Michael H. Bergin

Aerosol radiative, physical, and chemical properties in Beijing during June 1999

Beijing experiences air pollution such that the sky overhead is gray much of the time even on cloudless days. In order to understand the cause of this problem, the aerosol light scattering coefficient σ_(sp) and absorption coefficient σ_(ap) were measured under dry conditions (instrumental relative humidity 1.0 μm), the submicron aerosol was responsible for ∼80% of the light scattering at 530 nm. The largest contribution to the PM2.5 aerosol mass was due to organic compounds, which accounted for ∼30% of the mass. The contributions of sulfate, ammonium, and nitrate to the PM2.5 mass concentration were ∼15%, 5%, and 8%, respectively. Mineral aerosol contributed ∼16% to the PM2.5 aerosol mass. These data show that combustion-related particles rather than wind-blown dust dominated the light extinction budget during June 1999.

Microbiome of the upper troposphere: Species composition and prevalence, effects of tropical storms, and atmospheric implications

The composition and prevalence of microorganisms in the middle-to-upper troposphere (8–15 km altitude) and their role in aerosol-cloud-precipitation interactions represent important, unresolved questions for biological and atmospheric science. In particular, airborne microorganisms above the oceans remain essentially uncharacterized, as most work to date is restricted to samples taken near the Earth’s surface. Here we report on the microbiome of low- and high-altitude air masses sampled onboard the National Aeronautics and Space Administration DC-8 platform during the 2010 Genesis and Rapid Intensification Processes campaign in the Caribbean Sea. The samples were collected in cloudy and cloud-free air masses before, during, and after two major tropical hurricanes, Earl and Karl. Quantitative PCR and microscopy revealed that viable bacterial cells represented on average around 20% of the total particles in the 0.25- to 1-μm diameter range and were at least an order of magnitude more abundant than fungal cells, suggesting that bacteria represent an important and underestimated fraction of micrometer-sized atmospheric aerosols. The samples from the two hurricanes were characterized by significantly different bacterial communities, revealing that hurricanes aerosolize a large amount of new cells. Nonetheless, 17 bacterial taxa, including taxa that are known to use C1–C4 carbon compounds present in the atmosphere, were found in all samples, indicating that these organisms possess traits that allow survival in the troposphere. The findings presented here suggest that the microbiome is a dynamic and underappreciated aspect of the upper troposphere with potentially important impacts on the hydrological cycle, clouds, and climate.

The contributions of snow, fog, and dry deposition to the summer flux of anions and cations at Summit, Greenland

Experiments were performed during the period May–July of 1993 at Summit, Greenland. Aerosol mass size distributions as well as daily average concentrations of several anionic and cationic species were measured. Dry deposition velocities for SO42− were estimated using surrogate surfaces (symmetric airfoils) as well as impactor data. Real-time concentrations of particles greater than 0.5 μm and greater than 0.01 μm were measured. Snow and fog samples from nearly all of the events occurring during the field season were collected. Filter sampler results indicate that SO42− is the dominant aerosol anion species, with Na+, NH4+, and Ca2+ being the dominant cations. Impactor results indicate that MSA and SO42− have similar mass size distributions. Furthermore, MSA and SO42− have mass in both the accumulation and coarse modes. A limited number of samples for NH4+ indicate that it exists in the accumulation mode. Na, K, Mg, and Ca exist primarily in the coarse mode. Dry deposition velocities estimated from impactor samples and a theory for dry deposition to snow range from 0.017 cm/s +/− 0.011 cm/s for NH4+ to 0.110 cm/s +/− 0.021 cm/s for Ca. SO42− dry deposition velocity estimates using airfoils are in the range 0.023 cm/s to 0.062 cm/s, as much as 60% greater than values calculated using the airborne size distribution data. The rough agreement between the airfoil and impactor-estimated dry deposition velocities suggests that the airfoils may be used to approximate the dry deposition to the snow surface. Laser particle counter (LPC) results show that particles > 0.5 μm in diameter efficiently serve as nuclei to form fog droplets. Condensation nuclei (CN) measurements indicate that particles < 0.5 μm are not as greatly affected by fog. Furthermore, impactor measurements suggest that from 50% to 80% of the aerosol SO42− serves as nuclei for fog droplets. Snow deposition is the dominant mechanism transporting chemicals to the ice sheet. For NO3−, a species that apparently exists primarily in the gas phase as HNO3(g), 93% of the seasonal inventory (mass of a deposited chemical species per unit area during the season) is due to snow deposition, which suggests efficient scavenging of HNO3(g) by snowflakes. The contribution of snow deposition to the seasonal inventories of aerosols ranges from 45% for MSA to 76% for NH4+. The contribution of fog to the seasonal inventories ranges from 13% for Na+ and Ca2+ to 26% and 32% for SO42− and MSA. The dry deposition contribution to the seasonal inventories of the aerosol species is as low as 5% for NH4+ and as high as 23% for MSA. The seasonal inventory estimations do not take into consideration the spatial variability caused by blowing and drifting snow. Overall, results indicate that snow deposition of chemical species is the dominant flux mechanism during the summer at Summit and that all three deposition processes should be considered when estimating atmospheric concentrations based on ice core chemical signals.

MODELS OVERESTIMATE DIFFUSE CLEAR-SKY SURFACE IRRADIANCE: A CASE FOR EXCESS ATMOSPHERIC ABSORPTION

Radiative transfer models consistently overestimate surface diffuse downward irradiance in cloud-free atmospheres by 9 to 40% at two low altitude sites while correctly calculating direct-normal Solar irradiance. For known systematic and random measurement errors and for realistic aerosol optical properties, the discrepancy can be resolved by a reduction in the vertical aerosol optical thickness (AOT) inferred from sunphotometric measurements by an average 0.02 ± 0.01 for 32 cases examined, together with a compensating increase in a continuum-like atmospheric absorptance over the solar spectrum of ∼5.0% ± 3.0%. This phenomenon is absent at two high altitude sites, where models and measurements agree to within their mutual uncertainties. Examination of apparent AOT at several locations around the globe also indicates presence of such excess atmospheric absorption. The proposed absorption and corresponding reduction in AOT would have important consequences for climate prediction and remote sensing.

Aerosol chemical, physical, and radiative characteristics near a desert source region of northwest China during ACE‐Asia

in both sap and ssp, resulting from diurnal changes in the mixing height as well as from local combustion sources in the morning and dust sources in the afternoon. Two distinct populations of aerosol mass scattering efficiencies Escat_2.5, one for aerosols dominated by desert dust (� 1.0 m 2 g � 1 ) and the other for aerosols composed primarily of local pollutants (� 3.0 m 2 g � 1 ), are observed. During the field study there were three significant dust events that occurred for, on average, several days at a time. The most significant dust storm resulted in a 24-hour-average PM2.5 concentration (mass concentration of particles having aerodynamic diameters less than 2.5 mm) of 453 m gm � 3 and a peak ssp of 2510 Mm � 1 on 8 April. The mean PM2.5 mass concentration during the dust storm periods is approximately 169 m gm � 3 , about 4 times greater than the mean value of 44 m gm � 3 observed during local pollution periods. When local pollution is the dominant source of fine particulate mass, organic matter (OM) is the major chemical component, contributing 41% to the PM2.5 mass, followed by crustal material (29%), sulfate (17%), and elemental carbon (EC) (13%). During sand storm periods, � 51% of PM2.5 mass is crustal material, followed by CO3� (11%) and OM (9.5%). The element enrichment factors indicate that coal combustion, biomass burning, and mobile source emissions are important local pollution sources. Overall, our results indicate that in addition to dust, local pollution also has a significant influence on aerosol properties in the region. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 9320 Information Related to Geographic Region: Asia; KEYWORDS: aerosol, dust, ACE-Asia

Apportionment of light scattering and hygroscopic growth to aerosol composition

During a recent campaign at the NOAA CMDL monitoring station on Sable Island, Canada (43.93° N, 60.01° W) a dual-nephelometer humidigraph measured the hygroscopic growth factor of aerosol scattering, f RH (σ sp ), one of the key parameters necessary for estimating short-wave aerosol radiative forcing. Measurements revealed less growth for anthropogenically influenced aerosols than for marine, f RH (σ sp ) of 1.7 ± 0.1 vs. 2.7 ± 0.4, where f RH (σ sp ) = σ sp(85%) /σ sp(40%) . A combined measurement-modeling approach was used to estimate σ sp and its RH-dependence, based on the measured particle size distribution and composition. The model suggested that differences in the particle size distribution, assuming the same aerosol composition, could not explain the observed differences in f RH (σ sp ). We have confirmed with individual particle analysis, that aerosol composition was indeed responsible for the difference in f RH (σ sp ). As well, the scattering contribution of organic carbon for the influenced case is at least as much as sulfate aerosol.

Urban aerosol radiative properties: Measurements during the 1999 Atlanta Supersite Experiment

calculated with a Mie code yielding Eap = 9.5 ± 1.5 m 2 g 1 , while EC mass summed from the impactor stages in comparison to measured sap gives Eap = 9.3 ± 3.2 m 2 g 1 .M ie light-scattering calculations using inputs of measured mass and EC size distributions give geometric mean light scattering and absorption Dp = 0.54 and 0.13 mm, respectively, and show the dominance of the submicrometer diameter particles to light extinction in the urban environment. Based on the measured aerosol optical depth in Atlanta, da (500 nm) = 0.44 ± 0.22, and other radiative measurements, a best estimate of the average direct aerosol radiative forcing at the top of the atmosphere (a measure of the climate significance) is F= 11 ± 6 W m 2 in Atlanta. This value is an order of magnitude greater than global mean estimates for aerosols underscoring the influence of aerosol particles on radiative transfer in the urban environment. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0394 Atmospheric Composition and

Brown carbon in the continental troposphere

Little is known about the optical significance of light absorbing particulate organic compounds (i.e., brown carbon, BrC), including the importance relative to black carbon (BC) and influence on direct radiative forcing by aerosols. The vertical profile of BrC affects its radiative forcing, yet the distribution of BrC in the free troposphere is largely unknown. In this study, BrC absorption was directly measured in solvent extracts of particulate filters obtained from aircraft sampling over the continental USA. Excluding biomass burning plumes, BrC was observed throughout the tropospheric column (<13 km), and its prevalence increased relative to BC with increasing altitude, indicating contributions from secondary sources. Closure analysis showed good agreement between light absorption from BC plus BrC relative to measured total aerosol absorption. A radiative transfer model indicated that BrC absorption reduced top of atmosphere aerosol forcing by ~20%, suggesting that it is an important component of direct aerosol radiative forcing.

Composition and sources of carbonaceous aerosols at three contrasting sites in Hong Kong

[1] A significant fraction of the fine particulate matter in Hong Kong is made up of organic carbon. In order to quantitatively assess the contributions of various sources to carbonaceous aerosol in Hong Kong, a chemical mass balance (CMB) receptor model in combination with organic tracers was employed. Organic tracers including n-alkanes, polycyclic aromatic hydrocarbons (PAHs), steranes, hopanes, resin acids, cholesterol, levoglucosan, and picene in PM2.5 collected from three air monitoring sites located at roadside, urban, and rural areas in Hong Kong are quantified using gas chromatography-mass spectrometry (GC/MS) in the present study. Analyses of some overlapping species from two separate laboratories will be compared for the first time. Spatial and seasonal source contributions to organic carbon (OC) in PM2.5 from up to nine air pollution sources are assessed, including diesel engine exhaust, gasoline engine exhaust, meat cooking, cigarette smoke, biomass burning, road dust, vegetative detritus, coal combustion, and natural gas combustion. Diesel engine exhaust dominated fine organic carbon in Hong Kong (57 ± 13% at urban sites and 25 ± 2% at the rural site). Other sources that play an important role are meat cooking and biomass burning, which can account for as much as 14% of fine organic carbon. The primary sources identified by this technique explained 49%, 79%, and 94% of the measured fine organic carbon mass concentration at the rural, the urban, and the roadside sites, respectively. The unexplained fine OC is likely due to secondary organic aerosol formation.

Aerosol pollution in some Chinese cities (IUPAC Technical Report)

Emissions caused by the use of coal and by traffic have caused serious photochemical smog and aerosol pollution with unique characteristics in most Chinese cities. This report gives an overview of aerosol concentrations in China based on data obtained from both the literature and recent research by the authors. The results show that TSP (total suspended particulate) and PM-10 (particles with aerodynamic diameter 10 µm) concentrations frequently exceed the National Ambient Air Quality Standard and that ambient aerosol concentrations constitute a serious air pollution problem. PM-2.5 concentrations are also high and account for 60 % of the PM-10 mass. Organic carbon and sulfate are the most abundant components of PM-2.5, while crustal elements represent a minor portion.Nitrate concentrations are almost the same as sulfate in summertime, which implies that NOx control is very important in lowering fine particle concentrations and in improving air visibility. The chemical mass balance (CMB) method was applied in Beijing to identify the sources of PM-2.5. The main sources include fugitive dust, coal burning/industrial processes, traffic emissions, and secondary aerosol produced by atmospheric chemical conversion.