Earth's Radiation Balance Research Articles

Speciation and Sources of Brown Carbon in Precipitation at Seoul, Korea: Insights from Excitation-Emission Matrix Spectroscopy and Carbon Isotopic Analysis.

Brown carbon (BrC) plays a significant role in the Earth's radiative balance, yet its sources and chemical composition remain poorly understood. In this work, we investigated BrC in the atmospheric environment of Seoul by characterizing dissolved organic matter in precipitation using excitation-emission matrix (EEM) fluorescence spectroscopy coupled with parallel factor analysis (PARAFAC). The two independent fluorescent components identified by PARAFAC were attributed to humic-like substance (HULIS) and biologically derived material based on their significant correlations with measured HULIS isolated using solid-phase extraction and total hydrolyzable tyrosine. The year-long observation shows that HULIS contributes to 66 ± 13% of total fluorescence intensity of our samples on average. By using dual carbon (13C and 14C) isotopic analysis conducted on isolated HULIS, the HULIS fraction of BrC was found to be primarily derived from biomass burning and emission of terrestrial biogenic gases and particles (>70%), with minor contributions from fossil-fuel combustion. The knowledge derived from this study could contribute to the establishment of a characterizing system of BrC components identified by EEM spectroscopy. Our work demonstrates that, EEM fluorescence spectroscopy is a powerful tool in BrC study, on the basis of its chromophore resolving power, allowing investigation into individual components of BrC by other organic matter characterization techniques.

Atmospheric processes affecting the separation of volcanic ash and SO<sub>2</sub> in volcanic eruptions: inferences from the May 2011 Grímsvötn eruption

Abstract. The separation of volcanic ash and sulfur dioxide (SO2) gas is sometimes observed during volcanic eruptions. The exact conditions under which separation occurs are not fully understood but the phenomenon is of importance because of the effects volcanic emissions have on aviation, on the environment, and on the earth's radiation balance. The eruption of Grímsvötn, a subglacial volcano under the Vatnajökull glacier in Iceland during 21–28 May 2011 produced one of the most spectacular examples of ash and SO2 separation, which led to errors in the forecasting of ash in the atmosphere over northern Europe. Satellite data from several sources coupled with meteorological wind data and photographic evidence suggest that the eruption column was unable to sustain itself, resulting in a large deposition of ash, which left a low-level ash-rich atmospheric plume moving southwards and then eastwards towards the southern Scandinavian coast and a high-level predominantly SO2 plume travelling northwards and then spreading eastwards and westwards. Here we provide observational and modelling perspectives on the separation of ash and SO2 and present quantitative estimates of the masses of ash and SO2 that erupted, the directions of transport, and the likely impacts. We hypothesise that a partial column collapse or sloughing fed with ash from pyroclastic density currents (PDCs) occurred during the early stage of the eruption, leading to an ash-laden gravity intrusion that was swept southwards, separated from the main column. Our model suggests that water-mediated aggregation caused enhanced ash removal because of the plentiful supply of source water from melted glacial ice and from entrained atmospheric water. The analysis also suggests that ash and SO2 should be treated with separate source terms, leading to improvements in forecasting the movement of both types of emissions.

Some observations on the greenhouse effect at the Earth's surface

It is shown that the greenhouse gases carbon dioxide and water vapour reflect back to the surface, all IR radiation originating at the surface within their respective spectral bands. This reflection occurs in a very thin layer at the surface, not much over 12cm in thickness. Heat is lost from the surface by heat exchange with the atmosphere and by loss of radiation. About 52% of radiation leaves the surface in two principal window regions but this is not enough to account for the earth's equilibrium temperature. This window radiation seems to disappear quite quickly and is replaced by black body radiation. It is this which eventually contributes to the earth's radiation balance, and has to originate approximately between 40 and 50km altitude where the temperature is about correct, near 255K. Doubling the CO2 concentration increases the surface temperature by about 0.9°C and this need not have any influence higher up in the atmosphere. The surface temperature seems indeed to have no direct influence on the earth's external radiation balance.

Cloud Activation Potentials for Atmospheric α-Pinene and β-Caryophyllene Ozonolysis Products.

The formation of atmospheric cloud droplets due to secondary organic aerosol (SOA) particles is important for quantifying the Earth’s radiative balance under future, possibly warmer, climates, yet is only poorly understood. While cloud activation may be parametrized using the surface tension depression that coincides with surfactant partitioning to the gas–droplet interface, the extent to which cloud activation is influenced by both the chemical structure and reactivity of the individual molecules comprising this surfactant pool is largely unknown. We report herein considerable differences in the surface tension depression of aqueous pendant droplets that contain synthetically prepared ozonolysis products derived from α-pinene and β-caryophyllene, the most abundant of the monoterpenes and sesquiterpenes, respectively, that are emitted over the planet’s vast forest ecosystems. Oxidation products derived from β-caryophyllene were found to exhibit significantly higher surface activity than those prepared from α-pinene, with the critical supersaturation required for cloud droplet activation reduced by 50% for β-caryophyllene aldehyde at 1 mM. These considerable reductions in the critical supersaturation were found to coincide with free energies of adsorption that exceed ∼25 kJ/mol, or just one hydrogen bond equivalent, depending on the ammonium sulfate and oxidation product concentration in the solution. Additional experiments showed that aldehyde-containing oxidation products exist in equilibrium with hydrated forms in aqueous solution, which may modulate their bulk solubility and surface activity. Equilibration time scales on the order of 10–5 to 10–4 s calculated for micrometer-sized aerosol particles indicate instantaneous surface tension depression in the activation processes leading to cloud formation in the atmosphere. Our findings highlight the underlying importance of molecular structure and reactivity when considering cloud condensation activity in the presence of SOA particles.

Testing the sensitivity of past climates to the indirect effects of dust

AbstractMineral dust particles are important ice nuclei (IN) and as such indirectly impact Earth's radiative balance via the properties of cold clouds. Using the Community Earth System Model version 1.0.6, and Community Atmosphere Model version 5.1, and a new empirical parameterization for ice nucleation on dust particles, we investigate the radiative forcing induced by dust IN for different dust loadings. Dust emissions are representative of global conditions for the Last Glacial Maximum and the mid‐Pliocene Warm Period. Increased dust leads to smaller and more numerous ice crystals in mixed phase clouds, impacting cloud opacity, lifetime, and precipitation. This increases the shortwave cloud radiative forcing, resulting in significant surface temperature cooling and polar amplification—which is underestimated in existing studies relative to paleoclimate archives. Large hydrological changes occur and are linked to an enhanced dynamical response. We conclude that dust indirect effects could potentially have a significant impact on the model‐data mismatch that exists for paleoclimates.

The thin border between cloud and aerosol: Sensitivity of several ground based observation techniques

Cloud and aerosol are two manifestations of what it is essentially the same physical phenomenon: a suspension of particles in the air. The differences between the two come from the different composition (e.g., much higher amount of condensed water in particles constituting a cloud) and/or particle size, and also from the different number of such particles (10–10,000 particles per cubic centimeter depending on conditions). However, there exist situations in which the distinction is far from obvious, and even when broken or scattered clouds are present in the sky, the borders between cloud/not cloud are not always well defined, a transition area that has been coined as the “twilight zone”. The current paper presents a discussion on the definition of cloud and aerosol, the need for distinguishing or for considering the continuum between the two, and suggests a quantification of the importance and frequency of such ambiguous situations, founded on several ground-based observing techniques. Specifically, sensitivity analyses are applied on sky camera images and broadband and spectral radiometric measurements taken at Girona (Spain) and Boulder (Co, USA). Results indicate that, at these sites, in more than 5% of the daytime hours the sky may be considered cloudless (but containing aerosols) or cloudy (with some kind of optically thin clouds) depending on the observing system and the thresholds applied. Similarly, at least 10% of the time the extension of scattered or broken clouds into clear areas is problematic to establish, and depends on where the limit is put between cloud and aerosol. These findings are relevant to both technical approaches for cloud screening and sky cover categorization algorithms and radiative transfer studies, given the different effect of clouds and aerosols (and the different treatment in models) on the Earth's radiation balance.

Profuse activity of blue electrical discharges at the tops of thunderstorms

AbstractThunderstorm clouds may reach the lower stratosphere, affecting the exchange of greenhouse gases between the troposphere and stratosphere. This region of the atmosphere is difficult to access experimentally, and our knowledge of the processes taking place here is incomplete. We recently recorded color video footage of thunderstorms over the Bay of Bengal from the International Space Station. The observations show a multitude of blue, kilometer‐scale, discharges at the cloud top layer at ~18 km altitude and a pulsating blue discharge propagating into the stratosphere reaching ~40 km altitude. The emissions are related to the so‐called blue jets, blue starters, and possibly pixies. The observations are the first of their kind and give a new perspective on the electrical activity at the top of tropical thunderstorms; further, they underscore that thunderstorm discharges directly perturb the chemistry of the stratosphere with possible implications for the Earth's radiation balance.

Refractive Index and Absorption Attribution of Highly Absorbing Brown Carbon Aerosols from an Urban Indian City-Kanpur.

Atmospheric aerosols influence Earth’s radiative balance, having both warming and cooling effects. Though many aerosols reflect radiation, carbonaceous aerosols such as black carbon and certain organic carbon species known as brown carbon have the potential to warm the atmosphere by absorbing light. Black carbon absorbs light over the entire solar spectrum whereas brown carbon absorbs near-UV wavelengths and, to a lesser extent, visible light. In developing countries, such as India, where combustion sources are prolific, the influence of brown carbon on absorption may be significant. In order to better characterize brown carbon, we present experimental and modeled absorption properties of submicron aerosols measured in an urban Indian city (Kanpur). Brown carbon here is found to be fivefold more absorbing at 365 nm wavelength compared to previous studies. Results suggest ~30% of total absorption in Kanpur is attributed to brown carbon, with primary organic aerosols contributing more than secondary organics. We report the spectral brown carbon refractive indices along with an experimentally constrained estimate of the influence of aerosol mixing state on absorption. We conclude that brown carbon in Kanpur is highly absorbing in nature and that the mixing state plays an important role in light absorption from volatile species.

Physicochemical characteristics of black carbon aerosol and its radiative impact in a polluted urban area of China

AbstractBlack carbon (BC) aerosol plays an important role in the Earth's radiative balance. An intensive measurement campaign was conducted at Xi'an, China, from December 2012 to January 2013 to investigate the sources and physicochemical characteristics of refractory BC (rBC) and its direct radiative forcing at the surface. The overall average rBC concentration for the campaign was 8.0 ± 7.1 µg m−3. Source apportionment based on positive matrix factorization showed that traffic was the dominant rBC source (46.0%), followed by coal burning (33.9%) and biomass burning (20.1%). The rBC mass size distributions were monomodal and lognormal with larger mass median diameters for coal burning source (215 nm) compared with the traffic source (189 nm). Coal burning rBC was more strongly associated with sulfate than traffic rBC, suggesting a higher cloud condensation nuclei activity. The slope of a robust linear regression between rBC and carbon monoxide (CO) for all samples was 5.9 µg m−3 ppm−1, and the slope for the coal burning source (4.5 µg m−3 ppm−1) was larger than that for the traffic source (2.7 µg m−3 ppm−1). The net rBC emission during winter of 2009 was estimated to be 4.5 Gg based on the relationship between rBC and CO. A Tropospheric Ultraviolet and Visible radiation model showed that the average daytime value for the clear‐sky direct radiative forcing due to rBC from 23 December 2012 to 31 January 2013 was −47.7 ± 28.9 W m−2, which amounted to an average of 45.7% of the total surface atmospheric aerosol forcing.

Spatiotemporal Monitoring of CO<sub>2</sub> and CH<sub>4</sub> over Pakistan Using Atmospheric Infrared Sounder (AIRS)

Carbon dioxide (CO2), Methane (CH4) are two most potent greenhouse gases and are major source of climate change. Human activities particularly fossil fuels burning have caused considerable increase in atmospheric concentrations of greenhouse gases. CO2 contributes 60% of anthropogenic greenhouse effect whereas CH4 contributes 15%. Ice core records also show that the concentrations of Carbon dioxide and methane have increased substantially. The emission of these gases alters the Earth’s energy budget and are drivers of climate change. In the present study, atmospheric concentration of CO2 and CH4 over Pakistan is measured using Atmospheric Infrared Sounder (AIRS). Time series and time averaged maps are prepared to measure the concentrations of CO2 and CH4. The results show considerable increase in concentration of Carbon dioxide and methane. The substantial increase in these concentrations can affect human health, earth radiative balance and can damage crops.

Spatiotemporal Monitoring of CO<sub>2</sub> and CH<sub>4</sub> over Pakistan Using Atmospheric Infrared Sounder (AIRS)

Carbon dioxide (CO2), Methane (CH4) are two most potent greenhouse gases and are major source of climate change. Human activities particularly fossil fuels burning have caused considerable increase in atmospheric concentrations of greenhouse gases. CO2contributes 60% of anthropogenic greenhouse effect whereas CH4contributes 15%. Ice core records also show that the concentrations of Carbon dioxide and methane have increased substantially. The emission of these gases alters the Earth’s energy budget and are drivers of climate change. In the present study, atmospheric concentration of CO2and CH4over Pakistan is measured using Atmospheric Infrared Sounder (AIRS). Time series and time averaged maps are prepared to measure the concentrations of CO2and CH4. The results show considerable increase in concentration of Carbon dioxide and methane. The substantial increase in these concentrations can affect human health, earth radiative balance and can damage crops.

Parameterization of single-scattering albedo (SSA) and absorption Ångström exponent (AAE) with EC / OC for aerosol emissions from biomass burning

Abstract. Single-scattering albedo (SSA) and absorption Ångström exponent (AAE) are two critical parameters in determining the impact of absorbing aerosol on the Earth's radiative balance. Aerosol emitted by biomass burning represent a significant fraction of absorbing aerosol globally, but it remains difficult to accurately predict SSA and AAE for biomass burning aerosol. Black carbon (BC), brown carbon (BrC), and non-absorbing coatings all make substantial contributions to the absorption coefficient of biomass burning aerosol. SSA and AAE cannot be directly predicted based on fuel type because they depend strongly on burn conditions. It has been suggested that SSA can be effectively parameterized via the modified combustion efficiency (MCE) of a biomass burning event and that this would be useful because emission factors for CO and CO2, from which MCE can be calculated, are available for a large number of fuels. Here we demonstrate, with data from the FLAME-4 experiment, that for a wide variety of globally relevant biomass fuels, over a range of combustion conditions, parameterizations of SSA and AAE based on the elemental carbon (EC) to organic carbon (OC) mass ratio are quantitatively superior to parameterizations based on MCE. We show that the EC ∕ OC ratio and the ratio of EC ∕ (EC + OC) both have significantly better correlations with SSA than MCE. Furthermore, the relationship of EC ∕ (EC + OC) with SSA is linear. These improved parameterizations are significant because, similar to MCE, emission factors for EC (or black carbon) and OC are available for a wide range of biomass fuels. Fitting SSA with MCE yields correlation coefficients (Pearson's r) of ∼ 0.65 at the visible wavelengths of 405, 532, and 660 nm while fitting SSA with EC / OC or EC / (EC + OC) yields a Pearson's r of 0.94–0.97 at these same wavelengths. The strong correlation coefficient at 405 nm (r = 0.97) suggests that parameterizations based on EC / OC or EC / (EC + OC) have good predictive capabilities even for fuels in which brown carbon absorption is significant. Notably, these parameterizations are effective for emissions from Indonesian peat, which have very little black carbon but significant brown carbon (SSA = 0.990 ± 0.001 at 532 and 660 nm, SSA = 0.937 ± 0.011 at 405 nm). Finally, we demonstrate that our parameterization based on EC / (EC + OC) accurately predicts SSA during the first few hours of plume aging with data from Yokelson et al. (2009) gathered during a biomass burning event in the Yucatán Peninsula of Mexico.

Optical Properties of Secondary Organic Aerosol from cis-3-Hexenol and cis-3-Hexenyl Acetate: Effect of Chemical Composition, Humidity, and Phase.

Atmospheric aerosols play an important role in Earth's radiative balance directly, by scattering and absorbing radiation, and indirectly, by acting as cloud condensation nuclei (CCN). Atmospheric aerosol is dominated by secondary organic aerosol (SOA) formed by the oxidation of biogenic volatile organic compounds (BVOCs). Green leaf volatiles (GLVs) are a class of BVOCs that contribute to SOA, yet their role in the Earth's radiative budget is poorly understood. In this work we measured the scattering efficiency (at 450, 525, and 635 nm), absorption efficiency (between 190 and 900 nm), particle phase, bulk chemical properties (O:C, H:C), and molecular-level composition of SOA formed from the ozonolysis of two GLVs: cis-3-hexenol (HXL) and cis-3-hexenyl acetate (CHA). Both HXL and CHA produced SOA that was weakly absorbing, yet CHA-SOA was a more efficient absorber than HXL-SOA. The scatter efficiency of SOA from both systems was wavelength-dependent, with the stronger dependence exhibited by HXL-SOA, likely due to differences in particle size. HXL-SOA formed under both dry (10% RH) and wet (70% RH) conditions had the same bulk chemical properties (O:C), yet significantly different optical properties, which was attributed to differences in molecular-level composition. We have found that SOA derived from green leaf volatiles has the potential to affect the Earth's radiative budget, and also that bulk chemical properties can be insufficient to predict SOA optical properties.

The distribution of PM10 and PM2.5 carbonaceous aerosol in Baotou, China

Particulate matter (PM), including PM10 and PM2.5, is one of the major impacts on air quality, visibility, climate change, earth radiation balance, and public health. Organic carbon (OC) and elemental carbon (EC) are the major components of PM. 804 samples (PM10 and PM2.5) were simultaneously collected from six urban sites covering 3 districts in Baotou, in January, April, September, and November 2014. As to a long-term study on the effects of carbonaceous aerosol, data were collected annually at Environmental Protection Agency of Baotou (EPB). The concentrations of PM10 and PM2.5, the spatial distribution and content of OC and EC, the relationship between OC and EC, and the formation of secondary organic carbon (SOC) have been investigated. The findings indicated that the concentrations of these particle matter are higher than that in US or European standards. The average concentrations of OC in PM10 and PM2.5 follow the order: January > November > April > September; and for EC in PM10 and PM2.5 follow the order: January > November > September > April. Affected by metrological factors, it was indicated that high wind speed and low relative humidity were beneficial for removal of OC and EC in January and November. Pearson correlations and cluster analysis on OC and EC concentrations in PM10 and PM2.5 with gaseous pollutants (SO2, NO2, and CO) suggested that OC shared the same emission sources with SO2 and CO from combustion, while EC's sources mainly came from vehicles exhaust and combustion which contributed to NO2 as well. The OC concentration is mainly primary in warm months, while it appears secondary in cold months in Baotou. There is a common characteristic among the cities with higher SOC in winter, wherever the coal combustion can lead to the severe pollution. This work is important for the construction of the database of OC and EC concentrations in PM10 and PM2.5 at spatial and time intervals, and it can provide scientific suggestion for similar PM atmospheric pollutant control and air quality improvement in Baotou.

Carbonaceous aerosols in megacity Xi'an, China: Implications of thermal/optical protocols comparison

Carbonaceous aerosol is an important component that influences the environment, climate, and human health. Organic and elemental carbon (OC and EC) are the two main constituents of carbonaceous aerosols that have opposite, i.e., cooling versus warming, effects on the Earth's radiation balance. Knowledge on the variability of OC/EC splits measured by different thermal/optical protocols is useful for understanding the uncertainty in the climate models. This study shows good correlations within OC or EC (r2 > 0.83, P < 0.001) across the IMPROVE, IMPROVE_A, and EUSAAR_2 protocols for both ambient aerosol samples and biomass burning samples. However, EC concentrations differ by more than two folds, and OC/EC ratios differ up to a factor of 2.7. The discrepancies were attributed to the selection between the reflectance and transmittance corrections and the different peak inert-atmosphere temperature. The IMPROVE and IMPROVE_A protocols also quantified different char and soot concentrations, two subtypes of EC with distinct chemical and optical properties. Char, but not soot, was found to correlate with the humic-like substances (HULIS) content in the samples, suggesting that both char and HULIS originate mainly from biomass burning. A one-year (2012–2013) ambient aerosol monitoring in Xi'an, China, shows that OC, EC, and char displayed winter highs and summer lows, while soot had no seasonal trend. The char/soot ratios showed a “single peak” in winter, while OC/EC ratios exhibited “dual peak” feature due to the influence of secondary organic aerosol formation. In addition to commonly measured OC and EC, we recommend both char and soot from a common reference method to be considered in the chemical transport and climate models.

Bidirectional-reflectance measurements for various snow crystal morphologies

An understanding of snow optical properties is vital to accurately quantifying the effect of snow cover on the Earth's radiative energy balance. Central to these models is the need for accurate bidirectional reflectance data for various snow surface types. However, few studies in this area exist and none focus specifically on surface hoar—a well-known surface crystal type often responsible for avalanches.In this study, it is postulated that the bidirectional reflectance distribution of the snow's surface before and after surface hoar growth will be predictably and quantifiably different when viewed in the visible wavelengths. To test this hypothesis, a methodology for reliably growing surface hoar in a lab setting was developed. Temporal changes in crystal habit were documented using computed tomography and visible microscopic imaging. A spectrometer was used to measure bidirectional-reflectance factors (BRF) both before and after surface hoar growth.Analysis of the results revealed three primary conclusions: 1) Surface hoar growth is accompanied by a departure from Lambertian scattering. The effect is more apparent the larger the surface hoar grains. 2) The incident lighting and viewing geometries at which maximum and minimum BRF values occur are difficult to discern. 3) In the transition from rounded grains to surface hoar, spectral albedo (as calculated by averaging the BRF over a hemispheric solid angle at 510nm) decreases slightly.

Concentrations and light absorption characteristics of carbonaceous aerosol in PM 2.5 and PM 10 of Lhasa city, the Tibetan Plateau

Abstract Light absorption properties of carbonaceous aerosol strongly influence the Earth's radiative balance, yet the related knowledge is limited for the Tibetan Plateau (TP), the highest and largest plateau in the world. In this study, organic carbon (OC), elemental carbon (EC) and water soluble organic carbon (WSOC) of PM 2.5 and PM 10 of Lhasa collected from May 2013 to March 2014 were studied. It showed that daily-average concentrations of OC, EC and WSOC of PM 2.5 and PM 10 were lower than those of other megacities. Lhasa PM 2.5 was characterized by low OC/EC ratio (1.46 ± 0.55), which was similar to that of Lhasa roadside PM 2.5 (1.25 ± 0.45), reflecting mainly direct influence of primary emissions and less secondary formation. Hence, although Lhasa atmosphere is relatively clean, it is intensively influenced by local vehicle emissions. Mass absorption cross-section of EC (MAC EC ) for both PM 2.5 and PM 10 at 632 nm were 7.19 ± 1.19 m 2 g −1 and 7.98 ± 2.32 m 2 g −1 , respectively, both of which had similar variation patterns to OC/EC and secondary OC (SOC)/OC, indicating that the increase of MAC EC might be caused by coating with organic aerosol. Additionally, the loading of EC for both PM 2.5 and PM 10 showed logarithmic relationships with those of optical attenuation (ATN) of EC, implying that the shadowing effect enhanced logarithmic with increased EC concentration. MAC of WSOC at 365 nm for PM 2.5 (0.74 ± 0.22 m 2 g −1 ) and PM 10 (0.78 ± 0.21 m 2 g −1 ) were also close to reported values of other cities mainly influenced by fossil combustion. Additionally, attenuation at 365 nm of WSOC of both PM 2.5 and PM 10 showed the same relationship with their WSOC concentrations, implying no difference for light absorption properties of WSOC for these two grain sizes.

The Effects of Monomer Size Distribution on the Radiative Properties of Black Carbon Aggregates

Black carbon (BC), normally existing as aggregates, significantly affects the Earth radiative forcing, energy balance, and climate by scattering and absorbing both solar radiation and terrestrial emission. The BC particles are usually treated as fractal aggregates with same-sized monomers. However, experimental studies show that monomer diameters of BC normally obey a lognormal distribution ranging from 10 nm to over 100 nm. This study investigates the effects of monomer size distribution on the radiative properties of BC particles. The fractal aggregates are generated by a cluster–cluster aggregation (CCA) algorithm, and the Multiple Sphere T-Matrix (MSTM) method is used to simulate the radiative properties of randomly oriented aggregates. The integral radiative properties of aggregates with different-sized monomers have normal distributions with large standard deviations, and it requires to average radiative properties of over 60 aggregate realizations to represent their ensemble-averaged properties. Th...

Seasonal variation of secondary organic aerosol tracers in Central Tibetan Plateau

Abstract. Secondary organic aerosol (SOA) affects the earth's radiation balance and global climate. High-elevation areas are sensitive to global climate change. However, at present, SOA origins and seasonal variations are understudied in remote high-elevation areas. In this study, particulate samples were collected from July 2012 to July 2013 at the remote Nam Co (NC) site, Central Tibetan Plateau and analyzed for SOA tracers from biogenic (isoprene, monoterpenes and β-caryophyllene) and anthropogenic (aromatics) precursors. Among these compounds, isoprene SOA (SOAI) tracers represented the majority (26.6 ± 44.2 ng m−3), followed by monoterpene SOA (SOAM) tracers (0.97 ± 0.57 ng m−3), aromatic SOA (SOAA) tracer (2,3-dihydroxy-4-oxopentanoic acid, DHOPA, 0.25 ± 0.18 ng m−3) and β-caryophyllene SOA tracer (β-caryophyllenic acid, 0.09 ± 0.10 ng m−3). SOAI tracers exhibited high concentrations in the summer and low levels in the winter. The similar temperature dependence of SOAI tracers and isoprene emission suggested that the seasonal variation of SOAI tracers at the NC site was mainly influenced by the isoprene emission. The ratio of high-NOx to low-NOx products of SOAI (2-methylglyceric acid to 2-methyltetrols) was highest in the winter and lowest in the summer, due to the influence of temperature and relative humidity. The seasonal variation of SOAM tracers was impacted by monoterpenes emission and gas-particle partitioning. During the summer to the fall, temperature effect on partitioning was the dominant process influencing SOAM tracers' variation; while the temperature effect on emission was the dominant process influencing SOAM tracers' variation during the winter to the spring. SOAM tracer levels did not elevate with increased temperature in the summer, probably resulting from the counteraction of temperature effects on emission and partitioning. The concentrations of DHOPA were 1–2 orders of magnitude lower than those reported in the urban regions of the world. Due to the transport of air pollutants from the adjacent Bangladesh and northeastern India, DHOPA presented relatively higher levels in the summer. In the winter when air masses mainly came from northwestern India, mass fractions of DHOPA in total tracers increased, although its concentrations declined. The SOA-tracer method was applied to estimate secondary organic carbon (SOC) from these four precursors. The annual average of SOC was 0.22 ± 0.29 μgC m−3, with the biogenic SOC (sum of isoprene, monoterpenes and β-caryophyllene) accounting for 75 %. In the summer, isoprene was the major precursor with its SOC contributions of 81 %. In the winter when the emission of biogenic precursors largely dropped, the contributions of aromatic SOC increased. Our study implies that anthropogenic pollutants emitted in the Indian subcontinent could be transported to the TP and have an impact on SOC over the remote NC.

Evaluation of DMA Size Selection of Dry Dispersed Mineral Dust Particles

Mineral dust particles play a significant role in the Earth's radiative balance via direct interaction with solar radiation and indirectly through their ability to initiate cloud formation. Many field and laboratory studies utilize a differential mobility analyzer (DMA) for particle size selection. Here we evaluate the use of a DMA to size-segregate dry dispersed mineral dust particles. We examine the post-DMA size distribution using four different techniques: a scanning mobility particle sizer (SMPS) for mobility sizing, an optical particle sizer (OPS) for optical sizing, the Particle Analysis by Laser Mass Spectrometry (PALMS) instrument for vacuum aerodynamic sizing, and electron microscopy (EM) for geometric sizing. While the SMPS measured a narrow mobility size distribution at the DMA-selected diameter, the OPS, PALMS, and EM in most cases showed broader distributions and a smaller mode size than that selected by the DMA. These techniques also observed super-micrometer particles, often extending beyond the upper size limit of a typical SMPS scan. Complicating analysis, particle shape factor (χ) was observed to be a function of mobility size, ranging from 1.3 at 500 nm to 3.1 at 1000 nm. We conclude that mobility size selection of mineral dust particles using a DMA most often does not yield particles of the desired physical size or surface area. We suggest that attempts to size-select from a broad distribution of non-spherical particles require an independent measurement downstream of the DMA to verify the actual selected size.Copyright 2015 American Association for Aerosol Research