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The atmospheric global budget and distribution of C 1 –C 3 alkyl nitrates have been investigated using a global three-dimensional chemistry transport model, STOCHEM-CRI. Alkyl nitrates (RONO 2 ) are significant NO x reservoir species and the more detailed VOC oxidation mechanism (CRI v2-R5) leads to greater photochemical production. RONO 2 are significant sources of NO x in regions remote from NO x sources. The study shows that the global burden and the atmospheric life-time of C 1 –C 3 alkyl nitrates are 113 Gg and 9–10 days, respectively, which are in excellent agreement with estimates established by previous studies. The abundance of alkyl nitrates have been found to be higher in the continental atmosphere, with CH 3 ONO 2 mixing ratios up to 20 ppt over the Amazon rainforest. Up to 15, 10, 2, and 5 ppt of modelled CH 3 ONO 2 , C 2 H 5 ONO 2 , n-C 3 H 7 ONO 2 and i-C 3 H 7 ONO 2 have been found in the northern hemisphere over regions with large anthropogenic emissions of NO x and VOCs. The combination of atmospheric production and long-range transport led to high alkyl nitrate levels at high latitudes. The model performance for C 1 –C 3 alkyl nitrates was established using observations from nine flights and nine field campaigns. The comparison shows a tendency towards model under-prediction of the observations, particularly in the southern hemispheric marine boundary layer, possibly due to the absence of oceanic production mechanisms and air-sea exchange processes in the model. The discrepancies between model and observed seasonal cycles, especially of CH 3 ONO 2 , in both hemispheres are discussed. [ABSTRACT FROM AUTHOR]

The impact of including a more detailed VOC oxidation scheme (CRI v2-R5) with a multi-generational approach for simulating tropospheric acetone is investigated using a 3-D global model, STOCHEM-CRI. The CRI v2-R5 mechanism contains photochemical production of acetone from monoterpenes which account for 64% (46.8 Tg/yr) of the global acetone sources in STOCHEM-CRI. Both photolysis and oxidation by OH in the troposphere contributes equally (42%, each) and dry deposition contributes 16% of the atmospheric sinks of acetone. The tropospheric life-time and the global burden of acetone are found to be 18 days and 3.5 Tg, respectively, these values being close to those reported in the study of Jacob et al. (2002). A dataset of aircraft campaign measurements are used to evaluate the inclusion of acetone formation from monoterpenes in the CRI v2-R5 mechanism used in STOCHEM-CRI. The overall comparison between measurements and models show that the parameterised approach in STOCHEM-NAM (no acetone formation from monoterpenes) underpredicts the mixing ratios of acetone in the atmosphere. However, using a detailed monoterpene oxidation mechanism forming acetone has brought the STOCHEM-CRI into closer agreement with measurements with an improvement in the vertical simulation of acetone. The annual mean surface distribution of acetone simulated by the STOCHEM-CRI shows a peak over forested regions where there are large biogenic emissions and high levels of photochemical activity. Year-long observations of acetone and methanol at the Mace Head research station in Ireland are compared with the simulated acetone and methanol produced by the STOCHEM-CRI and found to produce good overall agreement between model and measurements. The seasonal variation of model and measured acetone levels at Mace Head, California, New Hampshire and Minnesota show peaks in summer and dips in winter, suggesting that photochemical production may have the strongest effect on its seasonal trend. [ABSTRACT FROM AUTHOR]

Abstract: Formaldehyde and acetaldehyde concentrations over the Eastern half of the United States are simulated with a 3-D air quality model to identify the most important chemical precursors under January and July conditions. We find that both aldehydes primarily result from photochemical production, although 25% or more result from direct emissions in urban areas during winter. Isoprene is the major precursor of formaldehyde in most areas during summer, contributing 20–60% of total production, with the magnitude being spatially variable. Other alkenes from anthropogenic and/or biogenic emissions dominate formaldehyde production in winter, contributing 60–85% of total formation, and are prominent contributors in summer. Alkenes, including biogenic alkenes, dominate acetaldehyde production during both seasons. These conclusions are based on the degradation of emitted VOCs described by the SAPRC07TB chemical mechanism, but even this detailed model has difficulty reproducing observed values better than a factor of 2. The substantial role of isoprene and other alkenes in aldehyde formation emphasizes that we examine and improve emission estimates of these compounds. Until we can estimate the emissions and understand the chemistry of VOC precursors to aldehyde formation with greater certainty, it will be difficult to accurately predict atmospheric concentrations of aldehydes and develop strategies to reduce their concentrations. [Copyright &y& Elsevier]

Abstract: The photochemical characteristics of ozone (O3) and its main precursors (e.g. NOx and VOCs) were investigated in relation to the photochemical O3 budget and O3–NOx–VOC sensitivity, based on a modeling approach using data sets observed during intensive field campaigns conducted under various environmental conditions (i.e. urban (Seoul), rural (Yang Pyeong), landfill (Dae Gu), coastal (Gwang Yang), and remote sites (Jeju Island). The production of O3 under most environmental conditions (especially, spring season at the urban site) was found to be more strongly sensitive to VOCs than NOx. At the landfill site and during summer at the rural and coastal sites, the production of O3 was partially sensitive to NOx. On the other hand, despite the remote air on Jeju Island, the production of O3 was mainly VOC-sensitive (high NOx levels), but in part NOx-sensitive (low NOx levels), due to the transport processes from different environmental regions (e.g. heavily and less polluted locations). [Copyright &y& Elsevier]

Abstract: The influence of ship emissions on ozone (O3) concentrations in a coastal area (CA) including Busan port, Korea was examined based on a numerical modeling approach during a high O3 episode. The analysis was performed by two sets of simulation scenarios: (1) with ship emissions (e.g., TOTAL case) and (2) without ship emissions (e.g., BASE case). A process analysis (PA) (the integrated processes rate (IPR) and integrated reaction rate (IRR) analyses) was used to evaluate the relative contributions of individual physical and chemical processes in O3 production in and around the CA (e.g., sites of Dong Sam (DS) and Dae Yeon (DY)). The model study suggested the possibility that pollutant gases emitted from the ships traversing Busan port can exert a direct impact on the O3 concentration levels in the CA. Largest impacts of ship emissions on the O3 concentrations were predicted at the coast (up to 15 ppb) and at inland locations (about 5 ppb) due to both the photochemical production of pollutant gases emitted from the ships and meteorological conditions. From the PA, the photochemical production of O3 (P(O3)) due to ship emissions in the CA was found to increase by a mean of 1.5 ppb h−1 (especially by ≥10 ppb h−1 at the DS site) during the day. [Copyright &y& Elsevier]

Measurement of ambient gas-phase total peroxides was performed at the summit of Mount Tai (Mt. Tai, 1534m above sea level) in central-eastern China during March 22–April 24 and June 16–July 20, 2007. The hourly averaged concentration of peroxides was 0.17ppbv (±0.16ppbv, maximum: 1.28ppbv) and 0.55ppbv (±0.67ppbv, maximum: 3.55ppbv) in the spring and summer campaigns, respectively. The average concentration of peroxides at Mt. Tai, which is in a heavily polluted region, was much lower than hydrogen peroxide measurements made at some rural mountain sites, suggesting that significant removal processes took place in this region. An examination of diurnal variation and a correlation analysis suggest that these removal processes could include chemical suppression of peroxide production due to the scavenging of peroxy and hydroxy radicals by high NOx, wet removal by clouds/fogs rich in dissolved sulfur dioxide which reacts quickly with peroxides, and photolysis. These sinks competed with photochemical sources of peroxides, resulting in different mean concentrations and diurnal pattern of peroxides in the spring and summer. A principal component analysis was conducted to quantify the major processes that influenced the variation of peroxide concentrations. This analysis shows that in the spring photochemical production was an important source of peroxides, and the major sink was scavenging during upslope transport of polluted and humid air from the lower part of the planetary boundary layer (PBL) and wet removal by synoptic scale clouds. During the summer, highly polluted PBL air (with high NOx) was often associated with very low peroxides due to the chemical suppression of HO2 by high NOx and wet-removal by clouds/fogs in this sulfur-rich atmosphere, especially during the daytime. Higher concentrations of peroxides, which often appeared at mid-nighttime, were mainly associated with subsidence of air masses containing relatively lower concentrations of NOy. [Copyright &y& Elsevier]

Oxalic acid is the dominant dicarboxylic acid (DCA), and it constitutes up to 50% of total atmospheric DCAs, especially in non-urban and marine atmospheres. A significant amount of particulate H2Ox/oxalate (Ox) occurred in the coarse particle fraction of a dichotomous sampler, the ratio of oxalate concentrations in the PM10 to PM2.5 fractions ranged from 1 to 2, with mean±sd being 1.4±0.2. These results suggest that oxalate does not solely originate in the gas phase and condense into particles. Gaseous H2Ox concentrations are much lower than particulate Ox concentrations and are well correlated with HNO3, HCHO, and O3, supporting a photochemical origin. Of special relevance to the Bay Region Atmospheric Chemistry Experiment (BRACE) is the extent of nitrogen deposition in the Tampa Bay estuary. Hydroxyl radical is primarily responsible for the conversion of NO2 to HNO3, the latter being much more easily deposited. Hydroxyl radical is also responsible for the aqueous phase formation of oxalic acid from alkenes. Hence, we propose that an estimate of obtained from H2Ox/Ox production rate and we accordingly show that the product of total oxalate concentration and NO2 concentration approximately predicts the total nitrate concentration during the same period. [Copyright &y& Elsevier]

Abstract: We report measurement results of ozone (O3) and carbon monoxide (CO) obtained from 5 July to 23 November 2003 at the summit of Mount Tai (1534m a.s.l, 36.25°N, 117.10°E) in the Shandong Peninsula of China. The study was carried out to gain insights into regional O3 pollution and air-mass transport in the highly populated North China (Huabei) Plains. The average mixing ratio was 58 (±16) ppbv for O3 and 393 (±223) ppbv for CO during the study period. The monthly variations in O3 and CO exhibited a similar pattern, i.e. high in summer and low in autumn. While such a seasonal pattern for O3 is a common phenomenon in many rural areas in the Northern Hemisphere and is mainly due to seasonal changes in solar radiation and temperature, the higher CO levels in summer were attributed to the difference in dynamic transport and the evolution of PBL heights in summer and autumn over this region. An examination of diurnal variations in O3 indicated evident daytime photochemical production, especially in summer. The evolution of PBL heights and mountain-valley breezes also had a large impact on the diurnal patterns of O3 and CO. O3 and CO showed a moderately good positive correlation () in July with a slope of 0.08ppbv/ppbv, which is much lower than the slope (0.3–0.4ppbv/ppbv) observed in North America. A back trajectory analysis showed that air masses mainly originated from the North China Plains or were re-circulating over the Shandong Peninsula, collectively accounting for 76% of the air masses sampled. Summertime air traveling in the lower troposphere over northern China had the highest concentrations of O3 and CO, revealing that the lower troposphere in northern China was significantly polluted in summer due to strong convection transporting PBL pollution to the free troposphere. The trajectories also showed notable contributions from eastern China, central China, and countries in northeast Asia such as Korea and Japan. [Copyright &y& Elsevier]

Abstract: Unique daily measurements of water-soluble organics in fine (<2μm) and coarse (>2μm) aerosols were conducted at Alert in the Canadian Arctic in winter to spring of 1992. They yield insight into photochemical production and loss of organics during long-range transport and ozone depletion events following polar sunrise. Comprehensive analyses of α, ω-dicarboxylic acids (C2–C12), ω-oxocarboxylic acids (C2–C9) and α-dicarbonyls (C2, C3) as well as pyruvic acid and aromatic (phthalic) diacid were conducted using GC and GC/MS techniques. Oxalic (C2) acid was generally the dominant diacid species in both fine and coarse fractions, followed by malonic (C3) and succinic (C4) acids. Concentrations of total diacids in the fine aerosol fraction (0.2–64ngm-3) were 5–60 times higher than those in the coarse fraction (0.01–3ngm-3). After polar sunrise in early-March, the total concentration of fine aerosol diacids increased by a factor of 3–5 while the coarse mode did not change significantly. From dark winter to sunlit spring, temporal changes in correlations and ratios of these water-soluble organics to vanadium and sulfate measured simultaneously suggest that atmospheric diacids and related organic compounds are largely controlled by long-range atmospheric transport of polluted air during winter, but they are significantly affected by photochemical production. The latter can occur in sunlight either during transport to the Arctic or during photochemical events associated with surface ozone depletion and bromine chemistry near Alert in spring. Conversion of gaseous precursors to particulate matter via photochemical oxidation was intensified at polar sunrise, resulting in a peak in the ratio of total diacids to V. During ozone depletion events, complex patterns are indicated in photochemical production and loss depending on the diacid compound. Unsaturated (maleic and phthalic) diacids were inversely correlated with particulate Br whereas saturated diacids (C2–C4) positively correlated with particulate Br. These results suggest that Br chemistry associated with ozone depletion leads to degradation of unsaturated diacids and to the production of smaller saturated diacids. [Copyright &y& Elsevier]

Atmospheric levels of formaldehyde and acetaldehyde as well as their diurnal and seasonal variations were investigated from 1994 to 1997 in downtown Rome during sunny and wind calm days. Hourly concentrations of formaldehyde ranged from 8 to 28 ppbV in summer and 7 to 17 ppbv in winter; acetaldehyde concentrations varied correspondingly within the 3–18 and 2–7 ppbv intervals. Percentages of both aldehydes photochemically produced were estimated through a simple relationship based upon the comparison of individual ratios of formaldehyde and acetaldehyde to toluene in ambient air and automobile emission. Photochemical production was found to weigh upon atmospheric levels for 80–90% in summer days. It dropped below 35% in the winter period, when direct emission from traffic largely predominated. Photochemical summer source was more efficient for acetaldehyde than for formaldehyde, especially in the early morning. The importance of formaldehyde as the major source of hydroxyl radicals in Rome was also assessed. [Copyright &y& Elsevier]