Observation and Analysis of Particle Nucleation at a Forest Site in Southeastern US

This study examines the characteristics of new particle formation at a forest site in southeastern US. Particle size distributions above a Loblolly pine plantation were measured between November 2005 and September 2007 and analyzed by event type and frequency, as well as in relation to meteorological and atmospheric chemical conditions. Nucleation events occurred on 69% of classifiable observation days. Nucleation frequency was highest in spring. The highest daily nucleation (class A and B events) frequency (81%) was observed in April. The average total particle number concentration on nucleation days was 8,684 cm−3 (10 < Dp < 250 nm) and 3,991 cm−3 (10 < Dp < 25 nm) with a mode diameter of 28 nm with corresponding values on non-nucleation days of 2,143 cm−3, 655 cm−3, and 44.5 nm, respectively. The annual average growth rate during nucleation events was 2.7 ± 0.3 nm·h−1. Higher growth rates were observed during summer months with highest rates observed in May (5.0 ± 3.6 nm·h−1). Winter months were associated with lower growth rates, the lowest occurring in February (1.2 ± 2.2 nm·h−1). Consistent with other studies, nucleation events were more likely to occur on days with higher radiative flux and lower relative humidity compared to non-nucleation days. The daily minimum in the condensation sink, which typically occurred 2 to 3 h after sunrise, was a good indicator of the timing of nucleation onset. The intensity of the event, indicated by the total particle number concentration, was well correlated with photo-synthetically active radiation, used here as a surrogate for total global radiation, and relative humidity. Even though the role of biogenic VOC in the initial nuclei formation is not understood from this study, the relationships with chemical precursors and secondary aerosol products associated with nucleation, coupled with diurnal boundary layer dynamics and seasonal meteorological patterns, suggest that H2SO4 and biogenic VOC play a role in nucleated particle growth at this site.


Introduction
Atmospheric aerosols contribute to acid rain, impair visibility, and alter the earth's radiation budget.Radiative effects can be direct by scattering incoming solar radiation and absorbing outgoing long-wave radiation, or they can be indirect by acting as cloud condensation nuclei to modify cloud micro-physical properties [1].However, large uncertainties in the understanding of aerosol formation mechanisms and the estimation of their effects on global climate persist [2].Aerosols can either be directly emitted (primary) from natural and anthropogenic sources, or formed from gas phase precursors (secondary).Aerosol production mechanisms, number concentrations, and chemical composition are highly dependent on geographical location, air mass origin, source type, meteorology, and atmospheric chemistry.
Nucleation is the process of forming dispersed nuclei from the homogeneous phase under super saturation of vapor.It may produce very high concentrations of particles with diameters < 10 nm [3].Nucleation involves the formation of initial nuclei and their growth through condensation and/or coagulation.A number of different nucleation mechanisms have been proposed [4,5], including binary water-sulfuric acid nucleation, ternary water-sulfuric acid-ammonia nucleation, ion-induced nucleation [6], and nucleation enhanced by organic acids [7].Depending on the prevailing chemical and meteorological conditions, newly formed particles may grow to larger sizes by condensation.Primary emission, nucleation, and subsequent growth of pre-existing particles by condensation and coagulation are responsible for maintaining atmospheric aerosol concentrations and size distributions [8].
Spatio-temporally resolved measurements of aerosol size distributions are essential for understanding the chemical and meteorological conditions leading to nucleation.Kulmala et al. [9,10] summarized nucleation studies spanning a broad range of geographical locations and ambient conditions.Nucleation events have been observed both in rural and urban environments [8,[11][12][13].Boy et al. [14], Gerrit et al. [15], and O'Dowd et al. [16] observed that new particle formation in coastal environments was positively correlated with heat flux and negatively correlated with RH or water vapor flux.In a recent study by Place et al. [17], nucleation events were associated with strong solar irradiance and boundary layer growth rate; and were driven either by anthropogenic activities or by a chemical species whose emission rates are similar to that of terpenes.Though the condensation of sulfuric acid (H 2 SO 4 ) could explain only a small fraction of the aerosol growth rate, sulfur dioxide (SO 2 ) is considered an important nucleation precursor [18].Relatively few studies have investigated particle nucleation in southeastern US [18][19][20][21], particularly over extended periods of time sufficient to examine seasonal variability.New particle formation was frequently observed in dryer air with low background aerosols [19].Kulmala et al. [9] observed nucleation events even in polluted environments if the ratio between precursor gases and pre-existing aerosols are higher.The vertical variation of aerosol particle size distributions shows nucleation primarily occurred above the canopy at a rural forested site in southern Indiana [22].This study also shows a significant association between nucleation mode particles and local meteorological conditions at the Indiana Forest site.
To better understand the frequency and characteristics of particle formation events in southeastern US, particle size distributions were measured above a Loblolly Pine canopy at Duke Forest, near Chapel Hill, NC, USA, between November 2005 and September 2007.This study identifies the nucleation events and investigates the corresponding physical properties (total number concentration, condensation sink, and particle growth rates), chemical properties, and the meteorological conditions favorable for nucleation.

Site Description
Particle size distribution, total particle number concentration, chemical, and meteorological measurements were conducted at Duke Forest, Chapel Hill, North Carolina.Duke Forest (35.98°N, 79.09°W), considered a suburban forest site, is surrounded by the cities of Chapel Hill (7 km to the south-southeast), Durham (17 km to the east-northeast), Raleigh (40 km to the southeast), and Burlington (33 km to the west-northwest).Interstate 40 passes approximately 2.4 km northeast of the site.A detailed description of the site is provided by Geron [23].Measurements were conducted over a Loblolly pine plantation with an approximate tree height of 18 m.

Measurements
The particle size distribution measurements were conducted using a Scanning Mobility Particle Sizer (SMPS, TSI, Shoreview, MN, USA) consisting of a TSI series 3080 Electrostatic Classifier with a 3081 Differential Mobility Analyzer (DMA), and a 3010 Condensation Particle Counter (CPC).A TSI Model 3025 CPC was used for a brief period early in the study.The instrument was operated at sheath and aerosol air flow rates of 10 and 1 Lpm, respectively, with a 0.0457 cm impactor inlet, establishing an effective particle size range of 7 to 305 nm (D p ).The quantitative analysis of this study included the size range 10 nm-250 nm.Multiple charge and diffusion loss corrections were applied using the TSI AIM software [24].The scanning routine was configured to yield 30-min average concentrations and size distributions.After accounting for periods of instrument malfunction and maintenance activities, a total of 364 observation days were available for analysis during the period November 2005-September 2007.
Prior to December 2006, particles were sampled from above the canopy (25 m above ground) by drawing air through 3/8" O.D. copper tubing at a flow rate of 30 Lpm into the climate controlled shelter in which the instruments were housed.Tubing was thermally insulated inside the shelter to avoid condensation.The potential for particle loss during transport through the copper tubing was tested by examining transmission of NaCl particles through a 10-m section of tubing oriented in the same configuration as the field system.Test results indicated negligible particle loss at the flow rates and tubing lengths employed in the field for particle sizes on the order of 30-50 nm D p .Because sufficient concentrations of smaller particles could not be generated during loss tests, potential diffusional loss of 10-nm particles was estimated as described by von der Weiden et al. [25] for turbulent flow.For the 3/8" O.D. tubing as configured in the field, losses of 10 nm particles may have been on the order of 25%-30%.Data were not corrected for this potential artifact.To accommodate additional measurements at the site, the above-canopy sampling system was changed to a 6" O.D. PVC pipe through which air was drawn to the shelter at a flow rate of 500 Lpm.Particle losses for this sampling configuration, also estimated using the approach of von der Weiden et al. [25], are negligible for the range of particle sizes investigated (i.e., 10-250 nm).Particle residence times in the low-and high-flow sampling configurations were ~3 and 40 s, respectively, with corresponding Reynold's numbers of 5,300 and 5,000.

Detection and Classification of Nucleation Events
The observation days are categorized into nucleation event classes based on 2D size distribution plots.For this study, we adopted the nucleation event classification scheme by Dal Maso et al. [26], Boy et al. [27], and Pryor et al. [22,28].The particle number concentration for each size bins (N i ) and for each observation days were plotted as a function of particle diameter and time.The periods of increasing N i were assessed to identify potential nucleation event classes A, B, C, or to categorize the day as non-nucleation (non-event) day.The event classes that are interrupted or that do not fit into the aforementioned categories are identified as unclassified events and are excluded from further analysis.The typical size distribution behavior for each of these event classes are shown in Figure 1.Non-Nucleation: Observation days with particle size distributions devoid of increased NM concentration and their subsequent growth to larger particles.
Unclassified: Days that do not fit into the described event classes or that had an interrupted size distribution data due to inclement weather or instrument outage or for which the growth rates are extremely fluctuating.

Condensation Sink (CS) and Growth Rate
The condensation sink (CS) quantifies the loss rate of molecules in the entire size spectrum due to condensation of condensable vapors on pre-existing aerosols [29,30].It is calculated as the integration of condensational loss of condensable vapors onto existing aerosols and is dependent on the molecular properties such as vapor phase diffusion and mean free path.The CS (cm −2 ) is calculated as follows: (1) where is the transitional correction factor [31,32], is the mid-point diameter, and is the number distribution of particles corresponding to different size classes with mid-point diameter between the size range 10 nm and 250 nm.The accommodation coefficient of unity was used to calculate .
Since we use 10 nm to 250 nm diameter range for quantitative analysis, the calculated value is an underestimation of the actual , though the contribution of particles with sizes outside the measured range is expected to be relatively small.
Growth rate indicates size changes of nucleated particles over time.A log-normal fit is applied to the data to identify the peaks in each size distribution.GR in nm•h −1 was calculated from the difference between midpoint diameters corresponding to the peaks in particle number concentration for each instant of time (∆ ) by dividing it with the corresponding difference in time (∆ .
Particle growth rate is a function of the condensable vapor concentration and the CS.GR increases significantly when the condensable vapor concentration is large and the CS is low [9].

Summary Statistics
Nucleation events were observed on approximately 69% (273 days) of days for days with complete measurements.The observed nucleation frequency at Duke Forest was significantly higher compared to that observed at forested areas elsewhere.From the total valid observations, 46% of the days at a mixed deciduous forest in southern Indiana [22], 35% of the days at a deciduous forest in central Virginia [19], 45% of the days [26] at a boreal forest in Hyytyala in Finland, and 53% of the days [11] at a forested site near Pittsburg, PA exhibited new particle formation characteristics.Month-to-month comparisons of event statistics are difficult as the total number of valid observation days in each month The hourly average growth rate (GR) (Figure 4(b)) illustrates diurnal patterns in the rate of change of nucleation mode particle size and is dependent on the condensable vapor concentration and the number concentration of pre-existing particles that act as CS.Growth rate begins to increase with the onset of nucleation and generally reaches its peak 4 to 5 h later due to increasing photochemistry.However, there is about an hour delay (Figure 4(a,b)) between the nucleation onset and increase in GR.The similar time lag (Figure 4(c)) observed between NM particle concentration and concentration of SO 2 indicates condensational growth.This temporal lag may be due to the two stages in nucleation-the formation of new particles, and their growth through condensation to detectable larger sizes.Sihto et al. [33] observed a 1.4-h time lag between the number concentration of 3-6 nm particles and sulfuric acid concentration.Not all nucleation events lead to new particle formation.If the nucleated species can survive against condensation and coagulation loss to pre-existing particles, and if condensable vapors are available for condensation onto nucleated species, new particle formation may occur [34].
The typical growth rates observed at continental sites are between 1 and 20 nm/h [9].The grand average growth rate during nucleation events was 2.7 ± 0.3 nm/h.The observed average growth rates are consistent with the range of the growth rates reported by Pryor et al. [22] (2.5 nm/h), and Place et al. [17] (2.7 nm/h).The particle growth rate at the Duke Forest site exhibits seasonal variability.The monthly average growth rate (Figure 6) was at its maximum during late spring and the early summer months, with the highest growth rate being in May (5.0 ± 3.6 nm/h) followed by June (4.1 ± 3.9 nm/h), and the minimum growth rate occurring during the winter months, with the lowest being in February (1.2 ± 2.2 nm/h).This observed seasonality is consistent with Qian et al. [35] (6.7 ± 4.8 nm/h for summer and 1.8 ± 1.9 nm/h for winter).Higher growth rates in summer months may be due to the higher oxidizing capacity of the atmosphere and subsequent oxidation rates of precursors, such as SO 2 , and other compounds, such as biogenic volatile organic compounds (VOC), that exhibit temperature or physiologically dependent (e.g., bud break or needle expansion) emission rates.
The particle growth rate depends on the availability of condensable vapor concentration and the new particle formation is observed when freshly nucleated particles can grow fast enough to be detected without being scavenged onto pre-existing aerosols.The condensable vapor species are produced from their precursor gases through oxidation by ozone, and hydroxyl (OH), and/or nitrate (NO 3 ) radicals [9].The concentration of these condensable vapor species depends on the concentration ratio between precursor gases and pre-existing aerosols [9] predominance of nucleation events on days with high PAR likely reflects a photochemical control, such days would also exhibit higher emission rates of biogenic VOC relative to low PAR days at a similar temperature.The highest particle growth rates were observed during warm months when VOC (mono-and sesquiterpene) emission rates from vegetation at this site are highest [48].It is noteworthy that Geron and Arnts [48] observed high monoterpene emission rates in the spring, which corresponds to period of most frequent Class A nucleation events.We found moderate positive correlation between the condensation sink and O 3 concentrations on nucleation days, suggesting that on days when nucleation occurred, rapid and sustained particle growth was associated with conditions (i.e., high O 3 ) that favor the oxidation of gas phase organic compounds.This pattern is consistent with the observed relationship between CS and the concentration of organic carbon in PM.
The frequency, seasonal distribution, and relationships between nucleation events and meteorology at short time scales are consistent with other studies in the US Relationships with chemical precursors and secondary aerosol products associated with nucleation, coupled with diurnal boundary layer dynamics and seasonal temperature trends, suggest that H 2 SO 4 and biogenic VOC play a role in nucleation at this site.However, without additional detailed measurements, isolation of the nucleation mechanism is not possible, thus inviting further study.Data from this study provide a dataset for testing of particle formation and dynamics in regional modeling tools such as the Community Multi-scale Air Quality Model (CMAQ) and add to a growing understanding of the frequency, intensity, and chemical and meteorological drivers of particle formation in southeastern US.Our results may also be used to guide further studies aimed at characterizing the chemical characteristics of nucleation, with a focus on H 2 SO 4 and organic precursors.
Figure left), a numbe

Table 1 .
Total number concentrations of fine mode (N) and nucleation mode (N 25 ) particles, peak diameter (D peak ), condensation sink (CS), and particle growth rate (GR) during the 4 March 2006 nucleation event.