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The role of ice in the formation of chemically active halogens in the environment requires a full understanding because of its role in atmospheric chemistry, including controlling the regional atmospheric oxidizing capacity in specific situations. In particular, ice and snow are important for facilitating multiphase oxidative chemistry and as media upon which marine algae live. This paper reviews the nature of environmental ice substrates that participate in halogen chemistry, describes the reactions that occur on such substrates, presents the field evidence for ice-mediated halogen activation, summarizes our best understanding of ice-halogen activation mechanisms, and describes the current state of modeling these processes at different scales. Given the rapid pace of developments in the field, this paper largely addresses advances made in the past five years, with emphasis given to the polar boundary layer. The integrative nature of this field is highlighted in the presentation of work from the molecular to the regional scale, with a focus on understanding fundamental processes. This is essential for developing realistic parameterizations and descriptions of these processes for inclusion in larger scale models that are used to determine their regional and global impacts.
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We use NAME, a trajectory model, to investigate the routes and timescales over which air parcels reach the tropical tropopause layer (TTL). Our aim is to assist the planning of aircraft campaigns focussed on improving knowledge of such transport. We focus on Southeast Asia and the Western Pacific which appears to be a particularly important source of air that enters the TTL. We first study the TTL above Borneo in November 2008, under neutral El Niño/Southern Oscillation (ENSO) conditions. Air parcels (trajectories) arriving in the lower TTL (below ~15 km) are most likely to have travelled from the boundary layer (BL; <1 km) above the West Pacific. Few air parcels found above ~16 km travelled from the BL in the previous 15 days. We then perform similar calculations for moderate El Niño (2006) and La Niña (2007) conditions and find year-to-year variability consistent with the phase of ENSO. Under El Niño conditions fewer air parcels travel from the BL to the TTL above Borneo. During the La Niña year, more air parcels travel from the BL to the mid and upper TTL (above ~15 km) than in the ENSO-neutral year, and again they do so from the BL above the West Pacific. We also find intra-month variability in all years, with day-to-day differences of up to an order of magnitude in the fraction of an idealised short-lived tracer travelling from the BL to the TTL above Borneo. These calculations were performed as a prelude to the SHIVA field campaign, which took place in Borneo during November 2011. So finally, to validate our approach, we consider measurements made in two previous campaigns. The features of vertical profiles of short-lived species observed in the TTL during CR-AVE and TC4 are in broad agreement with calculated vertical profiles of idealised short-lived tracers. It will require large numbers of observations to fully describe the statistical distribution of short-lived species in the TTL. This modelling approach should prove valuable in planning flights for the long-duration aircraft now capable of making such measurements.
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To show how remote-sensing products can be used to classify the entire CONUS domain into "geographical regions" and "chemical regimes", we analyzed the results of simulation from the Community Multiscale Air Quality (CMAQ) model version 4.7.1 over the Conterminous United States (CONUS) for August 2009. In addition, we observe how these classifications capture the weekly cycles of ground-level nitrogen oxide (NO<sub>x</sub>) and ozone (O<sub>3</sub>) at US EPA Air Quality System (AQS) sites. We use the Advanced Very High Resolution Radiometer (AVHRR) land use dominant categories and the Global Ozone Monitoring Experiment-2 (GOME-2) HCHO/NO<sub>2</sub> column density ratios to allocate geographical regions (i.e., "urban", "forest", and "other" regions) and chemical regimes (i.e., "NO<sub>x</sub>-saturated", "NO<sub>x</sub>-sensitive", and "mixed" regimes). We also show that CMAQ simulations using GOME-2 satellite-adjusted NO<sub>x</sub> emissions mitigate the discrepancy between the weekly cycles of NO<sub>x</sub> from AQS observations and that from CMAQ simulation results. We found geographical regions and chemical regimes do not show a one-to-one correspondence: the averaged HCHO / NO<sub>2</sub> ratios for AVHRR "urban" and "forest" regions are 2.1 and 4.0, which correspond to GOME-2 "mixed" and "NO<sub>x</sub>-sensitive" regimes, respectively. Both AQS-observed and CMAQ-simulated weekly cycles of NO<sub>x</sub> show high concentrations on weekdays and low concentrations on weekends, but with one- or two-day shifts of weekly high peaks in the simulated results, which eventually introduces the shifts in simulated weekly-low O<sub>3</sub> concentration. In addition, whereas the high weekend O<sub>3</sub> anomaly is clearly observable at sites over the GOME-2 NO<sub>x</sub>-saturated regime in both AQS and CMAQ, the weekend effect is not captured at sites over the AVHRR urban region because of the chemical characteristics of the urban sites (≈GOME-2 mixed regime). In addition, the weekend effect from AQS is more clearly discernible at sites above the GOME-2 NO<sub>x</sub>-saturated regime than at other sites above the CMAQ NO<sub>x</sub>-saturated regime, suggesting that the GOME-2-based chemical regime classification is more accurate than CMAQ-based chemical classification. Furthermore, the CMAQ simulations using the GOME-2-derived NO<sub>x</sub> emissions adjustment (decreasing from 462 Gg N to 426 Gg N over the US for August 2009) show large reductions of simulated NO<sub>x</sub> concentrations (particularly over the urban, or NO<sub>x</sub>-saturated, regime), and mitigates the large discrepancies between the absolute amount and the weekly pattern of NO<sub>x</sub> concentrations of the EPA AQS and those of the baseline CMAQ.
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Molecular hydrogen (H<sub>2</sub>), its stable isotope signature (δD), and the key combustion parameters carbon monoxide (CO), carbon dioxide (CO<sub>2</sub>), and methane (CH<sub>4</sub>) were measured from various combustion processes. H<sub>2</sub> in the exhaust of gas and oil-fired heaters and of waste incinerator plants was generally depleted compared to ambient intake air, while CO was significantly elevated. These findings contradict the often assumed co-occurring net H<sub>2</sub> and CO emissions in combustion processes and suggest that previous H<sub>2</sub> emissions from combustion may have been overestimated when scaled to CO emissions. For the gas and oil-fired heater exhausts, H<sub>2</sub> and δD generally decrease with increasing CO<sub>2</sub>, from ambient values of ~0.5 ppm and +130‰ to 0.2 ppm and −206‰, respectively. These results are interpreted as a combination of an isotopically light H<sub>2</sub> source from fossil fuel combustion and a D/H kinetic isotope fractionation of hydrogen in the advected ambient air during its partial removal during combustion. Diesel exhaust measurements from dynamometer test stand driving cycles show elevated H<sub>2</sub> and CO emissions during cold-start and some acceleration phases. While H<sub>2</sub> and CO emissions from diesel vehicles are known to be significantly less than those from gasoline vehicles (on a fuel-energy base), we find that their molar H<sub>2</sub>/CO ratios (median 0.026, interpercentile range 0.12) are also significantly less compared to gasoline vehicle exhaust. Using H<sub>2</sub>/CO emission ratios, along with CO global emission inventories, we estimate global H<sub>2</sub> emissions for 2000, 2005, and 2010. For road transportation (gasoline and diesel), we calculate 8.3 ± 2.2 Tg, 6.0 ± 1.5 Tg, and 3.8 ± 0.94 Tg, respectively, whereas the contribution from diesel vehicles is low (0.9–1.4%). Other fossil fuel emissions are believed to be negligible but H<sub>2</sub> emissions from coal combustion are unknown. For residential (domestic) emissions, which are likely dominated by biofuel combustion, emissions for the same years are estimated at 2.7 ± 0.7 Tg, 2.8 ± 0.7 Tg, and 3.0 ± 0.8 Tg, respectively. For biomass burning H<sub>2</sub> emissions, we derive a mole fraction ratio ΔH<sub>2</sub>/ΔCH<sub>4</sub> (background mole fractions subtracted) of 3.6 using wildfire emission data from the literature and support these findings with our wood combustion results. When combining this ratio with CH<sub>4</sub> emission inventories, the resulting global biomass burning H<sub>2</sub> emissions agree well with published global H<sub>2</sub> emissions, suggesting that CH<sub>4</sub> emissions may be a good proxy for biomass burning H<sub>2</sub> emissions.
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Ozone pollution represents a serious health and environmental problem. While ozone pollution is mostly produced by photochemistry in summer, elevated ozone concentrations can also be influenced by long range transport driven by the atmospheric circulation and stratospheric ozone intrusions. We analyze the role of large scale atmospheric circulation variability in the North Atlantic basin in determining surface ozone concentrations over Europe. Here, we show, using ground station measurements and a coupled atmosphere-chemistry model simulation for the period 1980–2005, that the North Atlantic Oscillation (NAO) does affect surface ozone concentrations – on a monthly timescale, over 10 ppbv in southwestern, central and northern Europe – during all seasons except fall. The commonly used NAO index is able to capture the link existing between atmospheric dynamics and surface ozone concentrations in winter and spring but it fails in summer. We find that the first Principal Component, computed from the time variation of the sea level pressure (SLP) field, detects the atmosphere circulation/ozone relationship not only in winter and spring but also during summer, when the atmospheric circulation weakens and regional photochemical processes peak. Given the NAO forecasting skill at intraseasonal time scale, the first Principal Component of the SLP field could be used as an indicator to identify areas more exposed to forthcoming ozone pollution events. Finally, our results suggest that the increasing baseline ozone in western and northern Europe during the 1990s could be related to the prevailing positive phase of the NAO in that period.
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Vertical transport by moist sub-grid scale processes such as deep convection is a well-known source of uncertainty in CO<sub>2</sub> source/sink inversion. However, a dynamical link between vertical transport, satellite based retrievals of column mole fractions of CO<sub>2</sub>, and source/sink inversion has not yet been established. By using the same offline transport model with meteorological fields from slightly different data assimilation systems, we examine sensitivity of frontal CO<sub>2</sub> transport and retrieved fluxes to different parameterizations of sub-grid vertical transport. We find that frontal transport feeds off background vertical CO<sub>2</sub> gradients, which are modulated by sub-grid vertical transport. The implication for source/sink estimation is two-fold. First, CO<sub>2</sub> variations contained in moist poleward moving air masses are systematically different from variations in dry equatorward moving air. Moist poleward transport is hidden from orbital sensors on satellites, causing a sampling bias, which leads directly to small but systematic flux retrieval errors in northern mid-latitudes. Second, differences in the representation of moist sub-grid vertical transport in GEOS-4 and GEOS-5 meteorological fields cause differences in vertical gradients of CO<sub>2</sub>, which leads to systematic differences in moist poleward and dry equatorward CO<sub>2</sub> transport and therefore the fraction of CO<sub>2</sub> variations hidden in moist air from satellites. As a result, sampling biases are amplified and regional scale flux errors enhanced, most notably in Europe (0.43 ± 0.35 PgC yr<sup>−1</sup>). These results, cast from the perspective of moist frontal transport processes, support previous arguments that the vertical gradient of CO<sub>2</sub> is a major source of uncertainty in source/sink inversion.
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The effect of photochemical ageing and initial precursor concentration on the composition and hygroscopic properties of secondary organic aerosol (SOA) formed during the chamber photo-oxidation of β-caryophyllene/NO<sub>x</sub> mixtures were investigated. Nucleation of β-caryophyllene SOA particles occurred almost immediately after oxidation was initiated and led to the formation of fresh SOA with a relatively simpler composition than has been reported for monoterpenes. The SOA yield values ranged from 9.5–26.7% and 30.4–44.5% using a differential mobility particle sizer (DMPS) and an aerosol mass spectrometer (AMS) mass based measurements, respectively. A total of 20 compounds were identified in the SOA by LC-MS/MS, with the most abundant compounds identified as β-caryophyllonic acid and β-caryophyllinic acid/β-nocaryophyllonic acid. The O:C and H:C elemental ratios of products identified in the condensed phase ranged from 0.20 to 1.00 and 1.00 to 2.00, with average values of 0.39 and 1.58, respectively. The increase in the O:C ratio was associated with a decrease in the saturation concentration of the identified compounds. The compounds identified in the lower initial concentration experiments were more oxidised compared to those that were found to be more abundant in the higher initial concentration experiments with average O:C ratios of 0.51 and 0.27, respectively. Photochemical ageing led to a more complex SOA composition with a larger contribution coming from lower molar mass, higher generation products, where both double bonds had been oxidised. This effect was more evident in the experiments conducted using the lower initial precursor concentration; a finding confirmed by the temporal behaviour of key organic mass fragment measured by an Aerosol Mass Spectrometer. Although the composition changed with both initial precursor concentration and ageing, this had no significant measurable effect on the hygroscopic properties of the SOA formed. The latter finding might have been influenced by the difference in pre-treatment of the semivolatile-containing particles prior to their measurements.
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The heterogeneous hydrolysis of dinitrogen pentoxide (N<sub>2</sub>O<sub>5</sub>) has typically been modeled as only producing nitric acid. However, recent field studies have confirmed that the presence of particulate chloride alters the reaction product to produce nitryl chloride (ClNO<sub>2</sub>) which undergoes photolysis to generate chlorine atoms and nitrogen dioxide (NO<sub>2</sub>). Both chlorine and NO<sub>2</sub> affect atmospheric chemistry and air quality. We present an updated gas-phase chlorine mechanism that can be combined with the Carbon Bond 05 mechanism and incorporate the combined mechanism into the Community Multiscale Air Quality (CMAQ) modeling system. We then update the current model treatment of heterogeneous hydrolysis of N<sub>2</sub>O<sub>5</sub> to include ClNO<sub>2</sub> as a product. The model, in combination with a comprehensive inventory of chlorine compounds, reactive nitrogen, particulate matter, and organic compounds, is used to evaluate the impact of the heterogeneous ClNO<sub>2</sub> production on air quality across the United States for the months of February and September in 2006. The heterogeneous production increases ClNO<sub>2</sub> in coastal as well as many in-land areas in the United States. Particulate chloride derived from sea-salts, anthropogenic sources, and forest fires activates the heterogeneous production of ClNO<sub>2</sub>. With current estimates of tropospheric emissions, it modestly enhances monthly mean 8-h ozone (up to 1–2 ppbv or 3–4%) but causes large increases (up to 13 ppbv) in isolated episodes. This chemistry also substantially reduces the mean total nitrate by up to 0.8–2.0 μg m<sup>−3</sup> or 11–21%. Modeled ClNO<sub>2</sub> accounts for up to 6% of the monthly mean total reactive nitrogen. Sensitivity results of the model suggest that heterogeneous production of ClNO<sub>2</sub> can further increase O<sub>3</sub> and reduce TNO<sub>3</sub> if elevated particulate-chloride levels are present in the atmosphere.
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During the POLARCAT-France airborne measurement campaign in spring 2008, several pollution plumes transported from mid-latitude regions were encountered. The study presented here focuses on air masses from two different geographic origins (Europe and Asia) and from 2 different source types (anthropogenic pollution and forest fires). A first case study is dedicated to a European air mass, which was repeatedly sampled and analysed during three consecutive days. Thereby, the evolution of the aerosol properties (size distributions, CO mixing ratio) is characterised and related processes are discussed. In particular, the role of coagulation, condensation and cloud processing in the evolution of the Aitken and the accumulation mode particles are contrasted. <br><br> A second case study focuses on European air masses impacted solely by biomass burning emissions and Asian air masses with contributions from both biomass burning and anthropogenic emissions. The analysis of aerosol modes highlight a similar behaviour for particle originating from biomass burning (from Europe as well as Asia). In comparison to the predominating aged accumulation mode in biomass burning particles, a still larger aerosol accumulation mode related to Asian anthropogenic emissions can be isolated. These findings corroborate the external mixing of such kind of aerosol size distributions. An electron microscopy study (coupled to X-ray elemental analysis) of particles illustrated soot-like inclusions in several samples. Within samples attributed to forest fire sources, the chemical signature is highly associated with the presence of potassium, which is a characteristic tracer element for biomass burning plumes. The single particle images suggest an internal mixing of sampled individual aerosol particles. Thus, particles are found externally mixed as demonstrated from particle size distributions while they appear internally mixed at the particle scale.
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The chemical composition of the boundary layer in snow covered regions is impacted by chemistry in the snowpack via uptake, processing, and emission of atmospheric trace gases. We use the coupled one-dimensional (1-D) snow chemistry and atmospheric boundary layer model MISTRA-SNOW to study the impact of snowpack chemistry on the oxidation capacity of the boundary layer. The model includes gas phase photochemistry and chemical reactions both in the interstitial air and the atmosphere. While it is acknowledged that the chemistry occurring at ice surfaces may consist of a true quasi-liquid layer and/or a concentrated brine layer, lack of additional knowledge requires that this chemistry be modeled as primarily aqueous chemistry occurring in a liquid-like layer (LLL) on snow grains. The model has been recently compared with BrO and NO data taken on 10 June–13 June 2008 as part of the Greenland Summit Halogen-HO<sub>x</sub> experiment (GSHOX). In the present study, we use the same focus period to investigate the influence of snowpack derived chemistry on OH and HO<sub>x</sub> + RO<sub>2</sub> in the boundary layer. We compare model results with chemical ionization mass spectrometry (CIMS) measurements of the hydroxyl radical (OH) and of the hydroperoxyl radical (HO<sub>2</sub>) plus the sum of all organic peroxy radicals (RO<sub>2</sub>) taken at Summit during summer 2008. Using sensitivity runs we show that snowpack influenced nitrogen cycling and bromine chemistry both increase the oxidation capacity of the boundary layer and that together they increase the mid-day OH concentrations. Bromine chemistry increases the OH concentration by 10–18% (10% at noon LT), while snow sourced NO<sub>x</sub> increases OH concentrations by 20–50% (27% at noon LT). We show for the first time, using a coupled one-dimensional snowpack-boundary layer model, that air-snow interactions impact the oxidation capacity of the boundary layer and that it is not possible to match measured OH levels without snowpack NO<sub>x</sub> and halogen emissions. Model predicted HONO compared with mistchamber measurements suggests there may be an unknown HONO source at Summit. Other model predicted HO<sub>x</sub> precursors, H<sub>2</sub>O<sub>2</sub> and HCHO, compare well with measurements taken in summer 2000, which had lower levels than other years. Over 3 days, snow sourced NO<sub>x</sub> contributes an additional 2 ppb to boundary layer ozone production, while snow sourced bromine has the opposite effect and contributes 1 ppb to boundary layer ozone loss.
