Long-range Transport Episodes Research Articles

Assessment of Low-Cost Particulate Matter Sensor Systems against Optical and Gravimetric Methods in a Field Co-Location in Norway

The increased availability of commercially-available low-cost air quality sensors combined with increased interest in their use by citizen scientists, community groups, and professionals is resulting in rapid adoption, despite data quality concerns. We have characterized three out-the-box PM sensor systems under different environmental conditions, using field colocation against reference equipment. The sensor systems integrate Plantower 5003, Sensirion SPS30 and Alphasense OCP-N3 PM sensors. The first two use photometry as a measuring technique, while the third one is an optical particle counter. For the performance evaluation, we co-located 3 units of each manufacturer and compared the results against optical (FIDAS) and gravimetric (KFG) methods for a period of 7 weeks (28 August to 19 October 2020). During the period from 2nd and 5th October, unusually high PM concentrations were observed due to a long-range transport episode. The results show that the highest correlations between the sensor systems and the optical reference are observed for PM1, with coefficients of determination above 0.9, followed by PM2.5. All the sensor units struggle to correctly measure PM10, and the coefficients of determination vary between 0.45 and 0.64. This behavior is also corroborated when using the gravimetric method, where correlations are significantly higher for PM2.5 than for PM10, especially for the sensor systems based on photometry. During the long range transport event the performance of the photometric sensors was heavily affected, and PM10 was largely underestimated. The sensor systems evaluated in this study had good agreement with the reference instrumentation for PM1 and PM2.5; however, they struggled to correctly measure PM10. The sensors also showed a decrease in accuracy when the ambient size distribution was different from the one for which the manufacturer had calibrated the sensor, and during weather conditions with high relative humidity. When interpreting and communicating air quality data measured using low-cost sensor systems, it is important to consider such limitations in order not to risk misinterpretation of the resulting data.

Added Value of Vaisala AQT530 Sensors as a Part of a Sensor Network for Comprehensive Air Quality Monitoring

Poor air quality influences the quality of life in the urban environment. The regulatory observation stations provide the backbone for the city administration to monitor urban air quality. Recently a suite of cost-effective air quality sensors has emerged to provide novel insights into the spatio-temporal variability of aerosol particles and trace gases. Particularly in low concentrations these sensors might suffer from issues related e.g., to high detection limits, concentration drifts and interdependency between the observed trace gases and environmental parameters. In this study we characterize the optical particle detector used in AQT530 (Vaisala Ltd.) air quality sensor in the laboratory. We perform a measurement campaign with a network of AQT530 sensors in Helsinki, Finland in 2020–2021 and present a long-term performance evaluation of five sensors for particulate (PM10, PM2.5) and gaseous (NO2, NO, CO, O3) components during a half-year co-location study with reference instruments at an urban traffic site. Furthermore, short-term (3–5 weeks) co-location tests were performed for 25 sensors to provide sensor-specific correction equations for the fine-tuning of selected pollutants in the sensor network. We showcase the added value of the verified network of 25 sensor units to address the spatial variability of trace gases and aerosol mass concentrations in an urban environment. The analysis assesses road and harbor traffic monitoring, local construction dust monitoring, aerosol concentrations from fireworks, impact of sub-urban small scale wood combustion and detection of long-range transport episodes on a city scale. Our analysis illustrates that the calibrated network of Vaisala AQT530 air quality sensors provide new insights into the spatio-temporal variability of air pollution within the city. This information is beneficial to, for example, optimization of road dust and construction dust emission control as well as provides data to tackle air quality problems arising from traffic exhaust and localized wood combustion emissions in the residential areas.

Unusually high birch (Betula spp.) pollen concentrations in Poland in 2016 related to long-range transport (LRT) and the regional pollen occurrence

In 2016, the highest birch (Betula spp.) pollen concentrations were recorded in Kraków (Poland) since the beginning of pollen observations in 1991. The aim of this study was to ascertain the reason for this phenomenon, taking the local sources of pollen in Poland and long-range transport (LRT) episodes associated with the pollen influx from other European countries into account. Three periods of higher pollen concentrations in Kraków in 2016 were investigated with the use of pollen data, phenological data, meteorological data and the HYSPLIT numerical model to calculate trajectories up to 4 days back (96 h) at the selected Polish sites. From 5 to 8 April, the birch pollen concentrations increased in Kraków up to 4000 Pollen/m3, although no full flowering of birch trees in the city was observed. The synoptic situation with air masses advection from the South as well as backward trajectories and the general birch pollen occurrence in Europe confirm that pollen was transported mainly from Serbia, Hungary, Austria, the Czech Republic, Slovakia, into Poland. The second analyzed period (13–14 April) was related largely to the local flowering of birches, while the third one in May (6–7 May) mostly resulted from the birch pollen transport from Fennoscandia and the Baltic countries. Unusual high pollen concentrations at the beginning of the pollen season can augment the symptomatic burden of birch pollen allergy sufferers and should be considered during therapy. Such incidents also affect the estimation of pollen seasons timing and severity.Graphical

Concentrations and biosphere–atmosphere fluxes of inorganic trace gases and associated ionic aerosol counterparts over the Amazon rainforest

Abstract. The Amazon rainforest presents a unique, natural laboratory for the study of surface–atmosphere interactions. Its alternation between a near-pristine marine-influenced atmosphere during the wet season and a vulnerable system affected by periodic intrusions of anthropogenic pollution during the dry season provides an opportunity to investigate some fundamental aspects of boundary-layer chemical processes. This study presents the first simultaneous hourly measurements of concentrations, fluxes, and deposition velocities of the inorganic trace gases NH3, HCl, HONO, HNO3, and SO2 as well as their water-soluble aerosol counterparts NH4+, Cl−, NO2-, NO3- and SO42- over the Amazon. Species concentrations were measured in the dry season (from 6 October to 5 November 2017), at the Amazon Tall Tower Observatory (ATTO) in Brazil, using a two-point gradient wet-chemistry instrument (GRadient of AErosols and Gases Online Registration, GRAEGOR) sampling at 42 and 60 m. Fluxes and deposition velocities were derived from the concentration gradients using a modified form of the aerodynamic gradient method corrected for measurement within the roughness sub-layer. Findings from this campaign include observations of elevated concentrations of NH3 and SO2 partially driven by long-range transport (LRT) episodes of pollution and the substantial influence of coarse Cl− and NO3- particulate on overall aerosol mass burdens. From the flux measurements, the dry season budget of total reactive nitrogen dry deposition at the ATTO site was estimated as −2.9 kg N ha-1a-1. HNO3 and HCl were deposited continuously at a rate close to the aerodynamic limit. SO2 was deposited with an average daytime surface resistance (Rc) of 28 s m−1, whilst aerosol components showed average surface deposition velocities of 2.8 and 2.7 mm s−1 for SO42- and NH4+, respectively. Deposition rates of NO3- and Cl− were higher at 7.1 and 7.8 mm s−1, respectively, reflecting their larger average size. The exchange of NH3 and HONO was bidirectional, with NH3 showing emission episodes in the afternoon and HONO in the early morning hours. This work provides a unique dataset to test and improve dry deposition schemes for these compounds for tropical rainforest, which have typically been developed by interpolation from conditions in temperate environments. A future campaign should focus on making similar measurements in the wet season in order to provide a complete view of the annual pattern of inorganic trace gas and coarse aerosol biosphere–atmosphere exchange over tropical rainforest.

Dust induced radiative perturbations during an episode of long-range dust transport over Delhi, India: a high-resolution regional NWP model study

Dust-induced modifications to the radiative fluxes/heating during a light-blocking haze episode over Delhi, India, 7–9th Nov 2017—triggered by long range transport from a severe Saudi Arabian dust storm—have been investigated with a regional NWP model, NCUM-R with prognostic dust-radiation feedbacks. The study employs ‘Double Radiation Calls’, wherein parallel runs of the radiation scheme ‘with’ (prognostic) and ‘without’ (diagnostic) dust radiative effects—while prognostic fields drive the forecast—isolate the dust-induced perturbations. The forecasted dust optical depth agreed spatially with the AOD from MODIS with Angstrom Exponent > 0.5, indicating that the dust was well mixed with the fine mode anthropogenic aerosols upwind. The ‘downward shortwave (SW) flux’ was diminished (upto − 12.9 Wm−2) in layers sampling (i) near-surface (L1Avg), (ii) well-mixed layer within the planetary boundary layer (PBL, L2Avg) and (iii) free-troposphere (FT, L3Avg). Dust-induced ‘Solar heating’ dominated in FT (upto 9.5 × 10–7 Ks−1) and the patches below (in L2Avg) exhibited a cooling, leading to thermal dipoles. The ‘upward longwave (LW) flux’ in FT was reduced and ‘LW heating’ prevailed in all levels—peak (2.5 × 10–6 Ks−1) in L2Avg—along with well-defined cooling zones in L1Avg. The dust–radiation interaction in turn influenced the boundary layer meteorology, manifested as (i) shallow PBLs that spatially correlate with dust-induced cooling of the boundary layer column, (ii) enhanced surface humidity and (iii) reduced visibility. The study is an instance of prognostic dust-radiation feedbacks improving the skill of NWP models in dust-laden regions.

Long-range air pollution transport in East Asia during the first week of the COVID-19 lockdown in China

Long-range transport (LRT) of air pollutants from East Asia during the northeast monsoon season impacts several downwind locations. In 2020, the initial COVID-19 lockdowns in China overlapped with Week 3 of the Chinese New Year (CNY) holiday, and an Asian outflow event. Thus, movement of the Chinese populace from city to city was already greatly reduced by the time of the LRT episode, although the reductions in industrial output are less clear. We found NO2 column concentrations were reduced by 24% during the CNY Week 3 this year compared to previous years. The attenuated transport event arrived to northern Taiwan with a PM2.5 concentration <45 μg m−3 and most often <35 μg m−3, which is 2–3 times lower than LRT episodes of similar back-trajectory and synoptic patterns. The whole episode persisted for about 60 h, longer than most LRT episodes from China to Taiwan. CMAQ v5.2.1 modeling of the LRT event with 100% emission and reduced emission scenarios, revealed emissions in China were approximately 50% less than normal periods. Due to the length of the episode and the significant reduction in emissions, Taiwan avoided a PM2.5 surplus of 19.2 μg m−3 on average during the episode, equivalent to a 0.5 μg m−3 reduction for the whole 3-month winter season. Employing the 100% emission model scenario and scaling up to the average episode hours each winter, the PM2.5 surplus delivered via plumes on the northeast monsoon is equivalent to a 0.5 μg m−3 surplus for the whole year.

Analysis of Brown Carbon Content and Evolution in Smokes from Siberian Forest Fires Using AERONET Measurements

The content and evolution of brown carbon (BrC) in smokes from Siberian forest fires were analyzed using measurements of the aerosol absorption optical depth (AAOD) at three Russian AERONET stations located in Tomsk, Zvenigorod, and Yekaterinburg. Estimates are obtained of the relative contribution of BrC in fine aerosol particles to the absorption of solar radiation at a wavelength of 440 nm (ηBrC) and, in particular, for an anomalous episode of long-range transport of smokes from Siberia to the European part of Russia in summer 2016. A considerable BrC content is found in smokes in Tomsk and Zvenigorod (where ηBrC is estimated to be 15 and 18% on average). It is noteworthy that no significant ηBrC values were revealed during passage of smokes from Siberian fires over Yekaterinburg. The ηBrC values were found to decrease with aerosol aging under sunlit conditions on the characteristic timescale of about 30 h. At the same time, the measurements in Zvenigorod indicate that the absorption properties of the organic component of smoke aerosol increase during a much longer evolution.

Characteristics of Aerosol Chemical Compositions and Size Distributions during a Long-Range Dust Transport Episode in an Urban City in the Yangtze River Delta

A long- and large-range heavy dust episode occurred from 3 to 8 May 2017 in China. To explore the impacts of this long-range dust transport episode on the chemical compositions and size distributions of urban aerosols, such instruments as an online analyzer for monitoring aerosols and gases (MARGA) and a wide-range particle spectrometer (WPS) were mainly used to monitor chemical components, such as PM2.5 and aerosol size distributions in the range of 10 nm to 10 μm, in Nanjing in this study. During the dust episode, the average concentrations of PM2.5 and PM10 and ions of Ca2+, Mg2+, Cl-, SO42−, NO3−, and NH4+ were 66.2, 233.9, and 1.1, 1.5, 1.1, 11.4, 7.8 and 4.4 μg·m−3, which were 4.4, 5.8, 3.7, 15, 1.38, 1.84, 1.66 and 1.83 times higher than the values observed before the episode and 2.2, 3.3, 5.5, 5.0, 1.57, 1.97, 1.39 and 1.69 times the levels after the episode. The dusts were demonstrated to have differential impacts on the water-soluble gases in the air. During the dust episode, the concentrations of HCl, SO2 and NH3 were comparably low, while the HNO2 and HNO3 concentrations were high. The diurnal variations in pollutants, including SO2, HNO3, Cl−, Ca2+, Mg2+, PM2.5 and PM10, were strongly impacted by the dust episode. However, those variations in NH3, NO3−, SO42− and NH4+ were only slightly influenced. Pollutants were distinctively featured in the various dust stages. The concentration of HNO2 was relatively high in the earliest stage but was substituted by those of SO2, PM10, PM2.5, Ca2+, Mg2+ HNO3 and Cl- in the explosion stage. The aerosol number concentrations exhibited unimodal distributions in the earliest and explosion stages but showed bimodal distributions in the duration and dissipation stages. Additionally, the aerosol size distributions were observed to shift to larger particle segments in different dust stages. The surface area concentrations exhibited four peaks in different dust stages and exhibited trimodal distributions in the non-dust episode. The surface area concentration of fine particles first increased during the earliest stage, while that of coarse particles first decreased during the dissipation stage.

Long-term observations of cloud condensation nuclei over the Amazon rain forest – Part 2: Variability and characteristics of biomass burning, long-range transport, and pristine rain forest aerosols

Abstract. Size-resolved measurements of atmospheric aerosol and cloud condensation nuclei (CCN) concentrations and hygroscopicity were conducted over a full seasonal cycle at the remote Amazon Tall Tower Observatory (ATTO, March 2014–February 2015). In a preceding companion paper, we presented annually and seasonally averaged data and parametrizations (Part 1; Pöhlker et al., 2016a). In the present study (Part 2), we analyze key features and implications of aerosol and CCN properties for the following characteristic atmospheric conditions: Empirically pristine rain forest (PR) conditions, where no influence of pollution was detectable, as observed during parts of the wet season from March to May. The PR episodes are characterized by a bimodal aerosol size distribution (strong Aitken mode with DAit ≈ 70 nm and NAit ≈ 160 cm−3, weak accumulation mode with Dacc ≈ 160 nm and Nacc≈ 90 cm−3), a chemical composition dominated by organic compounds, and relatively low particle hygroscopicity (κAit≈ 0.12, κacc ≈ 0.18). Long-range-transport (LRT) events, which frequently bring Saharan dust, African biomass smoke, and sea spray aerosols into the Amazon Basin, mostly during February to April. The LRT episodes are characterized by a dominant accumulation mode (DAit ≈ 80 nm, NAit ≈ 120 cm−3 vs. Dacc ≈ 180 nm, Nacc ≈ 310 cm−3), an increased abundance of dust and salt, and relatively high hygroscopicity (κAit≈ 0.18, κacc ≈ 0.35). The coarse mode is also significantly enhanced during these events. Biomass burning (BB) conditions characteristic for the Amazonian dry season from August to November. The BB episodes show a very strong accumulation mode (DAit ≈ 70 nm, NAit ≈ 140 cm−3 vs. Dacc ≈ 170 nm, Nacc ≈ 3400 cm−3), very high organic mass fractions (∼ 90 %), and correspondingly low hygroscopicity (κAit≈ 0.14, κacc ≈ 0.17). Mixed-pollution (MPOL) conditions with a superposition of African and Amazonian aerosol emissions during the dry season. During the MPOL episode presented here as a case study, we observed African aerosols with a broad monomodal distribution (D ≈ 130 nm, NCN,10 ≈ 1300 cm−3), with high sulfate mass fractions (∼ 20 %) from volcanic sources and correspondingly high hygroscopicity (κ&lt; 100 nm ≈ 0.14, κ&gt;100nm≈ 0.22), which were periodically mixed with fresh smoke from nearby fires (D ≈ 110 nm, NCN,10 ≈ 2800 cm−3) with an organic-dominated composition and sharply decreased hygroscopicity (κ&lt;150nm≈ 0.10, κ&gt;150nm≈ 0.20). Insights into the aerosol mixing state are provided by particle hygroscopicity (κ) distribution plots, which indicate largely internal mixing for the PR aerosols (narrow κ distribution) and more external mixing for the BB, LRT, and MPOL aerosols (broad κ distributions). The CCN spectra (CCN concentration plotted against water vapor supersaturation) obtained for the different case studies indicate distinctly different regimes of cloud formation and microphysics depending on aerosol properties and meteorological conditions. The measurement results suggest that CCN activation and droplet formation in convective clouds are mostly aerosol-limited under PR and LRT conditions and updraft-limited under BB and MPOL conditions. Normalized CCN efficiency spectra (CCN divided by aerosol number concentration plotted against water vapor supersaturation) and corresponding parameterizations (Gaussian error function fits) provide a basis for further analysis and model studies of aerosol–cloud interactions in the Amazon.

Influence of intense secondary aerosol formation and long-range transport on aerosol chemistry and properties in the Seoul Metropolitan Area during spring time: results from KORUS-AQ

Abstract. Non-refractory submicrometer particulate matter (NR-PM1) was measured in the Seoul Metropolitan Area (SMA), Korea, using an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) from 14 April to 15 June 2016, as a part of the Korea-US Air Quality Study (KORUS-AQ) campaign. This was the first highly time-resolved, real-time measurement study of springtime aerosol in SMA and the results reveal valuable insights into the sources and atmospheric processes that contribute to PM pollution in this region. The average concentration of submicrometer aerosol (PM1 = NR-PM1 + black carbon (BC)) was 22.1 µg m−3, which was composed of 44 % organics, 20 % sulfate, 17 % nitrate, 12 % ammonium, and 7 % BC. Organics had an average atomic oxygen-to-carbon (O ∕ C) ratio of 0.49 and an average organic mass-to-carbon (OM/OC) ratio of 1.82. Four distinct sources of OA were identified via positive matrix factorization (PMF) analysis of the HR-ToF-AMS data: vehicle emissions represented by a hydrocarbon-like OA factor (HOA; O ∕ C = 0.15; 17 % of OA mass), food cooking activities represented by a cooking-influenced OA factor (COA; O ∕ C = 0.19; 22 % of OA mass), and secondary organic aerosol (SOA) represented by a semi-volatile oxygenated OA factor (SV-OOA; O ∕ C = 0.44; 27 % of OA mass) and a low-volatility oxygenated OA factor (LV-OOA; O ∕ C = 0.91; 34 % of OA mass). Our results indicate that air quality in SMA during KORUS-AQ was influenced strongly by secondary aerosol formation, with sulfate, nitrate, ammonium, SV-OOA, and LV-OOA together accounting for 76 % of the PM1 mass. In particular, the formation of LV-OOA and sulfate was mainly promoted by elevated ozone concentrations and photochemical reactions during daytime, whereas SV-OOA and nitrate formation was contributed by both nocturnal processing of VOC and nitrogen oxides, respectively, and daytime photochemical reactions. In addition, lower nighttime temperature promoted gas-to-particle partitioning of semivolatile species and formation of SV-OOA and nitrate. During a period of 4 days (from 20 to 23 May ), LV-OOA increased dramatically and accounted for up to 41 % of the PM1 mass. This intense LV-OOA formation event was associated with large enhancements of both anthropogenic and biogenic VOCs (e.g., isoprene and toluene), high concentration of Ox ( = O3 + NO2), strong solar radiation, and stagnant conditions, suggesting that it was mainly driven by local photochemical formation. We have also investigated the formation and evolution mechanisms of severe haze episodes. Unlike the winter haze events which were mainly caused by intense local emissions coupled with stagnant meteorological conditions, the spring haze events appeared to be influenced by both regional and local factors. For example, there were episodes of long-range transport of plumes followed by calm meteorology conditions, which promoted the formation and accumulation of local secondary species, leading to high concentrations of PM. Overall, our results indicate that PM pollutants in urban Korea originate from complex emission sources and atmospheric processes and that the concentrations and composition of PM are controlled by various factors, including meteorological conditions, local anthropogenic emissions, and upwind sources.

Four years of highly time resolved measurements of elemental and organic carbon at a rural background site in Central Europe

Elemental carbon (EC) and organic carbon (OC) in fine atmospheric aerosols (PM2.5: aerodynamic diameter smaller than 2.5 μm) have been measured with a semi-automatic instrument during a 4-year survey at the National Atmospheric Observatory Košetice (NAOK), Czech Republic. Ground based measurements were performed from March 2013 to December 2016 with a field Semi-Continuous OCEC Aerosol Analyzer (Sunset Laboratory Inc., USA). The variation of EC and OC concentrations and the OC/EC ratio was characterized for different seasons and days of the week. During our survey, higher concentrations of EC and OC were observed in winter (0.83 ± 0.67 and 3.33 ± 2.28 μg m−3, respectively), and lower concentrations were recorded in summer (0.34 ± 0.18 and 2.30 ± 1.15 μg m−3, respectively). Inversely, the OC/EC ratio with mean value (5.1 ± 2.6) characteristic to rural background area was higher in summer (7.33 ± 3.23) in comparison to the other seasons. Since the data contain values below detection and quantification limits of the measuring device (i.e., censored values), statistical methods for censored data have been used in order to compare mean EC and OC concentrations between various seasons. It was found out that there is a significant difference between summer and the other seasons with the exception of mean OC concentrations at noon. In most cases, there was also a significant difference between winter and the other seasons. Moreover, it was found out that when dealing with OC concentrations, it is possible to replace censored values by a constant and still obtain reasonable results. In case of EC concentrations, the method based on censored distributions should be preferred when the sample size is small and the proportion of censored values is high. The diurnal variation of EC and OC is less pronounced in summer. During working days, the EC diurnal pattern displays a morning (between 6:00 and 10:00) and an afternoon/evening (between 18:00 and 22:00) peaks, while for OC, only the afternoon/evening peak is observed. These seasonal, diurnal and weekly variations of EC and OC concentrations and OC/EC ratio are probably related to variability in terms of emission sources (residential heating, traffic), transport characteristic and meteorological conditions. A weaker correlation between EC and OC in summer (r = 0.56) suggests additional sources and/or transport processes during other seasons. The elevated OC/EC ratio, the higher correlation between OC and O3, and the temperature and solar radiation during summer confirmed an increasing contribution of OC from secondary organic carbon (SOC) estimated as at least 59 ± 11% of total carbon in the PM2.5 using the EC tracer method. Backward trajectories of air masses arriving at 100 m AGL calculated in winter and summer show that higher pollution episodes of EC and OC are predominantly associated with continental air masses confined over Central Europe (about 79%), while lower EC and OC levels are mainly associated with episodes of long-range transport of marine air masses. Interestingly, the results reveal that in winter pollutants emitted during workdays could be accumulated above the region and influence the rural background air quality during some prolonged time of the weekend, especially on Saturday.

Numerical simulations of windblown dust over complex terrain: the Fiambalá Basin episode in June 2015

Abstract. On 13 June 2015, the London Volcanic Ash Advisory Centre (VAAC) warned the Buenos Aires VAAC about a possible volcanic eruption from the Nevados Ojos del Salado volcano (6879 m), located in the Andes mountain range on the border between Chile and Argentina. A volcanic ash cloud was detected by the SEVIRI instrument on board the Meteosat Second Generation (MSG) satellites from 14:00 UTC on 13 June. In this paper, we provide the first comprehensive description of this event through observations and numerical simulations. Our results support the hypothesis that the phenomenon was caused by wind remobilization of ancient pyroclastic deposits (ca. 4.5 ka Cerro Blanco eruption) from the Bolsón de Fiambalá (Fiambalá Basin) in northwestern Argentina. We have investigated the spatiotemporal distribution of aerosols and the emission process over complex terrain to gain insight into the key role played by the orography and the condition that triggered the long-range transport episode. Numerical simulations of windblown dust were performed using the ARW (Advanced Research WRF) core of the WRF (Weather Research and Forecasting) model (WRF-ARW) and FALL3D modeling system with meteorological fields downscaled to a spatial resolution of 2 km in order to resolve the complex orography of the area. Results indicate that favorable conditions to generate dust uplifting occurred in northern Fiambalá Basin, where orographic effects caused strong surface winds. According to short-range numerical simulations, dust particles were confined to near-ground layers around the emission areas. In contrast, dust aerosols were injected up to 5–6 km high in central and southern regions of the Fiambalá Basin, where intense ascending airflows are driven by horizontal convergence. Long-range transport numerical simulations were also performed to model the dust cloud spreading over northern Argentina. Results of simulated vertical particle column mass were compared with the MSG-SEVIRI retrieval product. We tested two numerical schemes: with the default configuration of the FALL3D model, we found difficulties to simulate transport through orographic barriers, whereas an alternative configuration, using a numerical scheme to more accurately compute the horizontal advection in abrupt terrains, substantially improved the model performance.

Particulate matter (PM) episodes at a suburban site in Hong Kong: evolution of PM characteristics and role of photochemistry in secondary aerosol formation

Abstract. Episodes with high concentrations of particulate matter (PM) across the seasons were investigated during four 1-month campaigns at a suburban site in Hong Kong. High-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) measurements revealed that both regional transport and secondary formation contributed to high PM levels during the episodes at this site. Based on distinct meteorological conditions, episodes were categorized into three types: liquid water content (LWC), solar irradiance (IR), and long-range transport (LRT). Despite the difference in meteorological conditions, all episodes were characterized by a high fraction of sulfate (45–56 %) and organics (23–34 %). However, aerosols in LWC episodes were less aged, consisting of the lowest fraction of secondary organic aerosol (SOA) and the highest fraction of small particles. Large particles mixed internally while freshly formed small particles mixed externally in LWC episodes. Aerosols in LRT episodes, by contrast, were the most aged and consisted of the highest proportion of low-volatility oxygenated organic aerosol (LVOOA) and the lowest proportion of small particles. Both small and large particles mixed externally in LRT episodes. The highest proportion of semi-volatile oxygenated organic aerosol (SVOOA) and a medium proportion of small particles were observed in IR episodes. Both small and large particles were likely externally mixed during IR episodes. Furthermore, aerosols experienced the most dramatic size increase and diurnal variation, with a time lag between SVOOA and LVOOA and a gradual increase in carbon oxidation state (OSc ≈ 2 × O : C − H : C). Five out of 10 episodes were of the IR type, further reflecting the importance of this type of episode. The evolution of aerosol components in one particular episode of the IR type, which exhibited a clear land–sea breeze pattern, was examined in detail. Sulfate and SOA due to photochemical aging were very efficiently produced during the course of 6 h. The “less-oxidized” SOA (SVOOA) was initially formed at a higher rate than the “more-oxidized” SOA (LVOOA). The SVOOA transformed to LVOOA at the later stage of photochemical aging. This transformation was further supported by mass spectral analysis, which showed an increase in the most oxidized ion (CO2+) and decreases in moderately oxidized ones (C2H3O+, C3H3O+ and C3H5O+). By measuring the physical and chemical properties of PM in a highly time-resolved manner, the current study was able to demonstrate the dynamic and complex nature of PM transformation during high-PM episodes.

Composition and origin of PM 10 in Cape Verde: Characterization of long-range transport episodes

Abstract A receptor modelling study was performed to identify source categories and their contributions to the PM10 total mass at the Cape Verde archipelago. Trajectory statistical methods were also used to characterize the main atmospheric circulation patterns causing the transport of air masses and to geographically identify the main potential source areas of each PM10 source category. Our findings point out that the variability of the PM10 levels at Cape Verde was prompted by the advections of African mineral dust. The mineral dust load was mainly composed by clay-silicates mineral derived elements (22% of the PM10 total mass on average) with lower amounts of carbonates (9%). A clear northward gradient was observed in carbonates concentration that illustrates the differences in the composition according to the source regions of mineral dust. Mineral dust was frequently linked to industrial emissions from crude oil refineries, fertilizer industries as well as oil and coal power plants, located in the northern and north-western coast of the African continent (29%). Sea salt was also registered in the PM10 mass during most part of the sampling period, with a lower impact in the PM10 levels than the mineral dust one (26%). Combustion aerosols (6%) reached the highest mean values in summer as a consequence of the emissions from local-regional sources. Biomass burning aerosols produced from October to November in sub-sahelian latitudes, had a clear influence in the content of elemental carbon (EC) recorded at Cape Verde but a small impact in the PM10 total mass levels. A minor contribution to the PM10 mass has been associated to secondary inorganic compounds-SIC. Namely, ammonium sulfate and nitrate (SIC 1–5%) and calcium sulfate and nitrate (SIC 2–3%). The main origin of SIC 1 was attributed to emissions of SO2 and NOx from industrial sources located in the northern and north-western African coast and from wildfires produced in the continent. SIC 2 had a clear regional origin in the summer period. However, in the winter period there were probably contributions of soil emissions of evaporate minerals from regions of eastern Algeria. The location of Cape Verde in the Atlantic Ocean at subtropical latitudes, and the absence of relevant local sources of anthropogenic atmospheric pollutants, becomes this archipelago, a perfect site to study the impact of external contributions on the background levels of PM10 registered over the north-eastern tropical Atlantic.

Is long range transport of pollen in the NW Mediterranean basin influenced by Northern Hemisphere teleconnection patterns?

Climatic oscillations triggered by the atmospheric modes of the Northern Hemisphere teleconnection patterns have an important influence on the atmospheric circulation at synoptic scale in Western Mediterranean Basin. Simultaneously, this climate variability could affect a variety of ecological processes. This work provides a first assessment of the effect of North Atlantic Oscillation (NAO), Arctic Oscillation (AO) and Western Mediterranean Oscillation (WeMO) on the atmospheric long-range pollen transport episodes in the North-Eastern Iberian Peninsula for the period 1994–2011. Alnus, Ambrosia, Betula, Corylus and Fagus have been selected as allergenic pollen taxa with potential long-range transport associated to the Northern Hemisphere teleconnection patterns in the Western Mediterranean Basin. The results showed an increase of long range pollen transport episodes of: (1) Alnus, Corylus and Fagus from Western and Central Europe during the negative phase of annual NAO and AO; (2) Ambrosia, Betula and Fagus from Europe during the negative phase of winter WeMO; (3) Corylus and Fagus from Mediterranean area during the positive phase of the annual AO; and (4) Ambrosia from France and Northern Europe during the positive phase of winter WeMO. Conversely, the positive phase of annual NAO and AO are linked with the regional transport of Alnus, Betula and Corylus from Western Iberian Peninsula. The positive phase of annual WeMO was also positively correlated with regional transport of Corylus from this area.

High time and mass resolved PTR-TOF-MS measurements of VOCs at an urban site of India during winter: Role of anthropogenic, biomass burning, biogenic and photochemical sources

This study is based on the high mass and time-resolved measurements of seven VOCs using a PTR-TOF-MS instrument at an urban site of India during winter 2013. Daily levels of OVOCs and aromatics were in the ranges of 3.5–37ppbv and 0.85–23ppbv, respectively with OVOCs accounted for up to 80% of total measured VOCs. The impact of long-range transport from the polluted Indo-Gangetic Plain and clean Thar desert was observed during the episodes of high and low VOCs, respectively. VOCs exhibited strong diurnal variations with peaks during morning and evening hours and lowest in the afternoon. Relatively elevated aromatics during evening hours coincided with the lowest-OVOCs indicating influence of fresh vehicular emissions. Emission ratios of isoprene and OVOCs with respect to benzene followed the diurnal cycles of temperature and solar flux indicating role of biogenic and photochemical processes, respectively. Correlation study of VOCs with benzene suggests major contribution from anthropogenic and also from biogenic and secondary sources to some extent. The higher emission ratios of ∆methanol/∆acetonitrile correspond to the episodes of long-range transport from biomass burning sources located in the Indo-Gangetic Plain (IGP). In addition to the pattern of emission, the diurnal and day-to-day variations of VOCs were influenced by the local meteorological conditions and depth of planetary boundary layer (PBL).

Seasonal and Diurnal Variations of Fluorescent Bioaerosol Concentration and Size Distribution in the Urban Environment

A recently introduced fluorescence based real-time bioaerosol instrument, BioScout, and an ultraviolet aerodynamic particle sizer (UVAPS) were used to study fluorescent bioaerosol particles (FBAP) in the Helsinki metropolitan area, Finland, during winter and summer. Two FBAP modes at 0.5–1.5 µm (fine) and 1.5–5 µm (coarse) were detected during the summer, whereas the fine mode dominated in the winter. The concentration and proportion of the coarse FBAP was high in summer (0.028 #/cm 3 , 23%) and low in winter (0.010 #/cm 3 , 6%). Snow cover and low biological activity were assumed to be the main reasons for the low coarse FBAP concentration in the wintertime. Both the fine and the coarse FBAP fraction typically increased at nighttime during the summer. Correlations between the BioScout and the UVAPS were high with the coarse (R = 0.83) and fine (R = 0.92) FBAP. The BioScout showed 2.6 and 9.7 times higher detection efficiencies for the coarse and fine FBAP, respectively, compared to the UVAPS. A long-range transport episode of particles from Eastern Europe increased the fine FBAP concentration by over two orders of magnitude compared to the clean period in the winter, but these FBAP probably also included fluorescent non-biological particles. Correlation analysis indicates that local combustion sources did not generate fluorescent non-biological particles that can disturb fine FBAP counting. The results provide information that can be used to estimate health risks and climatic relevance of bioaerosols in the urban environment.

Sources of carbonaceous aerosol in the free troposphere

In this study concentrations of organic (OC) and elemental carbon (EC) from free tropospheric (FT) fine particulate matter (PM) were measured from March to September, 2012 with a Semi-Continuous OC/EC carbon aerosol analyzer at the top of Mt. Bachelor (2.8 km a.s.l) in Central Oregon, U.S. The average concentrations of OC and EC in the FT were low (OC: 1.87 ± 6.10, EC: 0.07 ± 0.26 μg m−3; average ± SD) but much higher during specific pollution episodes. During springtime the highest OC and EC concentrations were measured for dry free tropospheric air masses, whereas during summertime the highest OC and EC concentrations were typically measured for more humid air masses that were uplifted from the boundary layer (BL). The highest OC and EC concentrations were measured during biomass burning episodes (3 h average OC: up to 146.0 μg m−3, EC up to 5.5 μg m−3). Elevated OC and EC concentrations were also measured during Asian Long Range Transport (LRT) episodes (OC: up to 3.6 μg m−3, EC up to 1.1 μg m−3). In addition, between episodes, an increase in OC was seen in the afternoon, possibly due to SOA formation. This SOA can then be exported to the FT via diurnal ventilation of the BL. For Asian LRT episodes the OC/EC ratios varied between 8 and 34, with an average of 17.9. For local biomass burning emissions OC/EC ratios were between 25 and 30. Higher OC/EC ratios (30–40) were observed for the biomass burning plumes originating from longer distances, possibly due to SOA formation.

Seasonal variations in the chemical composition of particulate matter: a case study in the Po Valley. Part I: macro-components and mass closure

The seasonal variability in the mass concentration and chemical composition of atmospheric particulate matter (PM10 and PM2.5) was studied during a 2-year field study carried out between 2010 and 2012. The site of the study was the area of Ferrara (Po Valley, Northern Italy), which is characterized by frequent episodes of very stable atmospheric conditions in winter. Chemical analyses carried out during the study allowed the determination of the main components of atmospheric PM (macro-elements, ions, elemental carbon, organic matter) and a satisfactory mass closure was obtained. Accordingly, chemical components could be grouped into the main macro-sources of PM: soil, sea spray, inorganic compounds from secondary reactions, vehicular emission, organics from domestic heating, organics from secondary formation, and other sources. The more significant seasonal variations were observed for secondary inorganic species in the fine fraction of PM; these species were very sensitive to air mass age and thus to the frequency of stable atmospheric conditions. During the winter ammonium nitrate, the single species with the highest concentration, reached concentrations as high as 30 μg/m(3). The intensity of natural sources was fairly constant during the year; increases in natural aerosols were linked to medium and long-range transport episodes. The ratio of winter to summer concentrations was roughly 2 for combustion product, close to 3 for secondary inorganic species, and between 2 and 3 for organics. The winter increase of organics was due to poorer atmospheric dispersion and to the addition of the emission from domestic heating. A similar winter to summer ratio (around 3) was observed for the fine fraction of PM.

Influence of background particulate matter (PM) on urban air quality in the Pacific Northwest

Elevated particulate matter concentrations due to Asian long-range transport (LRT) are frequently observed in the free troposphere (FT) above the Pacific Northwest, U.S. Transport of this aerosol from the FT to the boundary layer (BL) and its effect to local air quality remain poorly constrained. We used data collected at the Mount Bachelor observatory (MBO, 2.8 km a.s.l) and from ground stations in the Pacific Northwest to study transport of fine particulate matter (PM) from the FT to the BL. During Asian LRT episodes PM concentrations were clearly elevated above the corresponding monthly averages at MBO as well as at low elevation sites across Washington and Oregon. Also, a clear correlation between MBO and low elevation sites was observed, indicating that LRT episodes are seen in both the FT and BL. In addition, drum impactor measurements show that the chemical composition of PM at MBO was similar to that measured at the BL sites. Using a simple regression model, we estimate that during springtime, when the transport from Asia is most effective, the contribution of Asian sources to PM2.5 in clean background areas of the Pacific Northwest was on average 1.7 μg m−3 (representing approximately 50–80% of PM). The influence of LRT PM was also seen in measurement stations situated in the urban and urban background areas. However, the fraction of LRT PM was less pronounced (36–50% of PM) due to larger local emissions in the urban areas.