Particle Growth Events Research Articles

Aerosol chemistry and particle growth events at an urban downwind site in North China Plain

Abstract. The North China Plain (NCP) has experienced frequent severe haze pollution events in recent years. While extensive measurements have been made in megacities, aerosol sources, processes, and particle growth at urban downwind sites remain less understood. Here, an aerosol chemical speciation monitor and a scanning mobility particle sizer, along with a suite of collocated instruments, were deployed at the downwind site of Xingtai, a highly polluted city in the NCP, for real-time measurements of submicron aerosol (PM1) species and particle number size distributions during May and June 2016. The average mass concentration of PM1 was 30.5 (±19.4) µg m−3, which is significantly lower than that during wintertime. Organic aerosols (OAs) constituted the major fraction of PM1 (38 %), followed by sulfate (25 %) and nitrate (14 %). Positive matrix factorization with the multilinear engine version 2 showed that oxygenated OA (OOA) was the dominant species in OA throughout the study, on average accounting for 78 % of OA, while traffic and cooking emissions both accounted for 11 % of OA. Our results highlight that aerosol particles at the urban downwind site were highly aged and mainly from secondary formation. However, the diurnal cycle also illustrated the substantial influence of urban emissions on downwind sites, which are characterized by similar pronounced early morning peaks for most aerosol species. New particle formation and growth events were also frequently observed (58 % of the time) on both clean and polluted days. Particle growth rates varied from 1.2 to 4.9 nm h−1 and our results showed that sulfate and OOA played important roles in particle growth during clean periods, while OOA was more important than sulfate during polluted events. Further analyses showed that particle growth rates have no clear dependence on air mass trajectories.

Differentiating between particle formation and growth events in an urban environment

Abstract. Small aerosols at a given location in the atmosphere often originate in situ from new particle formation (NPF). However, they can also be produced and then transported from a distant location to the point of observation where they may continue to grow to larger sizes. This study was carried out in the subtropical urban environment of Brisbane, Australia, in order to assess the relative occurrence frequencies of NPF events and particle growth events with no NPF. We used a neutral cluster and air ion spectrometer (NAIS) to monitor particles and ions in the size range 2–42 nm on 485 days, and identified 236 NPF events on 213 days. The majority of these events (37 %) occurred during the daylight hours with just 10 % at night. However, the NAIS also showed particle growth with no NPF on many nights (28 %). Using a scanning mobility particle sizer (SMPS), we showed that particle growth continued at larger sizes and occurred on 70 % of nights, typically under high relative humidities. Most particles in the air, especially near coastal locations, contain hygroscopic salts such as sodium chloride that may exhibit deliquescence when the relative humidity exceeds about 75 %. The growth rates of particles at night often exceeded the rates observed during NPF events. Although most of these night time growth events were preceded by day time NPF events, the latter was not a prerequisite for growth. We conclude that particle growth in the atmosphere can be easily misidentified as NPF, especially when they are monitored by an instrument that cannot detect them at the very small sizes.

Particle growth in an isoprene-rich forest: Influences of urban, wildfire, and biogenic air masses

Growth of freshly nucleated particles is an important source of cloud condensation nuclei (CCN) and has been studied within a variety of environments around the world. However, there remains uncertainty regarding the sources of the precursor gases leading to particle growth, particularly in isoprene-rich forests. In this study, particle growth events were observed from the 14 total events (31% of days) during summer measurements (June 24 – August 2, 2014) at the Program for Research on Oxidants PHotochemistry, Emissions, and Transport (PROPHET) tower within the forested University of Michigan Biological Station located in northern Michigan. Growth events were observed within long-range transported air masses from urban areas, air masses impacted by wildfires, as well as stagnant, forested/regional air masses. Growth events observed during urban-influenced air masses were prevalent, with presumably high oxidant levels, and began midday during periods of high solar radiation. This suggests that increased oxidation of biogenic volatile organic compounds (BVOCs) likely contributed to the highest observed particle growth in this study (8 ± 2 nm h−1). Growth events during wildfire-influenced air masses were observed primarily at night and had slower growth rates (3 ± 1 nm h−1). These events were likely influenced by increased SO2, O3, and NO2 transported within the smoke plumes, suggesting a role of NO3 oxidation in the production of semi-volatile compounds. Forested/regional air mass growth events likely occurred due to the oxidation of regionally emitted BVOCs, including isoprene, monoterpenes, and sesquiterpenes, which facilitated multiday growth events also with slower rates (3 ± 2 nm h−1). Intense sulfur, carbon, and oxygen signals in individual particles down to 20 nm, analyzed by transmission electron microscopy with energy dispersive X-ray spectroscopy (TEM-EDX), suggest that H2SO4 and secondary organic aerosol contributed to particle growth. Overall, aerosol growth was frequently observed in a range of air masses (urban, wildfire, forested) and oxidant conditions (day vs. night), with rates ranging from 0.8 to 10.2 nm h−1.

Growth rates of fine aerosol particles at a site near Beijing in June 2013

Growth of fine aerosol particles is investigated during the Aerosol–CCN–Cloud Closure Experiment campaign in June 2013 at an urban site near Beijing. Analyses show a high frequency (∼ 50%) of fine aerosol particle growth events, and show that the growth rates range from 2.1 to 6.5 nm h−1 with a mean value of ∼ 5.1 nm h−1. A review of previous studies indicates that at least four mechanisms can affect the growth of fine aerosol particles: vapor condensation, intramodal coagulation, extramodal coagulation, and multi-phase chemical reaction. At the initial stage of fine aerosol particle growth, condensational growth usually plays a major role and coagulation efficiency generally increases with particle sizes. An overview of previous studies shows higher growth rates over megacity, urban and boreal forest regions than over rural and oceanic regions. This is most likely due to the higher condensational vapor, which can cause strong condensational growth of fine aerosol particles. Associated with these multiple factors of influence, there are large uncertainties for the aerosol particle growth rates, even at the same location.

Simultaneous measurements of particle number size distributions at ground level and 260 m on a meteorological tower in urban Beijing, China

Abstract. Despite extensive studies into the characterization of particle number size distributions at ground level, real-time measurements above the urban canopy in the megacity of Beijing have never been performed to date. Here we conducted the first simultaneous measurements of size-resolved particle number concentrations at ground level and 260 m in urban Beijing from 22 August to 30 September. Our results showed overall similar temporal variations in number size distributions between ground level and 260 m, yet periods with significant differences were also observed. Particularly, accumulation-mode particles were highly correlated (r2 = 0. 85) at the two heights, while Aitken-mode particles presented more differences. Detailed analysis suggests that the vertical differences in number concentrations strongly depended on particle size, and particles with a mobility diameter between 100 and 200 nm generally showed higher concentrations at higher altitudes. Particle growth rates and condensation sinks were also calculated, which were 3.2 and 3.6 nm h−1, and 2.8 × 10−2 and 2.9 × 10−2 s−1, at ground level and 260 m, respectively. By linking particle growth with aerosol composition, we found that organics appeared to play an important role in the early stage of the growth (09:00–12:00 LT) while sulfate was also important during the later period. Positive matrix factorization of size-resolved number concentrations identified three common sources at ground level and 260 m, including a factor associated with new particle formation and growth events (NPEs), and two secondary factors that represent photochemical processing and regional transport. Cooking emission was found to have a large contribution to small particles and showed much higher concentration at ground level than 260 m in the evening. These results imply that investigation of NPEs at ground level in megacities needs to consider the influences of local cooking emissions. The impacts of regional emission controls on particle number concentrations were also illustrated. Our results showed that regional emission controls have a dominant impact on accumulation-mode particles by decreasing gas precursors and particulate matter loadings, and hence suppressing particle growth. In contrast, the influences on Aitken particles were much smaller due to the enhanced new particle formation (NPF) events.

10-Month characterization of the aerosol number size distribution and related air quality and meteorology at the Bondville, IL Midwestern background site

The aerosol size distribution was measured at a rural continental U.S. location (Bondville, Illinois 40.05∘N and 88.37∘W) for the period July 2013–June 2014. The dehumidified size distribution in the range 3 nm to 2.5 μm was continuously recorded by two scanning mobility particle sizers and an aerodynamic particle sizer. The Illinois measurement site is significant because it is home to one of the longest records (20+ years) in North America of atmospheric aerosol, meteorology, and gas phase variables. It is a common site for model-measurement comparison of both chemical and aerosol microphysical variables. However, while the measurement record has aerosol number concentration and optical properties, the number size distribution has not been well-constrained. Measurement highlights include means in number concentration (3–2500 nm), aerosol optical depth, SO2, NH3, and PM2.5 of 6500 cm−3, 0.18, 0.87 ppb, 1.6 ppb, and 8.8 μg m−3, respectively. The mode in the mean size distribution was 31 nm (number) and 287 nm (volume). The mean size distribution was heavily influenced by nuclei mode particles, with 46% of particles smaller than 20 nm. New particle formation and growth events increase midday concentrations of 3–50 nm particles substantially, with the highest activity in spring months and lower activity in summer and winter. Short term elevations in particle number and mass associated with local agricultural burning in April are reported as well.

Aerosols physical properties at Hada Al Sham, western Saudi Arabia

This is the first time to clearly derive the comprehensive physical properties of aerosols at a rural background area in Saudi Arabia. Aerosol measurements station was established at a rural background area in the Western Saudi Arabia to study the aerosol properties. This study gives overview of the aerosol physical properties (PM10, PM2.5, black carbon and total number concentration) over the measurement period from November 2012 to February 2015. The average PM10 and PM2.5 concentrations were 95 ± 78 μg m−3 (mean ± STD, at ambient conditions) and 33 ± 68 μg m−3 (at ambient conditions), respectively. As expected PM10 concentration was dominated by coarse mode particles (PM10–PM2.5), most probably desert dust. Especially from February to June the coarse mode concentrations were high because of dust storm season. Aerosol mass concentrations had clear diurnal cycle. Lower values were observed around noon. This behavior is caused by wind direction and speed, during night time very calm easterly winds are dominating whereas during daytime the stronger westerly winds are dominating (sea breeze). During the day time the boundary layer is evolving, causing enhanced mixing and dilution leading to lower concentration. PM10 and PM2.5 concentrations were comparable to values measured at close by city of Jeddah. Black carbon concentration was about 2% and 6% of PM10 and PM2.5 mass, respectively. Total number concentration was dominated by frequent new particle formation and particle growth events. The typical diurnal cycle in particle total number concentration was clearly different from PM10 and PM2.5.

Chemical characterization of submicron aerosol and particle growth events at a national background site (3295 m a.s.l.) on the Tibetan Plateau

Abstract. Atmospheric aerosols exert highly uncertain impacts on radiative forcing and also have detrimental effects on human health. While aerosol particles are widely characterized in megacities in China, aerosol composition, sources and particle growth in rural areas in the Tibetan Plateau remain less understood. Here we present the results from an autumn study that was conducted from 5 September to 15 October 2013 at a national background monitoring station (3295 m a.s.l.) in the Tibetan Plateau. The submicron aerosol composition and particle number size distributions were measured in situ with an Aerodyne Aerosol Chemical Speciation Monitor (ACSM) and a Scanning Mobility Particle Sizer (SMPS). The average mass concentration of submicron aerosol (PM1) is 11.4 μg m−3 (range: 1.0–78.4 μg m−3) for the entire study, which is much lower than observed at urban and rural sites in eastern China. Organics dominated PM1, accounting for 43 % on average, followed by sulfate (28 %) and ammonium (11 %). Positive Matrix Factorization analysis of ACSM organic aerosol (OA) mass spectra identified an oxygenated OA (OOA) and a biomass burning OA (BBOA). The OOA dominated OA composition, accounting for 85 % on average, 17 % of which was inferred from aged BBOA. The BBOA contributed a considerable fraction of OA (15 %) due to the burning of cow dung and straw in September. New particle formation and growth events were frequently observed (80 % of time) throughout the study. The average particle growth rate is 2.0 nm h−1 (range: 0.8–3.2 nm h−1). By linking the evolution of particle number size distribution to aerosol composition, we found an elevated contribution of organics during particle growth periods and also a positive relationship between the growth rate and the fraction of OOA in OA, which potentially indicates an important role of organics in particle growth in the Tibetan Plateau.

Seasonality of new particle formation in Vienna, Austria – Influence of air mass origin and aerosol chemical composition

The impact of air mass origin and season on aerosol chemical composition and new particle formation and growth events (NPF events) in Vienna, Austria, is investigated using impactor samples from short-term campaigns and two long-term number size distribution datasets. The results suggest that air mass origin is most important for bulk PM concentrations, chemical composition of the coarse fraction (>1.5 μm) and the mass size distribution, and less important for chemical composition of the fine fraction (<1.5 μm). Continental air masses (crustal elements) were distinguished from air masses of marine origin (traces of sea salt). NPF events were most frequent in summer (22% of measurement days), and least frequent in winter (3% of measurement days). They were associated with above-average solar radiation and ozone concentrations, but were largely independent of PM2.5. Air mass origin was a secondary influence on NPF, largely through its association with meteorological conditions. Neither a strong dependence on the PM2.5 loading of the air masses, nor indications of a source area for NPF precursors outside the city were found.

Airborne and ground based CCN spectral characteristics: Inferences from CAIPEEX – 2011

Abstract A first time comprehensive study of Cloud Condensation Nuclei (CCN) and associated spectra from both airborne and ground campaigns of the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) conducted over the rain shadow region of Western Ghats during September and October 2011 is illustrated. Observations of CCN spectra during clean, polluted and highly polluted conditions indicated significant differences between airborne and ground observations. Vertical variation of CCN concentration is illustrated from airborne observations in the clean, polluted and highly polluted conditions with different air mass characteristics. The cloud base CCN number concentrations are three times less than that of the surface measurements at different supersaturations. Diurnal variations of the ground based CCN number concentration and activation diameter showed bimodality. Atmospheric mixing in the wet conditions is mainly through mechanical mixing. The dry conditions favored convective mixing and were dominated by more CCN than the wet conditions. New particle formation and growth events have been observed and were found more often on days with convective mixing. The average critical activation diameter (at 0.6% SS) observed at the ground is approximately 60 nm and availability of a large number of particles below this limit was due to the new particle formation. Observations give convincing evidence that the precipitable water and liquid water path is inversely proportional to surface CCN number concentration, and this relationship is largely dictated by the meteorological conditions.

Sensitivity of convection to observed variation in aerosol size distributions and composition at a rural site in the southeastern United States

We present a sensitivity analysis to determine the impact of variations in aerosol physical and chemical characteristics, as observed in the field in the southeastern United States, on convective cloud microphysics. Scenarios reflecting changes in aerosol properties, observed over the Piedmont of Virginia and associated with new particle formation and growth events, were evaluated using the two-dimensional version of the Goddard Cumulus Ensemble Model with detailed spectral-bin microphysics. Two aerosol size distributions represented early and late stages of particle growth, and two aerosol chemical compositions (norpinic acid and ammonium sulfate) represented extremes in aerosol hygroscopicity observed at the field site, for a total of four scenarios. The chosen compositions reflect inferred local changes in aerosol composition over short time scales. Variations in the aerosol size distribution and composition resulted in substantial variation in the total number of cloud condensation nuclei (CCN) produced in the four case studies. Cases with high CCN concentrations developed larger, more vigorous clouds with more precipitation generated by both warm and cold rain processes. Greater numbers of drops were propelled aloft and formed an extensive ice anvil that produced a large area of stratiform rain. Convection was enhanced by increasing aerosols despite decreases in precipitation efficiency. In contrast, lower CCN concentrations developed smaller clouds with suppressed cold rain processes and less total precipitation. The relatively small increase in CCN concentration associated with the increase in aerosol hygroscopicity resulted in an increase in accumulated modeled precipitation of 12 % after 180 min of simulation time in both high and low CCN cases. The increase in accumulated precipitation due to the substantial increase in CCN concentration associated with growth of the aerosol size distribution was 93 % for both aerosol compositions. The timing of the onset of precipitation was not affected by aerosol concentration or composition.

A Simplified Approach to Calculate Particle Growth Rate Due to Self-Coagulation, Scavenging and Condensation Using SMPS Measurements during a Particle Growth Event in New Delhi

The scanning mobility particle sizer (SMPS) technique is a widely employed technique to measure the particle number size distribution, and thus calculate the particle total growth rate. However, growth due to individual atmospheric processes needs to be known precisely and accurately to better model the secondary aerosol distribution. In this study, we use simplified analytical formulas to calculate the growth rates due to self-coagulation, coagulation scavenging and condensation processes of particle size distribution (9–425 nm) measured using SMPS. Firstly, total growth rate is determined from the regression fit of SMPS data plotted between the geometric mean diameter (GMD) of particle size (nm) versus time (hour) measured during a particle growth event. The SMPS measurements were conducted during November-December 2011 in New Delhi. The particle growth event days and non-event days were classified according to the protocol discussed elsewhere. Assuming that the particle number size distribution of a growing population can be described by a unimodal distribution and particles are neutral in the population, we calculated the growth rate due to self-coagulation (GRscoag), which is proportional to the total number of particles in the mode and mode peak diameter. Similarly, assuming that particles with mode peak size 25 nm and above act as a coagulation sink and grow due to scavenging of newly formed nucleation range particles (< 12 nm), we calculated the coagulation scavenging growth rate (GRscav) as a time derivative of the mode peak diameter, which is equivalent to the product of particle diameter and its coagulation sink. The condensation growth rate (GRcond) is calculated based on the assumption that total growth rate is the summation of the growth resulting of three physical processes: self-coagulation, coagulation scavenging and condensation. During the study period, three event days were recorded at the measurement site. To explain the growth rate calculation approach, which is presented here in detail, we have taken SMPS data of one event day (November 4, 2011) as an example (two other event days are also briefly discussed). On November 4, the total average growth rate was found to be 15.4 ± 11 nm/h, while the average GRscoag, GRscav and GRcond were calculated to be 3.8 ± 0.4 (with min and max values of 2.9–5.1 nm/h), 8.0 ± 6 (0.6–19.3 nm/h) and 3.6 nm/h, respectively. These growth rates are comparable to those reported for other urban sites around the world using different methods. This approach is simple, and growth by individual processes can be calculated without knowing several other parameters, which include vapor concentration of atmospheric constituents, heterogeneous processes, and complex modeling procedures.

Meteorological controls on particle growth events in Beltsville, MD, USA during July 2011

During summer 2011, particle growth events were investigated in Beltsville, Maryland, USA where anthropogenic and biogenic hydrocarbon emissions heavily influenced the lower atmosphere. Aerosol sampling was made from a 5-m tower during July 2011. Ultrafine particle growth events occurred on 4 of the 25 days sampled. Regional particle growth events occurred after the passage of synoptic cold fronts. During regional particle growth events temperature and relative humidity were 2° Celsius lower and 3 % less than on non-event days and the local wind direction came from the northeast. With average growth rates of 3.5 nm hr−1, regional particle growth events happened around 15:00 Universal Time Coordinated, shortly after increased mixing within the atmospheric boundary layer. Regional particle growth events were associated with cooler, drier, mostly sunny days that had low sulfur dioxide mixing ratio (~0.6 parts per billion by volume). Regional particle growth events occurred when the atmospheric boundary layer was dominated by free convection and had lower sensible heat fluxes compared to short-lived events or non-particle growth event days.

On the submicron aerosol distributions and CCN number concentrations in and around the Korean Peninsula

Abstract. Total number concentrations of particles having a diameter larger than 10 nm (NCN), cloud condensation nuclei at several supersaturation (S) values (NCCN) and number size distributions of particles with 10–414 nm diameter were measured in Seoul between 2004 and 2010. Overall average values of NCN and geometric mean diameter were 17 811 ± 5581 cm−3 and 48 ± 6 nm. Average NCCN at 0.4, 0.6 and 0.8% S were 4145 ± 2016, 5323 ± 2453 and 6067 ± 2780 cm−3 and corresponding NCCN / NCN were 0.26 ± 0.11, 0.33 ± 0.11 and 0.37 ± 0.12. There is a clear seasonal variation in aerosol concentration, which seems to be due to the monsoon. NCN and NCCN are also found to depend on the volume of traffic and the height of the planetary boundary layer, respectively. During aircraft campaigns in 2009 and 2011, NCN and NCCN at 0.6% S (N0.6%) were measured in and around the Korean Peninsula. During the 2011 campaign, the aerosol scattering coefficient was also measured. NCN and N0.6% in the lower altitudes were generally higher than at higher altitudes, except for cases when particle formation and growth events were thought to occur at higher altitudes. NCN and N0.6% generally show a positive correlation with aerosol scattering coefficients but this correspondence tends to vary with altitude. Occasional instances of low (&lt; 0.3) N0.6% / NCN in the boundary layer are demonstrated to be associated with particle formation and growth events. With the support of ground measurements, it is confirmed that a particle formation and growth event did indeed occur over the Yellow Sea on a flight day, and the areal extent of this event is estimated to be greater than 100 km × 450 km. With the combination of the current and several relevant previous studies, a composite map of NCN and NCCN in and around the Korean Peninsula is produced. Overall, the exhibited concentrations are typical of values measured over polluted regions elsewhere on the globe. Moreover, there is a generally decreasing trend from west to east over the region, implying that the region is constantly under the dominant influence of continental outflow.

Chemistry of new particle growth in mixed urban and biogenic emissions – insights from CARES

Abstract. Regional new particle formation and growth events (NPEs) were observed on most days over the Sacramento and western Sierra foothills area of California in June 2010 during the Carbonaceous Aerosols and Radiative Effect Study (CARES). Simultaneous particle measurements at both the T0 (Sacramento, urban site) and the T1 (Cool, rural site located ~40 km northeast of Sacramento) sites of CARES indicate that the NPEs usually occurred in the morning with the appearance of an ultrafine mode at ~15 nm (in mobility diameter, Dm, measured by a mobility particle size spectrometer operating in the range 10-858 nm) followed by the growth of this modal diameter to ~50 nm in the afternoon. These events were generally associated with southwesterly winds bringing urban plumes from Sacramento to the T1 site. The growth rate was on average higher at T0 (7.1 ± 2.7 nm h−1) than at T1 (6.2 ± 2.5 nm h−1), likely due to stronger anthropogenic influences at T0. Using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS), we investigated the evolution of the size-resolved chemical composition of new particles at T1. Our results indicate that the growth of new particles was driven primarily by the condensation of oxygenated organic species and, to a lesser extent, ammonium sulfate. New particles appear to be fully neutralized during growth, consistent with high NH3 concentration in the region. Nitrogen-containing organic ions (i.e., CHN+, CH4N+, C2H3N+, and C2H4N+) that are indicative of the presence of alkyl-amine species in submicrometer particles enhanced significantly during the NPE days, suggesting that amines might have played a role in these events. Our results also indicate that the bulk composition of the ultrafine mode organics during NPEs was very similar to that of anthropogenically influenced secondary organic aerosol (SOA) observed in transported urban plumes. In addition, the concentrations of species representative of urban emissions (e.g., black carbon, CO, NOx, and toluene) were significantly higher whereas the photo-oxidation products of biogenic VOCs (volatile organic compounds) and the biogenically influenced SOA also increased moderately during the NPE days compared to the non-event days. These results indicate that the frequently occurring NPEs over the Sacramento and Sierra Nevada regions were mainly driven by urban plumes from Sacramento and the San Francisco Bay Area, and that the interaction of regional biogenic emissions with the urban plumes has enhanced the new particle growth. This finding has important implications for quantifying the climate impacts of NPEs on global scale.

Observation of new particle formation and growth under cloudy conditions at Gosan Climate Observatory, Korea

Under cloudy conditions at the Gosan Climate Observatory (GCO), Korea, we observed distinct new particle formation and growth (NPF) events from simultaneous co-located measurements of aerosol and cloud profiles, cloud cover, shortwave radiation, and the number concentration and size distribution of aerosols. The high frequency of NPF was observed at GCO under decreased downwelling solar radiation caused by clouds. Although we observed about 15 and 40 % decreases in downwelling surface shortwave radiations, in the presence of thick mid-level (low-level) clouds, on January 28 and 30, 2012, respectively, distinct NPFs with a growth rate of 3.3 (3.9) nm h−1 were observed. We examined a 4-year series (May 2008 to April 2012) of continuous measurements of the size distribution of aerosol numbers and visually observed cloud cover. We found that approximately 13 % (i.e., 35 days out of 280 days) of total NPF events were observed under cloud-free conditions (i.e., cloud cover of 0/10). About 20 % (i.e., 57 days out of 280 days) of total NPF events occurred under mostly overcast conditions (i.e., cloud cover of 9/10–10/10). Although NPF events occurring under cloudy conditions were also found elsewhere, the frequency of NPF occurring at GCO seems much higher. The average value of relative humidity for the strong-NPF event days is lower than that of the weak and non-NPF event days for all cloud categories. No significant difference in the condensation sink was found among strong-, weak-, and non-NPF days, but the condensation sink showed a slight decreasing tendency with increasing cloudiness. Further investigations on precursor gases and preexisting aerosols under cloudy conditions are needed.

Observations on the formation, growth and chemical composition of aerosols in an urban environment.

The charge and chemical composition of ambient particles in an urban environment were determined using a neutral particle and air ion spectrometer and an aerodyne compact time-of-flight aerosol mass spectrometer. Particle formation and growth events were observed on 20 of the 36 days of sampling, with eight of these events classified as strong. During these events, peaks in the concentration of intermediate and large ions were followed by peaks in the concentration of ammonium and sulfate, which were not observed in the organic fraction. Comparison of days with and without particle formation events revealed that ammonium and sulfate were the dominant species on particle formation days while high concentrations of biomass burning OA inhibited particle growth. Analyses of the degree of particle neutralization lead us to conclude that an excess of ammonium enabled particle formation and growth. In addition, the large ion concentration increased sharply during particle growth, suggesting that during nucleation the neutral gaseous species ammonia and sulfuric acid react to form ammonium and sulfate ions. Overall, we conclude that the mechanism of particle formation and growth involved ammonia and sulfuric acid, with limited input from organics.

Comparative study of ultrafine atmospheric aerosol within a city

Particle number size distributions in a mobility diameter range of 6–1000 nm and size-resolved number concentrations were determined with a time resolution of 10 min for a near-city background, city centre, street canyon and road tunnel environments in Budapest. Median N6–100 concentrations for the sites listed were 3.1 × 103, 9.3 × 103, 19.4 × 103 and 123 × 103 cm−3, respectively. Contributions of the ultrafine (UF) particles (<100 nm) to the total particle number for all locations were rather large (up to 86%), and do not seem to vary substantially in time. Diurnal variations of the mean N6–100 concentrations had different patterns for both the various urban sites, and for workdays and weekends. Nucleation strength factor (NSF) was introduced for the first time to quantify the relative importance of new particle formation with respect to all sources of UF particles. During the daytime in summer, nucleation in the near-city background was a major production process of UF particles with a daily mean relative contribution of 42%. In the city centre and street canyon, the daily mean relative contributions of nucleation to the UF particles were 30% and 23%, respectively. Median particle diameters for the background, city centre, street canyon and road tunnel environments were 61, 42, 35 and 42 nm, respectively, so they were jointly influenced with the anthropogenic impact and aerosol ageing. Monthly mean frequency of new particle formation and growth events in the background seems somewhat larger, while it appears smaller for the street canyon in comparison to the city centre.

Aerosol measurements at a high-elevation site: composition, size, and cloud condensation nuclei activity

Abstract. Measurements of cloud condensation nuclei (CCN) concentrations, single particle composition and size distributions at a high-elevation research site from March 2011 are presented. The temporal evolution of detailed single particle composition is compared with changes in CCN activation on four days, two of which include new particle formation and growth events. Sulfate-containing particles dominated the single particle composition by number; biomass burning particles, sea salt particles, and particles containing organic components were also present. CCN activation largely followed the behavior of the sulfate-containing particle types; biomass burning particle types also likely contained hygroscopic material that impacted CCN activation. Newly formed particles also may contribute to CCN activation at higher supersaturation conditions. Derived aerosol hygroscopicity parameters from the size distribution and CCN concentration measurements are within the range of previous reports of remote continental kappa values.

New particle formation and growth associated with East-Asian long range transportation observed at Fukue Island, Japan in March 2012

Time-resolved data for size distribution, number concentration and chemical composition of atmospheric aerosols were taken from March 9 to 16, 2012 during Fukue Site Atmospheric Aerosol Observation Campaign (128.7°E, 32.8°N) organized by ASEPH (Impacts of Aerosols in East Asia on Plants and Human Health) project. A scanning mobility particle sizer was employed to continuously measure the particle size distribution from 14 nm to 670 nm and the total number concentration of particles (>3 nm) was measured by a condensation particle counter. Very large amount of polluted air mass transportation (PM2.5 > 65 μg m−3) was observed on March 11, associated with a sudden increase in particle number concentration having a mobility diameter from 100 to 300 nm. The transported air mass contained anthropogenic pollutants such as sulfate (>20 μg m−3), nitrate (>15 μg m−3) and black carbon (>3 μg m−3). Typical new particle formation and growth events were also observed on March 12–13. The onset of particle formation and nucleation bursts was identified at around noon and it is considered that these were nucleated species near the island area. It was found that some new particle formation events were associated with the long range transportation of polluted air mass from East Asian region.