Index Geophysics

Items

• Assessment of fossil fuel carbon dioxide and other anthropogenic trace gas emissions from airborne measurements over Sacramento, California in spring 2009

  Direct quantification of fossil fuel CO₂ (CO₂ff) in atmospheric samples can be used to examine several carbon cycle and air quality questions. We collected in situ CO₂, CO, and CH₄ measurements and flask samples in the boundary layer and free troposphere over Sacramento, California, USA, during two aircraft flights over and downwind of this urban area during spring of 2009. The flask samples were analyzed for Δ¹⁴CO₂ and CO₂ to determine the recently added CO₂ff mole fraction. A suite of greenhouse and other trace gases, including hydrocarbons and halocarbons, were measured in the same samples. Strong correlations were observed between CO₂ff and numerous trace gases associated with urban emissions. From these correlations we estimate emission ratios between CO₂ff and these species, and compare these with bottom-up inventory-derived estimates. Recent county level inventory estimates for carbon monoxide (CO) and benzene from the California Air Resources Board CEPAM database are in good agreement with our measured emission ratios, whereas older emissions inventories appear to overestimate emissions of these gases by a factor of two. For most other trace species, there are substantial differences (200–500%) between our measured emission ratios and those derived from available emission inventories. For the first flight, we combine in situ CO measurements with the measured CO:CO₂ff emission ratio of 14 ± 2 ppbCO/ppmCO₂ to derive an estimate of CO₂ff mole fraction throughout this flight, and also estimate the biospheric CO₂ mixing ratio (CO₂bio) from the difference of total and fossil CO₂. The resulting CO₂bio varies dramatically from up to 8 ± 2 ppm in the urban plume to −6 ± 1 ppm in the surrounding boundary layer air. Finally, we use the in situ estimates of CO₂ff mole fraction to infer total fossil fuel CO₂ emissions from the Sacramento region, using a mass balance approach. The resulting emissions are uncertain to within a factor of two due to uncertainties in wind speed and boundary layer height. Nevertheless, this first attempt to estimate urban-scale CO₂ff from atmospheric radiocarbon measurements shows that CO₂ff can be used to verify and improve emission inventories for many poorly known anthropogenic species, separate biospheric CO₂, and indicates the potential to constrain CO₂ff emissions if transport uncertainties are reduced.

• High solubility of SO2: evidence in an intensive fog event measured in the NCP region, China

  A field experiment was conducted in a heavy SO₂ pollution area located in north China plain (NCP). During the experiment, SO₂ and other air pollutants, liquid water content (LWC) of fog droplets, and basic meteorological parameters were measured. During the experiment, an intensive fog event occurred between 5 and 8 November 2009. During the fog period, the concentrations of SO₂ showed a strong variability, and the variability was closely correlated to the appearances of fogs and LWC. For example, the averaged concentration of SO₂ during the non-fog period was about 25 ppbv. By contrast, during the fog period, the concentration of SO₂ reduced to about 4–7 ppbv. The large reduction of SO₂ suggests that a majority of SO₂ (about 70–80%) had been converted from gas-phase to aqueous-phase, showing a high solubility of SO₂. However, according to the value of Henry Law constant, the solubility of SO₂ is modest, which cannot explain the measured large reduction of SO₂. This study highlights that the aqueous reactions of SO₂ in the droplets of fogs play important roles to enhance the solubility of SO₂. To account for the effect of aqueous reactions on the solubility of SO₂, an "effective" Henry Law constant of SO₂ is proposed in this study. The study shows that without considering aqueous reactions of SO₂ in fog droplets, the estimate of the partitioning of SO₂ in droplets is significantly lower than the measured values. By contrast, when the "effective" Henry Law constant is applied in the calculation, the calculated SO₂ concentrations are significantly improved, showing that the aqueous reactions of SO₂ play important roles in controlling the solubility of SO₂, and should be considered in model calculations.

• Observations of Saharan dust microphysical and optical properties from the Eastern Atlantic during NAMMA airborne field campaign

  As part of the international project entitled "African Monsoon Multidisciplinary Analysis (AMMA)", NAMMA (NASA AMMA) aimed to gain a better understanding of the relationship between the African Easterly Waves (AEWs), the Sahara Air Layer (SAL), and tropical cyclogenesis. The NAMMA airborne field campaign was based out of the Cape Verde Islands during the peak of the hurricane season, i.e., August and September 2006. Multiple Sahara dust layers were sampled during 62 encounters in the eastern portion of the hurricane main development region, covering both the eastern North Atlantic Ocean and the western Saharan desert (i.e., 5–22° N and 10–35° W). The centers of these layers were located at altitudes between 1.5 and 3.3 km and the layer thickness ranged from 0.5 to 3 km. Detailed dust microphysical and optical properties were characterized using a suite of in-situ instruments aboard the NASA DC-8 that included a particle counter, an Ultra-High Sensitivity Aerosol Spectrometer, an Aerodynamic Particle Sizer, a nephelometer, and a Particle Soot Absorption Photometer. The NAAMA sampling inlet has a size cut (i.e., 50% transmission efficiency size) of approximately 4 μm in diameter for dust particles, which limits the representativeness of the NAMMA observational findings. The NAMMA dust observations showed relatively low particle number densities, ranging from 268 to 461 cm⁻³, but highly elevated volume density with an average at 45 μm³ cm⁻³. NAMMA dust particle size distributions can be well represented by tri-modal lognormal regressions. The estimated volume median diameter (VMD) is averaged at 2.1 μm with a small range of variation regardless of the vertical and geographical sampling locations. The Ångström Exponent assessments exhibited strong wavelength dependence for absorption but a weak one for scattering. The single scattering albedo was estimated at 0.97 ± 0.02. The imaginary part of the refractive index for Sahara dust was estimated at 0.0022, with a range from 0.0015 to 0.0044. Closure analysis showed that observed scattering coefficients are highly correlated with those calculated from spherical Mie-Theory and observed dust particle size distributions. These values are generally consistent with literature values reported from studies with similar particle sampling size range.

• Detecting the influence of fossil fuel and bio-fuel black carbon aerosols on near surface temperature changes

  Past research has shown that the dominant influence on recent global climate changes is from anthropogenic greenhouse gas increases with implications for future increases in global temperatures. One mitigation proposal is to reduce black carbon aerosol emissions. How much warming can be offset by controlling black carbon is unclear, especially as its influence on past climate has not been previously unambiguously detected. In this study observations of near-surface warming over the last century are compared with simulations using a climate model, HadGEM1. In the simulations black carbon, from fossil fuel and bio-fuel sources (fBC), produces a positive radiative forcing of about +0.25 Wm⁻² over the 20th century, compared with +2.52 Wm⁻² for well mixed greenhouse gases. A simulated warming of global mean near-surface temperatures over the twentieth century from fBC of 0.14 ± 0.1 K compares with 1.06 ± 0.07 K from greenhouse gases, −0.58 ± 0.10 K from anthropogenic aerosols, ozone and land use changes and 0.09 ± 0.09 K from natural influences. Using a detection and attribution methodology, the observed warming since 1900 has detectable influences from anthropogenic and natural factors. Fossil fuel and bio-fuel black carbon is found to have a detectable contribution to the warming over the last 50 yr of the 20th century, although the results are sensitive to the period being examined as fBC is not detected for the later fifty year period ending in 2006. The attributed warming of fBC was found to be consistent with the warming from fBC unscaled by the detection analysis. This study suggests that there is a possible significant influence from fBC on global temperatures, but its influence is small compared to that from greenhouse gas emissions.

• Generation of hydrogen peroxide from San Joaquin Valley particles in a cell-free solution

  Epidemiological studies have shown a correlation between exposure to ambient particulate matter (PM) and adverse health effects. One proposed mechanism of PM-mediated health effects is the generation of reactive oxygen species (ROS) – e.g., superoxide (^•O₂^−), hydrogen peroxide (HOOH), and hydroxyl radical (^•OH) – followed by oxidative stress. There are very few quantitative, specific measures of individual ROS generated from PM, but this information would help to more quantitatively address the link between ROS and the health effects of PM. To address this gap, we quantified the generation of HOOH by PM collected at an urban (Fresno) and rural (Westside) site in the San Joaquin Valley (SJV) of California during summer and winter from 2006 to 2009. HOOH was quantified by HPLC after extracting the PM in a cell-free, phosphate-buffered saline (PBS) solution with or without 50 μM ascorbate (Asc). Our results show that the urban PM generally generates much more HOOH than the rural PM but that there is no apparent seasonal difference in HOOH generation. In nearly all of the samples the addition of a physiologically relevant concentration of Asc greatly enhances HOOH formation, but a few of the coarse PM samples were able to generate a considerable amount of HOOH in the absence of added Asc, indicating the presence of unknown reductants. Normalized by air volume, the fine PM (PM_2.5) generally makes more HOOH than the corresponding coarse PM (PM_cf, i.e., 2.5 to 10 μm), primarily because the mass concentration of PM_2.5 is much higher than that of PM_cf. However, normalized by PM mass, the coarse PM typically generates more HOOH than the fine PM. The amount of HOOH produced by SJV PM is reduced on average by (78 ± 15)% when the transition metal chelator desferoxamine (DSF) is added to the extraction solution, indicating that transition metals play a dominant role in HOOH generation. By measuring calibration curves of HOOH generation from copper, and quantifying copper concentrations in our particle extracts, we find that PBS-soluble copper is primarily responsible for HOOH production by the Fresno PM. Extrapolating our results to expected concentrations of PM-derived HOOH in human lung lining fluid suggests that typical daily PM exposures in the San Joaquin Valley are unlikely to cause HOOH-mediated acute health effects, but that very high PM events might lead to cytotoxic levels of pulmonary HOOH.

• Relating tropical ocean clouds to moist processes using water vapor isotope measurements

  We examine the co-variations of tropospheric water vapor, its isotopic composition and cloud types and relate these distributions to tropospheric mixing and distillation models using satellite observations from the Aura Tropospheric Emission Spectrometer (TES) over the summertime tropical ocean. Interpretation of these process distributions must take into account the sensitivity of the TES isotope and water vapor measurements to variations in cloud, water, and temperature amount. Consequently, comparisons are made between cloud-types based on the International Satellite Cloud Climatology Project (ISSCP) classification; these are clear sky, non-precipitating (e.g., cumulus), boundary layer (e.g., stratocumulus), and precipitating clouds (e.g. regions of deep convection). In general, we find that the free tropospheric vapor over tropical oceans does not strictly follow a Rayleigh model in which air parcels become dry and isotopically depleted through condensation. Instead, mixing processes related to convection as well as subsidence, and re-evaporation of rainfall associated with organized deep convection all play significant roles in controlling the water vapor distribution. The relative role of these moisture processes are examined for different tropical oceanic regions.

• Atmospheric ions and nucleation: a review of observations

  This review is based on ca. 260 publications, 93 of which included data on the temporal and spatial variation of the concentration of small ions (<1.6 nm in diameter) especially in the lower troposphere, chemical composition, or formation and growth rates of sub-3 nm ions. This information was collected on tables and figures. The small ions exist all the time in the atmosphere, and the average concentrations of positive and negative small ions are typically 200–2500 cm⁻³. However, concentrations up to 5000 cm⁻³ have been observed. The results are in agreement with observations of ion production rates in the atmosphere. We also summarised observations on the conversion of small ions to intermediate ions, which can act as embryos for new atmospheric aerosol particles. Those observations include the formation rates (<i>J</i>₂[ion]) of 2-nm intermediate ions, growth rates (GR[ion]) of sub-3 nm ions, and information on the chemical composition of the ions. Unfortunately, there were only a few studies which presented <i>J</i>₂[ion] and GR[ion]. Based on the publications, the formation rates of 2-nm ions were 0–1.1 cm⁻³ s⁻¹, while the total 2-nm particle formation rates varied between 0.001 and 60 cm⁻³ s⁻¹. Due to small changes in <i>J</i>₂[ion], the relative importance of ions in 2-nm particle formation was determined by the large changes in <i>J</i>₂[tot], and, accordingly the contribution of ions increased with decreasing <i>J</i>₂[tot]. Furthermore, small ions were observed to activate for growth earlier than neutral nanometer-sized particles and at lower saturation ratio of condensing vapours.

• Seasonal variation of trans-Pacific transport of carbon monoxide (CO) in the upper troposphere: MLS observations and GEOS-Chem and GEM-AQ simulations

  Multi-year Microwave Limb Sounder (MLS) carbon monoxide (CO) measurements at 215 hPa are employed to present a climatological view of seasonal variation of upper tropospheric trans-Pacific transport of Asian air pollution. The measurements show that the transport peaks in late boreal spring and early boreal summer. Although the strongest Asian air pollution outflow occurs in boreal summer, the "transport pathway" over the northeast Pacific is narrower in summer than in spring. Results from two tropospheric chemistry models GEOS-Chem and GEM-AQ are compared to MLS observations. Both models reproduce the strong trans-Pacific transport in boreal spring and summer well, but show different morphologies over Southeast Asia in winter and fall. A tagged CO simulation using GEOS-Chem indicates that Asian fossil fuel is the biggest source of upper tropospheric CO over the north Pacific in all seasons, excepting methane (CH₄) and non-methane hydrocarbons, although there are large fires in Southeast Asia in boreal spring and fall. A sensitivity test indicates that deep convection has a large effect on upper tropospheric CO abundances, increasing the abundances by more than 40%, over the north Pacific in boreal spring. In boreal summer, however, the increase is not significant over the north Pacific although it is large over continental Asia.

• Residual circulation trajectories and transit times into the extratropical lowermost stratosphere

  Transport into the extratropical lowermost stratosphere (LMS) can be divided into a slow part (time-scale of several months to years) associated with the global-scale stratospheric residual circulation and a fast part (time-scale of days to a few months) associated with (mostly quasi-horizontal) mixing (i.e. two-way irreversible transport, including extratropical stratosphere-troposphere exchange). The stratospheric residual circulation may be considered to consist of two branches: a deep branch more strongly associated with planetary waves breaking in the middle to upper stratosphere, and a shallow branch associated with synoptic and planetary scale waves breaking in the subtropical lower stratosphere. In this study the contribution due to the stratospheric residual circulation alone to transport into the LMS is quantified using residual circulation trajectories, i.e. trajectories driven by the (time-dependent) residual mean meridional and vertical velocities. This contribution represents the advective part of the overall transport into the LMS and can be viewed as providing a background onto which the effect of mixing has to be added. Residual mean velocities are obtained from a comprehensive chemistry-climate model as well as from reanalysis data. Transit times of air traveling from the tropical tropopause to the LMS along the residual circulation streamfunction are evaluated and compared to recent mean age of air estimates. A time-scale separation with much smaller transit times into the mid-latitudinal LMS than into polar LMS is found that is indicative of a separation of the shallow from the deep branch of the residual circulation. This separation between the shallow and the deep circulation branch is further manifested in a distinction in the aspect ratio of the vertical to meridional extent of the trajectories, the integrated mass flux along the residual circulation trajectories, as well as the stratospheric entry latitude of the trajectories. The residual transit time distribution reproduces qualitatively the observed seasonal cycle of youngest air in the extratropical LMS in fall and oldest air in spring.

• Modeling secondary organic aerosol formation from isoprene oxidation under dry and humid conditions

  A new model for the formation of secondary organic aerosol (SOA) from isoprene was developed. This model uses surrogate molecular species (hydroxy-hydroperoxides, tetrols, methylglyceric acid, organic nitrates) to represent SOA formation. The development of this model used available experimental data on yields and molecular composition of SOA from isoprene and methacrolein oxidation. This model reproduces the amount of particles measured in smog chambers under both low-NO_x and high-NO_x conditions. Under low-NO_x conditions, the model reproduces the transitional formation of hydroxy-hydroperoxides particles, which are photolyzed and lead to SOA mass decrease after reaching a maximum. Under high-NO_x conditions, particles are assumed to be formed mostly from the photo-oxidation of a PAN-type molecule derived from methacrolein (MPAN). This model successfully reproduces the complex NO_x-dependence of isoprene oxidation and suggests a possible yield increase under some high-NO_x conditions. Experimental data correspond to dry conditions (RH < 10%). However, particles formed from isoprene are expected to be highly hydrophilic, and isoprene oxidation products would likely partition between an aqueous phase and the gas phase at high humidity in the atmosphere. The model was extended to take into account the hydrophilic properties of SOA, which are relevant under atmospheric conditions, and investigate the effect of particulate liquid water on SOA formation. An important increase in SOA mass was estimated for humid conditions due to the hydrophilic properties. Experiments under high relative humidity conditions should be conducted to confirm the results of this study, which have implications for SOA modeling.