Coarse Mode Research Articles

Aerosol pH and its driving factors in Beijing

Abstract. Aerosol acidity plays a key role in secondary aerosol formation. The high-temporal-resolution PM2.5 pH and size-resolved aerosol pH in Beijing were calculated with ISORROPIA II. In 2016–2017, the mean PM2.5 pH (at relative humidity (RH) > 30 %) over four seasons was 4.5±0.7 (winter) > 4.4±1.2 (spring) > 4.3±0.8 (autumn) > 3.8±1.2 (summer), showing moderate acidity. In coarse-mode aerosols, Ca2+ played an important role in aerosol pH. Under heavily polluted conditions, more secondary ions accumulated in the coarse mode, leading to the acidity of the coarse-mode aerosols shifting from neutral to weakly acidic. Sensitivity tests also demonstrated the significant contribution of crustal ions to PM2.5 pH. In the North China Plain (NCP), the common driving factors affecting PM2.5 pH variation in all four seasons were SO42-, TNH3 (total ammonium (gas + aerosol)), and temperature, while unique factors were Ca2+ in spring and RH in summer. The decreasing SO42- and increasing NO3- mass fractions in PM2.5 as well as excessive NH3 in the atmosphere in the NCP in recent years are the reasons why aerosol acidity in China is lower than that in Europe and the United States. The nonlinear relationship between PM2.5 pH and TNH3 indicated that although NH3 in the NCP was abundant, the PM2.5 pH was still acidic because of the thermodynamic equilibrium between NH4+ and NH3. To reduce nitrate by controlling ammonia, the amount of ammonia must be greatly reduced below excessive quantities.

Optically effective complex refractive index of coated black carbon aerosols: from numerical aspects

Abstract. Aerosol complex refractive index (ACRI) is an important microphysical parameter used for the studies of modeling their radiative effects. With considerable uncertainties related to retrieval based on observations, a numerical study is a powerful method, if not the only one, to provide a better and more accurate understanding of retrieved optically effective ACRIs of aged black carbon (BC) particles. Numerical investigations of the optically effective ACRIs of polydisperse coated BC aggregates retrieved from their accurate scattering and absorption properties, which are calculated by the multiple-sphere T-matrix method (MSTM), without overall particle shape variations during retrieval, are carried out. The aim of this study is to evaluate the effects of aerosol microphysics, including shell∕core ratio Dp∕Dc, BC geometry, BC position inside coating, and size distribution, on retrieved optically effective ACRIs of coated BC particles. At odds with expectations, retrieved optically effective ACRIs of coated BC particles in coarse mode are not merely impacted by their chemical compositions and shell∕core ratio, being highly complicated functions of particle microphysics. However, in accumulation mode, the coated BC optically effective ACRI is dominantly influenced by particle chemical compositions and the shell∕core ratio. The popular volume-weighted average (VWA) method and effective medium theory (EMT) provide acceptable ACRI results for coated BC in accumulation mode, and the resulting uncertainties in particle scattering and absorption are both less than approximately 10 %. For coarse coated BC, the VWA and EMT, nevertheless, produce dramatically higher imaginary parts than those of optically effective ACRIs, significantly overestimating particle absorption by a factor of nearly 2 for heavily coated BC with a large BC fractal dimension or BC close to the coating boundary. Using the VWA could introduce significant overestimation in aged BC absorption analysis studies, and this may be one of the reasons why modeled aerosol optical depth is 20 % larger than observed, since it is widely employed in the state-of-the-art aerosol–climate models. We propose a simple new ACRI parameterization for fully coated BC with Dp/Dc≥2.0 in coarse mode, which can serve as a guide for the improvement of ACRIs of heavily coated BC, and its scattering and absorption errors are reduced by a factor of nearly 2 compared to the VWA. Our study indicates that a reliable estimate of the radiative effects of aged BC particles in coarse mode would require accounting for the optically effective ACRI, rather than the ACRI given by the VWA, in aerosol–climate models.

Pseudo-simultaneous measurements for the spatial-temporal characteristics of accumulation and coarse mode particles near an urban viaduct within street canyons

This study aimed to assess the effects of viaduct settings, meteorological and traffic conditions on the vertical and horizontal distributions of particle number concentrations (PNCs) and particle number size distributions (PNSDs) for accumulation and coarse mode particles around a viaduct in street canyons with different aspect ratios (building height/street width, H/W = 1 and 1.8). Two field measurements were conducted in a built-up block in Guangzhou, China, including vertical locations and various traffic hotspots. The PNCs in the wide street canyon decayed monotonously with increasing height, while those in the deep street canyon decreased first and then increased above the viaduct due to the impact of the viaduct structure and an extra elevated vehicle emission. The frequent stop-and-go behavior of vehicles resulted in 22.8% higher PNCs at the bus stop (BS) compared to the averaged PNCs far from the BS in the deep street canyon. The PNCs at the traffic intersection (TI) with favorable ventilation were slightly lower than those in the wide street canyon. The PNSDs at all traffic hotspots were similar in shape with the same peak values (0.28 and 1.6 μm) in accumulation and coarse modes. Under low wind speeds, the hourly averaged vertical profiles were consistent with those of daily averaged PNCs, while at higher wind speeds, no obvious rule was observed due to unsteady flow field. These findings are useful for understanding exposure in urban areas.

Global transport of dust emitted from different regions of the Sahara

An improved understanding of how emissions from different regions are transported globally is necessary to establish stronger causal relationships between particulate matter emissions and global scale circulation. This study uses the GEOS-Chem global chemical transport model to identify spatial differences in the transport of Saharan dust emitted from different regions in Northern Africa, during different seasons, and in different particle size modes. This study finds noticeable spatial differences in the transport of dust emitted from each region of the Sahara and during each season. Dust emitted from the northern regions of the Sahara undergoes 25% more latitudinal and 2% more longitudinal transport than dust emitted from the more southern regions. Dust originating from the central regions of the Sahara experiences 28% and 11% more longitudinal transport than dust from western and eastern regions, respectively. Dust emitted from the Sahara during the period from April to June travels 10%, 25%, and 17% percent further latitudinally, longitudinally, and vertically, respectively, than the annual average. As expected coarse mode particles travel shorter distances than fine mode particles. Rather than determining the regions which produce the most dust, we focus on identifying the regions producing the dust that is most extensively transported.

Evaluation of epifluorescence methods for quantifying bioaerosols in fine and coarse particulate air pollution

Despite being recognized as an important part of particulate matter (PM) air pollution and health risk, bioaerosols have not been quantified as extensively as other PM components for establishing PM standards and management strategies. The challenge lies partly in the lack of practical measurement methods. This study evaluated a filter-based, direct-staining fluorescence microscopy (DS-FM) method that may be adapted to routine air quality monitoring for bioaerosol concentration and size distribution. Through testing with bioaerosol standards made of bacterial cells and fungal spores, the method is shown to have precision, accuracy, detection limit, and dynamic range suitable for most ambient environments. DS-FM was demonstrated with PM samples from an arid urban location in Las Vegas, Nevada during the spring allergy season. Detectable bioaerosols ranged from 0.37 to 16 μm in geometric diameter and averaged 0.27 ± 0.23 cm−3 in number concentration with about 2/3 and 1/3 in the fine (≤2.5 μm) and coarse (>2.5 μm) mode, respectively. The bioaerosol mass, estimated from the size distribution and an assumed density, was mainly in the coarse mode and accounted for 17 ± 11% of PM10, 20 ± 13% of PM10-2.5, and 4 ± 3% of PM2.5 mass. Rain and high wind speeds appeared to elevate bioaerosol levels. Other advantages of DS-FM include low sample consumption and short turnaround times; a large amount of data can be generated by incorporating the measurement into current long-term air quality networks. Suggestions for using the data to inform bioaerosol origins, contributions, and public health impacts are discussed.

PM2.5 vertical variation during a fog episode in a rural area of the Yangtze River Delta, China

A dense radiation fog event occurred at the Shouxian site, Anhui Province, China, from the evening of January 2 to noon on January 3, 2017. During this event, vertical profiles of particulate matter (PM) and meteorological parameters within the lower troposphere (0–1000 m) were collected using a tethered balloon. This study assessed the evolution of the PM2.5 profile with the planetary boundary layer (PBL) structure and the effects of fog on the PM2.5 concentration. The results showed the following: (1) At the surface, the average diurnal variation in Aitken mode, accumulation mode and coarse mode particles had bimodal patterns before fog formation and was mainly influenced by diurnal variation in the mixing level depth (MLD). The aerosol number concentrations decreased remarkably, and the PM2.5 was strongly scavenged from 150 μg/m3 to 45 μg/m3 during the fog process. (2) In the vertical direction, the PM2.5 distribution was affected by the PBL height and the vertical fog structure. At 05:00 LT (local time) (i.e., early morning before the fog event), the PM2.5 concentration was slightly higher in the stable layer (260 μg/m3) than in the residual layer (200 μg/m3). At 14:00 LT (haze period), PM2.5 was well mixed below 500 m, with a concentration of 310 μg/m3. After 20:00 LT, when fog formed, PM2.5 was scavenged from the surface to the upper layers, and the scavenging height was controlled by the fog top height. (3) The vertical development of fog was promoted by turbulent mixing and radiation cooling at the fog top. Turbulent mixing enhanced the particle scavenging efficiency of fog droplets by the collision-coalescence process. The PM2.5 scavenging height was corresponded to the turbulence height. Therefore, turbulence development in the fog was the essential dynamic factor driving PM2.5 reduction.

The impact of sea-salt aerosols on particulate inorganic nitrogen deposition in the western Taiwan Strait region, China

Sea-salt aerosol (SSA) has significant impact on the formation of secondary nitrate and its dry deposition in the coastal region. In this study, size-segregated aerosols were collected for water-soluble ions analysis in the coastal cities and sea cruises in the western Taiwan Strait of China from 2016 to 2017 to investigate the impact of SSA on particulate inorganic N deposition. The size distributions of NH4+ were characterized by a unimodal pattern peaking at 0.44–1.0 μm while NO3− exhibited typical bimodal distributions peaking at 0.44–1.0 μm and 2.5–10 μm. Furthermore, the fine mode peaks of NO3− became insignificant or disappeared in summer and fall due to the volatilization and dissociation of NH4NO3 at higher temperatures. Elevated sea-salt contributions in the largest size bin (>16 μm) during sea cruises were attributed to vessel induced wave breaking, while there was no peak for NO3− at this range due to its short atmospheric lifetime. Although the concentrations of particulate NH4+-N showed higher levels than those of NO3−-N for both coastal and marine aerosols, the dry deposition fluxes of the particulate NO3−-N were significant higher than those of the particulate NH4+-N due to their different size distributions. The contribution of SSA to particulate inorganic N deposition was estimated on average to be 20.33% and 36.91% in the coastal and offshore region, respectively, assuming all of the chlorine depletion was caused by the reaction between nitric acid and SSA in the coarse mode. In addition, dry deposition fluxes using constant deposition velocities for fine/coarse particles and NH4+/NO3− were higher than those using size-dependent dry deposition velocities. Considering the increasing human activities at the coastal urban zone in the western Taiwan Strait, the atmospheric N deposition could have an important ecological impact on the marine environment that is already under pressure from riverine inputs and urban waste discharges.

Impact of New Year's Eve fireworks on the size resolved element distributions in airborne particles

New Year's Eve fireworks represent one of the most unusual atmospheric pollution events in the course of a year. A majority of particles aerosolized by burning of fireworks consist of metals and their compounds used in firework displays. In this study, ambient particulate matter was sampled using a 14 stage cascade impactor in two European cities during turn of the years 2016/17 and 2017/18. Concentrations of 33 elements were determined by inductively coupled plasma mass spectrometry. To assess the impact of New Year's Eve fireworks on ambient air quality, chemically resolved size distributions of particles with diameters between 15 nm and 10 μm collected during fireworks episodes were compared to ones collected in normal winter weeks. For some metals a distinct shift of their concentration maximum related to fireworks could be observed, which is in between the maxima for accumulation and coarse mode particles. Concentrations of these elements (Sr, Ba, Mg, Bi, Al, Cu and K) were also higher during weeks with fireworks episodes than during control weeks. Although New Year's Eve fireworks only take place once a year, these results show that air pollution caused by fireworks can be a potential health risk, especially for people with pre-existing diseases.

A Theoretical Analysis for Improving Aerosol-Induced CO2 Retrieval Uncertainties Over Land Based on TanSat Nadir Observations Under Clear Sky Conditions

Aerosols significantly affect carbon dioxide (CO2) retrieval accuracy and precision by modifying the light path. Hyperspectral measurements in the near infrared and shortwave infrared (NIR/SWIR) bands from the generation of new greenhouse gas satellites (e.g., the Chinese Global Carbon Dioxide Monitoring Scientific Experimental Satellite, TanSat) contain aerosol information for correction of scattering effects in the retrieval. Herein, a new approach is proposed for optimizing the aerosol model used in the TanSat CO2 retrieval algorithm to reduce CO2 uncertainties associated with aerosols. The weighting functions of hyperspectral observations with respect to elements in the state vector are simulated by a forward radiative transfer model. Using the optimal estimation method (OEM), the information content and each component of the CO2 column-averaged dry-air mole fraction (XCO2) retrieval errors from the TanSat simulations are calculated for typical aerosols which are described by Aerosol Robotic Network (AERONET) inversion products at selected sites based on the a priori and measurement assumptions. The results indicate that the size distribution parameters (reff, veff), real refractive index coefficient of fine mode (arf) and fine mode fraction (fmf) dominate the interference errors, with each causing 0.2–0.8 ppm of XCO2 errors. Given that only 4–7 degrees of freedom for signal (DFS) of aerosols can be obtained simultaneously and CO2 information decreases as more aerosol parameters are retrieved, four to seven aerosol parameters are suggested as the most appropriate for inclusion in CO2 retrieval. Focusing on only aerosol-induced XCO2 errors, forward model parameter errors, rather than interference errors, are dominant. A comparison of these errors across different aerosol parameter combination groups reveals that fewer aerosol-induced XCO2 errors are found when retrieving seven aerosol parameters. Therefore, the model selected as the optimal aerosol model includes aerosol optical depth (AOD), peak height of aerosol profile (Hp), width of aerosol profile (Hw), effective variance of fine mode aerosol (vefff), effective radius of coarse mode aerosol (reffc), coefficient a of the real part of the refractive index for the fine mode and coarse mode (arf and arc), with the lowest error of less than 1.7 ppm for all aerosol and surface types. For marine aerosols, only five parameters (AOD, Hp, Hw, reffc and arc) are recommended for the low aerosol information. This optimal aerosol model therefore offers a theoretical foundation for improving CO2 retrieval precision from real TanSat observations in the future.

Comparison of size-resolved hygroscopic growth factors of urban aerosol by different methods in Tianjin during a haze episode

Size-resolved hygroscopic growth factors of urban aerosol during a haze episode were measured using a Humidified Tandem Differential Mobility Analyzer (HTDMA) (gm(RH)). These factors were also derived from size-resolved particulate chemical composition combined with the κ-Köhler theory (gκ(RH)) and the thermodynamic model ISORROPIA-II running in forward mode (giso-f(RH)) and reverse mode (giso-r(RH)), respectively. In terms of agreement among these hygroscopic growth factors, gκ(RH) matched gm(RH) best, followed by giso-r(RH). In contrast, giso-f(RH) demonstrated a poorer agreement with gm(RH). The good consistency among gm(RH), gκ(RH), and giso-r(RH) was because they only focus on the physical hygroscopic process, whereas giso-f(RH) contains not only the direct influence of relative humidity (RH) on particle size but also the influence of gaseous precursor on the particle chemical composition, which indirectly affects the hygroscopicity of the particles. In this sense, size-resolved gκ(RH) and giso-r(RH) in a wide size range are more adequate to investigate the impact of RH on light scattering and aerosol radiative forcing. At RH = 80%, gκ(RH) for accumulation mode particles was 1.30–1.45 on polluted days and higher than that on clean days (1.2–1.3). Whereas on both polluted and clean days, gκ(RH) of ultrafine and coarse mode particles were generally lower than 1.25. The strong hygroscopicity of accumulation mode particles observed on polluted days can deteriorate visibility due to their high extinction efficiency.

The Fundamental Aerosol Models Over China Region: A Cluster Analysis of the Ground‐Based Remote Sensing Measurements of Total Columnar Atmosphere

AbstractTen fundamental aerosol models in China are derived from a cluster study based on the ground‐based remote sensing measurements of Sun‐sky radiometer Observation NETwork. The aerosol size distribution decomposition techniques are employed to yield individual fine and coarse mode size distribution functions with independent refractive indices. The total 10,773 records containing 18 kinds of aerosol microphysical parameters are used to yield 10 typical clusters with the verification of clustering robustness. Ten clusters suggest five typical fine particle aerosol models including urban polluted, secondary polluted, combined polluted, polluted fly ash, and continental background, as well as five coarse models including summer fly ash, winter fly ash, primary dust, transported dust, and background dust over China region. The representativeness and coappearance analyses again reveal five dominative aerosol patterns on the base of fundamental models. These models can be used in the chemical model simulation, satellite remote sensing, climate, and environment analyses.

Effect of air masses motion on the rapid change of aerosols in marine atmosphere

The impact of air masses motion on marine aerosol properties was investigated using an on-board single particle mass spectrometer (SPAMS) deployed for the determination of single particle size resolved chemical composition over Southeast China Sea. Two aerosol blooms (E1 and E2) were observed during the cruise. High average particle number count occurred in E1 (7320), followed by E2 (5850), which was more than 100–150 times of the average particle number count during normal periods. Particles were classified as four major sources, including continental source, shipping source, marine source, and transport source based on the mass spectral similarity. Transport source was identified as those particles with high particle number count occurred only during aerosol bloom period. Three sub-types of EC-Ca, OC-Ca, and Al-rich were classified as transport source. EC-Ca was the dominant particles of the transport source, accounting for more than 70% of the total particles in aerosol bloom events. A uni-modal size distribution in the size range of 0.1–2.0 μm was observed during normal period, while a bimodal distribution with a tiny mode (<0.3 μm) and a coarse mode between 0.4 and 0.6 μm was present during aerosol bloom. The variation of aerosol source is consistent with air masses back trajectories, for the reason that most of the long-range air trajectories are from the ocean, while short air trajectories originate in the continental regions, which means that air masses have a significant impact on the aerosol physical–chemical properties along their tracks.

Below-cloud scavenging of size-segregated aerosols and its effect on rainwater acidity and nutrient deposition: A long-term (2009–2018) and real-time observation over eastern Himalaya

The major removal pathway of atmospheric aerosols is the below cloud scavenging. The present study is the first-ever in the world, where long-term (2009–2018) as well as real-time observations on the below-cloud scavenging of ultrafine (<0.4 μm), superfine (0.4–1.0 μm) and coarse mode (>1 μm) aerosols have been made. The study was conducted with 919 rain events over a high altitude Himalayan station (27.01 °N, 88.15 °E, 2200 m amsl) in India. The other factors were normalized in order to investigate the “rain only” effect and therefore 919 rain events were screened and finally 165 events were studied. We determined threshold values of the rain rate (and duration) above which aerosols are scavenged in very high proportion (>75%) irrespective of the duration (and rain rate). These threshold values decrease as the aerosol size increases. For example, threshold rain rate decreases from ~17 mm h−1 to ~8 mm h−1 as the aerosol size increases from ultrafine to coarse mode. We also showed that how the rainwater acidity and the deposition flux of major inorganic nutrients (NH4+ + NO3− + SO42−) vary with the rain rate and duration. We observed that the rains either >12 mm h−1 or >80 min are all acidic. Maximum nutrients were accumulated in the ultrafine aerosols and hence the spectrum of the deposition flux of the nutrients (with rain rate and duration) was similar to the scavenging spectrum of ultrafine aerosol. Such long-term database enables us to quantitatively predict the aerosol scavenging, acid rains and nutrient deposition which showed excellent agreement with the observed results. Such quantitative prediction would in turn help the researchers to predict the rain-induced changes in air quality as well as any bio-geo chemical parameter. The present study bears paramount importance in Himalayan context as well as any ecologically-rich regions.

Field measurements on particle size distributions and emission characteristics of PM10 in a cement plant of China

The particle size distributions (PSDs) and the characteristics of the particulate matter (PM) from cement plants are of importance for the air pollution control strategies. The study focuses on the PSDs and emission characteristics of PM10 in the flue gas from the preheater and in the hot air out from the clinker cooler in a cement plant equipped with a conditioning tower, two raw mills, a gravity settler and two fabric filters. A Dekati low pressure impactor (DLPI) with a cyclone was performed to sample the PM10 at the inlet/outlet of the above devices. The PM10 at the outlet of SP boiler and at the inlet of the gravity settler both present a bi-modal distribution, with a fine mode at 0.1–0.4 μm and a coarse mode at 1.0–5.0 μm. The PM10 emitted from the flue gas and the hot air into the atmosphere both present a tri-modal distribution, with a fine mode at 0.2–0.5/0.06–0.2 μm, a middle mode at 0.5–2.5/0.2–0.6 μm and a coarse mode at 5.0–10.0/3.0–6.0 μm. The mass concentration of PM2.5 increases by 17.74% at the outlet of the raw mill due to the mechanical fragmentation and flow transport. The spraying and cooling of the conditioning tower promote the agglomeration and growth of fine particulates, and reduce the concentration proportion of PM2.5 to PM10 from 89.40 to 70.93%. The emission factors of PM10, PM2.5 and PM1 in the flue gas are 0.017–0.031, 0.005–0.015 and 0.002–0.007 kg/t, respectively; while they are 0.005, 0.003 and 0.001 in the hot air, respectively.

Optimal Estimation Retrieval of Aerosol Fine-Mode Fraction from Ground-Based Sky Light Measurements

In this paper, the feasibility of retrieving the aerosol fine-mode fraction (FMF) from ground-based sky light measurements is investigated. An inversion algorithm, based on the optimal estimation (OE) theory, is presented to retrieve FMF from single-viewing multi-spectral radiance measurements and to evaluate the impact of utilization of near-infrared (NIR) measurements at a wavelength of 1610 nm in aerosol remote sensing. Self-consistency tests based on synthetic data produced a mean relative retrieval error of 4.5%, which represented the good performance of the OE inversion algorithm. The proposed algorithm was also performed on real data taken from field experiments in Beijing during a haze pollution event. The correlation coefficients (R) for the retrieved aerosol volume fine-mode fraction (FMFv) and optical fine-mode fraction (FMFo) against AErosol RObotic NETwork (AERONET) products were 0.94 and 0.95 respectively, and the mean residual error was 4.95%. Consequently, the inversion of FMFv and FMFo could be well constrained by single-viewing multi-spectral radiance measurement. In addition, by introducing measurements of 1610 nm wavelength into the retrieval, the validation results showed a significant improvement in the R value for FMFo (from 0.89–0.94). These results confirm the high value of NIR measurements for the retrieval of coarse mode aerosols.

Impact of a Dust Event on the Size Distribution of Metal Elements in Atmospheric Aerosols at a Coastal Region and over the Ocean

Size-segregated aerosol samples were collected in the Qingdao coastal region and over the East Sea and the Pacific Northwest from March to April in 2016, and the concentration of metal elements (Al, Na, Fe, Mg, K, Cu, Pb, and Cd) were analyzed. The characteristics and the variation in the size distribution of the metal elements in the samples during dusty weather were discussed, as well as the sources and factors that influence the metals. The results indicated that the concentration of Al, Fe, K, Na, Cu, and Pb in the aerosols in the Qingdao coastal samples increased significantly by 85%-1400%, while the concentration of Cd decreased by 8%; the concentrations of Al, Fe, K, Cu, Pb, and Cd in the samples over the ocean increased by 163%-4580%, while the concentration of Na decreased by 62% during dusty weather. In addition, the size distribution of K changed from a double-peak to a coarse peak mode; that of Pb and Cd changed from a fine peak mode to a double-peak distribution. Cu still presented a double-peak distribution in which the proportion of coarse particles increased; Al, Fe, Mg, and Na showed little variation in their size distribution with a single peak in coarse mode. The dust event had different effects on the size distributions of the metal elements in the coastal and marine aerosol samples. The size distribution of the metal elements (Al, Fe, Mg, Na, and K) in the aerosols collected at the Qingdao coastal site had one peak of 2.1-3.3 μm during dusty weather; the peak moved to 3.3-4.7 μm for marine samples. In addition, the proportion of Al, Fe, Mg, Na, K, Pb, and Cd in the coarse mode decreased by 1%-35% for dust aerosol samples collected in Qingdao coastal region, whereas the proportion of these metals in coarse mode increased by 4%-33% for samples collected over the East Sea. This difference may be related to the hygroscopic growth, transport path, and height of the dust.

Different metrics (number, surface area, and volume concentration) of urban particles with varying sizes in relation to fractional exhaled nitric oxide (FeNO).

There have been increasing concerns on potential health effects of ultrafine particles (UFP); but little is known as to what are the most biologically relevant metrics for these particles that make up very little mass concentration. We examined a range of particle metrics (number, surface area, active surface area, and volume concentration) in relation to fractional exhaled nitric oxide (FeNO), a well-established biomarker of pulmonary inflammation. We conducted a panel study in 17 non-asthmatic children who attended schools and resided near a monitoring site at which particles in the size range of 3-800 nm were measured using a TDMPS and particles in the size range of 0.5 to 10 µm were measured using an APS. Particles were classified by size into the nucleation, Aitken, accumulation, or coarse mode, respectively, for calculating mode-specific number, surface area, active surface area, and volume concentrations. Each participating child was measured for FeNO daily for 30 days. We used linear mixed-effects models to assess the associations between various particle metrics and FeNO. In terms of number concentration, ambient particles in the Aitken mode and in the accumulation mode were significantly and positively associated with FeNO; but particles in the nucleation mode were significantly and negatively associated with FeNO. Moreover, UFP as a lump sum of both nucleation-mode and Aikten-mode particles did not show a significant association with FeNO. In terms of surface area concentration, ambient particles only in the accumulation mode were significantly and positively associated with FeNO. In terms of volume concentration, ambient particles in both the accumulation mode and the coarse mode were significantly and positively associated with FeNO. Analyses of the relationships between FeNO and metrics for particles deposited in the respiratory tract generated consistent findings, showing a negative association for the number concentration of deposited particles (driven by nucleation-mode particles), a positive association for the surface area concentration of deposited particles (driven by accumulation-mode particles), and a positive association for the volume concentration of deposited particles (driven by accumulation-mode and coarse-mode particles). Particles contributing largely to the surface area concentration and/or the volume concentration of ambient particles or particles deposited in the respiratory tract had a significant positive association with pulmonary inflammation. Nucleation-mode particles, that have large number concentrations but contribute little to the surface area or volume concentration of ambient or deposited particles, had a significant negative association with FeNO. This may indicate a different biological process or may simply be due to the negative and strong correlation between nucleation-mode and accumulation-mode particles. Given that particles in different modes may have different biological actions, measuring UFP as a whole may not necessarily be useful from a biological effect standpoint.

Abundance and Diurnal Trends of Fluorescent Bioaerosols in the Troposphere over Mt. Tai, China, in Spring

AbstractPrimary biological aerosol particles are ubiquitous in the global atmosphere and can affect cloud formation, deteriorate air quality, and cause human infections. Mt. Tai (1,534 m a.s.l.) is an elevated site in the North China Plain where atmospheric aerosols reflect both regional advection and long‐range transport. In this study, we deployed a Wideband Integrated Bioaerosol Sensor (WIBS‐4A) and collected total suspended particles and eight‐stage size‐segregated aerosol samples at the summit of Mt. Tai in spring from 21 March to 8 April 2017 to quantify the abundance, size distributions, and diurnal variations of fluorescent bioaerosols and to investigate the effect of different fluorescence thresholds of WIBS for ambient bioaerosol recognition. During the whole sampling period, the number concentration of fluorescent particles (&gt;0.8 μm) was 647 ± 533 L−1, accounting for 26.9% ± 10.0% by number of the total particles in that size range. Three‐dimensional excitation‐emission matrix fluorescence of water‐soluble organic matter in size‐segregated aerosols shows that humic‐like substances (HULIS) are mainly in the fine mode (&lt;2.1 μm) while protein‐like substances are mainly in the coarse mode (&gt;2.1 μm). From the diurnal variation, it is shown that bioaerosols can undergo transformation during long‐range transport and contribute to HULIS. For bioaerosol recognition, we find that 6σ‐threshold can lead to better classification of fluorescent aerosol particles for fungal spores. Overall, our results constrain the abundance of primary bioaerosols in the troposphere over East Asia and elucidate the processes for their evolution via mountain/valley breezes and long‐range transport.

Inferring Fine-Mode and Coarse-Mode Aerosol Complex Refractive Indices from AERONET Inversion Products over China

Detailed knowledge of the complex refractive indices (m) of fine- and coarse-mode aerosols is important for enhancing understanding of the effect of atmospheric aerosol on climate. However, studies on obtaining aerosol modal m values are particularly scarce. This study proposes a method for inferring m values of fine- and coarse-mode aerosol using the inversion products from the AERONET ground-based aerosol robotic network. By identifying the aerosol type, modal m values are constrained and then inferred based on a maximum likelihood method. Numerical tests showed that compared with the reference values, our method slightly overestimates the real parts of the refractive indices (n), but underestimates the imaginary parts (k) by 2.11% ± 11.59% and 8.4% ± 26.42% for fine and coarse modes, respectively. We applied this method to 21 AERONET sites around China, which yielded annual mean m values of (1.45 ± 0.04) + (0.0109 ± 0.0046)i and (1.53 ± 0.01) + (0.0039 ± 0.0011)i for fine- and coarse-mode aerosols, respectively. It is observed that the fine mode n decreased from 1.53 to 1.39 with increasing latitude, while fine mode k values were generally larger than 0.008 over most of China. The coarse-mode n and k ranged from 1.52 to 1.56 and from 0.002 to 0.006, respectively.

Model simulation of ammonium and nitrate aerosols distribution in the Euro-Mediterranean region and their radiative and climatic effects over 1979–2016

Abstract. Aerosols play an important role in Europe and the Mediterranean area where different sources of natural and anthropogenic particles are present. Among them ammonium and nitrate (A&amp;amp;N) aerosols may have a growing impact on regional climate. In this study, their representation in coarse and fine modes has been introduced in the prognostic aerosol scheme of the ALADIN-Climate regional model. This new aerosol scheme is evaluated over Europe and the Mediterranean Sea, using two twin simulations over the period 1979–2016 with and without A&amp;amp;N aerosols. This evaluation is performed at local and regional scales, using surface stations and satellite measurements. Despite an overestimate of the surface nitrate concentration, the model is able to reproduce its spatial pattern including local maxima (Benelux, Po Valley). Concerning the simulated aerosol optical depth (AOD), the inclusion of A&amp;amp;N aerosols significantly reduces the model bias compared to both AERONET stations and satellite data. Our results indicate that A&amp;amp;N aerosols can contribute up to 40 % of the total AOD550 over Europe, with an average of 0.07 (550 nm) over the period 2001–2016. Sensitivity studies suggest that biases still present are related to uncertainties associated with the annual cycle of A&amp;amp;N aerosol precursors (ammonia and nitric acid). The decrease in sulfate aerosol production over Europe since 1980 produces more free ammonia in the atmosphere leading to an increase in A&amp;amp;N concentrations over the studied period. Analyses of the different aerosol trends have shown for the first time to our knowledge that, since 2005 over Europe, A&amp;amp;N AOD550 and A&amp;amp;N shortwave (SW) direct radiative forcing (DRF) are found to be higher than sulfate and organics, making these the species with the highest AOD and the highest DRF. On average over the period 1979–2016, the A&amp;amp;N DRF is found to be about −1.7 W m−2 at the surface and −1.4 W m−2 at the top of the atmosphere (TOA) in all sky conditions over Europe, with regional maxima located at the surface over the Po Valley (−5 W m−2). Finally, the dimming effect of A&amp;amp;N aerosols is responsible for a cooling of about −0.2∘ C over Europe (summer), with a maximum of −0.4 ∘C over the Po Valley. Concerning precipitation, no significant impact of A&amp;amp;N aerosols has been found.