To investigate the sources and formation mechanisms of carbonaceous aerosols, a major contributor to severe particulate air pollution, radiocarbon (.sup.14 C) measurements were conducted on aerosols ...sampled from November 2015 to November 2016 in Xi'an, China. Based on the .sup.14 C content in elemental carbon (EC), organic carbon (OC) and water-insoluble OC (WIOC), contributions of major sources to carbonaceous aerosols are estimated over a whole seasonal cycle: primary and secondary fossil sources, primary biomass burning, and other non-fossil carbon formed mainly from secondary processes. Primary fossil sources of EC were further sub-divided into coal and liquid fossil fuel combustion by complementing .sup.14 C data with stable carbon isotopic signatures.
The recently developed time-of-flight aerosol chemical speciation monitor with a capture vaporizer and a PM2.5 aerodynamic lens (TOF-ACSM-CV-PM2.5) aims to improve the collection efficiency and ...chemical characterization of aerosol particles with a diameter smaller than 2.5 µm. In this study, comprehensive cross-comparisons were performed between real-time online measurements and offline filter analysis with 24 h collection time. The goal was to evaluate the capabilities of the TOF-ACSM-CV-PM2.5 lens, as well as the accuracy of the TOF-ACSM-CV-PM2.5. The experiments were conducted at Cabauw Experimental Site for Atmospheric Research (CESAR) during the RITA-2021 campaign. The non-refractory fine particulate matter (PM1.0 and PM2.5) was measured by two collocated TOF-ACSM-CV-PM2.5 instruments by placing them behind a PM2.5 and PM1.0 inlet, respectively. A comparison between the ACSMs and PM2.5 and PM1.0 filter samples showed a much better accuracy than ±30 % less given in the previous reports, with average differences less than ±10 % for all inorganic chemical species. In addition, the ACSMs were compared to the Monitoring Instrument for Aerosol and Gas (MARGA) (slope between 0.78 and 0.97 for inorganic compounds, R2≥ 0.93) and a mobility particle size spectrometer (MPSS), measuring the particle size distribution from around 10 to 800 nm (slope was around 1.00, R2= 0.91). The intercomparison of the online measurements and the comparison between the online and offline measurements indicated a low bias (< 10 % for inorganic compounds) and demonstrated the high accuracy and stability of the TOF-ACSM-CV-PM2.5 lens for the atmospheric observations of particle matter. The two ACSMs exhibited an excellent agreement, with differences less than 7 %, which allowed a quantitative estimate of PM1.0 vs. PM2.5 chemical composition. The result showed that the PM1.0 accounted for about 70 %–80 % of the PM2.5 on average. The NO3 mass fraction increased, but the organic carbon (OC) mass fraction decreased from PM1.0 to PM2.5, indicating the size dependence on chemical composition.
To investigate the sources and evolution of haze pollution in different
seasons, long-term (from 15 August to 4 December 2015) variations in
chemical composition of PM1 were characterized in Beijing, ...China.
Positive matrix factorization (PMF) analysis with a multi-linear engine (ME-2)
resolved three primary and two secondary organic aerosol (OA) sources, including
hydrocarbon-like OA (HOA), cooking OA (COA), coal combustion OA (CCOA),
local secondary OA (LSOA) and regional SOA (RSOA). The sulfate source
region analysis implies that sulfate was mainly transported at a large
regional scale in late summer, while local and/or nearby sulfate formation
may be more important in winter. Meanwhile, distinctly different
correlations between sulfate and RSOA or LSOA (i.e., better correlation with
RSOA in late summer, similar correlations with RSOA and LSOA in autumn, and
close correlation with LSOA in early winter) confirmed the regional
characteristic of RSOA and local property of LSOA. Secondary aerosol species
including secondary inorganic aerosol (SIA – sulfate, nitrate, and ammonium) and SOA (LSOA and RSOA)
dominated PM1 during all three seasons. In particular, SOA contributed
46 % to total PM1 (with 31 % as RSOA) in late summer, whereas SIA
contributed 41 % and 45 % to total PM1 in autumn and early winter,
respectively. Enhanced contributions of secondary species (66 %–76 % of
PM1) were also observed in pollution episodes during all three seasons,
further emphasizing the importance of secondary formation processes in haze
pollution in Beijing. Combining chemical composition and meteorological
data, our analyses suggest that both photochemical oxidation and
aqueous-phase processing played important roles in SOA formation during all
three seasons, while for sulfate formation, gas-phase photochemical
oxidation was the major pathway in late summer, aqueous-phase reactions were
more responsible during early winter and both processes had contributions
during autumn.
Nitrated aromatic compounds (NACs) are a group of key chromophores for brown carbon (light-absorbing organic carbon, i.e., BrC) aerosol, which affects radiative forcing. The chemical composition and ...sources of NACs and their contributions to BrC absorption, however, are still not well understood. In this study, PM2.5-bound NACs in Xi'an, Northwest China, were investigated for 112 daily PM2.5 filter samples from 2015 to 2016. Both the total concentrations and contributions from individual species of NACs show distinct seasonal variations. The seasonally averaged concentrations of NACs are 2.1 (spring), 1.1 (summer), 12.9 (fall), and 56 ng m-3 (winter). Thereinto, 4-nitrophenol is the major NAC component in spring (58 %). The concentrations of 5-nitrosalicylic acid and 4-nitrophenol dominate in summer (70 %), and the concentrations of 4-nitrocatechol and 4-nitrophenol dominate in fall (58 %) and winter (55 %). The NAC species show different seasonal patterns in concentrations, indicating differences in emissions and formation pathways. Source apportionment results using positive matrix factorization (PMF) further show large seasonal differences in the sources of NACs. Specifically, in summer, NACs were highly influenced by secondary formation and vehicle emissions (∼ 80 %), while in winter, biomass burning and coal combustion contributed the most (∼ 75 %). Furthermore, the light absorption contributions of NACs to BrC are wavelength-dependent and vary greatly by season, with maximum contributions at ∼ 330 nm in winter and fall and∼ 320 nm in summer and spring. The differences in the contribution to light absorption are associated with the higher mass fractions of 4-nitrocatechol (λmax= 345 nm) and 4-nitrophenol (λmax= 310 nm) in fall and winter, 4-nitrophenol in spring, and 5-nitrosalicylic acid (λmax= 315 nm) and 4-nitrophenol in summer. The mean contributions of NACs to BrC light absorption at a wavelength of 365 nm in different seasons are 0.14 % (spring), 0.09 % (summer), 0.36 % (fall), and 0.91 % (winter), which are about 6–9 times higher than their mass fractional contributions of carbon in total organic carbon. Our results indicate that the composition and sources of NACs have profound impacts on the BrC light absorption.
To mitigate haze pollution in China, a better
understanding of the sources of carbonaceous aerosols is required due to the
complexity in multiple emissions and atmospheric processes. Here we combined
...the analysis of radiocarbon and the stable isotope 13C to investigate
the sources and formation of carbonaceous aerosols collected in two Chinese
megacities (Beijing and Xi'an) during severe haze events of a “red alarm”
level from December 2016 to January 2017. The haze periods with daily
PM2.5 concentrations as high as ∼ 400 µg m−3
were compared to subsequent clean periods (i.e., PM2.5 less than median concentrations during the winter 2016/2017) with PM2.5 concentrations below 100 µg m−3 in Xi'an and below 20 µg m−3 in Beijing. In Xi'an, liquid fossil fuel combustion was
the dominant source of elemental carbon (EC; 44 %–57 %), followed by
biomass burning (25 %–29 %) and coal combustion (17 %–29 %). In
Beijing, coal combustion contributed 45 %–61 % of EC, and biomass
burning (17 %–24 %) and liquid fossil fuel combustion (22 %–33 %)
contributed less. Non-fossil sources contributed 51 %–56 % of organic
carbon (OC) in Xi'an, and fossil sources contributed 63 %–69 % of OC in
Beijing. Secondary OC (SOC) was largely contributed by non-fossil sources in
Xi'an (56±6 %) and by fossil sources in Beijing (75±10 %), especially during haze periods. The fossil vs. non-fossil
contributions to OC and EC did not change drastically during haze events in
both Xi'an and Beijing. However, compared to clean periods, the contribution
of coal combustion to EC during haze periods increased in Xi'an and
decreased in Beijing. During clean periods, primary OC from biomass burning
and fossil sources constituted ∼ 70 % of OC in Xi'an and
∼ 53 % of OC in Beijing. From clean to haze periods, the
contribution of SOC to total OC increased in Xi'an but decreased in
Beijing, suggesting that the contribution of secondary organic aerosol formation
to increased OC during haze periods was more efficient in Xi'an than in
Beijing. In Beijing, the high SOC fraction in total OC during clean periods
was mainly due to an elevated contribution from non-fossil SOC. In Xi'an, a
slight day–night difference was observed during the clean period with
enhanced fossil contributions to OC and EC during the day. This day–night
difference was negligible during severe haze periods, likely due to the enhanced
accumulation of pollutants under stagnant weather conditions.
Aerosol formation acts as a sink for gas-phase
atmospheric species that controls their atmospheric lifetime and
environmental effects. To investigate aerosol formation and evolution in the
...Netherlands, a hybrid positive matrix factorization (PMF) analysis was
conducted using observations from May, June, and September 2021 collected in
the rural site of Cabauw in the central part of the Netherlands. The hybrid input matrix
consists of the full organic mass spectrum acquired from a time-of-flight
aerosol chemical speciation monitor (ToF-ACSM), ACSM inorganic species
concentrations, and binned particle size distribution concentrations from a
scanning mobility particle sizer (SMPS). These hybrid PMF analyses discerned
four factors that describe aerosol composition variations: two size-driven
factors that are related to new particle formation (NPF) and growth (F4 and
F3), as well as two bulk factors driven by composition, not size (F2 and F1). The
distribution of chemical species across these factors shows that different
compounds are responsible for nucleation and growth of new particles. The
smallest-diameter size factor (F4) contains ammonium sulfate and organics
and typically peaks during the daytime. Newly formed particles, represented
by F4, are mainly correlated with wind from the southwesterly–westerly and
easterly sectors that transport sulfur oxides (SOx), ammonia
(NH3), and organic precursors to Cabauw. As the particles grow from F4
to F3 and to bulk factors, nitrate and organics play an increasing role, and
the particle loading diurnal cycle shifts from daytime to a nighttime
maximum. Greater organics availability makes secondary organic aerosol (SOA)
more influential in summertime aerosol growth, principally due to volatility
differences produced by seasonal variation in photooxidation and
temperature.
Landscape fires are a significant contributor to atmospheric burdens of greenhouse gases and aerosols. Although many studies have looked at biomass burning products and their fate in the atmosphere, ...estimating and tracing atmospheric pollution from landscape fires based on atmospheric measurements are challenging due to the large variability in fuel composition and burning conditions. Stable carbon isotopes in biomass burning (BB) emissions can be used to trace the contribution of C3 plants (e.g. trees or shrubs) and C4 plants (e.g. savanna grasses) to various combustion products. However, there are still many uncertainties regarding changes in isotopic composition (also known as fractionation) of the emitted carbon compared to the burnt fuel during the pyrolysis and combustion processes. To study BB isotope fractionation, we performed a series of laboratory fire experiments in which we burned pure C3 and C4 plants as well as mixtures of the two. Using isotope ratio mass spectrometry (IRMS), we measured stable carbon isotope signatures in the pre-fire fuels and post-fire residual char, as well as in the CO2, CO, CH4, organic carbon (OC), and elemental carbon (EC) emissions, which together constitute over 98 % of the post-fire carbon. Our laboratory tests indicated substantial isotopic fractionation in combustion products compared to the fuel, which varied between the measured fire products. CO2, EC, and residual char were the most reliable tracers of the fuel 13C signature. CO in particular showed a distinct dependence on burning conditions; flaming emissions were enriched in 13C compared to smouldering combustion emissions. For CH4 and OC, the fractionation was the other way round for C3 emissions (13C-enriched) and C4 emissions (13C-depleted). This indicates that while it is possible to distinguish between fires that were dominated by either C3 or C4 fuels using these tracers, it is more complicated to quantify their relative contribution to a mixed-fuel fire based on the δ13C signature of emissions. Besides laboratory experiments, we sampled gases and carbonaceous aerosols from prescribed fires in the Niassa Special Reserve (NSR) in Mozambique, using an unmanned aerial system (UAS)-mounted sampling set-up. We also provided a range of C3:C4 contributions to the fuel and measured the fuel isotopic signatures. While both OC and EC were useful tracers of the C3-to-C4 fuel ratio in mixed fires in the lab, we found particularly OC to be depleted compared to the calculated fuel signal in the field experiments. This suggests that either our fuel measurements were incomprehensive and underestimated the C3:C4 ratio in the field or other processes caused this depletion. Although additional field measurements are needed, our results indicate that C3-vs.-C4 source ratio estimation is possible with most BB products, albeit with varying uncertainty ranges.
Black carbon measurements using an integrating sphere Hitzenberger, R.; Dusek, U.; Berner, A.
Journal of Geophysical Research: Atmospheres,
27 August 1996, Letnik:
101, Številka:
D14
Journal Article, Conference Proceeding
Recenzirano
An integrating sphere was used to determine the black carbon (BC) content of aerosol filter samples dissolved in chloroform (method originally described by Heintzenberg 1982). The specific absorption ...coefficient Ba (equal to absorption per mass) of the samples was also measured using the sphere as an integrating detector for transmitted light. Comparing the Ba of ambient samples taken in Vienna, Austria, to the BC concentrations measured on the dissolved filters, a value of approximately 6 m2/g was found to be a reasonable value for the Ba of the black carbon found at the site. The size dependence of Ba of a nebulized suspension of soot was measured using a rotating impactor, and a reasonable agreement between measured and calculated values was found.
We measured the radioactive carbon isotope 14C (radiocarbon) in various fractions of the carbonaceous aerosol sampled between February 2011 and March 2012 at the Cesar Observatory in the Netherlands. ...Based on the radiocarbon content in total carbon (TC), organic carbon (OC), water-insoluble organic carbon (WIOC), and elemental carbon (EC), we estimated the contribution of major sources to the carbonaceous aerosol. The main source categories were fossil fuel combustion, biomass burning, and other contemporary carbon, which is mainly biogenic secondary organic aerosol material (SOA). A clear seasonal variation is seen in EC from biomass burning (ECbb), with lowest values in summer and highest values in winter, but ECbb is a minor fraction of EC in all seasons. WIOC from contemporary sources is highly correlated with ECbb, indicating that biomass burning is a dominant source of contemporary WIOC. This suggests that most biogenic SOA is water soluble and that water-insoluble carbon stems mainly from primary sources. Seasonal variations in other carbon fractions are less clear and hardly distinguishable from variations related to air mass history. Air masses originating from the ocean sector presumably contain little carbonaceous aerosol from outside the Netherlands, and during these conditions measured carbon concentrations reflect regional sources. In these situations absolute TC concentrations are usually rather low, around 1.5 µg m−3, and ECbb is always very low ( ∼ 0.05 µg m−3), even in winter, indicating that biomass burning is not a strong source of carbonaceous aerosol in the Netherlands. In continental air masses, which usually arrive from the east or south and have spent several days over land, TC concentrations are on average by a factor of 3.5 higher. ECbb increases more strongly than TC to 0.2 µg m−3. Fossil EC and fossil WIOC, which are indicative of primary emissions, show a more moderate increase by a factor of 2.5 on average. An interesting case is fossil water-soluble organic carbon (WSOC, calculated as OC-WIOC), which can be regarded as a proxy for SOA from fossil precursors. Fossil WSOC has low concentrations when regional sources are sampled and increases by more than a factor of 5 in continental air masses. A longer residence time of air masses over land seems to result in increased SOA concentrations from fossil origin.
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