A13C-0260
Atmospheric Observations of Aerosol Sizes, Sulfuric Acid and Ammonia Measured in Kent, Ohio
Atmospheric particles affect atmospheric composition, cloud formation, global radiation budget, and human health. Nucleation is a gas-to-particle conversion process in which new particles form directly from gas phase species and is a key process that controls particle number concentrations. The most common feature of the new particle formation events is a substantial increase of number concentrations of nucleation mode particles reaching up to 105-106 cm-3 in the condensable vapor-laden air. There are several nucleation mechanisms for tropopsheric aerosol formation, but it is unclear which nucleation process dominates. In particular, observations and modeling studies show that ammonia can be important for atmospheric nucleation in the boundary layer, but simultaneous measurements of aerosol sizes and precursors including sulfuric acid and ammonia are critically lacking. In order to overcome these shortcomings, we conduct atmospheric observations of new particle formation in Kent, OH. We have measured aerosol sizes and concentrations for particles in the size range from 3-102 nm semi- continuously from December 2005 and for particles from 3-1000 nm continuously from September 2007 in Kent State campus, Kent, OH (with an inlet placed at ~11.5 m above ground level). We also simultaneously measure sulfuric acid and ammonia, two most important inorganic aerosol precursors, with two chemical ionization mass spectrometers (CIMS) from August 2008. Kent, located in Northeastern Ohio, is relatively rural itself, but is also surrounded by several urban cities within 40 miles. Because of the combination of its relatively rural environment (hence low surface areas of aerosol particles), active vegetation (organic and NH3 emissions), and possible transport of aerosol precursors from the surrounding urban and industrialized areas, Kent is a unique location to make new particle formation studies. So far, most of new particle formation observations made typically in US were at relatively polluted areas (e.g., Atlanta, Pittsburgh, and Detroit). We will present our ground-based, long-term measurements of aerosol sizes and concentrations and discuss how aerosol precursors (H2SO4 and NH3), RH, temperature and photochemistry affect new particle formation in this rather rural Midwestern town, by comparing with previous new particle formation observations made in other sites (e.g., EPA supersites in relatively polluted atmosphere and in Finland boreal forest).
A13C-0261
Parameterization of Aerosol Particle Growth Factors as a Function of Relative Humidity
Data collected as part of BAQS-Met in Southern Ontario, Canada during summer 2007 was used to study particle growth factors as a function of relative humidity using a Hygroscopic Tandem Differential Mobility Analyzer (HTDMA). Dry particle diameters of 50 nm, 100 nm, and 150 nm were used. The HTDMA was scanned between 50% and 85% RH over one hour cycles. A simple model was used to fit particle growth factors as a function of RH. We considered both a two component case in which particles were assumed to consist of organic material and ammonium sulfate, and a three component case in which ammonium bisulfate was added. The fits make it possible to retrieve the relative amounts of the three phases and the hygroscopicity parameter of the organic matter. Aerosol mass spectrometer (AMS) data was used to independently estimate the organic and sulfate fractions. In most cases the organic fraction from the fits was in a reasonable agreement with the AMS data. However, when the AMS organic fraction was low, there was a tendency for the fits to overestimate it and to assign the organic fraction a very high hygroscopicity parameter. Including a small amount of ammonium bisulfate significantly improved the results.
A13C-0262
Particle Nucleation And Growth Under Polluted Conditions Over The North China Plain During Spring And Summer Seasons
The particle nucleation and growth is a key process to determine size distribution of aerosols and the CCN number concentrations in atmosphere. The number-size distribution of aerosol particles (10-10000 nm diameter) and some trace gases were measured in spring and summer seasons in North China during 2006- 2007. The measurement sites were in urban Ji'nan, Shandong Province, a rural area downwind of urban Beijing, and the summit of Mount Tai in central-eastern China. All the three sites are situated in the populated and polluted North China Plain. Intensive new particle formation processes were observed on 60 percent of days at Ji'nan and 30 percent in rural Beijing. Regional nucleation processes were observed on 20 percent of the days near Beijing with an average particle growth rate of 4.4 nm•h-1. At Mount Tai, particle growth events were observed on 65 percent of days. During these growth events, newly formed particles of size 10 to 20 nm appeared around noon and grew at an averaged rate of 3.2 nm•h-1, reaching sizes between 60 to 80 nm in the evening. The intensive particle formation in surface and growth process at the top of the Planetary Boundary layer (PBL) suggests that the particle formation exists in the whole PBL in North China. Using the measured growth rates and condensation sinks, the concentration of condensable vapors was estimated to vary from 1.33E+06 cm-1 s-1 in rural Beijing to 7.21E+05 cm-1 s-1 at the summit of Mt Tai. These values are higher than those observed similar sites in previous studies.
A13C-0263
Abundance of Biogenic and Anthropogenic SOA Tracers Compounds in Fine Particles during the Nucleation in Forests Experiment (NIFTy)
The Nucleation in Forests field experiment was conducted in May 2008 at the Morgan-Monroe State forest in Indiana to observe the nature and occurrence of new particle formation. As part of this experiment filter samples were collected at mid-canopy height using a MOUDI-110. The MOUDI sampler collected 10 size fractions from 0.056 um to 5.6 um. The filter samples were collected for 24 hours and analyzed by gas chromatography ion trap mass spectrometry for several anthropogenic and biogenic secondary organic aerosol tracer compounds including 2-methyltetrols from isoprene oxidation. The objective of the size resolved PM sampling was to determine if the growth of small particles that occurs during nucleation events was driven by biogenic or anthropogenic VOC photooxidation products. Results from this analysis will be presented.
A13C-0264
Aerosol Production in a Mixed Deciduous/Coniferous Forest
Aerosols are of fundamental concern because of their impacts on air quality, human health and radiative forcing. Recent studies have focused on secondary organic aerosol (SOA) production due to oxidation of volatile organic compounds (VOCs), and more importantly biogenic-VOCs (BVOCs), in particular, isoprene. However, the SOA precursors are not well understood because the mechanisms have shown that isoprene oxidation can contribute to aerosol production through multiple generation oxidation products. For terpenes, it is more likely that primary or secondary oxidation products lead to particle formation. In the present study, we measured the aerosol size distribution, along with O3, HOx, NOx, NOy and BVOCs, in a mixed deciduous forest that is undergoing successional transition to a conifer-dominated species mix. This study was conducted in a rural forest environment in northern Michigan as a part of the summer 2008 PROPHET campaign at the University of Michigan Biological Station (UMBS). We examine here the potential BVOC contribution to aerosol formation. A TSI, inc. Scanning Mobility Particle Sizer (SMPS) was used to measure aerosol number density in the size range, 15 nm < x < 711 nm and a Proton Transfer Reaction – Linear Ion Trap (PTR-LIT) mass spectrometer for quantifying isoprene and other BVOCs, including methyl vinyl ketone and methacrolein, and total monoterpenes. Preliminary results show periods of new particle production. Here we use a unique set of BVOC, HOx, NOx, NOy, O3 and meteorological data to examine conditions leading to new particle production.
A13C-0265
A Total OH Loss Rate Measurement Technique Using Select Ion Chemical Ionization Mass Spectrometry (SICIMS)
Typically tropospheric chemical models overestimate [OH] when compared to measurements. This discrepancy is usually attributed to incompletely modeling OH sinks due to a lack of measurements. The determination of OH reactivity provides an independent means to quantify the overall contribution of these unmeasured trace species to OH oxidation. Therefore, a new instrument is being developed to measure the total loss rate of OH to chemical species in ambient air. A standard flow tube scheme is the basis for this design in which OH is generated by flowing a N2/H2O mixture past an Hg lamp. The OH is then turbulently mixed into ambient air that is drawn into the flow tube with OH concentrations measured using Select Ion Chemical Ionization Mass Spectrometry (SICIMS). This instrument has been deployed as a part of several field experiments including the Nucleation in Forests (NIFTY). Additionally this instrument has been intercompared with other OH reactivity techniques data from these intercomparisons and NIFTY will be discussed.
A13C-0266
Measurement of Particle Concentrations above a Forest and Surrounding Farmland
Very little information is currently available regarding the spatial scales on which nucleation occurs in the near surface layer. As part of the May 2008 Nucleation In ForesTs (NIFTY) campaign, an unmanned aerial vehicle (UAV) system was flown across the study area of the Morgan-Monroe State Forest (MMSF). The Clarkson Vector-P UAV carried a TSI Model 3007 Condensation Particle Counter (CPC) and a Grimm Model 1.109 Universal Aerosol Spectrometer (UAS). The CPC provides the count of ultrafine particles while the UAS gives counts of large particles (0.25 to 20 ƒÝm). These measurements were designed to supplement the extensive measurements being made on the 46 m instrumented tower. Because of the FAA restrictions of line-of-sight observation of the UAV by ground observers, we were limited in the spatial extent and were limited to studying altitudes below 1200 feet. However, these restrictions still permitted us to make a series of measurements of the spatial and vertical extent of particle concentrations to support the ground level composition measurements to better understand nucleation and growth in this rural area of southwestern Indiana.
A13C-0267
Measurement of SOA Precursor Compounds in the Nucleation In ForesTs (NIFTY) Campaign
As part of the Nucleation In ForesTs (NIFTY) campaign, volatile organic compounds (VOCs) were quantified in the Morgan-Monroe State Forest (MMSF) in southwestern Indiana in May 2008. This facet of the study was to assess the role of secondary organic aerosol (SOA) precursors (biogenic and anthropogenic) in particle nucleation. Two complementary sampling schemes were deployed in the deciduous forest. A series of multi- sorbant tubes and small canisters collected samples at two and six hour intervals, respectively. The resulting samples were analyzed by GC/MS for biogenic (ƒÑ- and ƒÒ-pinene, isoprene) and anthropogenic (substituted aromatics) VOCs. The daily evolution and diurnal variation of VOC concentrations will be presented. The role of these VOCs in particle formation and growth will also be discussed.
A13C-0268
Spatial Variability in Ultrafine Particles at Five Sites in Southern Ontario, Canada During BAQS-Met 2007 Campaign: Implication of Regional Nucleation Events
Ultrafine particle (UFP) measurements were simultaneously conducted in five urban and rural areas; Toronto, Egbert, Ridgetown, Bear Creek, and Harrow in Ontario, Canada as part of the Border Air Quality and Meteorology Study (BAQS-Met 2007). The number and size distributions of UFP in the size range of 14 to 93 nm were measured by a Fast Mobility Particle Sizer (FMPS, TSI 3091) and four Scanning Mobility Particle Sizers (SMPS) from June 19 to July 8, 2007. Continuous measurements of PM2.5 mass and gaseous pollutants (NH3, CO, SO2, NOx, O3) were also performed at the sites. The highest particle concentrations were observed at the rural site in Harrow, whereas lower concentrations were found in Toronto, a metropolitan area. The variability of hourly averaged UFP concentrations were significantly higher at the Harrow and Bear Creek sites indicating low background levels and frequent occurrence of strong particle nucleation events. Regional particle nucleation events were classified by assessing particle formation and growth rates at the five monitoring sites. The influences of pre-existing particles, gaseous pollutants and meteorological parameters on the classified particle nucleation events were evaluated. Spatial variability of UFP was examined by using the coefficient of divergence (COD) of each size bin of UFP. The COD provides relative measure of homogeneity in UPF concentrations in the southern Ontario. On average, the COD value was lower between Harrow and Bear Creek than between Harrow and Ridgetown. The COD values for smaller particles in the size range (14nm-22nm) tended to be higher than the levels for larger particles (25nm-93nm) suggesting size dependent homogeneity. In order to regional distribution and dispersion of UFP with distance, cumulative semivariogram (CSV) technique was used for UFP number concentrations at five sites. Detailed results over the measurement periods will be presented and discussed.
A13C-0269
Relating Ground-Based Aerosol Size Distributions and Vertical Mixing: Mexico City and Other Case Studies
In March 2006, during the MILAGRO field campaign in Mexico City, observations at the T0 location could often be characterized by morning conditions with high particle mass concentrations, low mixing heights, and good correlation between particle number and carbon dioxide, indicative that particle number is controlled by primary emissions. Using this correlation between particle number and carbon dioxide, an average number based-emission factor of 3.4 × 1013 #/vehicle-km has been determined. In the afternoon, the CO2 level drops during ventilation of the daily polluted layer, and the coupling between CO2 and particle number breaks down, with particle number sometimes increasing as CO2 decreases. New particle formation events were observed both simultaneous to and separately from the afternoon ventilation. While the distinction between morning conditions dominated by primary emissions and afternoon conditions dominated by mixing and photochemical particle production is particularly strong in Mexico City because of local meteorology and intense sources, we show that this pattern is common to several locations. Furthermore, the observed particle size distribution after ventilation of the nighttime inversion is a weighted average of nocturnal capping inversion and the morning boundary layer. Finally, using a vertically resolved new particle formation model, we test the hypothesis that growth rates (high in Mexico City at up to 15nm per hour) can be influenced by boundary layer growth rates in addition to condensable species concentrations, under certain assumptions about the vertical profile of new particle formation.
A13C-0270
Laboratory-Measured Sulfuric Acid-Water Binary Homogeneous Nucleation from the SO2 + OH Reaction
Binary homogeneous nucleation (BHN) of sulphuric acid and water (H2SO4/H2O) is one of the most important atmospheric nucleation processes, but laboratory observations of this nucleation process are very limited and there are also large discrepancies between different laboratory studies. The difficulties associated with these experiments include wall loss of H2SO4 and uncertainties in estimation of H2SO4 concentrations ([H2SO4]) involved in nucleation. We have developed a new laboratory nucleation setup to study H2SO4/H2O BHN kinetics and provide relatively constrained [H2SO4] needed for nucleation. H2SO4 is produced from the SO2 + OH --> HSO3 reaction and OH radicals are produced from water vapor UV absorption. The residual [H2SO4] were measured at the end of the nucleation reactor with a chemical ionization mass spectrometer (CIMS). Wall loss factors (WLFs) of H2SO4 were estimated by assuming that wall loss is diffusion limited and these calculated WLFs were in good agreement with simultaneous measurements of the initial and residual [H2SO4] with two CIMSs. The nucleation zone was estimated from numerical simulations based on the measured aerosol sizes (particle diameter, Dp) and [H2SO4]. The measured BHN rates (J) ranged from 0.01-220 cm-3 s-1 at the initial and residual [H2SO4] from 108-1010 cm-3, a temperature of 288 K and relative humidity (RH) from 11-23%; J increased with increasing [H2SO4] and RH. The measured J was proportional to sulfuric acid concentration with the second to tenth powers and proportional to RH with the eleventh to fifteenth powers. These power dependences are consistent with other laboratory studies under similar [H2SO4] and RH, but different from atmospheric field observations which showed that particle number concentrations are often linearly dependent on [H2SO4]. These results, together with a higher [H2SO4] threshold (108-109 cm-3) needed to produce the unit J measured from the laboratory studies compared to the atmospheric conditions (106-107 cm-3), imply that H2SO4/H2O BHN alone is insufficient to explain atmospheric aerosol formation and growth. Particle growth rates estimated from the measured aerosol size distributions, residence times (tr), and [H2SO4] were 100-500 nm hour-1, much higher than those seen from atmospheric field observations, because of the higher [H2SO4] used in our study. The measured aerosol sizes were proportional to log RH, showing a weak dependence of aerosol growth on RH. But the RH effects were more pronounced at higher sulfuric acid concentration, indicating that RH is more important for aerosol growth in the H2SO4 rich environment. These results are summarized in these two publications.
A13C-0271
Sulfuric acid and OH in a deciduous forest
New particle formation in the atmosphere is a global phenomenon. Gas phase sulfuric acid is proposed to play a decisive role in the formation of these new particles. Hydroxyl radical, on the other hand, is the main daytime oxidant for e.g. various organic compounds, which also can contribute the formation and subsequent growth of atmospheric aerosol particles. We will present sulfuric acid and hydroxyl radical measurement data obtained using Chemical Ionization Mass Spectrometry technique at a deciduous forest site. This study was part of "Nucleation in ForestTs" (NIFTY) campaign held in Morgan Monroe State Forest near Bloomington, IN, in May 2008.