Anthropogenic Non-methane Hydrocarbons Research Articles

Strong relations of peroxyacetyl nitrate (PAN) formation to alkene and nitrous acid during various episodes

Peroxyacetyl nitrate (PAN) is one of the critical secondary pollutants in photochemical smog. This study investigated the relationship between PAN and PAN precursors with the Regional Atmospheric Chemical Mechanism version 2 model in six episodes recorded in Zhengzhou. In all episodes, peroxyacetyl radical (PA) was primarily produced by acetaldehyde oxidation, with more than 70% contributions. In photochemical episodes and photochemical-haze co-occurring episodes (combined episodes), methylglyoxal secondarily contributes 8.1%–10.6% to PA while in haze pollution, the propagation of other radicals to PA is the second most important source (12.0%–19.1%). Among anthropogenic non-methane hydrocarbons, alkene restricted PAN formation as first-generation precursors, with the relative incremental reactivity of PAN (RIRPAN) more than 0.6 during three-type episodes. Nitrous acid (HONO) also played important role in PAN formation. Especially during photochemical episodes, RIRPAN(HONO) reached 0.79, which was comparable to the RIRPAN value of alkene. Through sensitivity analysis of the relative formation of PAN to O3 (the amount of PAN generated when 100 ppb O3 formed), HONO was identified as the key precursor of PAN in haze pollution by promoting the oxidation of NMHC, while alkene predominated the relative formation of PAN to O3 in photochemical and combined pollution through producing acetaldehyde. The sensitivity of PAN to HONO is obviously enhanced with higher NOx/VOC ratios during photochemical and combined pollution.

Ambient Non-Methane Hydrocarbons (NMHCs) Measurements in Baoding, China: Sources and Roles in Ozone Formation

Ambient non-methane hydrocarbons (NMHCs) are important precursors of ozone (O3) and secondary organic aerosol (SOA). Online and offline measurements of NMHCs were conducted during September 2015 in Baoding, Hebei province of China, in order to investigate their sources and roles in ozone formation. Average levels of total NMHCs online measured at the urban site were 44.5 ± 26.7 ppb. Aromatics was the largest contributor to NMHCs levels and OH reactivity, with fraction of 27.1% and 35.9%, respectively. Based on offline measurements at eight sites, we found that toluene, ethylbenzene, and m,p-xylene displayed the highest level at the site close to automobile manufacturing factories, followed by downwind receptor sites and other sites. Positive matrix factorization (PMF) model was then used to analyze NMHCs sources. Four factors were identified, including traffic-related emission, automobile manufacturing coating, biogenic emission, and NG/LPG usage and background. Average relative contribution of automobile manufacturing coating to NMHCs levels during the entire online measurement period was 33.4%, and this value increased to 42% during two O3 pollution days. Sensitivity of O3 formation to NMHCs and NOX during an O3 pollution episode were analyzed using a box model based on observations. Relative incremental reactivity (RIR) results suggested that O3 formation was in NOx-titration regime (i.e., highly NMHCs-limited regime). Further scenario analyses on relationship of O3 formation with reduction of NOx and anthropogenic NMHCs (AHC) indicated that AHC and NOx should be reduced by a ratio greater than two and three to achieve 5% and 10% O3 control objectives, respectively. The largest RIR value for anthropogenic NMHC species was from xylenes, which were also an important contributor to SOA formation and dominantly from sources related to automobile manufacturing coating and traffic emission. This means reducing NMHCs emission from automobile manufacturing coating and traffic emission should be given priority for synergetic control of O3 and PM2.5.

The weekday/weekend ozone differences induced by the emissions change during summer and autumn in Guangzhou, China

Weekday/weekend mixing ratios of ozone (O3) and the O3 precursors of non-methane hydrocarbons (NMHCs) and nitrogen oxides (NOx) were recorded at the Guangzhou Panyu Atmospheric Composition Station (GPACS), a suburban site in Guangzhou, during the summer (June, July, and August) and autumn (September, October, and November) of 2011. In both summer and autumn, weekday/weekend O3 differences in the morning and at midday largely depend on how much the O3 precursors are affected by anthropogenic emissions. In the mornings (6:00–9:00 LT), pollutants (i.e. NOx and NMHCs) were more strongly influenced by vehicular emissions in autumn than in summer. In autumn, O3 titration and lower NOx on weekends in NMHCs-limited regimes lead to more rapid O3 production, which resulted in the O3 weekend effect during autumn morning. No O3 weekend effect occurred on summer mornings because O3 formation was in a NOx-limited regime, although O3 titration still existed. At midday (10:00–16:00 LT), the increase of biogenic NMHCs emissions reversed the sensitivity of O3 production from NMHCs-to NOx-sensitive. The weekday/weekend diurnal pattern of vehicular sources was the same at midday, more intense other human industrial activities in autumn not only gave rise to the higher mixing ratios of high-reactive anthropogenic NMHCs (e.g. aromatics) on weekdays, but also could affected the temperature in the city, leading to higher isoprene mixing ratio on weekdays. All these factors are likely to contribute to the O3 weekday effect in autumn. Meanwhile no weekday O3 effect occurred in summer due to the low-intensity industrial anthropogenic activities. Our results show that high-reactive NMHCs and NOx control can be effective for reducing peak O3 mixing ratios in Guangzhou. Further investigation based on numerical models is required to reach more robust conclusions.

Trends of non-methane hydrocarbons (NMHC) emissions in Beijing during 2002–2013

Abstract. Non-methane hydrocarbons (NMHCs) play a critical role in the photochemical production of ozone (O3) and organic aerosols. Obtaining an accurate understanding on temporal trends of NMHC emissions is essential for predicting air quality changes and evaluating the effectiveness of current control measures. In this study, we evaluated temporal trends of anthropogenic NMHC emissions during August in Beijing based on ambient measurements during selected summer periods at an urban site in Beijing from 2002 to 2013. In the contrast to the results from the most recent inventory (Multi-resolution Emission Inventory for China, MEIC), which reported that anthropogenic NMHC emissions during August increased by 28% from 2004 to 2012, whereas mixing ratios of NMHCs measured at this urban site decreased by 37% during the same time period. A positive matrix factorization (PMF) model was applied to these NMHC measurements for source apportionment. The results showed that the relative contributions of vehicular exhaust and gasoline evaporation to measured NMHC concentrations decreased by 66% during August from 2004 to 2012, comparable to the relative decline of 67% for transportation-related NMHC emissions reported by the MEIC inventory. This finding indicates that the implementation of stricter emissions standards for new vehicles and specific control measures for in-use vehicles has been effective for reducing transportation-related NMHC emissions. In addition, the PMF results suggested that there were no significant temporal changes in NMHC concentrations from solvent use and industry during August from 2004 to 2012, in contrast with the rapid rate of increase (8.8% yr−1) reported by the MEIC inventory. To re-evaluate the NMHC emissions trends for solvent use and industry, annual variations in NMHC / NOx ratios were compared between ambient measurements at the PKU site and the MEIC inventory. In contrast to the significant rise in NMHC / NOx ratios from the MEIC inventory, the measured NMHC / NOx ratios declined by 14% during August from 2005 to 2012. The inferred NMHC / NOx ratios based on PMF results exhibited a comparable decline of 11% to measured ratios. These results indicate that the increase rate for NMHC emissions from solvent use and industry in Beijing might be overestimated in the current inventory; therefore, additional research is necessary to verify the NMHC emission trends for this source.

Spatial and seasonal variability of measured anthropogenic non-methane hydrocarbons in urban atmospheres: Implication on emission ratios

Continuous measurements of a wide range of non-methane hydrocarbons (NMHC) have been performed since 2001 in Paris megacity and three French medium-sized cities (Grenoble, Marseille, and Strasbourg). After a careful verification of the data measured, the ambient concentrations are used to analyze the spatial and seasonal variability of the anthropogenic NMHC and determine the present NMHC emission ratios relative to acetylene, a useful metric to evaluate and constraint emission inventories. We show that NMHC urban composition is consistent between all cities with no industrial influence and characteristic of the urban emission mixtures, which are mostly dominated by vehicle exhaust emissions. In winter, the urban NMHC composition generally shows an enhancement in combustion-derived products (alkenes, acetylene), C2–C3 alkanes and benzene, which presumes seasonal changes in emission ratio values. Present emission ratios of NMHC relative to acetylene are determined in Paris and Strasbourg both in summer and winter. They generally compare within a factor of two except for C7–C9 aromatics in Paris. On a seasonal basis, summertime emission ratios are three times higher than wintertime ones while they stay constant for combustion derived product (alkenes) and benzene. The unburned gasoline fraction (alkanes and C7–C9 aromatics) shows the maximum difference up to a factor of seven. These findings suggest that the emission ratios reflect seasonal changes in emissions and can be a useful metric to constraint temporally resolved emission inventories at different time of the year.

Aromatic hydrocarbons as ozone precursors before and after outbreak of the 2008 financial crisis in the Pearl River Delta region, south China

In the second half of 2008 China's highly industrialized Pearl River Delta (PRD) region was hard‐hit by the financial crisis (FC). This study reports volatile organic compounds measured in the PRD during November–December in both 2007 before the FC and 2008 after the FC. While total mixing ratios of non‐methane hydrocarbons (NMHCs) on average were only about 7% lower from 40.2 ppbv in 2007 to 37.5 ppbv in 2008, their ozone formation potentials (OFPs) dropped about 30%, resulting from about 55% plummet of aromatic hydrocarbons (AHs) against a greater than 20% increase of total alkanes/alkenes. The elevated alkanes and alkenes in 2008 could be explained by greater emissions from vehicle exhausts and LPG combustion due to rapid increase of vehicle numbers and LPG consumption; the drop of AHs could be explained by reduced emissions from industries using AH‐containing solvents due to the influence of the FC, as indicated by much lower ratios of toluene to benzene and of xylenes/trichloroethylene/tetrachloroethylene to carbon monoxide (CO) in 2008. Source apportionment by positive matrix factorization (PMF) also revealed much less contribution of industry solvents to total anthropogenic NMHCs and particularly to toluene and xylenes in 2008 than in 2007. Based on PMF reconstructed source contributions, calculated OFPs by industrial emissions were responsible for 40.8% in 2007 in contrast to 18.4% in 2008. Further investigation into local industry output statistics suggested that the plummet of AHs in 2008 should be attributed to small enterprises, which contributed largely to ambient AHs due to their huge numbers and non‐existent emission treatment, but were much more influenced by the FC.

Seasonal and Diurnal Variations of Atmospheric Non-Methane Hydrocarbons in Guangzhou, China

In recent decades, high ambient ozone concentrations have become one of the major regional air quality issues in the Pearl River Delta (PRD) region. Non-methane hydrocarbons (NMHCs), as key precursors of ozone, were found to be the limiting factor in photochemical ozone formation for large areas in the PRD. For source apportioning of NMHCs as well as ozone pollution control strategies, it is necessary to obtain typical seasonal and diurnal patterns of NMHCs with a large pool of field data. To date, few studies have focused on seasonal and diurnal variations of NMHCs in urban areas of Guangzhou. This study explored the seasonal variations of most hydrocarbons concentrations with autumn maximum and spring minimum in Guangzhou. The diurnal variations of most anthropogenic NMHCs typically showed two-peak pattern with one at 8:00 in the morning and another at 20:00 in the evening, both corresponding to traffic rush hours in Guangzhou, whereas isoprene displayed a different bimodal diurnal curve. Propene, ethene, m, p-xylene and toluene were the four largest contributors to ozone formation in Guangzhou, based on the evaluation of individual NMHCs’ photochemical reactivity. Therefore, an effective strategy for controlling ozone pollution may be achieved by the reduction of vehicle emissions in Guangzhou.

Seasonal characteristics of tropical marine boundary layer air measured at the Cape Verde Atmospheric Observatory

Observations of the tropical atmosphere are fundamental to the understanding of global changes in air quality, atmospheric oxidation capacity and climate, yet the tropics are under-populated with long-term measurements. The first three years (October 2006–September 2009) of meteorological, trace gas and particulate data from the global WMO/Global Atmospheric Watch (GAW) Cape Verde Atmospheric Observatory Humberto Duarte Fonseca (CVAO; 16° 51′ N, 24° 52′ W) are presented, along with a characterisation of the origin and pathways of air masses arriving at the station using the NAME dispersion model and simulations of dust deposition using the COSMO-MUSCAT dust model. The observations show a strong influence from Saharan dust in winter with a maximum in super-micron aerosol and particulate iron and aluminium. The dust model results match the magnitude and daily variations of dust events, but in the region of the CVAO underestimate the measured aerosol optical thickness (AOT) because of contributions from other aerosol. The NAME model also captured the dust events, giving confidence in its ability to correctly identify air mass origins and pathways in this region. Dissolution experiments on collected dust samples showed a strong correlation between soluble Fe and Al and measured solubilities were lower at high atmospheric dust concentrations. Fine mode aerosol at the CVAO contains a significant fraction of non-sea salt components including dicarboxylic acids, methanesulfonic acid and aliphatic amines, all believed to be of oceanic origin. A marine influence is also apparent in the year-round presence of iodine and bromine monoxide (IO and BrO), with IO suggested to be confined mainly to the surface few hundred metres but BrO well mixed in the boundary layer. Enhanced CO2 and CH4 and depleted oxygen concentrations are markers for air-sea exchange over the nearby northwest African coastal upwelling area. Long-range transport results in generally higher levels of O3 and anthropogenic non-methane hydrocarbons (NMHC) in air originating from North America. Ozone/CO ratios were highest (up to 0.42) in relatively fresh European air masses. In air heavily influenced by Saharan dust the O3/CO ratio was as low as 0.13, possibly indicating O3 uptake to dust. Nitrogen oxides (NOx and NOy) show generally higher concentrations in winter when air mass origins are predominantly from Africa. High photochemical activity at the site is shown by maximum spring/summer concentrations of OH and HO2 of 9 × 106 molecule cm−3 and 6 × 108 molecule cm−3, respectively. After the primary photolysis source, the most important controls on the HOx budget in this region are IO and BrO chemistry, the abundance of HCHO, and uptake of HOx to aerosol.

Multi-year (2004–2008) record of nonmethane hydrocarbons and halocarbons in New England: seasonal variations and regional sources

Abstract. Multi-year time series records of C2-C6 alkanes, C2-C4 alkenes, ethyne, isoprene, C6-C8 aromatics, trichloroethene (C2HCl3), and tetrachloroethene (C2Cl4) from canister samples collected during January 2004–February 2008 at the University of New Hampshire (UNH) AIRMAP Observatory at Thompson Farm (TF) in Durham, NH are presented. The objectives of this work are to identify the sources of nonmethane hydrocarbons (NMHCs) and halocarbons observed at TF, characterize the seasonal and interannual variability in ambient mixing ratios and sources, and estimate regional emission rates of NMHCs. Analysis of correlations and comparisons with emission ratios indicated that a ubiquitous and persistent mix of emissions from several anthropogenic sources is observed throughout the entire year. The highest C2-C8 anthropogenic NMHC mixing ratios were observed in mid to late winter. Following the springtime minimums, the C3-C6 alkanes, C7-C8 aromatics, and C2HCl3 increased in early to mid summer, presumably reflecting enhanced evaporative emissions. Mixing ratios of C2Cl4 and C2HCl3 decreased by 0.7±0.2 and 0.3±0.05 pptv/year, respectively, which is indicative of reduced usage and emissions of these halogenated solvents. Emission rates of C3-C8 NMHCs were estimated to be 109 to 1010 molecules cm−2 s−1 in winter 2006. The emission rates extrapolated to the state of New Hampshire and New England were ~2–60 Mg/day and ~12–430 Mg/day, respectively. Emission rates of benzene, toluene, ethylbenzene, xylenes, and ethyne in the 2002 and 2005 EPA National Emissions Inventories were within ±50% of the TF emission rates.

Nonmethane hydrocarbons at Pico Mountain, Azores: 1. Oxidation chemistry in the North Atlantic region

Measurements of nonmethane hydrocarbons (NMHC) at the Pico Mountain observatory at 2225 m asl on Pico Island, Azores, Portugal, from August 2004 to August 2005 (in part overlapping with the field campaign of the International Consortium on Atmospheric Research on Transport and Transformation study) were used to investigate NMHC sources and seasonal oxidation chemistry in the central North Atlantic region. Levels of anthropogenic NMHC were characteristic of the marine free troposphere. Their concentrations were low compared to continental sites at higher northern latitudes, but higher than data reported from a similarly located Pacific mountain site at Mauna Loa Observatory, Hawaii. These higher NMHC levels are indicative of a greater influence of the adjacent continents on air composition at Pico. Substantially enhanced NMHC concentrations during the summers of 2004 and 2005 were attributed to long‐range transport of biomass burning plumes originating from fires in northern Canada, Alaska, and Siberia. This finding exemplifies the continuing impact of biomass burning plumes on atmospheric composition and chemistry many days downwind of these emission sources. Seasonal cycles with lower NMHC concentrations and lower ratios of more reactive to less reactive NMHC during summer reflect the higher degree of photochemical processing occurring during transport. The NMHC concentrations indicate no significant role of chlorine atom oxidation on NMHC. Ozone above 35 ppbv was measured at Pico Mountain throughout all seasons. Enhanced ozone levels were observed in air that had relatively “fresh” photochemical signatures (e.g., ln [propane]/[ethane] > −2.5). During spring‐summer air that was more processed (“older” air with ln [propane]/[ethane] < −2.5) on average had lower ozone levels (down to <20 ppbv). This relationship indicates that conditions in the lower free troposphere over the mid–North Atlantic during the spring and summer lead to net photochemical ozone destruction while air is photochemically aging during transport to Pico. This behavior contrasts to that in the mid–North Pacific where other recent studies have found that the photochemistry is more nearly ozone neutral.

Nonmethane hydrocarbon and oxy hydrocarbon measurements during the 2002 New England Air Quality Study

Nonmethane hydrocarbons (NMHCs) and oxy hydrocarbons (oxy HCs) were measured aboard the National Oceanic and Atmospheric Administration research vessel Ronald H. Brown during the New England Air Quality Study from 13 July to 10 August 2002 by an online dual gas chromatographic instrument with two separate analytical columns equipped, respectively, with flame ionization and mass spectrometer detectors. Measurements, taken each half hour, included C2 to C10 alkanes, C2 to C5 alkenes, alcohols and ketones, C6 to C9 aromatics, and biogenic volatile compounds including six monoterpenes, isoprene and its immediate oxidation products methacrolein and methylvinylketone. All compounds have been categorized by their contribution to the OH loss rate calculated for 298K and 1 atm. Large temporal variability was observed for all compounds. Airflow from the Providence, Rhode Island/Boston, Massachusetts, urban corridor northeast to the New Hampshire coast was usually heavily laden with NMHCs and oxy HCs of anthropogenic origin. Comparison of specific compound ratios with automotive tunnel studies suggested that these were predominantly mobile source emissions. When such flow occurred during daylight hours, these urban plumes were accompanied by increases in ozone in the 80 to 120 ppbv range. About equally as often, much less chemically mature NMHC plumes were encountered near the New Hampshire coast. Ozone was titrated out of these latter plumes, and the unusually high mixing ratios of C4 and C5 alkenes suggested that their source was partly gasoline vapor release rather than mobile source emissions. In the New England coastal region explored, in spite of the large anthropogenic NMHC input during periods of offshore flow, OH loss with hydrocarbons was frequently dominated by compounds of biogenic origin. During periods of cleaner marine air inflow the OH loss rate was dominated by reaction with methane and with oxy HCs, predominantly acetone, formaldehyde, and acetaldehyde.

Characterising sources and sinks of rural VOC in eastern France

Fifty non-methane hydrocarbons (NMHC) and seventeen carbonyl compounds were measured at a French rural site from 1997 to 2001, as part of the EMEP programme. Data handling was based on an original source–receptor approach. First, the examination of the levels and trends was completed by the comparison of the seasonal distribution of rural and urban VOC/acetylene ambient ratios. This analysis has shown that most of the compounds derived from mixing and photochemical transformation of mid-range transported urban pollutants from the downwind urban area. Then, identified sources and sinks were temporally apportioned. Urban air masses mixing explains, at least, 80% of the wintertime levels of anthropogenic NMHC and isoprene. In summer, photochemistry dominates the day-to-day distribution of anthropogenic NMHC whilst summertime isoprene is also controlled by in-situ biogenic emissions. Then, the results of C 1–C 3 carbonyls were discussed with respect to their direct biogenic and anthropogenic emissions and photochemical production through the {carbonyl/auto-exhaust tracers} emission ratio. Diluted vehicle exhaust emissions mainly contribute to the total content of lower aldehydes in winter while other processes control lower ketones. Secondary production is predominant in summer with at least a 50% high intensity. Its dependence upon temperature and radiation is also demonstrated. Finally, the importance of the primary and secondary biogenic production of acetone and formaldehyde is assessed. In particular, biogenic contribution would explain 37 ± 25% of acetone levels in summer.

Ground-Based and Airborne Measurements of Nonmethane Hydrocarbons in BERLIOZ: Analysis and Selected Results

During the Berlin Ozone Experiment BERLIOZ in July–August 1998 quasi-continuous measurements ofC2–C12 nonmethane hydrocarbons (NMHCs) were carried out at 10 sites in and around the city of Berlin using on-line gas-chromatographic systems (GCs) with a temporal resolution of 20–120 minutes. Additional airborne NMHCmeasurements were made using canister sampling on three aircraft and an on-line GC system on a fourth aircraft. The ground based data are analyzed to characterize the different sites and to identify the influence of emissions from Berlin on its surroundings. Benzene mixing ratios at the 4 rural sites were rather low (<0.5 ppbv). Berlin (and the surrounding highway ring) was identified as the main source of anthropogenic NMHCs at Eichstadt and Blossin, whilst other sources were important at the furthermost site Menz. The median toluene/benzene concentration ratio in Berlin was 2.3 ppbv/ppbv, agreeing well with measurements in other German cities. As expected, the ratios at the background sites decreased with increasing distance to Berlin and were usually around one or below. On 20 and 21 July, the three northwesterly sites were situated downwind of Berlin and thus were influenced by its emissions. Considering the distance between the sites and the windspeed, the city plume was observed at reasonable time scales, showing decreasing toluene/benzene ratios of 2.3, 1.6 and 1.3 with increasing distance from Berlin. Isoprene was the only biogenic NMHC measured at BERLIOZ. It was themost abundant compound at the background sites on the hotter days, dominating the local NMHC reactivity with averaged contributions to the total OH loss rate of 51% and 70% at Pabstthum and Blossin, respectively. Emissionratios (relative to CO and to the sum of analysed NMHCs) were derived from airborne measurements. The comparison with an emission inventory suggests traffic-related emissions to be the predominating source of the considered hydrocarbon species. Problems were identified with the emission inventory for propane, ethene and pentanes.

Statistical study of the diurnal variation of modeled and measured NMHC contributions

The aim of the EVA project is to investigate the precision of modeled emission inventories for a city based on experimental data from two field campaigns in March and October 1998 (Slemr et al., J. Atmos. Chem., 2001). According to the results of the emission inventory model (Kühlwein et al., Atmos. Environ., this issue) for the campaigns, the measured nonmethane hydrocarbons (NMHCs) should originate predominantly from evaporation processes. In this paper these model results are discussed from a meteorological point of view. The results from atmospheric dispersion calculations with a Gaussian plume model are compared with measurements of NMHCs and CO at a site few kilometers downwind of the city of Augsburg. The dispersion model results point out that the correlations of the individual NMHCs against CO lead to different qualities of correlation for NMHCs co-emitted with CO (i.e. from combustion processes) and NMHCs which are not co-emitted with CO (i.e. from evaporation processes). However, using the correlation coefficients r 2, as a measure of the quality of the correlation, the correlations of several NMHCs for which strong evaporation emissions are predicted from the emission inventory model do not show remarkable differences in the r 2 when compared with the correlations for species predominantly emitted from combustion processes. Thus, the NMHC emissions are found to be dominated by emissions from combustion processes and the influence of evaporation processes on anthropogenic NMHC emissions seems to be considerably overestimated by the emission model. This is in agreement with the results of Mannschreck et al. (Atmos. Environ., 2001) which are based on the NMHC composition of the Augsburg city plume.

Non-methane hydrocarbon variability during the FIELDVOC'94 campaign in Portugal

C2–C6 non-methane hydrocarbons (NMHC), including isoprene, were measured in a forested area of Portugal from June 20 to July 12, 1994 as part of the FIELDVOC'94 project. The day-to-day variability of the measured NMHC was clearly linked to the air mass origin. The longest-lived ethane did not show any significant diurnal variation. The diurnal variability of the C3–C5 alkanes and of acetylene presented low amplitude attributed to the changeable wind direction between northwest and southeast. The short-lived alkenes showed a well-defined diurnal variation with maximum mixing ratios during night-time and minimum during daytime resulting from both dynamical and photochemical sinks. Isoprene presented a very different trend with minima of 0.1 ppbv at night and maxima up to 12 ppbv occurring between 16:00 and 18:00 (UT). In continental air masses, the contribution of isoprene to the daytime ozone production was 3- to 6-fold greater than that of the other NMHC. In air masses of marine origin, the effect of isoprene on ozone and hydroxyl radicals was significantly reduced relative to the anthropogenic NMHC, which were dominating at night. In all cases, isoprene was a more efficient consumer of nitrate radicals than these NMHC. The observed decrease of isoprene at night was consistent with the measured nitrate radical mixing ratios of a few pptv.

Airborne measurements of isoprene, CO, and anthropogenic hydrocarbons and their implications

Measurements of the mixing ratios of light hydrocarbons (≥C6) were made aboard the National Oceanic and Atmospheric Administration Lockheed Orion WP‐3 aircraft during June and July of 1994 and 1995 during a series of flights over the region around Nashville, Tennessee. The measurements were carried out as part of the Nashville/Middle Tennessee Study of the Southern Oxidant Study (SOS) whose purpose was to describe the sources, variation, and distribution of ozone and its precursors in the southeastern United States during the summer season. The spatial (altitude and latitude) distribution of isoprene and anthropogenic nonmethane hydrocarbons (NMHCs) is described. The isoprene distribution measured within the planetary boundary layer over the region is compared to the recent inventories. The correlations between the various anthropogenic hydrocarbons are used to demonstrate the combined influence of OH photochemistry and dilution on their concentrations. Finally, the distributions of those NMHCs that were measured are compared to those of CO and methane and discussed in terms their implications for odd hydrogen photochemistry and our understanding of regional ozone production. In the boundary layer, OH reactions are dominated by isoprene in the southern part of the region explored and by CO and methane in the northern part. The other measured NMHCs are seen to play only a minor role in ozone production in the nonurban atmosphere.

Analysis of radical propagation efficiency to assess ozone sensitivity to hydrocarbons and NOx: 1. Local indicators of instantaneous odd oxygen production sensitivity

We used a simple trajectory model and a three‐dimensional grid model to evaluate several indicators that can be used to predict the sensitivity of odd oxygen production (P(Ox)) to changes in emissions of anthropogenic nonmethane hydrocarbons (VOC) and nitrogen oxides (NOx = NO + NO2). To perform the evaluation, we augmented the model to include new diagnostic outputs such as rates of P(Ox), rates of conversion of NOx to unreactive nitrogen, and radical chain length. We used the new diagnostic outputs to explain the model‐predicted sensitivity of P(Ox) to changes in VOC and NOx emissions. We found that the ozone ridgeline, which distinguishes between NOx‐limited and radical‐limited conditions, is determined by a ridgeline of maximum OH chain length. We examined the radical propagation reactions which affect OH chain length, and we developed four indicators related to radical propagation efficiency: I(HC,NO2) which approximates the fraction of OH that reacts with hydrocarbons; I(NO,RO2) which approximates the fraction of HO2 that reacts with NO; the ratio of [O3]/[NOx] which affects the balance between radical initiation and propagation; and [HO2] which is the dominant term in P(H2O2)/P(HNO3). Each of these indicators distinguishes conditions in which instantaneous P(Ox) is primarily sensitive to either NOx emissions reductions or VOC emissions reductions.

Biogenic and anthropogenic fluxes of non-methane hydrocarbons over an urban-imapcted forest, Frankfurter Stadtwald, Germany

In an urban-impacted oak/beech/pine forest (Frankfurter Stadtwald, 50° 04′ 06″ N; 8° 40′ 17″ E) trace gas distributions and fluxes of anthropogenic and biogenic non-methane hydrocarbons (NMHC) were determined for a bright weather period in August 1995. In general, ozone peaked at 70 ppb in the early afternoon. NO and NO x reach values of up to 25 ppb under low wind conditions and local automobile traffic. Anthropogenic NMHC dominate with up to 8.0 ppb in the air above the forest. The dominating biogenic NMHC in ambient air above the forest was isoprene with peak values of 1.5 ppb during daytime. The flux-gradient relationship with specific adapted and validated stability functions for this forest was used for calculating NMHC-fluxes. Transfer times of up to 100 s require a correction of the mixing ratios for HO-radical chemistry occurring along the gradient between 22 and 51 m for high reactive substances such as isoprene. The specific situation in the Frankfurter Stadtwald with high road traffic inside the forest (up to 10,000 vehicles per hour) lead to sometimes significant emission of anthropogenic NMHC as exhaust plumes were spread in the trunk space. Isoprene fluxes were high and amounted to 3.5 nmol m −2 ground area s −1 due to the high percentage of oaks growing in the forest but were at the lower end of estimates made by current biogenic NMHC emission inventories. The high isoprene emission flux and ambient air mixing ratios underscore the importance of isoprene daytime and nighttime chemistry for the Frankfurt area.

Inverse modeling of the global CO cycle: 2. Inversion of 13C/12C and 18O/16O isotope ratios

Following a three‐dimensional inverse modeling study on atmospheric CO mixing ratios [Bergamaschi et al., this issue], we present the expansion of our model for the treatment of the stable isotope ratios 13C/12C and 18O/16O in carbon monoxide. The individual isotopomers were included in the model as independent tracers, and the inversion scheme was extended, allowing the simultaneous optimization (with respect to observational data) of modeled atmospheric mixing and isotope ratios. Observational data of 13C/12C and 18O/16O ratios were taken from a set of five globally distributed sites, all of which exhibit pronounced seasonal cycles and which as a whole clearly define large latitudinal gradients. Incorporation of 13C/12C ratios in the inversion resulted in a clear constraining of the total amount of CO arising from CH4 oxidation. The present study suggests an average CO yield of 86% (80–88%), provided that (1) the reaction of CH4+Cl has no significant impact on the δ 13C of the resulting CO and (2) CO from the ocean helps to balance the δ13C in the Southern Hemisphere. Otherwise, a further reduced CO yield would be necessary, as low as 71% in case of an average kinetic isotope effect of 1.013 [Lowe et al., 1999] or 69% in case of a δ13C value of −25.0‰ for the oceanic source. Despite a lack of experimental investigations on 18O/16O in CO arising from CH4 or nonmethanehydrocarbons (NMHC) oxidation, inclusion of 18O/16O into the inversion gives further significant constraints on the global CO cycle. It is shown that apart from the large technological CO source, additional sources with dominant emissions in the Northern Hemisphere are required, such as biogenic emissions (either direct or via oxidation of biogenic NMHCs) or anthropogenic NMHCs, in order to reproduce observed atmospheric 18O/16O ratios. The extended inversion scheme allows CO budgets to be derived, which reproduce, within two standard deviations of observational data, simultaneously CO mixing ratios (from the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory network), 13C/12C, and 18O/16O ratios. Incorporation of stable isotopes renders inversion results much more robust compared to inversions of CO mixing ratios only.

Impact of Non-Methane Hydrocarbons on Tropospheric Chemistry and the Oxidizing Power of the Global Troposphere: 3-Dimensional Modelling Results

The impact of natural and anthropogenicnon-methane hydrocarbons (NMHC) on troposphericchemistry is investigated with the global,three-dimensional chemistry-transport model MOGUNTIA.This meteorologically simplified model allows theinclusion of a rather detailed scheme to describeNMHC oxidation chemistry. Comparing model resultscalculated with and without NMHC oxidation chemistryindicates that NMHC oxidation adds 40–60% to surfacecarbon monoxide (CO) levels over the continents andslightly less over the oceans. Free tropospheric COlevels increase by 30–60%. The overall yield of COfrom the NMHC mixture considered is calculated to beabout 0.4 CO per C atom. Organic nitrate formationduring NMHC oxidation, and their transport anddecomposition affect the global distribution of NO x and thereby O3 production. The impact of theshort-lived NMHC extends over the entire tropospheredue to the formation of longer-lived intermediateslike CO, and various carbonyl and carboxyl compounds.NMHC oxidation almost doubles the net photochemicalproduction of O3 in the troposphere and leads to20–80% higher O3 concentration inNO x -rich boundarylayers, with highest increases over and downwind ofthe industrial and biomass burning regions. Anincrease by 20–30% is calculated for the remotemarine atmosphere. At higher altitudes, smaller, butstill significant increases, in O3 concentrationsbetween 10 and 60% are calculated, maximizing in thetropics. NO from lightning also enhances the netchemical production of O3 by about 30%, leading to asimilar increase in the global mean OH radicalconcentration. NMHC oxidation decreases the OH radicalconcentrations in the continental boundary layer withlarge NMHC emissions by up to 20–60%. In the marineboundary layer (MBL) OH levels can increase in someregions by 10–20% depending on season and NO x levels.However, in most of the MBL OH will decrease by10–20% due to the increase in CO levels by NMHCoxidation chemistry. The large decreases especiallyover the continents strongly reduce the markedcontrasts in OHconcentrations between land and oceanwhich are calculated when only the backgroundchemistry is considered. In the middle troposphere, OHconcentrations are reduced by about 15%, although dueto the growth in CO. The overall effect of thesechanges on the tropospheric lifetime of CH4 is a 15%increase from 6.5 to 7.4 years. Biogenic hydrocarbonsdominate the impact of NMHC on global troposphericchemistry. Convection of hydrocarbon oxidationproducts: hydrogen peroxides and carbonyl compounds,especially acetone, is the main source of HO x in theupper troposphere. Convective transport and additionof NO from lightning are important for the O3 budgetin the free troposphere.