Dimethylsulfide (DMS) emitted from the ocean is a biogenic precursor gas for sulfur dioxide (SO2) and non-sea-salt sulfate aerosols (SO42−). During the VAMOS-Ocean-Cloud-Atmosphere-Land Study ...Regional Experiment (VOCALS-REx) in 2008, multiple instrumented platforms were deployed in the Southeastern Pacific (SEP) off the coast of Chile and Peru to study the linkage between aerosols and stratocumulus clouds. We present here observations from the NOAA Ship Ronald H. Brown and the NSF/NCAR C-130 aircraft along ~20° S from the coast (70° W) to a remote marine atmosphere (85° W). While SO42− and SO2 concentrations were distinctly elevated above background levels in the coastal marine boundary layer (MBL) due to anthropogenic influence (~800 and 80 pptv, respectively), their concentrations rapidly decreased west of 78° W (~100 and 25 pptv). In the remote region, entrainment from the free troposphere (FT) increased MBL SO2 burden at a rate of 0.05 ± 0.02 μmoles m−2 day−1 and diluted MBL SO42 burden at a rate of 0.5 ± 0.3 μmoles m−2 day−1, while the sea-to-air DMS flux (3.8 ± 0.4 μmoles m−2 day−1) remained the predominant source of sulfur mass to the MBL. In-cloud oxidation was found to be the most important mechanism for SO2 removal and in situ SO42− production. Surface SO42− concentration in the remote MBL displayed pronounced diel variability, increasing rapidly in the first few hours after sunset and decaying for the rest of the day. We theorize that the increase in SO42− was due to nighttime recoupling of the MBL that mixed down cloud-processed air, while decoupling and sporadic precipitation scavenging were responsible for the daytime decline in SO42−.
The air quality and climate forcing impacts of atmospheric aerosols in a metropolitan region depend on the amount, composition, and size of the aerosol transported into the region; the input and ...removal of aerosols and aerosol precursors within the region; and the subsequent chemical processing in the atmosphere. These factors were studied in the Houston‐Galveston‐Gulf of Mexico region, aboard the NOAA R/V Ronald H. Brown during the Texas Air Quality Study and Gulf of Mexico Atmospheric Composition and Climate Study (TexAQS/GoMACCS 2006). The aerosol measured in the Gulf of Mexico during onshore flow (low radon concentrations indicating no contact with land for several days) was highly impacted by Saharan dust and what appear to be ship emissions (acidic sulfate and nitrate). Mean (median) mass concentrations of the total submicrometer and supermicrometer aerosol were 6.5 (4.6) μg m−3 and 17.2 (8.7) μg m−3, respectively. These mass loadings of “background” aerosol are much higher than typically observed in the marine atmosphere and thus have a substantial impact on the radiative energy balance over the Gulf of Mexico and particulate matter (PM) loadings (air quality) in the Houston‐Galveston area. As this background aerosol moved onshore, local urban and industrial sources added an organic rich submicrometer component (66% particulate organic matter (POM), 20% sulfate, 14% elemental carbon) but no significant supermicrometer aerosol. The resulting aerosol had mean (median) mass concentrations of the total submicrometer and supermicrometer aerosol of 10.0 (9.1) μg m−3 and 16.8 (11.2) μg m−3, respectively. These air masses, with minimal processing of urban emissions contained the highest SO2/(SO2 + SO4=) ratios and the highest hydrocarbon‐like organic aerosol to total organic aerosol ratios (HOA/POM). In contrast, during periods of offshore flow, the aerosol was more processed and, therefore, much richer in oxygenated organic aerosol (OOA). Mean (median) mass concentrations of the total submicrometer and supermicrometer aerosol were 20.8 (18.6) μg m−3 and 7.4 (5.0) μg m−3, respectively. Sorting air masses based on their trajectories and time over land provides a means to examine the effects of transport and subsequent chemical processing. Understanding and parameterizing these processes is critical for the chemical transport modeling that forms the basis for air quality forecasts and radiative forcing calculations.
Particles generated by combustion of fossil fuels contribute to climate forcing by absorbing and scattering visible light. Residential combustion takes place in homes for heating or cooking purposes ...and is thought to contribute a large fraction of the global burden of anthropogenic primary particles. We present optical properties and size distributions of particulate matter emitted from three types of coal burned in residential combustors: bituminous coal, hard coal briquettes, and lignite. Emissions from these coals differ significantly and can be partially explained by differences in coal composition. For bituminous coal, particulate matter emission factors are somewhat greater than those used in current emission inventories. We observe particles for which the light absorption is weak and has a strong spectral dependence. For hard coal briquettes and lignite, emitted light absorption is low, and based on our measurements, current inventories of light‐absorbing aerosols significantly overestimate the contribution from these sources. Hard coal briquettes produce very few particles in the optically active size range. For all coals tested the size distributions required to represent the average of the emitted particles are broader than atmospheric size distributions, with geometric standard deviations between 2.2 and 3.0.
Aircraft-based measurements of aerosol light-scattering coefficient, cloud condensation nuclei (CCN) measured directly or by proxy, and aerosol chemical composition are reported for three different ...years in the region of the large stratocumulus deck off the California coast. Receptor modeling is used to differentiate the contributions of the main aerosol sources to the light scattering and CCN activity. The contribution of anthropogenic sources to the two climatically important aerosol parameters (for direct and indirect forcing) varied from year to year but, on average, was found to be 67% in the case of CCN concentration and 57% in the case of light-scattering coefficient.
The Around the Americas expedition was a 25,000 mile sailing circumnavigation of the North and South American continents, in coastal waters, that took place from June 2009 to June 2010. The broad ...geographical span of the voyage made it possible to measure marine aerosol optical depths in regions where surface measurements are not frequently taken. These were measured with a handheld Microtops II Sun photometer. In this study we compare these measurements with the ocean aerosol product from the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua and Terra platforms. Results for aerosol optical depth (AOD) show a strong relationship between both measurements, with most values from MODIS falling within published expectations. However, MODIS values are biased high relative to surface observations for small optical depth values. There appears to be a relationship between these discrepancies in measurements and surface wind speed, with a group of values showing overestimation at wind speeds near and over 6 m/s and a second, smaller group showing underestimation for calmer conditions. For derived Ångström exponents, it is found that higher differences occur at low AOD. No relationship between these differences and wind speed is found.
Key Points
Low AOD values for MODIS are biased high when compared to Microtops
Bias is attributable to surface wind speeds
No trends are observed for Ångström exponent
Hourly averaged particle size distributions measured at a centrally located urban site in Seattle were analyzed through the application of bilinear positive matrix factorization (PMF) and Unmix to ...study underlying size distributions and their daily patterns. A total of 1051 samples each with 16 size intervals from 20 to 400 nm were obtained from a differential mobility particle sizer operating between December 2000 and February 2001. Both PMF and Unmix identify four similar underlying factors in the size distributions. Factor 1 is an accumulation mode particle size spectrum that shows a regular nocturnal pattern, and factor 2 is a larger particle distribution. Factor 3 is assigned as a traffic-related particle distribution, based on its correlations with accompanying gas-phase measurements, and has a regular weekday-high rush-hour pattern. Factor 4 is a traffic-related particle size distribution that has a regular rush-hour pattern on weekdays as well as weekends. Conditional probability functions (CPF) were computed using wind profiles and factor contributions. The results of CPF analysis suggest that these factors are correlated with surrounding particle sources of wood burning, secondary aerosol, diesel emissions, and motor vehicle emissions.
Measurements were made onboard the NOAA R/V Discoverer during the First Aerosol Characterization Experiment (ACE 1) to understand the optical properties of a minimally perturbed natural aerosol ...system in terms of its chemical and physical properties. ACE 1 took place during November and December of 1995 in the Southern Ocean region south of Australia. Reported here are observations at a wavelength of 550 nm of the submicron and supermicron aerosol scattering coefficient, σsp; single scattering albedo, ω0; and the hemispheric backscattered fraction and mass scattering efficiencies of non‐sea‐salt sulfate, sea salt, and the total aerosol. Also presented is the Ångström exponent, å, for the 450 and 700 nm wavelength pair. Variations in these parameters were found to be a strong function of the relative concentrations and size distributions of the dominant aerosol chemical components. Both the submicron and supermicron aerosol mass were composed primarily of water‐soluble ionic species. This is in agreement with an experiment‐average single scattering albedo of 0.99 (−0.4, +1.0%). Of the submicron ionic mass, 80±10% was sea salt, 16±8% was non‐sea‐salt sulfate, and 4±3% was methanesulfonate. Sea salt composed 99±0.7% of the supermicron ionic mass. The magnitude of scattering by both submicron and supermicron aerosol was controlled by sea salt. The backscattered fraction for the submicron aerosol averaged 0.11±0.02 and was controlled by the tailing of coarse mode sea‐salt mass into the submicron size range. Calculated mass scattering efficiencies for submicron non‐sea‐salt sulfate ion averaged 1.5±0.74 m2 g−1 (at 30 to 45% relative humidity) with the highest values corresponding to continentally influenced air masses where sulfate aerosol surface mean diameters and surface area concentrations were the largest. Mass scattering efficiencies for submicron sea salt were higher (averaging 4.2±0.96 m2 g−1) due to the tailing of coarse mode sea salt into the particle size range most efficient for light scattering. Given the similar lifetimes of submicron non‐sea‐salt sulfate and sea salt in the marine boundary layer, it is evident that sea salt controls the aerosol optical properties in this Southern Ocean region.
Aerosol samples were collected and cloud condensation nuclei (CCN) concentrations at five supersaturations were measured along and off the central California coast within the cloud-topped, marine ...boundary layer from aircraft flights during August 2007. Receptor modelling has been applied to estimate the natural versus anthropogenic source contribution of cloud condensation nuclei in this region, a region of climatically important marine stratocumulus. The results suggest that anthropogenic CCN accounted for about 50% of the CCN active at 0.3% supersaturation in this region during the measurement period.
We assess the consistency (closure) between solar beam attenuation by aerosols and water vapor measured by airborne Sun photometry and derived from airborne in situ and ship‐based lidar measurements ...during the April 2001 Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia). The airborne data presented here were obtained aboard the Twin Otter aircraft. Comparing aerosol extinction σep(550 nm) from four different techniques shows good agreement for the vertical distribution of aerosol layers. However, the level of agreement in absolute magnitude of the derived aerosol extinction varied among the aerosol layers sampled. The σep(550 nm) computed from airborne in situ size distribution and composition measurements shows good agreement with airborne Sun photometry in the marine boundary layer but is considerably lower in layers dominated by dust if the particles are assumed to be spherical. The σep(550 nm) from airborne in situ scattering and absorption measurements are about ∼13% lower than those obtained from airborne Sun photometry during 14 vertical profiles. Combining lidar and the airborne Sun photometer measurements reveals the prevalence of dust layers at altitudes up to 10 km with layer aerosol optical depth (from 3.5 to 10 km altitude) of ∼0.1 to 0.2 (500 nm) and extinction‐to‐backscatter ratios of 59–71 sr (523 nm). The airborne Sun photometer aboard the Twin Otter reveals a relatively dry atmosphere during ACE‐Asia with all water vapor columns <1.5 cm and water vapor densities ρw < 12 g/m3. Comparing layer water vapor amounts and ρw from the airborne Sun photometer to the same quantities measured with aircraft in situ sensors leads to a high correlation (r2 = 0.96), but the Sun photometer tends to underestimate ρw by 7%.
Aircraft-based measurements of aerosol hygroscopicity, both in the form of size-resolved, diameter growth factors and in the dependence of particle light scattering on relative humidity, are ...presented for the marine boundary layer of the southern California coastal zone. The chemical composition of the aerosol is collated with the hygroscopicity data, both to examine the mechanism for the increase in aerosol hygroscopicity with altitude and as input for receptor type modeling. The data suggest an increase in aerosol hygroscopicity with altitude, possibly associated with oxidation of organic films. The receptor modeling suggests three distinct aerosol types/sources for this venue: marine, biomass burning and pollution. Model output is used in regression analyses to derive a prognostic mixing rule for the hygroscopicity of aerosol with these three sources. The mixing rule demonstrated substantial prognostic power for submicron hygroscopicity but essentially none for supermicron.
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