Low-volatility Oxygenated Organic Aerosol Research Articles

Comprehensive the seasonal characterization of atmospheric submicron particles at urban sites in the North China Plain

To comprehensively investigate the seasonal variations in submicron particles (PM1 = NR-PM1 + BC) composition, sources and chemical processes, an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS), along with a multiangle absorption photometer (MAAP), was deployed to observe at urban sites in the North China Plain from October 1, 2015, to July 31, 2016. The PM1 average mass were 36.4 ± 39.0, 32.9 ± 20.7, 40.4 ± 52.4 and 99.0 ± 88.7 μg m−3 in spring, summer, autumn and winter, respectively. On the whole, the OA accounted for the largest component of PM1 during the four seasons, with the highest contribution in winter, accounting for 49% on average, followed by autumn (47%), spring (40%) and summer (31%). The secondary inorganic aerosol (SIA) mass concentration in summer was the lowest but contributed the most to PM1 in summer (62%), and it was higher than that in spring (46%), autumn (43%) and winter (41%). Positive matrix factorization (PMF) was used to differentiate the high-resolution mass spectra (HRMS) of organic aerosol (OA) into six factors, including two secondary OA (highly oxidized, low-volatility oxygenated organic aerosols (LV-OOA) and less oxidized, semi-volatile oxygenated OA (SV-OOA)) and four primary OA (including cooking-related OA (COA), hydrocarbon-like OA (HOA), biomass burning OA (BBOA) and coal combustion OA (CCOA)). OOA (LV-OOA + SV-OOA) exhibited the highest OA loading in summer (59%), while POA (CCOA + COA + HOA) exhibited the highest OA loading in winter (62%). The OA exhibited similar bimodal diurnal patterns during the four seasons, the nitrate and sulfate seasonal diurnal variations slightly differed, and the peak value occurred at night in winter and in the morning during the other seasons. The trend of the LV-OOA and SV-OOA diurnal variation in spring, autumn and winter appeared small difference, and the summer vary differently compared to other seasons. However, the diurnal mass concentration in winter was much higher than other seasons. The COA diurnal pattern peaked at noon and at night during the four seasons, corresponding to the peak hours of dining activities. The HOA, BBOA and CCOA seasonal diurnal patterns showed low concentrations during the daytime and high concentrations at night. OA on HPD was 6, 7 and 7 times higher than that on CD in spring, autumn and winter. The SIA concentration significantly increased on HPD in spring (69.3 μg m−3, 52%), autumn (80.6 μg m−3, 50%) and winter (82.4 μg m−3, 43%). The organic nitrate average mass concentrations were 0.6 ± 0.5, 0.7 ± 0.2, 0.8 ± 1.2 and 2.5 ± 2.3 μg m−3 in spring, summer, autumn and winter, respectively. The organic nitrate diurnal pattern appeared at night was higher than that during the daytime in spring, autumn and winter. The opposite diurnal pattern emerged in summer. The average mass concentration of polycyclic aromatic hydrocarbons (PAHs) during the entire observation period reached 73.2 ± 154.2 ng m−3, and the seasonal average mass were 30.4 ± 30.6, 10.7 ± 4.0, 40.5 ± 10.3 and 220.3 ± 246.7 ng m−3 in spring, summer, autumn and winter, respectively. The highest diurnal values in spring, summer, autumn and winter were 44.2, 11.7, 71.6 and 366.8 ng m−3, respectively.

Real-time single particle characterization of oxidized organic aerosols in the East China Sea

Knowledge of the chemical characteristics and sources of organic aerosols (OA) over marine is needed for evaluating their effects on climate change and air quality. Here, a quadrupole aerosol chemical speciation monitor (Q-ACSM) and a single-particle aerosol mass spectrometry (SPAMS) were synchronously employed to investigate the chemical composition, mixing state, and oxidation degree of oxidized organic aerosols (OOA) in PM1 over the East China Sea (ECS) from 3 to 27 June 2017. Both aerosol mass spectrometers demonstrated that a higher oxidation state of OOA in aerosol particles could be generated during marine air mass-dominated periods (MDP) than that generated during land air mass-dominated periods (LDP). Two OOA factors including semi-volatile oxidized organic aerosol (SV-OOA) and low-volatility oxidized organic aerosol (LV-OOA) were distinguished based on Q-ACSM. Fifty-seven percent of the total detected particles with obvious signals of organic markers were identified as oxidized organic carbon (OOC) particles via SPAMS and further divided into lower oxidized organic carbon (LOOC) particles and more oxidized organic carbon (MOOC) particles. All OOC-containing particles were clustered into seven particle subgroups. The EC and K subgroups dominated the LOOC and MOOC particles, respectively, during periods controlled by land air masses, indicating that notable OOC formation was influenced by continental sources. OOA with higher oxygen states were found to dominate near ports. This suggested that OOA chemical characteristics over the ESC are seriously affected by continental, ship, and port emissions, which should be synergistically considered in evaluating their effects on solar radiation transfer and cloud processes.

Eight years of sub-micrometre organic aerosol composition data from the boreal forest characterized using a machine-learning approach

Abstract. The Station for Measuring Ecosystem–Atmosphere Relations (SMEAR) II, located within the boreal forest of Finland, is a unique station in the world due to the wide range of long-term measurements tracking the Earth–atmosphere interface. In this study, we characterize the composition of organic aerosol (OA) at SMEAR II by quantifying its driving constituents. We utilize a multi-year data set of OA mass spectra measured in situ with an Aerosol Chemical Speciation Monitor (ACSM) at the station. To our knowledge, this mass spectral time series is the longest of its kind published to date. Similarly to other previously reported efforts in OA source apportionment from multi-seasonal or multi-annual data sets, we approached the OA characterization challenge through positive matrix factorization (PMF) using a rolling window approach. However, the existing methods for extracting minor OA components were found to be insufficient for our rather remote site. To overcome this issue, we tested a new statistical analysis framework. This included unsupervised feature extraction and classification stages to explore a large number of unconstrained PMF runs conducted on the measured OA mass spectra. Anchored by these results, we finally constructed a relaxed chemical mass balance (CMB) run that resolved different OA components from our observations. The presented combination of statistical tools provided a data-driven analysis methodology, which in our case achieved robust solutions with minimal subjectivity. Following the extensive statistical analyses, we were able to divide the 2012–2019 SMEAR II OA data (mass concentration interquartile range (IQR): 0.7, 1.3, and 2.6 µg m−3) into three sub-categories – low-volatility oxygenated OA (LV-OOA), semi-volatile oxygenated OA (SV-OOA), and primary OA (POA) – proving that the tested methodology was able to provide results consistent with literature. LV-OOA was the most dominant OA type (organic mass fraction IQR: 49 %, 62 %, and 73 %). The seasonal cycle of LV-OOA was bimodal, with peaks both in summer and in February. We associated the wintertime LV-OOA with anthropogenic sources and assumed biogenic influence in LV-OOA formation in summer. Through a brief trajectory analysis, we estimated summertime natural LV-OOA formation of tens of ng m−3 h−1 over the boreal forest. SV-OOA was the second highest contributor to OA mass (organic mass fraction IQR: 19 %, 31 %, and 43 %). Due to SV-OOA's clear peak in summer, we estimate biogenic processes as the main drivers in its formation. Unlike for LV-OOA, the highest SV-OOA concentrations were detected in stable summertime nocturnal surface layers. Two nearby sawmills also played a significant role in SV-OOA production as also exemplified by previous studies at SMEAR II. POA, taken as a mix of two different OA types reported previously, hydrocarbon-like OA (HOA) and biomass burning OA (BBOA), made up a minimal OA mass fraction (IQR: 2 %, 6 %, and 13 %). Notably, the quantification of POA at SMEAR II using ACSM data was not possible following existing rolling PMF methodologies. Both POA organic mass fraction and mass concentration peaked in winter. Its appearance at SMEAR II was linked to strong southerly winds. Similar wind direction and speed dependence was not observed among other OA types. The high wind speeds probably enabled the POA transport to SMEAR II from faraway sources in a relatively fresh state. In the event of slower wind speeds, POA likely evaporated and/or aged into oxidized organic aerosol before detection. The POA organic mass fraction was significantly lower than reported by aerosol mass spectrometer (AMS) measurements 2 to 4 years prior to the ACSM measurements. While the co-located long-term measurements of black carbon supported the hypothesis of higher POA loadings prior to year 2012, it is also possible that short-term (POA) pollution plumes were averaged out due to the slow time resolution of the ACSM combined with the further 3 h data averaging needed to ensure good signal-to-noise ratios (SNRs). Despite the length of the ACSM data set, we did not focus on quantifying long-term trends of POA (nor other components) due to the high sensitivity of OA composition to meteorological anomalies, the occurrence of which is likely not normally distributed over the 8-year measurement period. Due to the unique and realistic seasonal cycles and meteorology dependences of the independent OA subtypes complemented by the reasonably low degree of unexplained OA variability, we believe that the presented data analysis approach performs well. Therefore, we hope that these results encourage also other researchers possessing several-year-long time series of similar data to tackle the data analysis via similar semi- or unsupervised machine-learning approaches. This way the presented method could be further optimized and its usability explored and evaluated also in other environments.

Variation in chemical composition and sources of PM2.5 during the COVID-19 lockdown in Delhi

The Government of India (GOI) announced a nationwide lockdown starting 25th March 2020 to contain the spread of COVID-19, leading to an unprecedented decline in anthropogenic activities and, in turn, improvements in ambient air quality. This is the first study to focus on highly time-resolved chemical speciation and source apportionment of PM2.5 to assess the impact of the lockdown and subsequent relaxations on the sources of ambient PM2.5 in Delhi, India. The elemental, organic, and black carbon fractions of PM2.5 were measured at the IIT Delhi campus from February 2020 to May 2020. We report source apportionment results using positive matrix factorization (PMF) of organic and elemental fractions of PM2.5 during the different phases of the lockdown. The resolved sources such as vehicular emissions, domestic coal combustion, and semi-volatile oxygenated organic aerosol (SVOOA) were found to decrease by 96%, 95%, and 86%, respectively, during lockdown phase-1 as compared to pre-lockdown. An unforeseen rise in O3 concentrations with declining NOx levels was observed, similar to other parts of the globe, leading to the low-volatility oxygenated organic aerosols (LVOOA) increasing to almost double the pre-lockdown concentrations during the last phase of the lockdown. The effect of the lockdown was found to be less pronounced on other resolved sources like secondary chloride, power plants, dust-related, hydrocarbon-like organic aerosols (HOA), and biomass burning related emissions, which were also swayed by the changing meteorological conditions during the four lockdown phases. The results presented in this study provide a basis for future emission control strategies, quantifying the extent to which constraining certain anthropogenic activities can ameliorate the ambient air. These results have direct relevance to not only Delhi but the entire Indo-Gangetic plain (IGP), citing similar geographical and meteorological conditions common to the region along with overlapping regional emission sources. Summary of main findingsWe identify sources like vehicular emissions, domestic coal combustion, and semi-volatile oxygenated organic aerosol (SVOOA) to be severely impacted by the lockdown, whereas ozone levels and, in turn, low-volatility oxygenated organic aerosols (LVOOA) rise by more than 95% compared to the pre-lockdown concentrations during the last phase of the lockdown. However, other sources resolved in this study, like secondary chloride, power plants, dust-related, hydrocarbon-like organic aerosols (HOA), and biomass burning related emissions, were mainly driven by the changes in the meteorological conditions rather than the lockdown.

Characterization of submicron particles by time-of-flight aerosol chemical speciation monitor (ToF-ACSM) during wintertime: aerosol composition, sources, and chemical processes in Guangzhou, China

Abstract. Particulate matter (PM) pollution in China is an emerging environmental issue which policy makers and the public have increasingly paid attention to. In order to investigate the characteristics, sources, and chemical processes of PM pollution in Guangzhou, field measurements were conducted from 20 November 2017 to 5 January 2018, with a time-of-flight aerosol chemical speciation monitor (ToF-ACSM) and other collocated instruments. Mass concentrations of non-refractory submicron particulate matter (NR-PM1) measured by the ToF-ACSM correlated well with those of PM2.5 or PM1.1 measured by filter-based methods. The organic mass fraction increased from 45 % to 53 % when the air switched from non-pollution periods to pollution episodes (EPs), indicating significant roles of organic aerosols (OAs) during the whole study. Based on the mass spectra measured by the ToF-ACSM, positive matrix factorization (PMF) with the multilinear engine (ME-2) algorithm was performed to deconvolve OA into four factors, including hydrocarbon-like OA (HOA, 12 %), cooking OA (COA, 18 %), semi-volatile oxygenated OA (SVOOA, 30 %), and low-volatility oxygenated OA (LVOOA, 40 %). Furthermore, we found that SVOOA and nitrate were significantly contributed from local traffic emissions while sulfate and LVOOA were mostly attributed to regional pollutants. Comparisons between this work and other previous studies in China show that secondary organic aerosol (SOA) fraction in total OA increases spatially across China from the north to the south. Two distinctly opposite trends for NR-PM1 formation were observed during non-pollution periods and pollution EPs. The ratio of secondary PM (SPM = SVOOA + LVOOA + sulfate + nitrate + ammonium) to primary PM (PPM = HOA + COA + chloride), together with peroxy radicals RO2∗ and ozone, increased with increasing NR-PM1 concentration during non-pollution periods, while an opposite trend of these three quantities was observed during pollution EPs. Furthermore, oxidation degrees of both OA and SOA were investigated using the f44∕f43 space and the results show that at least two OOA factors are needed to cover a large range of f44 and f43 in Guangzhou. Comparisons between our results and other laboratory studies imply that volatile organic compounds (VOCs) from traffic emissions, in particular from diesel combustion and aromatic compounds, are the most likely SOA precursors in Guangzhou. Peroxy radical RO2∗ was used as a tracer for SOA formed through gas-phase oxidation. For non-pollution periods, SOA concentration was reasonably correlated with RO2∗ concentration during both daytime and nighttime, suggesting that gas-phase oxidation was primarily responsible for SOA formation. However, there was no correlation between SOA and RO2∗ in pollution EPs, suggesting a dramatically changed mechanism for SOA formation. This conclusion can also be supported by different features of SOA in a van Krevelen diagram between non-pollution periods and pollution EPs. Furthermore, for pollution EPs, when NR-PM1 mass concentration was divided into six segments, in each segment except for the lowest one SOA concentration was correlated moderately with RO2∗ concentration, suggesting that gas-phase oxidation still plays important roles in SOA formation. The intercepts of the above linear regressions, which likely correspond to the extent of other mechanisms (i.e., heterogeneous and multiphase reactions), increase with increasing NR-PM1 mass concentration. Our results suggest that while gas-phase oxidation contributes predominantly to SOA formation during non-pollution periods, other mechanisms such as heterogeneous and multiphase reactions play more important roles in SOA formation during pollution EPs than gas-phase oxidation.

Comparison of dimension reduction techniques in the analysis of mass spectrometry data

Abstract. Online analysis with mass spectrometers produces complex data sets, consisting of mass spectra with a large number of chemical compounds (ions). Statistical dimension reduction techniques (SDRTs) are able to condense complex data sets into a more compact form while preserving the information included in the original observations. The general principle of these techniques is to investigate the underlying dependencies of the measured variables by combining variables with similar characteristics into distinct groups, called factors or components. Currently, positive matrix factorization (PMF) is the most commonly exploited SDRT across a range of atmospheric studies, in particular for source apportionment. In this study, we used five different SDRTs in analysing mass spectral data from complex gas- and particle-phase measurements during a laboratory experiment investigating the interactions of gasoline car exhaust and α-pinene. Specifically, we used four factor analysis techniques, namely principal component analysis (PCA), PMF, exploratory factor analysis (EFA) and non-negative matrix factorization (NMF), as well as one clustering technique, partitioning around medoids (PAM). All SDRTs were able to resolve four to five factors from the gas-phase measurements, including an α-pinene precursor factor, two to three oxidation product factors, and a background or car exhaust precursor factor. NMF and PMF provided an additional oxidation product factor, which was not found by other SDRTs. The results from EFA and PCA were similar after applying oblique rotations. For the particle-phase measurements, four factors were discovered with NMF: one primary factor, a mixed-LVOOA factor and two α-pinene secondary-organic-aerosol-derived (SOA-derived) factors. PMF was able to separate two factors: semi-volatile oxygenated organic aerosol (SVOOA) and low-volatility oxygenated organic aerosol (LVOOA). PAM was not able to resolve interpretable clusters due to general limitations of clustering methods, as the high degree of fragmentation taking place in the aerosol mass spectrometer (AMS) causes different compounds formed at different stages in the experiment to be detected at the same variable. However, when preliminary analysis is needed, or isomers and mixed sources are not expected, cluster analysis may be a useful tool, as the results are simpler and thus easier to interpret. In the factor analysis techniques, any single ion generally contributes to multiple factors, although EFA and PCA try to minimize this spread. Our analysis shows that different SDRTs put emphasis on different parts of the data, and with only one technique, some interesting data properties may still stay undiscovered. Thus, validation of the acquired results, either by comparing between different SDRTs or applying one technique multiple times (e.g. by resampling the data or giving different starting values for iterative algorithms), is important, as it may protect the user from dismissing unexpected results as “unphysical”.

Trajectory-based analysis on the source areas and transportation pathways of atmospheric particulate matter over Eastern Finland

In this study, we utilize aerosol mass spectrometer (AMS) and multi-angle absorption photometer (MAAP) measurements to assess the most predominant source regions of various atmospheric aerosol constituents transported to Eastern Finland. The non-refractory composition data from the AMS were measured during three intensive measurements campaigns in autumn 2012, 2014 and 2016, whereas the continuous long-term measurements on equivalent black carbon were conducted in 2012–2017. According to observations, the highest concentrations of particulate organics, sulphate, ammonium and black carbon originated from western parts of Russia and Eastern Europe, whereas Central Europe showed lesser contribution. In addition, exceptionally high concentrations of sulphate and ammonium were associated with the Timan-Pechora basin located in north-western Russia. Assumingly, this phenomenon could be linked to intensive gas flaring activities taking place in the area. We also performed positive matrix factorization analysis of organic fraction measured by the AMS. The trajectory analysis revealed increased concentration fields (CF) for low-volatility oxygenated organic aerosol (LVOOA) and hydrocarbon-like organic aerosol (HOA) in the same areas as for sulphate and ammonium. Meanwhile, the CF of semi-volatile oxygenated organic aerosol (SVOOA) suggested a local origin. To summarize, our results suggest that Western Russia and Eastern Europe are the most important source regions of several long-range transported aerosol constituents for Eastern Finland. Besides influencing the air quality and aerosol chemical composition on a local scale, these regions may also play a crucial role as the pollutants are transported further north, towards the vulnerable Arctic region.

Composition and origin of PM<sub>2.5</sub> aerosol particles in the upper Rhine valley in summer

Abstract. We conducted a 6-week measurement campaign in summer 2016 at a rural site about 11 km north of the city of Karlsruhe in southwest Germany in order to study the chemical composition and origin of aerosols in the upper Rhine valley. In particular, we deployed a single-particle mass spectrometer (LAAPTOF) and an aerosol mass spectrometer (AMS) to provide complementary chemical information on aerosol particles smaller than 2.5 µm. For the entire measurement period, the total aerosol particle mass was dominated by sodium salts, contributing on average (36±27) % to the total single particles measured by the LAAPTOF. The total particulate organic compounds, sulfate, nitrate, and ammonium contributed on average (58±12) %, (22±7) %, (10±1) %, and (9±3) % to the total non-refractory particle mass measured by the AMS. Positive matrix factorization (PMF) analysis for the AMS data suggests that the total organic aerosol (OA) consisted of five components, including (9±7) % hydrocarbon-like OA (HOA), (16±11) % semi-volatile oxygenated OA (SV-OOA), and (75±15) % low-volatility oxygenated OA (LV-OOA). The regional transport model COSMO-ART was applied for source apportionment and to achieve a better understanding of the impact of complex transport patterns on the field observations. Combining field observations and model simulations, we attributed high particle numbers and SO2 concentrations observed at this rural site to industrial emissions from power plants and a refinery in Karlsruhe. In addition, two characteristic episodes with aerosol particle mass dominated by sodium salts particles comprising (70±24) % of the total single particles and organic compounds accounting for (77±6) % of total non-refractory species, respectively, were investigated in detail. For the first episode, we identified relatively fresh and aged sea salt particles originating from the Atlantic Ocean more than 800 km away. These particles showed markers like m∕z 129 C5H7NO3+, indicating the influence of anthropogenic emissions modifying their composition, e.g. from chloride to nitrate salts during the long-range transport. For a 3 d episode including high organic mass concentrations, model simulations show that on average (74±7) % of the particulate organics at this site were of biogenic origin. Detailed model analysis allowed us to find out that three subsequent peaks of high organic mass concentrations originated from different sources, including local emissions from the city and industrial area of Karlsruhe, regional transport from the city of Stuttgart (∼64 km away), and potential local night-time formation and growth. Biogenic (forest) and anthropogenic (urban) emissions were mixed during transport and contributed to the formation of organic particles. In addition, topography, temperature inversion, and stagnant meteorological conditions also played a role in the build-up of higher organic particle mass concentrations. Furthermore, the model was evaluated using field observations and corresponding sensitivity tests. The model results show good agreement with trends and concentrations observed for several trace gases (e.g. O3, NO2, and SO2) and aerosol particle compounds (e.g. ammonium and nitrate). However, the model underestimates the number of particles by an order of magnitude and underestimates the mass of organic particles by a factor of 2.3. The discrepancy was expected for particle number since the model does not include all nucleation processes. The missing organic mass indicates either an underestimated regional background or missing sources and/or mechanisms in the model, like night-time chemistry. This study demonstrates the potential of combining comprehensive field observations with dedicated transport modelling to understand the chemical composition and complex origin of aerosols.

Characteristics and sources of fine organic aerosol over a big semi-arid urban city of western India using HR-ToF-AMS

Highly time-resolved measurements of non-refractory submicron particulate matter (NR-PM1) were performed using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) over a big urban city (Ahmedabad, 23.0°N, 72.6°E, 49 m amsl) of western India during the post-monsoon season. All the components of NR-PM1 [i.e., organic aerosol (OA)SO42−, NO3−, NH4+ and Cl−] showed a strong diurnal variation with the overall dominance of OA (58%) in NR-PM1, and the OA composition was found to be a mixture of fresh and aged species. O/C varied from 0.04 to 0.91 whereas, H/C varied from 1.34 to 2.26. A strong diurnal variation in O/C caused a large variability in OM/OC ratio (ranging from 1.2 to 2.3) during the study period, which suggests that a constant conversion factor for OM estimation (from OC) can be a source of large uncertainty in their load assessment over the study region. Furthermore, the observed slope (−0.78 ± 0.01) of Van Krevelen (VK) diagram suggests that plausible major functional groups of ambient oxidized OA could be the mixture of alcohols and carboxylic groups. Further, a drastic change in NR-PM1 concentrations and composition was observed during Diwali, a festival when huge amount of firecrackers are burnt across India. Three distinct sources of OA [i.e., primary OA (POA), semi-volatile oxygenated OA (SV-OOA), and low volatility OOA (LV-OOA)] were identified via positive matrix factorization (PMF) analysis. The OA mass was dominated by POA (42%), followed by LV-OOA (33%) and SV-OOA (25%). Diurnal variations in PMF factors suggest that OOA (SV-OOA + LV-OOA) were high during early morning and afternoon hours; whereas, POA was most abundant during traffic rush hours. Observations also revealed that the high OA loading events were dominated by POA during the study period. This study provides new insights on the atmospheric aging of OA, for the first time, over western India, which would be helpful in understanding the formation mechanism of secondary OA over this region.

Chemistry of new particle growth during springtime in the Seoul metropolitan area, Korea

New particle formation and growth events (NPEs) were frequently observed (17 out of 60 days) during April 14 to June 15, 2016 in the Seoul metropolitan area (SMA). In this study, we investigated the chemical mechanisms of new particle growth based on measurements conducted using an aerodyne high resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) and a scanning mobility particle sizer (SMPS). Both instruments were deployed as a part of the KORUS-AQ campaign (Korea-US Air Quality study). NPEs usually started around noon time between ∼11:00 and 14:00 with the appearance of an ultrafine mode peaking between ∼20 and 30 nm (in mobility diameter, Dm, measured by the SMPS operating in the range 18–947 nm) followed by the growth of this modal diameter to 50–100 nm during the next ∼6–18 h. The growth rate of NPEs during the study was on average 4.48 ± 1.39 nm/h. Comparing to the non-NPE days in SMA, NPEs occurred under the conditions of lower concentration of preexisting particles, higher ozone (48 vs 30 ppb), stronger solar radiation (2.53 vs1.20 MJ/m2), and drier air (34 vs 65%). The HR-ToF-AMS size-resolved aerosol composition measurements show that LV-OOA (low volatility oxidized organic aerosol) and sulfate were major contributors to the growth of new particles at the initial stage of NPE which mostly occurred during daytime and that the later growth which extended into nighttime was mainly contributed by semi-volatile condensable species such as nitrate and SV-OOA (semi-volatile oxygenated organic aerosol). Generally new particles grew to a modal size of ∼80 nm (12 out of 17 NPEs) over the course of an event, however, particles could grow to larger than 100 nm when nitrate concentration was high whereas particle growth was limited to ∼ 50 nm when nitrate, SV-OOA or sulfate were low.

Yearlong variability of oxidative potential of particulate matter in an urban Mediterranean environment

The oxidative potential (OP) of fine and coarse fractions of ambient aerosols was studied in the urban environment of Athens, Greece. OP was quantified using a dithiothreitol (DTT) assay, applied to the water soluble fraction of aerosol that was extracted from 361 fine and 84 coarse mode of 24-h and 12-h filter samples over a one-year period. During the cold period, samples were collected on a 12-h basis, to assess the impact of night-time biomass burning emissions from domestic heating on OP. The chemical characteristics of aerosols were measured in parallel using an Aerosol Chemical Speciation Monitoring (ACSM) and a 7-wavelength Aethalometer. A source apportionment analysis on the ACSM data resulted in the identification of organic aerosol (OA) factors on a seasonal basis. A good correlation of OP with NO3−, NH4+, BC (Black Carbon), Organics and LV-OOA (low volatility oxygenated OA) was found during winter, revealing the importance of combustion and aging processes for OP. During the summertime, a good correlation between OP and SO4−2 and NH4+indicates its association with regional aerosol – thus the importance of oxidative aging that reduces its association with any characteristic source. Multiple regression analysis during winter revealed that highly oxygenated secondary aerosol (LV-OOA) and, to a lesser extent, fresh biomass burning (BBOA) and fossil fuel (HOA) organic aerosol, are the prime contributors to the OP of fine aerosol, with extrinsic toxicities of 54 ± 22 pmol min−1 μg−1, 28 ± 7 and 17 ± 4 pmol min−1μg−1, respectively. In summer, OP cannot be attributed to any of the identified components and corresponds to a background aerosol value. In winter however, the regression model can reproduce satisfactorily the water soluble DTT activity of fine aerosol, providing a unique equation for the estimation of aerosol OP in an urban Mediterranean environment.

Fireworks: A major source of inorganic and organic aerosols during Christmas and New Year in Mexico city

Abstract The fireworks activity in Christmas and New Year is the culprit of a substantial increase in the particles pollution during those festivity dates in Mexico City. The fireworks' thrilling explosions fill the air with small particles full of metals and organic and inorganic species, and noxious gases such as sulfur dioxide, nitrogen dioxide, carbon monoxide and nitrous acid (HONO). The stagnant atmosphere at night accumulates the fireworks haze under a shallow boundary layer. By sunrise, once the fireworks activity has stopped, the photolysis of the abundant HONO accumulated throughout the night, leads to a rapid production of nitrate (NO3−), ammonium (NH4+), sulfate (SO42−) and low volatility secondary organic aerosols. This was the pattern observed during the winter of 2013–2014, when online measurements of water-soluble inorganic ions in particles smaller than 2.5 μm (PM2.5), non-refractory ions and organics in particles smaller than 1 μm (PM1), equivalent black carbon, and above-mentioned gases were conducted. The geometric average of the sum of the three major inorganic ions, NO3−, NH4+ and SO42− accounted for 25% (28%) and 34% (37%) of the total PM2.5 mass during the fireworks activity at night and the following morning on Christmas Day (New Year's Day), respectively. Similarly, their sum in the non-refractory fraction accounted for 37% (49%) and 47% (53%) of the total NR-PM1 mass concentration, while the organic fraction accounted for 56% (44%) and 49% (44%). Biomass burning organic aerosols and semi-volatile oxygenated organic aerosols (BBOA and SV-OOA, respectively) dominated the organic fraction during the fireworks period at night, while the low-volatility oxygenated organic aerosols (LV-OOA) increased substantially their concentration along the post-firework hours the following morning. Concentrations of PM2.5 > 100 μg m−3 and PM1 > 50 μg m−3 were observed on both days, with maximums of 157 and 137 μg m−3 for PM2.5 and PM1, respectively. The meteorological conditions helped to reduce the fireworks impact by late afternoon on both days, limiting the firework episodes to 22 h. The SO42− and NO3− related to the fireworks were not able to offset the deficit of NH4+, and thus the acidity nature of the particles during the mild winter of Mexico City did not change.

Combined effects of boundary layer dynamics and atmospheric chemistry on aerosol composition during new particle formation periods

Abstract. Characterizing aerosol chemical composition in response to meteorological changes and atmospheric chemistry is important to gain insights into new particle formation mechanisms. A BAECC (Biogenic Aerosols – Effects on Clouds and Climate) campaign was conducted during the spring 2014 at the SMEAR II station (Station for Measuring Forest Ecosystem–Aerosol Relations) in Finland. The particles were characterized by a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). A PBL (planetary boundary layer) dilution model was developed to assist interpreting the measurement results. Right before nucleation events, the mass concentrations of organic and sulfate aerosol species were both decreased rapidly along with the growth of PBL heights. However, the mass fraction of sulfate aerosol of the total aerosol mass was increased, in contrast to a decrease for the organic mass fraction. Meanwhile, an increase in LVOOA (low-volatility oxygenated organic aerosol) mass fraction of the total organic mass was observed, in distinct comparison to a reduction of SVOOA (semi-volatile OOA) mass fraction. Our results demonstrate that, at the beginning of nucleation events, the observed sulfate aerosol mass was mainly driven by vertical turbulent mixing of sulfate-rich aerosols between the residual layer and the newly formed boundary layer, while the condensation of sulfuric acid (SA) played a minor role in interpreting the measured sulfate mass concentration. For the measured organic aerosols, their temporal profiles were mainly driven by dilution from PBL development, organic aerosol mixing in different boundary layers and/or partitioning of organic vapors, but accurate measurements of organic vapor concentrations and characterization on the spatial aerosol chemical composition are required. In general, the observed aerosol particles by AMS are subjected to joint effects of PBL dilution, atmospheric chemistry and aerosol mixing in different boundary layers. During aerosol growth periods in the nighttime, the mass concentrations of organic aerosols and organic nitrate aerosols were both increased. The increase in SVOOA mass correlated well with the calculated increase in condensed HOMs' (highly oxygenated organic molecules) mass. To our knowledge, our results are the first atmospheric observations showing a connection between increase in SVOOA and condensed HOMs during the nighttime.

Characterization of submicron aerosol chemical composition and sources in the coastal area of Central Chile

Chemical characteristics and the sources of submicron particles (<1 μm in diameter) were investigated in Valle Alegre, the coastal area of Central Chile. The chemical composition of particles was studied by using a Soot particle Aerosol Mass Spectrometer and Multi-Angle Absorption Photometer. Submicron particles were dominated by organics (42% of mass) and sulfate (39% of mass) while the mass fractions of ammonium, nitrate and black carbon were much smaller (13, 2 and 4% of mass, respectively). Additionally, several metals (V, Zn, Fe, Cd, Cu, K, Na and Mg) were detected in submicron particles and also some of their inorganic salts (e.g. NaCl+, MgCl2+, CaCl2+, KCl+ and KNO3+). The sources of particles were examined by using Positive Matrix Factorization (PMF). Organic aerosol (OA) was divided into five factors by using PMF; hydrocarbon-like OA (HOA), biomass burning OA (BBOA), low-volatility oxygenated OA (LV-OOA), semi-volatile OA (SV-OOA) and marine oxygenated OOA (MOOA). Oxygenated factors (LV-OOA; SV-OOA and MOOA) comprised 75% of total OA with LV-OOA being the dominant factor (38% of OA). Sulfate had two major sources in Valle Alegre; ∼70% of sulfate was related to anthropogenic sources through the oxidation of gas phase SO2 whereas ∼24% of sulfate was associated with biogenic origin related to the oxidation of dimethyl sulfide in the marine environment. Regarding total submicron particle mass (campaign-average 9.5 μg m−3), the contribution of anthropogenic sources was at least as large as that of biogenic origin.

Chemical characteristics of brown carbon in atmospheric particles at a suburban site near Guangzhou, China

Abstract. Light-absorbing organic carbon (or brown carbon, BrC) in atmospheric particles has received much attention for its potential role in global radiative forcing. While a number of field measurement campaigns have differentiated light absorption by black carbon (BC) and BrC, the chemical characteristics of BrC are not well understood. In this study, we present co-located real-time light absorption and chemical composition measurements of atmospheric particles to explore the relationship between the chemical and optical characteristics of BrC at a suburban site downwind of Guangzhou, China, from November to December 2014. BrC and BC contributions to light absorption were estimated using measurements from a seven-wavelength aethalometer, while the chemical composition of non-refractory PM1 was measured with a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). Using the absorption Ångström exponent (AAE) method, we estimated that BrC contributed 23.6 % to the total aerosol absorption at 370 nm, 18.1 % at 470 nm, 10.7 % at 520 nm, 10.7 % at 590 nm, and 10.5 % at 660 nm. Biomass burning organic aerosol (BBOA) has the highest mass absorption coefficient among sources of organic aerosols. Its contribution to total brown carbon absorption coefficient decreased but that of low-volatility oxygenated organic aerosol (LVOOA) increased with increasing wavelength, suggesting the need for wavelength-dependent light absorption analysis for BrC in association with its chemical makeup. Clear correlations of N-containing ion fragments with absorption coefficient were observed. These correlations also depended on their degrees of unsaturation/cyclization and oxygenation. While the current study relates light absorption by BrC to ion fragments, more detailed chemical characterization is warranted to constrain this relationship.

Nitrate-driven urban haze pollution during summertime over the North China Plain

Abstract. Compared to the severe winter haze episodes in the North China Plain (NCP), haze pollution during summertime has drawn little public attention. In this study, we present the highly time-resolved chemical composition of submicron particles (PM1) measured in Beijing and Xinxiang in the NCP region during summertime to evaluate the driving factors of aerosol pollution. During the campaign periods (30 June to 27 July 2015, for Beijing and 8 to 25 June 2017, for Xinxiang), the average PM1 concentrations were 35.0 and 64.2 µg m−3 in Beijing and Xinxiang. Pollution episodes characterized with largely enhanced nitrate concentrations were observed at both sites. In contrast to the slightly decreased mass fractions of sulfate, semivolatile oxygenated organic aerosol (SV-OOA), and low-volatility oxygenated organic aerosol (LV-OOA) in PM1, nitrate displayed a significantly enhanced contribution with the aggravation of aerosol pollution, highlighting the importance of nitrate formation as the driving force of haze evolution in summer. Rapid nitrate production mainly occurred after midnight, with a higher formation rate than that of sulfate, SV-OOA, or LV-OOA. Based on observation measurements and thermodynamic modeling, high ammonia emissions in the NCP region favored the high nitrate production in summer. Nighttime nitrate formation through heterogeneous hydrolysis of dinitrogen pentoxide (N2O5) enhanced with the development of haze pollution. In addition, air masses from surrounding polluted areas during haze episodes led to more nitrate production. Finally, atmospheric particulate nitrate data acquired by mass spectrometric techniques from various field campaigns in Asia, Europe, and North America uncovered a higher concentration and higher fraction of nitrate present in China. Although measurements in Beijing during different years demonstrate a decline in the nitrate concentration in recent years, the nitrate contribution in PM1 still remains high. To effectively alleviate particulate matter pollution in summer, our results suggest an urgent need to initiate ammonia emission control measures and further reduce nitrogen oxide emissions over the NCP region.

Simultaneous aerosol mass spectrometry and chemical ionisation mass spectrometry measurements during a biomass burning event in the UK: insights into nitrate chemistry

Abstract. Over the past decade, there has been an increasing interest in short-term events that negatively affect air quality such as bonfires and fireworks. High aerosol and gas concentrations generated from public bonfires or fireworks were measured in order to understand the night-time chemical processes and their atmospheric implications. Nitrogen chemistry was observed during Bonfire Night with nitrogen containing compounds in both gas and aerosol phases and further N2O5 and ClNO2 concentrations, which depleted early next morning due to photolysis of NO3 radicals and ceasing production. Particulate organic oxides of nitrogen (PONs) concentrations of 2.8 µg m−3 were estimated using the m ∕ z 46 : 30 ratios from aerosol mass spectrometer (AMS) measurements, according to previously published methods. Multilinear engine 2 (ME-2) source apportionment was performed to determine organic aerosol (OA) concentrations from different sources after modifying the fragmentation table and it was possible to identify two PON factors representing primary (pPON_ME2) and secondary (sPON_ME2) contributions. A slight improvement in the agreement between the source apportionment of the AMS and a collocated AE-31 Aethalometer was observed after modifying the prescribed fragmentation in the AMS organic spectrum (the fragmentation table) to determine PON sources, which resulted in an r2 = 0.894 between biomass burning organic aerosol (BBOA) and babs_470wb compared to an r2 = 0.861 obtained without the modification. Correlations between OA sources and measurements made using time-of-flight chemical ionisation mass spectrometry with an iodide adduct ion were performed in order to determine possible gas tracers to be used in future ME-2 analyses to constrain solutions. During Bonfire Night, strong correlations (r2) were observed between BBOA and methacrylic acid (0.92), acrylic acid (0.90), nitrous acid (0.86), propionic acid, (0.85) and hydrogen cyanide (0.76). A series of oxygenated species and chlorine compounds showed good correlations with sPON_ME2 and the low volatility oxygenated organic aerosol (LVOOA) factor during Bonfire Night and an event with low pollutant concentrations. Further analysis of pPON_ME2 and sPON_ME2 was performed in order to determine whether these PON sources absorb light near the UV region using an Aethalometer. This hypothesis was tested by doing multilinear regressions between babs_470wb and BBOA, sPON_ME2 and pPON_ME2. Our results suggest that sPON_ME2 does not absorb light at 470 nm, while pPON_ME2 and LVOOA do absorb light at 470 nm. This may inform black carbon (BC) source apportionment studies from Aethalometer measurements, through investigation of the brown carbon contribution to babs_470wb.

Source apportionment of submicron organic aerosol collected from Atlanta, Georgia, during 2014–2015 using the aerosol chemical speciation monitor (ACSM)

The Aerodyne Aerosol Chemical Speciation Monitor (ACSM) was redeployed at the Jefferson Street (JST) site in downtown Atlanta, Georgia (GA) for 1 year (March 20, 2014–February 08, 2015) to chemically characterize non-refractory submicron particulate matter (NR-PM1) in near real-time and to assess whether organic aerosol (OA) types and amounts change from year-to-year. Submicron organic aerosol (OA) mass spectra were analyzed by season using multilinear engine (ME-2) to apportion OA subtypes to potential sources and chemical processes. A suite of real-time collocated measurements from the Southeastern Aerosol Research and Characterization (SEARCH) network was compared with ME-2 factor solutions to aid in the interpretation of OA subtypes during each season. OA tracers measured from high-volume filter samples using gas chromatography interfaced with electron ionization-mass spectrometry (GC/EI-MS) also aided in identifying OA sources. The initial application of ME-2 to the yearlong ACSM dataset revealed that OA source apportionment by season was required to better resolve sporadic OA types. Spring and fall OA mass spectral datasets were separated into finer periods to capture potential OA sources resulting from non-homogeneous emissions during transitioning periods. NR-PM1 was highest in summer (16.7 ± 8.4 μg m−3) and lowest in winter (8.0 ± 5.7 μg m−3), consistent with prior studies. OA dominated NR-PM1 mass (56–74% on average) in all seasons. Hydrocarbon-like OA (HOA) from primary emissions was observed in all seasons, averaging 5–22% of total OA mass. Strong correlations of HOA with carbon monoxide (CO) (R = 0.71–0.88) and oxides of nitrogen (NOx) (R = 0.55–0.79) indicated that vehicular traffic was the likely source. Biomass burning OA (BBOA) was observed in all seasons, with lower contributions (2%) in summer and higher in colder seasons (averaging 8–20% of total OA mass). BBOA correlated strongly with levoglucosan (R = 0.78–0.95) during colder seasons, which supports that BBOA is likely derived from fresh biomass/residential burning. However, weaker correlation with levoglucosan (R = 0.38) in summer suggested a more aged aerosol. During warmer seasons, OA from the reactive uptake of isoprene epoxydiols (IEPOX) onto acidic sulfate aerosol was resolved by ME-2 (denoted as IEPOX-OA), averaging 25–29% of the total OA mass. Temporal variation of IEPOX-OA was nearly coincident with that of 91Fac OA (a factor dominated by a distinct ion at m/z 91). The largest contribution of IEPOX-OA to total OA (29%) was found in summer, whereas the largest contribution of 91Fac to total OA (24%) occurred in early fall. Moderate negative correlation between IEPOX-OA and aerosol acidity was observed during late spring (−0.67) and summer (−0.42), consistent with laboratory studies showing that IEPOX-OA is enhanced in the presence of acidic aerosols. Finally, the largest OA mass in all seasons (46–70% of total OA) was derived from oxygenated OA denoted as low-volatility oxygenated OA (LV-OOA) and semi-volatile oxygenated OA (SV-OOA).

Organic carbon at a remote site of the western Mediterranean Basin: sources and chemistry during the ChArMEx SOP2 field experiment

Abstract. The ChArMEx (Chemistry and Aerosols Mediterranean Experiments) SOP2 (special observation period 2) field campaign took place from 15 July to 5 August 2013 in the western Mediterranean Basin at Ersa, a remote site in Cape Corse. During the campaign more than 80 volatile organic compounds (VOCs), including oxygenated species, were measured by different online and offline techniques. At the same time, an exhaustive description of the chemical composition of fine aerosols was performed with an aerosol chemical speciation monitor (ACSM). Low levels of anthropogenic VOCs (typically tens to hundreds of parts per trillion for individual species) and black carbon (0.1–0.9 µg m−3) were observed, while significant levels of biogenic species (peaking at the ppb level) were measured. Furthermore, secondary oxygenated VOCs (OVOCs) largely dominated the VOC speciation during the campaign, while organic matter (OM) dominated the aerosol chemical composition, representing 55 % of the total mass of non-refractory PM1 on average (average of 3.74 ± 1.80 µg m−3), followed by sulfate (27 %, 1.83 ± 1.06 µg m−3), ammonium (13 %, 0.90 ± 0.55 µg m−3) and nitrate (5 %, 0.31 ± 0.18 µg m−3). Positive matrix factorization (PMF) and concentration field (CF) analyses were performed on a database containing 42 VOCs (or grouped VOCs), including OVOCs, to identify the covariation factors of compounds that are representative of primary emissions or chemical transformation processes. A six-factor solution was found for the PMF analysis, including a primary and secondary biogenic factor correlated with temperature and exhibiting a clear diurnal profile. In addition, three anthropogenic factors characterized by compounds with various lifetimes and/or sources have been identified (long-lived, medium-lived and short-lived anthropogenic factors). The anthropogenic nature of these factors was confirmed by the CF analysis, which identified potential source areas known for intense anthropogenic emissions (north of Italy and southeast of France). Finally, a factor characterized by OVOCs of both biogenic and anthropogenic origin was found. This factor was well correlated with submicron organic aerosol (OA) measured by an aerosol chemical speciation monitor (ACSM), highlighting the close link between OVOCs and organic aerosols; the latter is mainly associated (96 %) with the secondary OA fraction. The source apportionment of OA measured by ACSM led to a three-factor solution identified as hydrogen-like OA (HOA), semi-volatile oxygenated OA (SV-OOA) and low volatility OOA (LV-OOA) for averaged mass concentrations of 0.13, 1.59 and 1.92 µg m−3, respectively. A combined analysis of gaseous PMF factors with inorganic and organic fractions of aerosols helped distinguish between anthropogenic continental and biogenic influences on the aerosol- and gas-phase compositions.

Internally mixed black carbon in the Indo-Gangetic Plain and its effect on absorption enhancement

We present the systematic analysis of individual black carbon (BC) mixing state and its impact on radiative forcing from an urban Indian city, Kanpur, located in Indo-Gangetic Plain (IGP). Simultaneous measurements using Single Particle Soot Photometer (SP2), Photo-Acoustic Soot Spectrometer (PASS-3) and High-Resolution Time-of-Flight Aerosol Mass Spectrometer (AMS) were conducted from 8 January 2015 to 28 February 2015 at Kanpur. BC mass and number concentrations varied between 0.7 and 17μg/m3 and 277–5866#/cm3 with a mean of 4.06μg/m3 and 1314#/cm3, respectively. The diurnal variation of BC mass concentration showed a traffic hour peak during both the morning and late night. The mean fraction of “thickly coated BC” particles (fTCBC) was found to be 61.6%, indicating that a large fraction of BC particles was internally mixed. The fTCBC increased after sunrise with a peak at about noontime, indicating that the formation of secondary organic aerosol under active photochemistry can enhance organic coating on a core of black carbon. High-resolution positive matrix factorization (HR-PMF) factors showed distinct characteristics with fTCBC. While primary organic aerosols like cooking organic aerosols (COA) and biomass burning organic aerosols (BBOA) were negatively correlated with fTCBC (r=−0.78 and −0.51, respectively), aged low volatile oxygenated organic aerosol (LVOOA) was forming a coating over BC (r=0.6). Similar positive correlation of fTCBC with inorganic species like ammonium (r=0.58) and nitrate (r=0.47) further suggested that BC appears to be largely coated with LVOOA, ammonium, and nitrate. A positive correlation between the fTCBC and the mass absorption cross-section at 781nm (MAC781) was also observed (r=0.58). Our results suggest that the observed fTCBC could amplify the MAC781 approximately by a factor of 1.8, which may catalyze the positive radiative forcing in the IGP.