SECONDARY ORGANIC AEROSOL FORMATION POTENTIAL IN SOUTH GEORGIA
Venus Dookwah.
ABSTRACT
Organic aerosols comprise a significant fraction of the total atmospheric particle loading and have strong correlations to climatic and health effects. Ambient aerosol is comprised of both primary and secondary components. The fraction of secondary organic aerosol was estimated for three cities in south Georgia by using ambient data collected and estimates of background organic carbon/elemental carbon ratio. Nonparametric sign correlations comparing estimated secondary organic carbon with another secondary photo-oxidation product, ozone, supported this method of quantifying secondary organic carbon. Secondary organic carbon is estimated to have contributed over 70% of total organic mass on 50% of sampled days at Columbus, 83% of sampled days at Augusta and 100% of days sampled at Macon.
Estimates of the amount of secondary organic aerosol potentially contributed by species found in mobile emissions in these cities were then determined using fractional aerosol coefficients. The main contributor of secondary organic aerosol (SOA) production in each city is toluene. It accounts for 48 % of the potential mobile emissions SOA loading. These results are relevant to ozone and PM2.5 abatement strategies.
INTRODUCTION
Approximately 10-70 percent of the total dry fine atmospheric particulate matter, is organic material [Turpin et al 2000]. PM2.5 is a US EPA regulated pollutant and the current National Ambient Air Quality Standard for PM2.5 is :
· Annual arithmetic mean of 15 mg m-3
· 24 hour average of 65 mg m-3
One of the main reasons that PM2.5 is regulated is because of its correlation to adverse human health effects such as cardiopulmonary disease, morbidity and mortality [Pope et al 1995]. PM2.5 bypasses our respiratory defenses, such as the ciliated mucous linings, and is speculated to being easily absorbed into the lining of the respiratory pathway. The organic constituent of PM2.5 is of particular significance in the mechanism of effecting hazardous health effects since it is capable of reacting synergistically with trace metals present on the same particle [Ron Wyzga EPA Supersite meeting]. The resulting potentially harmful redox reactions are one of the main reasons that the organic component of aerosols, which usually averages around 30-40 percent, requires study.
Another undesirous effect of PM2.5 relates to the possible effects of this pollutant on agricultural production. Fine particles affect the flux of solar radiation passing through the atmosphere by scattering and absorbing radiation. This can result in reduced downward photosynthetically active radiation (PAR) resulting in reduced crop yield [Chamedies et al, 1998]. For regions whose economies are strongly tied to agricultural yields, such as China, this phenomenon can have serious implications.
Organics in aerosols can modify the thermodynamic and chemical properties of atmospheric particles thereby, altering the role played by these particles in the atmosphere. According to Saxena et al, 1995, particle phase organics can alter the hygroscopic properties of the atmospheric particles. They reported that for non-urban locations organics enhance water absorption whereas for urban locations, the presence of organics inhibits water absorption of atmospheric particles.
Aerosols which serve as nuclei upon which water vapor condenses in the atmosphere are called cloud condensation nuclei (CCN). As explained by solute effects, for small particles, the higher the water solubility or wettability of an aerosol, the lower the supersaturation at which it can serve as CCN [Wallace and Hobbs]. Hence, the hygroscopic properties of organic aerosols are indeed important in this respect.
The optical and chemical properties of atmospheric particles and their ability to act as cloud condensation nuclei (CCN) depend strongly upon their affinity for water [Saxena et al 1995]. The albedo and radiative properties of clouds are determined largely by the number density of cloud condensation nuclei. Novakov and Penner, 1993, reported that organic aerosols accounted for a major part of both the total aerosol number concentration and the CCN fraction and the role played by organic aerosols was at least as important as sulphate aerosols in determining the climate effect of clouds.
Dickerson et al. 1997, reported that UV scattering by aerosols can have a substantial positive impact on the production of ground level ozone. Aerosol scattering of UV radiation was found to increase calculated boundary layer ozone mixing ratios by 20 ppbv or more and UV absorbing aerosol reduced calculated ozone mixing ratios by up to 24 ppbv.
In summary, organic aerosols are significant because:
- they can contain toxins which can cause deleterious health effects, if inhaled
as the majority of fine aerosols are too small to be efficiently trapped in bronchial passages and can reach the lungs and be absorbed into the mucous lining
- visibility and climate forcing issues are strongly influenced by organic species
- they play a role in cloud condensation nuclei, thereby affecting precipitation patterns which affects the hydrological cycle
- they contribute to photochemical reactions affecting tropospheric ozone formation and removal of atmospheric oxidizing species such as OH, O3, and NO3.
Even though a significant fraction of atmospheric aerosols consists of organic substances, little is known about source-reaction pathways and chemical composition of this organic fraction. One main reason for this lack of knowledge is due to the fact that organic particulate matter is really a complex aggregate of a wide variety of compounds which have varying chemical and thermodynamic properties [Saxena and Hildermann, 1996]. Further complications are due to the presence of multiple phases of the organics, that is, volatile, semi-volatile, and particle phases, which can interchange depending on the prevailing ambient meteorological conditions and species concentrations. Also, no single analytical technique can analyze the entire range of organics present in aerosols [Turpin et al, 2000].
SOURCES OF ORGANIC AEROSOL
Primary organic aerosol particles are emitted directly into the atmosphere by a variety of sources such as forest fires, biomass burning, oil refineries, chemical plants, pulp and paper industries, vehicular emissions, producers and users of paints and solvents, meat cooking and various agricultural activities, to name a few. Some primary aerosols are emitted from many sources, for example, n-nonadecane (C19) can be emitted from automobiles, road dust, vegetation, natural gas appliances, asphalt, boilers and wood burning [Seinfeld & Pandis, 1998]. Some primary organics are emitted by one specific type of activity and are, therefore, called tracer compounds or marker species for this particular activity type, for example,
SOURCE / TRACER COMPOUND / REFERENCEMeat cooking
Cigarette smoke
Biogenic sources / Cholesterol
Anteisoalkanes
C27, C29, C31, C33, n-alkanes / Rogge et al 1991
Rogge et al 1994
Mazurek & Simoneit 1984
Simoneit 1984
Rogge et al 1993
Secondary organic aerosols (SOA), like ozone, are formed as byproducts of gas-phase photochemical oxidation of volatile organic compounds (VOCs), but whereas the oxidation of most VOCs results in ozone formation, SOA is generally formed from the oxidation of low vapor pressure VOCs, that is, those comprised of six or more carbon atoms [Griffin et al. 1999 ; Grosjean and Seinfeld 1989]. Thus, for calculations of secondary formation potential estimates, isoprene, benzene and all aliphatic compounds with six or less carbon atoms, are not considered in this study.
Secondary organic aerosol is formed in the atmosphere by the oxidation of volatile organic gases by oxidants such as OH radical, ozone and the nitrate radical. Oxidation products which have low volatilities can condense onto existing particles in order to establish equilibrium between the gas and aerosol phases, thereby forming secondary organic aerosol via heterogeneous nucleation. Homogeneous nucleation is also a possible SOA formation mechanism. For example, a stable reaction product of cyclohexene-ozone oxidation is adipic acid. Assuming that for every 1 ppb of cyclohexene oxidation with ozone, 0.01 ppb of adipic acid is formed. The saturation mixing ratio of adipic acid is 0.08 ppb, which, based on the previous assumption requires 8 ppb of cyclohexene to be oxidized by ozone. When the adipic acid mixing ratio reaches saturation (0.08 ppb), then further cyclohexene-ozone reaction will lead to supersaturation of the gas phase adipic acid and the excess will condense onto any available aerosol particles or homogeneously nucleate resulting in SOA production. SOA production, therefore, involves two stages:
- gas phase oxidation of parent VOC, which is a chemical reaction and
- partitioning of the oxidation product between gas and particulate phases, which is a physicochemical process.
- The chemical reaction pathways involved in stage 1 are complex and not fully understood and the physicochemical processes leading to gas-to-particle partitioning are also unclear but are speculated to involve absorption, adsorption or some combination of these two processes.
Under peak photochemical smog conditions, when non-attainment of ozone and PM2.5 usually occurs, as much as eighty (80) percent of the observed organic particulate carbon can be secondary in origin [Turpin & Huntzicker, 1995].
Organic particulate matter can be speciated using a number of analytical techniques such as :
· Gas Chromatography-Mass Spectroscopy [Rogge et al 1993]
· Gas Chromatography-Flame Ionization Detector [Mazurek et al 1997]
· Carbon isotope analysis [Johnson and Dawson, 1993; Kaplan and Gordon, 1994; Hildemann et al., 1994]
· Fourier Transform Infrared Spectroscopy (FTIR) [Mylonas et al., 1991; Pickle et al., 1990]
· High Pressure Liquid Chromatography-Ultraviolet/Visible detector [Gorzelska et al., 1992]
· MALDI – Matrix Assisted Laser Desorption/Ionization [Mansoori et al., 1996]
· Thermal Desorption Particle Beam-Mass Spectroscopy [Ziemann and Tobias, 1999]
However, no analytical method by itself is able to distinguish between primary and secondary organic material. This is due to the fact that some secondary products can also be emitted by primary sources, for example, adipic acid is a by product of the cyclohexene-ozone oxidation but is also emitted from meat cooking and wood burning sources [Seinfeld and Pandis, 1998]. Hence, species can be identified but whether their source is primary or secondary really cannot be determined by analytical methods only. Additional assumptions must be used to make an estimate of the relative contribution of primary and secondary organics to total PM2.5 mass.
Knowledge of the estimated secondary organic aerosol formation potential and the main precursor species which contribute most to this fraction can lead to the institution of better controls, especially during summertime periods when photochemical conditions are ideal and exceedences are observed, and can mean the difference between attainment and non-attainment.
Because of the complexity of SOA reaction pathways, the vast number of products formed by photochemical oxidation of primary aerosol, and the costly analytical methods required for speciation, indirect methods for quantitative assessment of SOA have become very useful.
Literature review reveals three main empirical methods of estimating the secondary organic aerosol (SOA) component of PM:
· OC/EC ratios [Turpin and Huntzicker, 1991]
· Fractional Aerosol Coefficient method (FAC) [Grosjean, 1992]
· Gas/Particle Partitioning method [Pankow, 1994; Odum et al., 1996]
The first method will be used in this study to estimate the contribution of SOA to total PM2.5 mass in metropolitan cities in south Georgia and the second method will be used to estimate the relative species contribution of compounds found in mobile emissions of these cities to SOA formation.
OC/EC Ratio Method
Elemental carbon, (EC), is predominantly formed through combustion processes and is emitted into the atmosphere in particulate form. It is, therefore, a good tracer for primary carbonaceous aerosol of combustion origin. Organic aerosol can be emitted directly in particulate form (primary organic aerosol) or formed in the atmosphere from products of photochemical oxidation of precursor reactive gases called Volatile Organic Carbon (VOCs) or Reactive Organic Gases (ROGs) by various authors. The latter aerosol type is called secondary organic aerosol (SOA).
This method is based on the observation that background OC/EC ratios are much smaller than OC/EC ratios found during peak photochemical periods. This is expected since EC is unaffected by photochemical oxidation reactions whereas primary OC is the precursor of secondary OC. By participating in oxidation reactions, the OC fraction is increased resulting in an increased OC/EC ratio.
In order for this method to be used for secondary OC estimation, an estimate of the primary OC/EC ratio is first needed. OC/EC emissions vary from source to source and hence the primary OC/EC ratio will be influenced by local sources, meteorology, as well as diurnal and seasonal fluctuations in emissions. Therefore, it is only possible to determine the range in which the primary ratio is likely to fall rather than using a specific OC/EC ratio. This range will be determined by using the lowest evening/nightime average OC/EC ratio observed for each period and location studied. The rationale for this will be discussed later in this paper.
Experimental Procedure
The data used in this study were obtained during the “Fall Line Air Quality Study” (FAQS) in summer 2000. The FAQS project was initiated in response to observed poor air quality in Augusta, Macon and Columbus, which are metropolitan areas located south of Georgia’s Fall Line. Table 1 provides details on the days during which poor air quality was observed in these cities.
Table 1
Number of days with peak 8-hour averaged ozone concentrations exceeding 0.08 ppmv, 1997-1999.
Site 1997 1998 1999
Augusta 5 14 8
Macon 12 18 18
Columbus – Airport 1 8 9
Columbus – Crime Lab 2 8 13
Table 2
Site Period of sampling No. of OC/EC samples taken
Macon – Sandy Beach Park June 11-21 (11) 24 hr & (2) 12 hr
Augusta – Ft.Gordon June 25-July 10 (13) 24 hr & (3) 12 hr
Columbus – North Water Works July 13-23 (10) 24 hr & (2) 12 hr
Facility
Location of sites
Sandy Beach Park, Macon – 10 miles West of downtown Macon.
Ft. Gordon, Augusta – 12 miles SW of downtown Augusta
Lakeside High School, Augusta – 12 miles NW of downtown Augusta
North Water Works, Columbus – 4 miles N of downtown Columbus
Oxbow Learning Center, Columbus – 5 miles S of downtown Columbus.
EXPERIMENTAL
Ambient VOC samples were collected four (4) times daily, at each of the sampling sites during the sampling period, using evacuated canisters. The times selected for taking the VOC samples were ~ 0:00, 08:00, 12:00 and 17:00. The VOC samples were analyzed by The University of California, Irvine using gas chromatography / mass spectroscopy (GC/MS).
The days during which sampling was conducted, and the number of samples taken at each site is detailed in Table 2. OC/EC sampling was achieved using an insulated, temperature controlled particle composition monitoring sampling box and pump. A typical sampling setup can be seen in Figure 1.