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We have measured emission factors for 19 trace gas species and particulate matter (PM<sub>2.5</sub>) from 14 prescribed fires in chaparral and oak savanna in the southwestern US, as well as conifer forest understory in the southeastern US and Sierra Nevada mountains of California. These are likely the most extensive emission factor field measurements for temperate biomass burning to date and the only published emission factors for temperate oak savanna fuels. This study helps to close the gap in emissions data available for temperate zone fires relative to tropical biomass burning. We present the first field measurements of the biomass burning emissions of glycolaldehyde, a possible precursor for aqueous phase secondary organic aerosol formation. We also measured the emissions of phenol, another aqueous phase secondary organic aerosol precursor. Our data confirm previous observations that urban deposition can impact the NO<sub>x</sub> emission factors and thus subsequent plume chemistry. For two fires, we measured both the emissions in the convective smoke plume from our airborne platform and the unlofted residual smoldering combustion emissions with our ground-based platform. The smoke from residual smoldering combustion was characterized by emission factors for hydrocarbon and oxygenated organic species that were up to ten times higher than in the lofted plume, including high 1,3-butadiene and isoprene concentrations which were not observed in the lofted plume. This should be considered in modeling the air quality impacts for smoke that disperses at ground level. We also show that the often ignored unlofted emissions can significantly impact estimates of total emissions. Preliminary evidence suggests large emissions of monoterpenes in the residual smoldering smoke. These data should lead to an improved capacity to model the impacts of biomass burning in similar temperate ecosystems.
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The volcanic eruption of Grimsvötn in Iceland in May 2011 affected surface-layer air quality at several locations in Northern Europe. In Helsinki, Finland, the main pollution episode lasted for more than 8 h around the noon of 25 May. We characterized this episode by relying on detailed physical, chemical and optical aerosol measurements. The analysis was aided by air mass trajectory calculations, satellite measurements, and dispersion model simulations. During the episode, volcanic ash particles were present at sizes from less than 0.5 μm up to sizes >10 μm. The mass mean diameter of ash particles was a few μm in the Helsinki area, and the ash enhanced PM<sub>10</sub> mass concentrations up to several tens of μg m<sup>−3</sup>. Individual particle analysis showed that some ash particles appeared almost non-reacted during the atmospheric transportation, while most of them were mixed with sea salt or other type of particulate matter. Also sulfate of volcanic origin appeared to have been transported to our measurement site, but its contribution to the aerosol mass was minor due the separation of ash-particle and sulfur dioxide plumes shortly after the eruption. The volcanic material had very little effect on PM<sub>1</sub> mass concentrations or sub-micron particle number size distributions in the Helsinki area. The aerosol scattering coefficient was increased and visibility was slightly decreased during the episode, but in general changes in aerosol optical properties due to volcanic aerosols seem to be difficult to be distinguished from those induced by other pollutants present in a continental boundary layer. The case investigated here demonstrates clearly the power of combining surface aerosol measurements, dispersion model simulations and satellite measurements in analyzing surface air pollution episodes caused by volcanic eruptions. None of these three approaches alone would be sufficient to forecast, or even to unambiguously identify, such episodes.
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A mesospheric front was observed with an all-sky airglow imager on the night of 9–10 July 2007 at Ferraz Station (62° S, 58° W), located on King George island on the Antarctic Peninsula. The observed wave propagated from southwest to northeast with a well defined wave front and a series of crests behind the main front. The wave parameters were obtained via a 2-D Fourier transform of the imager data providing a horizontal wavelength of 33 km, an observed period of 6 min, and a horizontal phase speed of 92 m s<sup>−1</sup>. Simultaneous mesospheric winds were measured with a medium frequency (MF) radar at Rothera Station (68° S, 68° W) and temperature profiles were obtained from the SABER instrument on the TIMED satellite. These wind and temperature profiles were used to estimate the propagation environment of the wave event. A wavelet technique was applied to the wind in the plane of wave propagation at the OH emission height spanning three days centered on the front event to define the dominant periodicities. Results revealed a dominance of near-inertial periods, and semi-diurnal and terdiurnal tides suggesting that the ducting structure enabling mesospheric front propagation occurred on large spatial scales. The observed tidal motions were used to reconstruct the winds employing a least-squares method, which were then compared to the observed ducting environment. Results suggest an important contribution of large-scale winds to the ducting structure, but with buoyancy frequency variations in the vertical also expected to be important. These results allow us to conclude that the wave front event was supported by a duct including contributions from both winds and temperature.
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Balloon-borne observations of ozone from the South Pole Station have been reported to reach ozone mixing ratios below the detection limit of about 10 ppbv at the 70 hPa level by late September. After reaching a minimum, ozone mixing ratios increase to above 1 ppmv on the 70 hPa level by late December. While the basic mechanisms causing the ozone hole have been known for more than 20 yr, the detailed chemical processes determining how low the local concentration can fall, and how it recovers from the minimum have not been explored so far. Both of these aspects are investigated here by analysing results from the Chemical Lagrangian Model of the Stratosphere (CLaMS). As ozone falls below about 0.5 ppmv, a balance is maintained by gas phase production of both HCl and HOCl followed by heterogeneous reaction between these two compounds in these simulations. Thereafter, a very rapid, irreversible chlorine deactivation into HCl can occur, either when ozone drops to values low enough for gas phase HCl production to exceed chlorine activation processes or when temperatures increase above the polar stratospheric cloud (PSC) threshold. As a consequence, the timing and mixing ratio of the minimum ozone depends sensitively on model parameters, including the ozone initialisation. The subsequent ozone increase between October and December is linked mainly to photochemical ozone production, caused by oxygen photolysis and by the oxidation of carbon monoxide and methane.
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Many important atmospheric aerosol processes depend on the chemical composition of the aerosol, e.g. water uptake and particle cloud interactions. Atmospheric ageing processes, such as oxidation reactions, significantly and continuously change the chemical composition of aerosol particles throughout their lifetime. These ageing processes are often poorly understood. In this study we utilize an aerosol flow tube set up and an ultra-high resolution mass spectrometer to explore the effect of relative humidity (RH) in the range of <5–90% on the ozonolysis of maleic acid aerosol which is employed as model organic aerosol system. Due to the slow reaction kinetics relatively high ozone concentrations of 160–200 ppm were used to achieve an appreciable degree of oxidation of maleic acid. The effect of oxidative ageing on the hygroscopicity of maleic acid particles is also investigated using an electrodynamic balance and thermodynamic modelling. RH has a profound effect on the oxidation of maleic acid particles. Very little oxidation is observed at RH < 50% and the only observed reaction products are glyoxylic acid and formic acid. In comparison, when RH > 50% there are about 15 oxidation products identified. This increased oxidation was observed even when the particles were exposed to high humidities long after a low RH ozonolysis reaction. This result might have negative implications for the use of water as an extraction solvent for the analysis of oxidized organic aerosols. These humidity-dependent differences in the composition of the ozonolyzed aerosol demonstrate that water is both a key reactant in the oxidation scheme and a determinant of particle phase and hence diffusivity. The measured chemical composition of the processed aerosol is used to model the hygroscopic growth, which compares favourably with water uptake results from the electrodynamic balance measurements. A reaction mechanism is presented which takes into account the RH dependent observations. This study emphasises the importance of studying the combined effects of several atmospheric parameters such as oxidants and RH to accurately describe the complex oxidation scheme of organic aerosols.
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The budget of atmospheric secondary organic aerosol (SOA) is very uncertain, with recent estimates suggesting a global source of between 12 and 1820 Tg (SOA) a<sup>−1</sup>. We used a dataset of aerosol mass spectrometer (AMS) observations from 34 different surface locations to evaluate the GLOMAP global chemical transport model. The standard model simulation (which included SOA from monoterpenes only) underpredicted organic aerosol (OA) observed by the AMS and had little skill reproducing the variability in the dataset. We simulated SOA formation from biogenic (monoterpenes and isoprene), lumped anthropogenic and lumped biomass burning volatile organic compounds (VOCs) and varied the SOA yield from each precursor source to produce the best overall match between model and observations. We assumed that SOA is essentially non-volatile and condenses irreversibly onto existing aerosol. Our best estimate of the SOA source is 140 Tg (SOA) a<sup>−1</sup> but with a large uncertainty range which we estimate to be 50–380 Tg (SOA) a<sup>−1</sup>. We found the minimum in normalised mean error (NME) between model and the AMS dataset when we assumed a large SOA source (100 Tg (SOA) a<sup>−1</sup>) from sources that spatially matched anthropogenic pollution (which we term antropogenically controlled SOA). We used organic carbon observations compiled by Bahadur et al. (2009) to evaluate our estimated SOA sources. We found that the model with a large anthropogenic SOA source was the most consistent with these observations, however improvement over the model with a large biogenic SOA source (250 Tg (SOA) a<sup>−1</sup>) was small. We used a dataset of <sup>14</sup>C observations from rural locations to evaluate our estimated SOA sources. We estimated a maximum of 10 Tg (SOA) a<sup>−1</sup> (10 %) of the anthropogenically controlled SOA source could be from fossil (urban/industrial) sources. We suggest that an additional anthropogenic source is most likely due to an anthropogenic pollution enhancement of SOA formation from biogenic VOCs. Such an anthropogenically controlled SOA source would result in substantial climate forcing. We estimated a global mean aerosol direct effect of −0.26 ± 0.15 Wm<sup>−2</sup> and indirect (cloud albedo) effect of −0.6<sup>+0.24</sup><sub>−0.14</sub> Wm<sup>−2</sup> from anthropogenically controlled SOA. The biogenic and biomass SOA sources are not well constrained with this analysis due to the limited number of OA observations in regions and periods strongly impacted by these sources. To further improve the constraints by this method, additional OA observations are needed in the tropics and the Southern Hemisphere.
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The main purposes of the present paper are not only to investigate linear long-term trends of Aerosol Optical Thickness (AOT) at 443 and 555 nm over regions in Europe and South China, but also to show the uncertainty caused by cloud disturbance in the trend analysis of cloud-free aerosol. These research areas are the densely urbanised and often highly polluted regions. The study uses the Bremen AErosol Retrieval (BAER) and Sea-viewing Wide Field-of-view Sensor (SeaWiFS) data for AOT retrievals in the specified regions from October 1997 to May 2008. In order to validate the individually retrieved AOTs and the corresponding trends, AErosol RObotic NETwork (AERONET) level 2.0 data have been used. The retrieved AOTs were in good agreement with those of AERONET (0.79 ≤ <i>R</i> ≤ 0.88, 0.08 ≤ RMSD ≤ 0.13). The contamination of the aerosol retrievals and/or AERONET observations by thin clouds can significantly degrade the AOT and lead to statistically non-representative monthly-means, especially during cloudy seasons. Therefore an inter-correction method has been developed and applied. The "corrected" trends for both BAER SeaWiFS and AERONET AOT were similar and showed in average a relative difference of ∼25.19%. In general terms, negative trends (decrease of aerosol loading) were mainly observed over European regions, with magnitudes up to −0.00453 and −0.00484 yr<sup>−1</sup> at 443 and 555 nm, respectively. In contrast, the trend in Pearl River Delta was positive, most likely attributed to rapid urbanization and industrialization. The magnitudes of AOT increased by +0.00761 and +0.00625 yr<sup>−1</sup> respectively at 443 and 555 nm.
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We present results from two field deployments of a unique tandem differential mobility analyzer (TDMA) configuration with two primary capabilities: identifying alternative stable or meta-stable ambient aerosol hydration states associated with hysteresis in aerosol hydration behavior and determining the actual Ambient hydration State (AS-TDMA). This data set is the first to fully classify the ambient hydration state of aerosols despite recognition that hydration state significantly impacts the roles of aerosols in climate, visibility and heterogeneous chemistry. The AS-TDMA was installed at a site in eastern Tennessee on the border of Great Smoky Mountains National Park for projects during the summer of 2006 and winter of 2007–2008. During the summer, 12% of the aerosols sampled in continuous AS-TDMA measurements were found to posses two possible hydration states under ambient conditions. In every case, the more hydrated of the possible states was occupied. The remaining 88% did not posses multiple possible states. In continuous measurements during the winter, 49% of the aerosols sampled possessed two possible ambient hydration states; the remainder possessed only one. Of those aerosols with multiple possible ambient hydration states, 65% occupied the more hydrated state; 35% occupied the less hydrated state. This seasonal contrast is supported by differences in the fine particulate (PM<sub>2.5</sub>) composition and ambient RH as measured during the two study periods. In addition to seasonal summaries, this work includes case studies depicting the variation of hydration state with changing atmospheric conditions.
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Sea-land and mount-valley circulations are the dominant mesoscale synoptic systems affecting the Beijing area during summertime. Under the influence of these two circulations, the prevailing wind is southwesterly from afternoon to midnight, and then changes to northeasterly till forenoon. In this study, surface ozone (O<sub>3</sub>), carbon monoxide (CO), nitric oxide (NO), nitrogen dioxide (NO<sub>2</sub>), nitrogen oxide (NO<sub>x</sub>) and non-methane hydrocarbons (NMHCs) were measured at four sites located along the route of prevailing wind, including two upwind urban sites (Fengtai "FT" and Baolian "BL"), an upwind suburban site (Shunyi "SY") and a downwind rural site (Shangdianzi "SDZ") during 20 June–16 September 2007. The purpose is to improve our understanding of ozone photochemistry in urban and rural areas of Beijing and the influence of urban plumes on ozone pollution in downwind rural areas. It is found that ozone pollution was synchronism in the urban and rural areas of Beijing, coinciding with the regional-scale synoptic processes. Due to the high traffic density and local emissions, the average levels of reactive gases NO<sub>x</sub> and NMHCs at the non-rural sites were much higher than those at SDZ. The level of long-lived gas CO at SDZ was comparable to, though slightly lower than, at the urban sites. We estimate the photochemical reactivity (<i>L</i><sub>OH</sub> and the ozone formation potential (OFP) in the urban (BL) and rural (SDZ) areas using measured CO and NMHCs. The OH loss rate coefficient (<i>L</i><sub>OH</sub> by total NMHCs at the BL and SDZ sites are estimated to be 50.7 s<sup>-1</sup> and 15.8 s<sup>-1</sup>, respectively. While alkenes make a major contribution to the <i>L</i><sub>OH</sub>, aromatics dominate OFP at both urban and rural sites. With respect to the individual species, CO has the largest ozone formation potential at the rural site, and at the urban site aromatic species are the leading contributors. While the O<sub>3</sub> diurnal variations at the four sites are typical for polluted areas, the ozone peak values are found to lag behind one site after another along the route of prevailing wind from SW to NE. Intersection analyses of trace gases reveal that polluted air masses arriving at SDZ were more aged with both higher O<sub>3</sub> and O<sub>x</sub> concentrations than those at BL. The results indicate that urban plume can transport not only O<sub>3</sub> but its precursors, the latter leading more photochemical O<sub>3</sub> production when being mixed with background atmosphere in the downwind rural area.
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Sensitivity analysis of atmospheric models is necessary to identify the processes that lead to uncertainty in model predictions, to help understand model diversity through comparison of driving processes, and to prioritise research. Assessing the effect of parameter uncertainty in complex models is challenging and often limited by CPU constraints. Here we present a cost-effective application of variance-based sensitivity analysis to quantify the sensitivity of a 3-D global aerosol model to uncertain parameters. A Gaussian process emulator is used to estimate the model output across multi-dimensional parameter space, using information from a small number of model runs at points chosen using a Latin hypercube space-filling design. Gaussian process emulation is a Bayesian approach that uses information from the model runs along with some prior assumptions about the model behaviour to predict model output everywhere in the uncertainty space. We use the Gaussian process emulator to calculate the percentage of expected output variance explained by uncertainty in global aerosol model parameters and their interactions. To demonstrate the technique, we show examples of cloud condensation nuclei (CCN) sensitivity to 8 model parameters in polluted and remote marine environments as a function of altitude. In the polluted environment 95 % of the variance of CCN concentration is described by uncertainty in the 8 parameters (excluding their interaction effects) and is dominated by the uncertainty in the sulphur emissions, which explains 80 % of the variance. However, in the remote region parameter interaction effects become important, accounting for up to 40 % of the total variance. Some parameters are shown to have a negligible individual effect but a substantial interaction effect. Such sensitivities would not be detected in the commonly used single parameter perturbation experiments, which would therefore underpredict total uncertainty. Gaussian process emulation is shown to be an efficient and useful technique for quantifying parameter sensitivity in complex global atmospheric models.
