. NMHC mixing ratios are among the lowest reported for the continental boundary layer. CO mixing ratios are higher than marine measurements at these latitudes. Biomass burning may contribute significantly to mixing ratios of CO and NMHC's. Calculations based on the reactivity of the OH radical with NMHC's and CO indicate that NMHC's often initially consume more OH than CO.

Methane and nonmethane hydrocarbon concentrations in the North and South Atlantic marine boundary layer

Methane and nonmethane hydrocarbon (NMHC) data were collected in the Atlantic marine boundary layer from 40°N to 32°S latitude during October and November 1980. Average volume mixing ratios for CH4 of 1.69±0.02 ppmv were obtained north of the intertropical convergence zone (ITCZ) and 1.60±0.02 ppmv south of the ITCZ. The hemispheric CH4 gradient was very sharp, decreasing rapidly between 14°N and 10°N latitude. Gaseous NMHC concentrations (based on response as CH4) were measured simultaneously and found to lie consistently within the standard error for the individual measurements (±0.02 ppmv) after correction for water vapor interference. The correction for water vapor response was large, frequently as much as 80% of the raw NMHC signal. Application of the student's t distribution test to the NMHC data set established that the mean northern and southern hemispheric NMHC mixing ratios measured, 0.016 and 0.010 ppmv, respectively, were statistically different at the 95% confidence level. These results indicate volatile NMHC mixing ratios average less than 0.02 ppmv over the Atlantic and represent the first set of semicontinuous NMHC measurements spanning the North and South Atlantic marine boundary layer.