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We have used an off-line 3-D chemical transport model (CTM) to investigate the 11-yr solar cycle response in tropical stratospheric ozone. The model is forced with European Centre for Medium-Range Weather Forecasts (ECMWF) (re)analysis (ERA-40/operational and ERA-Interim) data for the 1979–2005 time period. We have compared the modelled solar response in ozone to observation-based data sets that are constructed using satellite instruments such as Total Ozone Mapping Spectrometer (TOMS), Solar Backscatter UltraViolet instrument (SBUV), Stratospheric Aerosol and Gas Experiment (SAGE) and Halogen Occultation Experiment (HALOE). A significant difference is seen between simulated and observed ozone during the 1980s, which is probably due to inhomogeneities in the ERA-40 reanalyses. In general, the model with ERA-Interim dynamics shows better agreement with the observations from 1990 onwards than with ERA-40. Overall both standard model simulations are partially able to simulate a "double peak"-structured ozone solar response with a minimum around 30 km, and these are in better agreement with HALOE than SAGE-corrected SBUV (SBUV/SAGE) or SAGE-based data sets. In the tropical lower stratosphere (TLS), the modelled solar response with time-varying aerosols is amplified through aliasing with a volcanic signal, as the model overestimates ozone loss during high aerosol loading years. However, the modelled solar response with fixed dynamics and constant aerosols shows a positive signal which is in better agreement with SBUV/SAGE and SAGE-based data sets in the TLS. Our model simulations suggests that photochemistry contributes to the ozone solar response in this region. The largest model-observation differences occur in the upper stratosphere where SBUV/SAGE and SAGE-based data show a significant (up to 4%) solar response whereas the standard model and HALOE do not. This is partly due to a positive solar response in the ECMWF upper stratospheric temperatures which reduces the modelled ozone signal. The large positive upper stratospheric solar response seen in SBUV/SAGE and SAGE-based data can be reproduced in model runs with fixed dynamical fields (i.e. no inter-annual meteorological changes). As these runs effectively assume no long-term temperature changes (solar-induced or otherwise), it should provide an upper limit of the ozone solar response. Overall, full quantification of the solar response in stratospheric ozone is limited by differences in the observed data sets and by uncertainties in the solar response in stratospheric temperatures.
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Residential wood burning is becoming an increasingly important cause of air quality problems since it has become a popular source of alternative energy to fossil fuel. In order to characterize the contribution of residential wood burning to local particle pollution, a field campaign was organized at the village of Seiffen (Saxony, Germany). During this campaign, an Aerosol Mass Spectrometer (AMS) was deployed in parallel to a PM<sub>1</sub> high volume filter sampler. The AMS mass spectra were analyzed using Positive Matrix Factorization (PMF) to obtain detailed information about the organic aerosol (OA). Biomass-burning organic aerosol (BBOA), Hydrocarbon-like organic aerosol (HOA), and Oxygenated Organic Aerosol (OOA) were identified and represented 20%, 17% and 62% of total OA, respectively. Additionally, Polycyclic Aromatic Hydrocarbons (PAH) were measured by the AMS with an average concentration of 10 ng m<sup>−3</sup> and short term events of extremely high PAH concentration (up to 500 ng m<sup>−3</sup>) compared to the mean PAH value were observed during the whole measurement period. A comparison with the results from PM<sub>1</sub> filter samples showed that the BBOA factor and the AMS PAH are good indicators of the total concentration of the different monosaccharide anhydrides and PAH measured on the filter samples. Based on its low correlation with CO and the low car traffic, the HOA factor was considered to be related to residential heating using liquid fuel. An influence of the time of the week (week vs. weekend) on the diurnal profiles of the different OA components was observed. The weekdays were characterized by two maxima; a first one early in the morning and a stronger one in the evening. During the weekend days, the different OA components principally reached only one maximum in the afternoon. Finally, the PAH emitted directly from residential wood combustion was estimated to represent 1.5% of the total mass of the BBOA factor and around 62% of the total PAH concentration measured at Seiffen. This result highlights the important contribution of residential wood combustion to air quality and PAH emissions at the sampling place, which might have a significant impact on human health. Moreover, it also emphasizes the need for a better time resolution of the chemical characterization of toxic particulate compounds in order to provide more information on variations of the different sources through the days as well as to better estimate the real human exposure.
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We have investigated an aerosol processing and evolution event from 21–22 July during the summer 2009 Field Intensive Study at Queens College in New York City (NYC). The evolution processes are characterized by three consecutive stages: (1) aerosol wet scavenging, (2) nighttime nitrate formation, and (3) photochemical production and evolution of secondary aerosol species. Our results suggest that wet scavenging of aerosol species tends to be strongly related to their hygroscopicities and also mixing states. The scavenging leads to a significant change in bulk aerosol composition and average carbon oxidation state because of scavenging efficiencies in the following order: sulfate > low-volatility oxygenated organic aerosol (LV-OOA) > semi-volatile OOA (SV-OOA) > hydrocarbon-like OA (HOA). The second stage involves a quick formation of nitrate from heterogeneous reactions at nighttime. During the third stage, simultaneous increases of sulfate and SV-OOA were observed shortly after sunrise, indicating secondary aerosol formation. Organic aerosols become highly oxidized in ~ half day as the result of photochemical processing, consistent with previously reported results from the CO-tracer method (OA/ΔCO). The photochemical reactions appear to progress gradually associated with a transformation of SV- OOA to low-volatility species based on the evolution trends of oxygen-to-carbon (O/C) ratio, relationship between f44 (fraction of <i>m/z</i> 44 in OA) and f43 (fraction of <i>m/z</i> 43 in OA), and size evolution of OOA and HOA. Aerosols appear to become more internally mixed during the processing. Our results suggest that functionalization by incorporation of both C and O plays a major role in the early period of OA oxidation (O/C < 0.5). Our results also show that photochemical production of LV-OOA during this event is approximately 2–3 h behind of sulfate production, which might explain, sometimes, the lack of correlations between LV-OOA and sulfate, two secondary aerosol species which often exist in internal mixtures over regional scales.
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Soils are the major terrestrial reservoir of persistent organic pollutants, and thus net volatilization from soil, when it happens, may exert a control on the atmospheric occurrence and variability of organic pollutants. Here, we report and discuss the concentrations of legacy organochlorine pesticides (OCPs) such as hexachlorobenzene (HCB), hexaclorocyclohexanes (HCH) and dichlorodiphenyltrichloroethane (DDT) in the atmosphere and in soils, their measured fugacities in soil, the soil-air partition coefficients (<i>K</i><sub>SA</sub>) and soil-air fugacity ratios (<i>f</i><sub>s</sub>/<i>f</i><sub>a</sub>) in rural background areas of N-NE Spain and N-NW England. Four sampling campaigns were carried out in Spain and UK to assess seasonal variability and differences between sampling sites. <i>K</i><sub>SA</sub> values were significantly dependent on soil temperature and soil organic matter quantity, and to a minor extent on organic matter type. HCH isomers and DDT metabolites in soil are close to equilibrium with the overlying atmosphere at rural background areas of Spain with a tendency to volatilize and deposit during warm and cold periods, respectively. The mixture of HCH and DDT found in the atmosphere is clearly strongly influenced by the mixture of HCH and DDT which escapes from soil, with significant correlations between them (<i>r</i><sup>2</sup> ranging between 0.63–0.76 and p-level<0.001 for the Ebro sampling sites), thus suggesting a close coupling of air and soil concentrations, demonstrating that net volatilization from soil control the atmospheric levels of OCPs in the Northern Spain background atmosphere. Conversely, soils at rural UK sites were usually a sink for atmospheric DDT and HCH, but not for HCB. The negative statistically significant relationship found between log <i>K</i><sub>SA</sub> and the log (<i>f</i><sub>s</sub>/<i>f</i><sub>a</sub>) ratio, suggests that high latitude regions, due to the high soil organic matter content and lower temperatures, will act as larger traps and accumulate more atmospheric OCPs. Thus, the extent to which soils are secondary sources to the atmosphere is currently dependent on the reservoir potential of soils for OCPs and shows a marked seasonality in their strength.
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Within the European Aerosol Cloud Climate and Air Quality Interactions (EUCAARI) project, the Meteo France research aircraft ATR-42 was operated from Rotterdam airport during May 2008, to perform scientific flights dedicated to the investigation of aerosol-cloud interactions. Therein, the objective of this study is to illustrate the impact of cloud processing on the aerosol particles physical and chemical properties. The presented results are retrieved from measurements during a double-flight mission from Rotterdam (Netherlands) to Newcastle (UK) and back using data measured with compact Time of Flight Aerosol Mass Spectrometer (cToF-AMS) and Scanning Mobility Particle Sizer (SMPS). Cloud-related measurements during these flights were performed over the North Sea within as well as in close vicinity of a marine stratocumulus cloud layer. Particle physical and chemical properties observed in the close vicinity (V), below and above the stratocumulus cloud show strong differences. Firstly, measurements at constant altitude above the cloud layer show decreasing mass concentrations with decreasing horizontal distance (210–0 km) to the cloud layer by a factor up to 7, whereas below the cloud and by same means of distance, the mass concentrations merely decrease by a factor of 2 on average. Secondly, the averaged aerosol size distributions, observed above and below the cloud layer, are of bimodal character with pronounced minima between Aitken and accumulation mode which is potentially the consequence of cloud processing. Finally, the chemical composition of aerosol particles is strongly dependent on the location relative to the cloud layer (vicinity or below/above cloud). In general, the nitrate and organic fractions decrease with decreasing distance to the cloud, in the transit from cloud–free conditions towards the cloud boundaries. The decrease of nitrate and organic compounds ranges at a factor of three to ten, affecting sulfate and ammonium compounds to be increasingly abundant in the aerosol chemical composition while approaching the cloud layer. Finally, the chemical composition of non-refractory evaporated cloud droplets measured within the cloud shows increased fractions of nitrate and organics (with respect to concentrations found below clouds), but also large amounts of sulfate, thus, related to activation of particles, made up of soluble compounds.
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Biomass burning emission inventories serve as critical input for atmospheric chemical transport models that are used to understand the role of biomass fires in the chemical composition of the atmosphere, air quality, and the climate system. Significant progress has been achieved in the development of regional and global biomass burning emission inventories over the past decade using satellite remote sensing technology for fire detection and burned area mapping. However, agreement among biomass burning emission inventories is frequently poor. Furthermore, the uncertainties of the emission estimates are typically not well characterized, particularly at the spatio-temporal scales pertinent to regional air quality modeling. We present the Wildland Fire Emission Inventory (WFEI), a high resolution model for non-agricultural open biomass burning (hereafter referred to as wildland fires, WF) in the contiguous United States (CONUS). The model combines observations from the MODerate Resolution Imaging Spectroradiometer (MODIS) sensors on the Terra and Aqua satellites, meteorological analyses, fuel loading maps, an emission factor database, and fuel condition and fuel consumption models to estimate emissions from WF. <br></br> WFEI was used to estimate emissions of CO (ECO) and PM<sub>2.5</sub> (EPM<sub>2.5</sub>) for the western United States from 2003–2008. The uncertainties in the inventory estimates of ECO and EPM<sub>2.5</sub> (<i>u</i><sub>ECO</sub> and <i>u</i><sub>EPM<sub>2.5</sub></sub>, respectively) have been explored across spatial and temporal scales relevant to regional and global modeling applications. In order to evaluate the uncertainty in our emission estimates across multiple scales we used a figure of merit, the half mass uncertainty, <i>ũ</i><sub>EX</sub> (where X = CO or PM<sub>2.5</sub>), defined such that for a given aggregation level 50% of total emissions occurred from elements with <i>u</i><sub>EX</sub> <i>ũ</i><sub>EX</sub>. The sensitivity of the WFEI estimates of ECO and EPM<sub>2.5</sub> to uncertainties in mapped fuel loading, fuel consumption, burned area and emission factors have also been examined. <br></br> The estimated annual, domain wide ECO ranged from 436 Gg yr<sup>−1</sup> in 2004 to 3107 Gg yr<sup>−1</sup> in 2007. The extremes in estimated annual, domain wide EPM<sub>2.5</sub> were 65 Gg yr<sup>−1</sup> in 2004 and 454 Gg yr<sup>−1</sup> in 2007. Annual WF emissions were a significant share of total emissions from non-WF sources (agriculture, dust, non-WF fire, fuel combustion, industrial processes, transportation, solvent, and miscellaneous) in the western United States as estimated in a national emission inventory. In the peak fire year of 2007, WF emissions were ~20% of total (WF + non-WF) CO emissions and ~39% of total PM<sub>2.5</sub> emissions. During the months with the greatest fire activity, WF accounted for the majority of total CO and PM<sub>2.5</sub> emitted across the study region. Uncertainties in annual, domain wide emissions was 28% to 51% for CO and 40% to 65% for PM<sub>2.5</sub>. Sensitivity of <i>ũ</i><sub>ECO</sub> and <i>ũ</i><sub>EPM<sub>2.5</sub></sub> to the emission model components depended on scale. At scales relevant to regional modeling applications (Δ<i>x</i> = 10 km, Δ<i>t</i> = 1 day) WFEI estimates 50% of total ECO with an uncertainty <133% and half of total EPM<sub>2.5</sub> with an uncertainty <146%. <i>ũ</i><sub>ECO</sub> and <i>ũ</i><sub>EPM<sub>2.5</sub></sub> are reduced by more than half at the scale of global modeling applications (Δ<i> x</i> = 100 km, Δ<i> t</i> = 30 day) where 50% of total emissions are estimated with an uncertainty <50% for CO and <64% for PM<sub>2.5</sub>. Uncertainties in the estimates of burned area drives the emission uncertainties at regional scales. At global scales <i>ũ</i><sub>ECO</sub> is most sensitive to uncertainties in the fuel load consumed while the uncertainty in the emission factor for PM<sub>2.5</sub> plays the dominant role in <i>ũ</i><sub>EPM<sub>2.5</sub></sub>. Our analysis indicates that the large scale aggregate uncertainties (e.g. the uncertainty in annual CO emitted for CONUS) typically reported for biomass burning emission inventories may not be appropriate for evaluating and interpreting results of regional scale modeling applications that employ the emission estimates. When feasible, biomass burning emission inventories should be evaluated and reported across the scales for which they are intended to be used.
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We examine the distribution of aerosols and associated optical/radiative properties in the Gangetic-Himalayan region from simultaneous radiometric measurements over the Indo-Gangetic Plains (IGP) and the foothill/southern slopes of the Himalayas during the 2009 pre-monsoon season. Enhanced dust transport extending from the Southwest Asian arid regions into the IGP, results in seasonal mean (April–June) aerosol optical depths of over 0.6 – highest over Southern Asia. The influence of dust loading is greater over the Western IGP as suggested by pronounced coarse mode peak in aerosol size distribution and spectral single scattering albedo (SSA). Transported dust in the IGP, driven by prevailing westerly airmass, is found to be more absorbing (SSA<sub>550 nm</sub><0.9) than the near-desert region in Northwestern (NW) India suggesting mixing with carbonaceous aerosols in the IGP. On the contrary, significantly reduced dust transport is observed over eastern IGP and foothill/elevated Himalayan slopes in Nepal where strongly absorbing haze is prevalent, as indicated by lower SSA (0.85–0.9 at 440–1020 nm), suggesting presence of more absorbing aerosols compared to IGP. Additionally, our observations show a distinct diurnal pattern of aerosols with characteristic large afternoon peak, from foothill to elevated mountain locations, associated with increased upslope transport of pollutants – that likely represent large-scale lifting of absorbing aerosols along the elevated slopes during pre-monsoon season. In terms of radiative impact of aerosols, over the source region of NW India, diurnal mean reduction in solar radiation fluxes was estimated to be 19–23 Wm<sup>−2</sup> at surface (12–15% of the surface solar insolation). Furthermore, based on limited observations of aerosol optical properties during the pre-monsoon period and comparison of our radiative forcing estimates with published literature, there exists a general spatial heterogeneity in the regional aerosol forcing, associated with the absorbing aerosol distribution over northern India, with both diurnal mean surface forcing and forcing efficiency over the IGP exceeding that over Northwestern India. Finally, the role of the seasonal progressive buildup of aerosol loading and water vapor is investigated in the observed net aerosol radiative effect over Northwestern India. The radiative impact of water vapor is found to amplify the net regional aerosol radiative forcing suggesting that the two exert forcing in tandem leading to enhanced surface cooling. It is suggested that water vapor contribution should be taken into account while assessing aerosol forcing impact for this region and other seasonally similar environments.
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The number concentration of activated CCN (<i>N</i><sub>a</sub>) is the most fundamental microphysical property of a convective cloud. It determines the rate of droplet growth with cloud depth and conversion into precipitation-sized particles and affects the radiative properties of the clouds. However, measuring <i>N</i><sub>a</sub> is not always possible, even in the cores of the convective clouds, because entrainment of sub-saturated ambient air deeper into the cloud lowers the concentrations by dilution and may cause partial or total droplet evaporation, depending on whether the mixing is homogeneous or extreme inhomogeneous, respectively. <br><br> Here we describe a methodology to derive <i>N</i><sub>a</sub> based on the rate of cloud droplet effective radius (<i>R</i><sub>e</sub>) growth with cloud depth and with respect to the cloud mixing with the entrained ambient air. We use the slope of the tight linear relationship between the adiabatic liquid water mixing ratio and <i>R</i><sub>e</sub><sup>3</sup> (or <i>R</i><sub>v</sub><sup>3</sup>) to derive an upper limit for <i>N</i><sub>a</sub> assuming extreme inhomogeneous mixing. Then we tune <i>N</i><sub>a</sub> down to find the theoretical relative humidity that the entrained ambient air would have for each horizontal cloud penetration, in case of homogeneous mixing. This allows us to evaluate both the entrainment and mixing process in the vertical dimension in addition to getting a better estimation for <i>N</i><sub>a</sub>. <br><br> We found that the derived <i>N</i><sub>a</sub> from the entire profile data is highly correlated with the independent CCN measurements from below cloud base. Moreover, it was found that mixing of sub-saturated ambient air into the cloud at scales of ~100 m and above is inclined towards the extreme inhomogeneous limit, i.e. that the time scale of droplet evaporation is significantly smaller than that for turbulent mixing. This means that ambient air that entrains the cloud is pre-moistened by total evaporation of cloud droplets before it mixes deeper into the clouds where it can hardly change the droplet size distribution, hence <i>R</i><sub>e</sub> remains close to its adiabatic value at any given cloud depth. However, the tendency towards the extreme inhomogeneous mixing appeared to slightly decrease with altitude, possibly due to enhanced turbulence and larger cloud drops aloft. <br><br> Quantifying these effects, based on more examples from other projects and high resolution cloud models is essential for improving our understanding of the interactions between the cloud and its environment. These interactions may play an important role in cloud dynamics and microphysics, by affecting cloud depth and droplet size spectra, for example, and may therefore influence the cloud precipitation formation processes.
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A nearly global statistical analysis of vertical backscatter and extinction profiles of cirrus clouds collected by the CALIOP lidar, on-board of the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation, is presented. <br><br> Statistics on frequency of occurrence and distribution of bulk properties of cirrus clouds in general and, for the first time, of horizontally homogeneous (on a 5-km field of view) cirrus clouds only are provided. Annual and seasonal backscatter profiles (BSP) are computed for the horizontally homogeneous cirri. Differences found in the day/night cases and for midlatitudes and tropics are studied in terms of the mean physical parameters of the clouds from which they are derived. <br><br> The relationship between cloud physical parameters (optical depth, geometrical thickness and temperature) and the shape of the BSP is investigated. It is found that cloud geometrical thickness is the main parameter affecting the shape of the mean CALIOP BSP. Specifically, cirrus clouds with small geometrical thicknesses show a maximum in mean BSP curve located near cloud top. As the cloud geometrical thickness increases the BSP maximum shifts towards cloud base. Cloud optical depth and temperature have smaller effects on the shape of the CALIOP BSPs. In general a slight increase in the BSP maximum is observed as cloud temperature and optical depth increase. <br><br> In order to fit mean BSPs, as functions of geometrical thickness and position within the cloud layer, polynomial functions are provided. The impact on satellite radiative transfer simulations in the infrared spectrum when using either a constant ice-content (IWC) along the cloud vertical dimension or an IWC profile derived from the BSP fitting functions is evaluated. It is, in fact, demonstrated that, under realistic hypotheses, the mean BSP is linearly proportional to the IWC profile.
