Sources of particulate organic carbon (OC) with different
volatility have rarely been investigated, despite the significant importance
for better understanding of the atmospheric processes of organic ...aerosols.
In this study we develop a radiocarbon-based (14C) approach for source
apportionment of more volatile OC (mvOC) and apply to ambient aerosol
samples collected in winter in six Chinese megacities. mvOC is isolated by
desorbing organic carbon from the filter samples in helium (He) at 200 ∘C in a custom-made aerosol combustion system for 14C
analysis. Evaluation of this new isolation method shows that the isolated
mvOC amount agrees very well with the OC1 fraction (also desorbed at 200 ∘C in He) measured by a thermal–optical analyzer using the
EUSAAR_2 protocol. The mvOC, OC and elemental carbon (EC) of
13 combined PM2.5 samples in six Chinese cities are analyzed
for 14C to investigate their sources and formation mechanisms. The
relative contribution of fossil sources to mvOC is 59±11 %,
consistently larger than the contribution to OC (48±16 %) and
smaller than that to EC (73±9 %), despite large differences in
fossil contributions in different cities. The average difference in the
fossil fractions between mvOC and OC is 13 % (range of 7 %–25 %),
similar to that between mvOC and EC (13 %, with a range 4 %–25 %). Secondary OC
(SOC) concentrations and sources are modeled based on the
14C-apportioned OC and EC and compared with concentrations and sources
of mvOC. SOC concentrations (15.4±9.0 µg m−3) are
consistently higher than those of mvOC (3.3±2.2 µg m−3),
indicating that only a fraction of SOC is accounted for by the more volatile
carbon fraction desorbed at 200 ∘C. The fossil fraction in SOC is
43 % (10 %–70 %), lower than that in mvOC (59 %, with a range of 45 %–78 %). Correlation between mvOC and SOC from nonfossil sources (mvOCnf
vs. SOCnf) and from fossil sources (mvOCfossil vs.
SOCfossil) is examined to further explore sources and formation
processes of mvOC and SOC.
This study reports emission of organic particulate matter by light-duty vehicles (LDVs) and heavy-duty vehicles (HDVs) in the city of São Paulo, Brazil, where vehicles run on three different fuel ...types: gasoline with 25 % ethanol (called gasohol, E25), hydrated ethanol (E100), and diesel (with 5 % biodiesel). The experiments were performed at two tunnels: Jânio Quadros (TJQ), where 99 % of the vehicles are LDVs, and RodoAnel Mário Covas (TRA), where up to 30 % of the fleet are HDVs. Fine particulate matter (PM2.5) samples were collected on quartz filters in May and July 2011 at TJQ and TRA, respectively. The samples were analyzed by thermal-desorption proton-transfer-reaction mass spectrometry (TD-PTR-MS) and by thermal–optical transmittance (TOT). Emission factors (EFs) for organic aerosol (OA) and organic carbon (OC) were calculated for the HDV and the LDV fleet. We found that HDVs emitted more PM2.5 than LDVs, with OC EFs of 108 and 523 mg kg−1 burned fuel for LDVs and HDVs, respectively. More than 700 ions were identified by TD-PTR-MS and the EF profiles obtained from HDVs and LDVs exhibited distinct features. Unique organic tracers for gasoline, biodiesel, and tire wear have been tentatively identified. nitrogen-containing compounds contributed around 20 % to the EF values for both types of vehicles, possibly associated with incomplete fuel burning or fast secondary production. Additionally, 70 and 65 % of the emitted mass (i.e. the OA) originates from oxygenated compounds from LDVs and HDVs, respectively. This may be a consequence of the high oxygen content of the fuel. On the other hand, additional oxygenation may occur during fuel combustion. The high fractions of nitrogen- and oxygen-containing compounds show that chemical processing close to the engine / tailpipe region is an important factor influencing primary OA emission. The thermal-desorption analysis showed that HDVs emitted compounds with higher volatility, and with mainly oxygenated and longer chain hydrocarbons than LDVs.
Biomass burning (BB) emits large quantities of greenhouse gases (GHG) and aerosols that impact the climate and adversely affect human health. Although much research has focused on quantifying BB ...emissions on regional to global scales, field measurements of BB emission factors (EFs) are sparse, clustered and indicate high spatio-temporal variability. EFs are generally calculated from ground or aeroplane measurements with respective potential biases towards smouldering or flaming combustion products. Unmanned aerial systems (UAS) have the potential to measure BB EFs in fresh smoke, targeting different parts of the plume at relatively low cost. We propose a light-weight UAS-based method to measure EFs for carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) as well as PM2.5 (TSI Sidepak AM520) and equivalent black carbon (eBC, microAeth AE51) using a combination of a sampling system with Tedlar bags which can be analysed on the ground and with airborne aerosol sensors. In this study, we address the main challenges associated with this approach: (1) the degree to which a limited number of samples is representative for the integral smoke plume and (2) the performance of the lightweight aerosol sensors. While aerosol measurements can be made continuously in a UAS set-up thanks to the lightweight analysers, the representativeness of our Tedlar bag filling approach was tested during prescribed burning experiments in the Kruger National Park, South Africa. We compared fire-averaged EFs from UAS-sampled bags for savanna fires with integrated EFs from co-located mast measurements. Both measurements matched reasonably well with linear R2 ranging from 0.81 to 0.94. Both aerosol sensors are not factory calibrated for BB particles and therefore require additional calibration. In a series of smoke chamber experiments, we compared the lightweight sensors with high-fidelity equipment to empirically determine specific calibration factors (CF) for measuring BB particles. For the PM mass concentration from a TSI Sidepak AM520, we found an optimal CF of 0.27, using a scanning mobility particle sizer and gravimetric reference methods, although the CF varied for different vegetation fuel types. Measurements of eBC from the Aethlabs AE51 aethalometer agreed well with the multi-wavelength aethalometer (AE33) (linear R2 of 0.95 at λ=880 nm) and the wavelength corrected multi-angle absorption photometer (MAAP, R2 of 0.83 measuring at λ=637 nm). However, the high variability in observed BB mass absorption cross-section (MAC) values (5.2±5.1 m2 g−1) suggested re-calibration may be required for individual fires. Overall, our results indicate that the proposed UAS set-up can obtain representative BB EFs for individual savanna fires if proper correction factors are applied and operating limitations are well understood.
Secondary organic aerosol (SOA) plays a central role in air pollution and climate. However, the description of the sources and mechanisms leading to SOA is elusive despite decades of research. While ...stable isotope analysis is increasingly used to constrain sources of ambient aerosol, in many cases it is difficult to apply because neither the isotopic composition of aerosol precursors nor the fractionation of aerosol forming processes is well characterised. In this paper, SOA formation from ozonolysis of α-pinene – an important precursor and perhaps the best-known model system used in laboratory studies – was investigated using position-dependent and average determinations of 13C in α-pinene and advanced analysis of reaction products using thermal-desorption proton-transfer-reaction mass spectrometry (PTR-MS). The total carbon (TC) isotopic composition δ13C of the initial α-pinene was measured, and the δ13C of the specific carbon atom sites was determined using position-specific isotope analysis (PSIA). The PSIA analysis showed variations at individual positions from −6.9 to +10. 5 ‰ relative to the bulk composition. SOA was formed from α-pinene and ozone in a constant-flow chamber under dark, dry, and low-NOx conditions, with OH scavengers and in the absence of seed particles. The excess of ozone and long residence time in the flow chamber ensured that virtually all α-pinene had reacted. Product SOA was collected on two sequential quartz filters. The filters were analysed offline by heating them stepwise from 100 to 400 °C to desorb organic compounds that were (i) detected using PTR-MS for chemical analysis and to determine the O : C ratio, and (ii) converted to CO2 for 13C analysis. More than 400 ions in the mass range 39–800 Da were detected from the desorbed material and quantified using a PTR-MS. The largest amount desorbed at 150 °C. The O : C ratio of material from the front filter increased from 0.18 to 0.25 as the desorption temperature was raised from 100 to 250 °C. At temperatures above 250 °C, the O : C ratio of thermally desorbed material, presumably from oligomeric precursors, was constant. The observation of a number of components that occurred across the full range of desorption temperatures suggests that they are generated by thermal decomposition of oligomers. The isotopic composition of SOA was more or less independent of desorption temperature above 100 °C. TC analysis showed that SOA was enriched in 13C by 0.6–1.2 ‰ relative to the initial α-pinene. According to mass balance, gas-phase products will be depleted relative to the initial α-pinene. Accordingly, organic material on the back filters, which contain adsorbed gas-phase compounds, is depleted in 13C in TC by 0.7 ‰ relative to the initial α-pinene, and by 1.3 ‰ compared to SOA collected on the front filter. The observed difference in 13C between the gas and particle phases may arise from isotope-dependent changes in the branching ratios in the α-pinene + O3 reaction. Alternatively, some gas-phase products involve carbon atoms from highly enriched and depleted sites, as shown in the PSIA analysis, giving a non-kinetic origin to the observed fractionations. In either case, the present study suggests that the site-specific distribution of 13C in the source material itself governs the abundance of 13C in SOA.
We developed a system that links a thermal desorption oven for aerosol filter samples to an isotope ratio mass spectrometer (IRMS) via a combustion interface. Organic compounds are desorbed from the ...filter sample in He at 7 temperature steps between 100 and 400 degree C and subsequently oxidized to CO2, which is analyzed for delta 13C. We tested the system for isotopic fractionation, reproducibility and linearity using organic test compounds and aerosol filter samples. An oxalic acid standard with known delta 13C value shows no isotopic fractionation when it is desorbed at temperatures well above its melting point. When organic acids are heated at a temperature close to their melting point only a fraction of the compound (10-80%) evaporates which shows depleted delta 13C values at this temperature step. At the next higher temperature step, when the rest of the compound is desorbed, delta 13C values are enriched. This effect is stronger for compounds with a lower molecular weight than for compounds with a higher molecular weight. In ambient samples this should only have a moderate effect on the overall delta 13C value, since hundreds of different compounds are desorbed at each temperature step. Choosing different values for temperature steps does not strongly change the delta 13C values for ambient aerosol filter samples. For typical ambient samples the reproducibility lies below plus or minus 0.3ppt for oven temperatures below 200 degree C and below plus or minus 0.5ppt for oven temperatures above 200 degree C. Tests with oxalic acid and ambient filter samples show that the IRMS is linear for peak areas in the operational range of 1-20 V s.
This paper presents first results obtained with a measurement system designed to measure the humidity-dependent water uptake by deposited aerosol samples. Aerosol in the size range 0.06-16 μm ...aerodynamic equivalent diameter is sampled with a nine stage cascade impactor. The individual impaction foils are exposed to elevated relative humidities in a chamber containing an aqueous solution of CaCl
2
of specified concentration. The mass increase of the deposits is measured with a balance above the chamber. Only equilibrium values of m/m
0
(i.e., wet mass/dry mass) for increasing humidities can be obtained. The two samples discussed here showed a strongly size-selective growth pattern. Deposits with accumulation range particles have larger m/m
0
than those with coarse mode particles (e.g., sample 1: 2.6 vs 1.33 at 90% humidity; sample 2: 2.35 vs 1.16). Although the two samples had been obtained at the same time of the year, the growth patterns are different, which might have been caused by the meteorological situation. Calculations of optical parameters (i.e., extinction coefficient and single scattering albedo) showed that in both cases, extinction (and therefore also the optical depth) and single scattering albedo increased with humidity. From the changes of the single scattering albedo it could be deduced that for the dry aerosol heating effects probably dominated over cooling effects, while for high humidities cooling predominated.
We present a method to investigate cloud condensation nuclei (CCN) concentrations and activation efficiencies as a function of two independent variables, aerosol particle size and water vapor ...supersaturation. To date, most ambient CCN measurements have been made as the integral (total) CCN concentration as a function of water vapor supersaturation only. However, since CCN properties of aerosol particles are strongly dependent on particle size, as well as on chemical composition, which commonly varies with particle size, more detailed measurements can provide additional important information about the CCN activation. With size-resolved measurements, the effect of particle size on CCN activity can be kept constant, which makes it possible to directly assess the influence of particle chemistry. The instrumental set-up consists of a differential mobility analyzer (DMA) to select particles of a known size, within a narrow size range. A condensation nuclei (CN) counter (condensation particle counter, CPC) is used to count the total number of particles in that size range, and a CCN counter is used to count the number of CCN as a function of supersaturation, in that same size range. The activation efficiency, expressed as CCN/CN ratios, can thus directly be calculated as a function of particle size and supersaturation. We present examples of the application of this technique, using salt and smoke aerosols produced in the laboratory as well as ambient aerosols.
The static (parallel-plate thermal-gradient) diffusion chamber (SDC) was one of the first instruments designed to measure cloud condensation nuclei (CCN) concentrations as a function of ...supersaturation. It has probably also been the most widely used type of CCN counter. This paper describes the detailed experimental characterization of a SDC CCN counter, including calibration with respect to supersaturation and particle number concentration. In addition, we investigated the proposed effect of lowered supersaturation because of water vapor depletion with increasing particle concentration. The results obtained gives a larger understanding why and in which way it is necessary to calibrate the SDC CCN counter. The calibration method is described in detail as well. The method can, in parts, be used for calibrations also for other types of CCN counters. We conclude the following: 1) it is important to experimentally calibrate SDC CCN counters with respect to supersaturation, and not only base the supersaturation on the theoretical description of the instrument; 2) the number concentration calibration needs to be performed as a function of supersaturation, also for SDC CCN counter using the photographic technique; and 3) we observed no evidence that water vapor depletion lowered the supersaturation.
Experimental and theoretical uncertainties in the measurement of cloud condensation nuclei (CCN) with a continuous-flow thermal-gradient CCN counter from Droplet Measurement Technologies (DMT-CCNC) ...have been assessed by model calculations and calibration experiments with ammonium sulfate and sodium chloride aerosol particles in the diameter range of 20-220 nm. Experiments have been performed in the laboratory and during field measurement campaigns, extending over a period of more than one year and covering a wide range of operating conditions (650-1020 hPa ambient pressure, 0.5-1.0 L min super(− 1) aerosol flow rate, 20-30 degree C inlet temperature, 4-34 K m super(− 1) temperature gradient). For each set of conditions, the effective water vapor supersaturation (S sub(eff)) in the CCNC was determined from the measured CCN activation spectra and Koehler model calculations. High measurement precision was achieved under stable laboratory conditions, where relative variations of S sub(eff) in the CCNC were generally less than plus or minus 2%. During field measurements, however, the relative variability increased up to plus or minus 5-7%, which can be mostly attributed to variations of the CCNC column top temperature with ambient temperature. To assess the accuracy of the Koehler models used to calculate S sub(eff), we have performed a comprehensive comparison and uncertainty analysis of the various Koehler models and thermodynamic parameterizations commonly used in CCN studies. For the relevant supersaturation range (0.05-2%), the relative deviations between different modeling approaches were as high as 25% for (NH sub(4)) sub(2)SO sub(4) deviations were mostly caused by the different parameterizations for the activity of water in aqueous solutions of (NH sub(4)) sub(2)SO sub(4) and NaCl (activity parameterization, osmotic coefficient, and van't Hoff factor models). The uncertainties related to the model parameterizations of water activity clearly exceeded the CCNC measurement precision. Relative deviations caused by different ways of calculating or approximating solution density and surface tension did not exceed 3% for (NH sub(4)) sub(2)SO sub(4) measurement precision under well-defined operating conditions and should not be neglected in studies aimed at high accuracy. To ensure comparability of results, we suggest that CCN studies should always report exactly which Koehler model equations and parameterizations of solution properties were used. Substantial differences between the CCNC calibration results obtained with (NH sub(4)) sub(2)SO sub(4) experimental conditions (relative deviations of S sub(eff) up to ~10%) indicate inconsistencies between widely used activity parameterizations derived from electrodynamic balance (EDB) single particle experiments (Tang and Munkelwitz, 1994; Tang, 1996) and hygroscopicity tandem differential mobility analyzer (HTDMA) aerosol experiments (Kreidenweis et al., 2005). Therefore, we see a need for further evaluation and experimental confirmation of preferred data sets and parameterizations for the activity of water in dilute aqueous (NH sub(4)) sub(2)SO sub(4) were also used to test the CCNC flow model of Lance et al.~(2006), which describes the dependence of S sub(eff) on temperature, pressure, and flow rate in the CCN counter. This model could be applied after subtraction of a near-constant temperature offset and derivation of an instrument-specific thermal resistance parameter (R sub(T)≈ 1.8 K W super(− 1)). At S sub(eff)>0.1% the relative deviations between the flow model and experimental results were mostly less than 5%, when the same Koehler model approach was used. At S sub(eff) less than or equal to .1%, however, the deviations exceeded 20%, which can be attributed to non-idealities which also caused the near-constant temperature offset. Therefore, we suggest that the CCNC flow model can be used to extrapolate calibration results, but should generally be complemented by calibration experiments performed under the relevant operating conditions - during field campaigns as well as in laboratory studies.
PDF
