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Ground-based aerosol measurements made in June 2010 within Sacramento urban area (site T0) and at a 40-km downwind location (site T1) in the forested Sierra Nevada foothills area are used to investigate the evolution of multispectral optical properties as the urban aerosols aged and interacted with biogenic emissions. Along with black carbon and non-refractory aerosol mass and composition observations, spectral absorptio (β<sub>abs</sub>), scattering (β<sub>sca</sub>), and extinction (β<sub>ext</sub>) coefficients for wavelengths ranging from 355 to 1064 nm were measured at both sites using photoacoustic (PA) instruments with integrating nephelometers and using cavity ring-down (CRD) instruments. The daytime average Ångström exponent of absorption (AEA) was ~1.6 for the wavelength pair 405 and 870 nm at T0, while it was ~1.8 for the wavelength pair 355 and 870 nm at T1, indicating a modest wavelength-dependent enhancement of absorption at both sites throughout the study. The measured and Mie theory calculations of multispectral β<sub>sca</sub> showed good correlation (<i>R</i><sup>2</sup>=0.85–0.94). The average contribution of supermicron aerosol (mainly composed of sea salt particles advected in from the Pacific Ocean) to the total scattering coefficient ranged from less than 20% at 405 nm to greater than 80% at 1064 nm. From 22 to 28 June, secondary organic aerosol mass increased significantly at both sites due to increased biogenic emissions coupled with intense photochemical activity and air mass recirculation in the area. During this period, the short wavelength scattering coefficients at both sites gradually increased due to increase in the size of submicron aerosols. At the same time, BC mass-normalized absorption cross-section (MAC) values for ultraviolet wavelengths at T1 increased by ~60% compared to the relatively less aged urban emissions at the T0 site. In contrast, the average MAC values for 870 nm wavelength were identical at both sites. These results suggest formation of moderately brown secondary organic aerosols formed in biogenically-influenced urban air.
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Pinonaldehyde oxidation by OH radicals under low-NO<sub>x</sub> conditions produces significant secondary organic aerosol (SOA) mass yields. Under concurrent UV illumination, mass yields are lower than high-NO<sub>x</sub> yields published earlier by our group. However, when OH radicals are produced via dark ozonolysis the SOA mass yields are comparable at high and low NO<sub>x</sub>. Because pinonaldehyde is a major first-generation gas-phase product of α-pinene oxidation by either ozone or OH radicals, its potential to form SOA serves as a molecular counterpoint to bulk SOA aging experiments involving SOA formed from α-pinene. Both the general tendency for aging reactions to produce more SOA and the sensitivity of the low-NO<sub>x</sub> products to UV photolysis observed in the bulk clearly occur for this single species as well. Photochemical oxidation of pinonaldehye and analogous first-generation terpene oxidation products are potentially a significant source of additional SOA in biogenically influenced air masses.
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Strong positive relationships between cloud fraction (<i>f</i><sub>c</sub>) and aerosol optical depth (τ) have been reported. Data retrieved from the MODerate resolution Imaging Spectroradiometer (MODIS) instrument show positive <i>f</i><sub>c</sub>–τ relationships across most of the globe. A global mean <i>f</i><sub>c</sub> increase of approximately 0.2 between low and high τ conditions is found for both ocean and land. However, these relationships are not necessarily due to cloud–aerosol interactions. Using state-of-the-art Monitoring Atmospheric Composition and Climate (MACC) reanalysis-forecast τ data, which should be less affected by retrieval artefacts, it is demonstrated that a large part of the observed <i>f</i><sub>c</sub>–τ signal may be due to cloud contamination of satellite-retrieved τ. For longer MACC forecast time steps of 24 h, which likely contain less cloud contamination, some negative <i>f</i><sub>c</sub>–τ relationships are found. The global mean <i>f</i><sub>c</sub> increase between low and high τ conditions is reduced to 0.1, suggesting that cloud contamination may account for approximately one half of the satellite-retrieved increase in <i>f</i><sub>c</sub>. ECHAM5-HAM general circulation model (GCM) simulations further demonstrate that positive <i>f</i><sub>c</sub>–τ relationships may arise due to covariation with relative humidity. Widespread negative simulated <i>f</i><sub>c</sub>–τ relationships in the tropics are shown to arise due to scavenging of aerosol by convective precipitation. Wet scavenging events are likely poorly sampled in satellite-retrieved data, because the properties of aerosol below clouds cannot be retrieved. Quantifying the role of wet scavenging, and assessing GCM representations of this important process, remains a challenge for future observational studies of aerosol–cloud–precipitation interactions.
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Thermal infrared radiances from the Tropospheric Emission Spectrometer (TES) between 10 and 15 μm contain significant carbon dioxide (CO<sub>2</sub>) information, however the CO<sub>2</sub> signal must be separated from radiative interference from temperature, surface and cloud parameters, water, and other trace gases. Validation requires data sources spanning the range of TES CO<sub>2</sub> sensitivity, which is approximately 2.5 to 12 km with peak sensitivity at about 5 km and the range of TES observations in latitude (40° S to 40° N) and time (2005–2011). We therefore characterize Tropospheric Emission Spectrometer (TES) CO<sub>2</sub> version 5 biases and errors through comparisons to ocean and land-based aircraft profiles and to the CarbonTracker assimilation system. We compare to ocean profiles from the first three Hiaper Pole-to-Pole Observations (HIPPO) campaigns between 40° S and 40° N with measurements between the surface and 14 km and find that TES CO<sub>2</sub> estimates capture the seasonal and latitudinal gradients observed by HIPPO CO<sub>2</sub> measurements. Actual errors range from 0.8–1.8 ppm, depending on the campaign and pressure level, and are approximately 1.6–2 times larger than the predicted errors. The bias of TES versus HIPPO is within 1 ppm for all pressures and datasets; however, several of the sub-tropical TES CO<sub>2</sub> estimates are lower than expected based on the calculated errors. Comparisons to land aircraft profiles from the United States Southern Great Plains (SGP) Atmospheric Radiation Measurement (ARM) between 2005 and 2011 measured from the surface to 5 km to TES CO<sub>2</sub> show good agreement with an overall bias of −0.3 ppm to 0.1 ppm and standard deviations of 0.8 to 1.0 ppm at different pressure levels. Extending the SGP aircraft profiles above 5 km using AIRS or CONTRAIL measurements improves comparisons with TES. Comparisons to CarbonTracker (version CT2011) show a persistent spatially dependent bias pattern and comparisons to SGP show a time-dependent bias of −0.2 ppm yr<sup>−1</sup>. We also find that the predicted sensitivity of the TES CO<sub>2</sub> estimates is too high, which results from using a multi-step retrieval for CO<sub>2</sub> and temperature. We find that the averaging kernel in the TES product corrected by a pressure-dependent factor accurately reflects the sensitivity of the TES CO<sub>2</sub> product.
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A new method to retrieve ice cloud asymmetry parameters from multi-directional polarized reflectance measurements is applied to measurements of the airborne Research Scanning Polarimeter (RSP) obtained during the CRYSTAL-FACE campaign in 2002. The method assumes individual hexagonal ice columns and plates serve as proxies for more complex shapes and aggregates. The closest fit is searched in a look-up table of simulated polarized reflectances computed for cloud layers that contain individual, randomly oriented hexagonal columns and plates with a virtually continuous selection of aspect ratios and distortion. The asymmetry parameter, aspect ratio and distortion of the hexagonal particle that leads to the best fit with the measurements are considered the retrieved values. Two cases of thick convective clouds and two cases of thinner anvil cloud layers are analyzed. Median asymmetry parameters retrieved by the RSP range from 0.76 to 0.78, and are generally smaller than those currently assumed in most climate models and satellite retrievals. In all cases the measurements indicate roughened or distorted ice crystals, which is consistent with previous findings. Retrieved aspect ratios in three of the cases range from 0.9 to 1.6, indicating compact particles dominate the cloud-top shortwave radiation. Retrievals for the remaining case indicate plate-like ice crystals with aspect ratios around 0.3. The RSP retrievals are qualitatively consistent with the CPI images obtained in the same cloud layers. Retrieved asymmetry parameters are compared to those determined in situ by the Cloud Integrating Nephelometer (CIN). For two cases, the median values of asymmetry parameter retrieved by CIN and RSP agree within 0.01, while for the two other cases RSP asymmetry parameters are about 0.03–0.05 greater than those obtained by the CIN. Part of this bias might be explained by vertical variation of the asymmetry parameter or ice shattering on the CIN probe, or both.
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Two semi-empirical models were developed for the Antarctic stratosphere to relate the shift of species within total chlorine (Cl<sub>y</sub> = HCl + ClONO<sub>2</sub> + HOCl + 2 × Cl<sub>2</sub> + 2×Cl<sub>2</sub>O<sub>2</sub> + ClO + Cl) into the active forms (here: ClO<sub>x</sub> = 2×Cl<sub>2</sub>O<sub>2</sub> + ClO), and to relate the rate of ozone destruction to ClO<sub>x</sub>. These two models provide a fast and computationally inexpensive way to describe the inter- and intra-annual evolution of ClO<sub>x</sub> and ozone mass deficit (OMD) in the Antarctic spring. The models are based on the underlying physics/chemistry of the system and capture the key chemical and physical processes in the Antarctic stratosphere that determine the interaction between climate change and Antarctic ozone depletion. They were developed considering bulk effects of chemical mechanisms for the duration of the Antarctic vortex period and quantities averaged over the vortex area. The model equations were regressed against observations of daytime ClO and OMD providing a set of empirical fit coefficients. Both semi-empirical models are able to explain much of the intra- and inter-annual variability observed in daily ClO<sub>x</sub> and OMD time series. This proof-of-concept paper outlines the semi-empirical approach to describing the evolution of Antarctic chlorine activation and ozone depletion.
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Simulated multi-model "diversity" in aerosol direct radiative forcing estimates is often perceived as a measure of aerosol uncertainty. However, current models used for aerosol radiative forcing calculations vary considerably in model components relevant for forcing calculations and the associated "host-model uncertainties" are generally convoluted with the actual aerosol uncertainty. In this AeroCom Prescribed intercomparison study we systematically isolate and quantify host model uncertainties on aerosol forcing experiments through prescription of identical aerosol radiative properties in twelve participating models. <br><br> Even with prescribed aerosol radiative properties, simulated clear-sky and all-sky aerosol radiative forcings show significant diversity. For a purely scattering case with globally constant optical depth of 0.2, the global-mean all-sky top-of-atmosphere radiative forcing is −4.47 Wm<sup>−2</sup> and the inter-model standard deviation is 0.55 Wm<sup>−2</sup>, corresponding to a relative standard deviation of 12%. For a case with partially absorbing aerosol with an aerosol optical depth of 0.2 and single scattering albedo of 0.8, the forcing changes to 1.04 Wm<sup>−2</sup>, and the standard deviation increases to 1.01 W<sup>−2</sup>, corresponding to a significant relative standard deviation of 97%. However, the top-of-atmosphere forcing variability owing to absorption (subtracting the scattering case from the case with scattering and absorption) is low, with absolute (relative) standard deviations of 0.45 Wm<sup>−2</sup> (8%) clear-sky and 0.62 Wm<sup>−2</sup> (11%) all-sky. <br><br> Scaling the forcing standard deviation for a purely scattering case to match the sulfate radiative forcing in the AeroCom Direct Effect experiment demonstrates that host model uncertainties could explain about 36% of the overall sulfate forcing diversity of 0.11 Wm<sup>−2</sup> in the AeroCom Direct Radiative Effect experiment. <br><br> Host model errors in aerosol radiative forcing are largest in regions of uncertain host model components, such as stratocumulus cloud decks or areas with poorly constrained surface albedos, such as sea ice. Our results demonstrate that host model uncertainties are an important component of aerosol forcing uncertainty that require further attention.
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The cryosphere is an important component of global organic contaminant cycles. Snow is an efficient scavenger of atmospheric organic pollutants while a seasonal snowpack, sea ice, glaciers and ice caps are contaminant reservoirs on time scales ranging from days to millennia. Important physical and chemical processes occurring in the various cryospheric compartments impact contaminant cycling and fate. A variety of interactions and feedbacks also occur within the cryospheric system, most of which are susceptible to perturbations due to climate change. In this article, we review the current state of knowledge regarding the transport and processing of organic contaminants in the global cryosphere with an emphasis on the role of a changing climate. Given the complexity of contaminant interactions with the cryosphere and limitations on resources and research capacity, interdisciplinary research and extended collaborations are essential to close identified knowledge gaps and to improve our understanding of contaminant fate under a changing climate.
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Measurements of mixing ratio profiles of stratospheric bromine monoxide (BrO) were made using observations of BrO rotational line emission at 650.179 GHz by a balloon-borne SIS (superconductor-insulator-superconductor) submillimeterwave heterodyne limb sounder (SLS). The balloon was launched from Ft. Sumner, New Mexico (34° N) on 22 September 2011. Peak mid-day BrO abundance varied from 16 ± 2 ppt at 34 km to 6 ± 4 ppt at 16 km. Corresponding estimates of total inorganic bromine (Br<sub>y</sub>), derived from BrO vmr (volume mixing ratio) using a photochemical box model, were 21 ± 3 ppt and 11 ± 5 ppt, respectively. Inferred Br<sub>y</sub> abundance exceeds that attributable solely to decomposition of long-lived methyl bromide and other halons, and is consistent with a contribution from bromine-containing very short lived substances, Br<sub>y</sub><sup>VSLS</sup>, of 4 ppt to 8 ppt. These results for BrO and Br<sub>y</sub> were compared with, and found to be in good agreement with, those of other recent balloon-borne and satellite instruments.
