Mathematical modeling of atmospheric fine particle-associated primary organic compound concentrations

An atmospheric transport model has been used to explore the relationship between source emissions and ambient air quality for individual particle phase organic compounds present in primary aerosol source emissions. An inventory of fine particulate organic compound emissions was assembled for the Los Angeles area in the year 1982. Sources characterized included noncatalyst- and catalyst-equipped autos, diesel trucks, paved road dust, tire wear, brake lining dust, meat cooking operations, industrial oil-fired boilers, roofing tar pots, natural gas combustion in residential homes, cigarette smoke, fireplaces burning oak and pine wood, and plant leaf abrasion products. These primary fine particle source emissions were supplied to a computer-based model that simulates atmospheric transport, dispersion, and dry deposition based on the time series of hourly wind observations and mixing depths. Monthly average fine particle organic compound concentrations that would prevail if the primary organic aerosol were transported without chemical reaction were computed for more than 100 organic compounds within an 80 km × 80 km modeling area centered over Los Angeles. The monthly average compound concentrations predicted by the transport model were compared to atmospheric measurements made at monitoring sites within the study area during 1982. The predicted seasonal variation and absolute values of the concentrations of the more stable compounds are found to be in reasonable agreement with the ambient observations. While model predictions for the higher molecular weight polycyclic aromatic hydrocarbons (PAH) are in agreement with ambient observations, lower molecular weight PAH show much higher predicted than measured atmospheric concentrations in the particle phase, indicating atmospheric decay by chemical reactions or evaporation from the particle phase. The atmospheric concentrations of dicarboxylic acids and aromatic polycarboxylic acids greatly exceed the contributions that are due to direct emissions from primary sources, confirming that these compounds are principally formed by atmospheric chemical reactions

Determination of Organic Compounds Present in Airborne Particulate Matter

Fine organic aerosol samples (d_p ≤ 2.1 µm) were collected systematically during the entire year 1982 at four urban sites in the greater Los Angeles area and at one remote station: West Los Angeles, Downtown Los Angeles, Pasadena, Rubidoux, and San Nicolas Island. Samples were taken at 6-day intervals and composited to form monthly sample sets. The aerosol sample composites were subjected to high resolution gas chromatography (HRGC) and gas chromatography/rnass spectrometry (GC/MS). The objective is to quantify the abundance and seasonal variation of individual organic compounds that may be diagnostic for the contribution of particular emission sources to the ambient organic complex. More than 80 organic compounds are quantified, including the series of a-alkanes, g-alkanoic acids, n-alkenoic acids, n-alkanals, and aliphatic dicarboxylic acids, as well as aromatic polycarboxylic acids, diterpenoids, polycyclic aromatic hydrocarbons, polycyclic aromatic ketones and quinones, nitrogen-containing organic compounds, and other organics. Primary organic aerosol constituents are readily identified, revealing an annual pattern with high winter concentrations and low summer concentrations in the Los Angeles area. In contrast, dicarboxylic acids of likely secondary origin show a reverse pattern, with high concentration in late spring/early summer. The total ambient annual average dicarboxylic acids concentration shows a steady increase when moving in the prevailing summer downwind direction from the most western urban sampling site (West Los Angeles) to the farthest eastern sampling location (Rubidoux), with an increase from 199 ng m^(-3) at West Los Angeles to 312 ng m^(-3) at Rubidoux. The occurrence of aromatic polycarboxylic acids in the fine particulate matter is discussed in detail in this study, including possible sources and formation pathways. The total aromatic polycarboxylic acid concentration reveals elevated summer concentrations when compared to the annual concentration cycle, indicating increased formation or/and emissions in summertime. Polycyclic aromatic hydrocarbons (PAH's), without exception, show low summer and high winter concentrations; whereas, polycyclic aromatic ketones (PAK's) and quinones (PAQ's) show slightly increased input/formation during early summer, indicating possible atmospheric chemical reactions involving PAH's as precursor compounds. Molecular markers characteristic of wood smoke are identified, and their concentrations change by season in close agreement with prior estimates of the seasonal use of wood as a fuel. The total mass concentration of identified aerosol organic compounds ranges from about 650 ng m^(-3) (West LA) to about 760 ng m^(-3) (Downtown LA) on an annual basis. Subdividing the total identified masses into their single compound classes reveals that n-alkanoic acids and aliphatic dicarboxylic acids make up the main portions quantified, followed by aromatic polycarboxylic acids, n-alkanes, diterpenoid acids, and polycyclic aromatic hydrocarbons. This compilation of fine organic aerosol data on a molecular level provides an extensive catalog of the organic compounds quantified, covering an entire year. Further research is underway to characterize the organic aerosol released by primary emission sources in the Los Angeles area. That study will not only provide complete characterizations of these emissions sources on a molecular basis, but in addition will enable the identification and quantification of additional organic compounds in the same airborne particle samples which otherwise would have gone unidentified in the complexity of the organic matrix inherent in fine airborne particle samples. In the future, these data from the monitoring network can be used to evaluate the predictions of mathematical models for the atmospheric transport and reaction of organic aerosol constituents defined at a molecular level

Contribution of primary aerosol emissions from vegetation-derived sources to fine particle concentrations in Los Angeles

Field measurements of the n-alkanes present in fine atmospheric aerosols show a predominance of odd carbon numbered higher molecular weight homologues (C_(27)–C_(33)) that is characteristic of plant waxes. Utilizing a local leaf wax n-alkane profile in conjunction with an air quality model, it is estimated that, at most, 0.2–1.0 μg m^(−3) of the airborne fine particulate matter (d_p < 2.1 μm) present in the Los Angeles basin could originate from urban vegetative detritus; this corresponds to approximately 1–3% of the total ambient fine aerosol burden. However, some of the observed vegetation aerosol fingerprint in the Los Angeles air may be due in part to emissions from food cooking rather than plant detritus. Seasonal trends in the ambient n-alkane patterns are examined to seek further insight into the relative importance of anthropogenic versus natural sources of vegetation-derived fine particulate matter

Molecular tracers for sources of atmospheric carbon particles : measurements and model predictions

Carbonaceous compounds are the largest contributor to the fine particulate matter in the atmosphere of urban areas. However, little is known about the concentrations, seasonal patterns, ambient chemical formation/destruction and source/receptor relationships that govern the individual compounds present in this complex organic mixture. The objective of the present research is to characterize the particulate organic compounds present in source emissions and in ambient air and to use those data to evaluate methods for computing source contributions to ambient pollutant concentrations. Airborne fine particulate matter samples collected at 4 urban sites within the Los Angeles basin during 1982 were analyzed by gas chromatography/mass spectrometry. More than 100 individual organic compounds were identified. Primary organic aerosol constituents including n-alkanes, n-alkanoic acids, polycyclic aromatic hydrocarbons, hopanes, and steranes reveal a seasonal pattern with high winter and low summer concentrations. Aliphatic dicarboxylic acids possibly formed by atmospheric reactions show a reverse pattern, with high concentrations in late spring/early summer. Next, fine particulate emissions from major urban sources were characterized. The sources tested were responsible for more than 80% of the fine carbonaceous aerosol emitted to the Los Angeles atmosphere. The identification of organic compounds that act as markers for the presence of effluents from particular source types was emphasized. It was found that fossil petroleum compounds such as hopanes and steranes can be used to trace vehicular fine particulate emissions in the urban atmosphere, iso- and anteiso-alkanes are useful tracers for cigarette smoke, cholesterol is a likely tracer for meat smoke aerosol, C29 - C33 odd carbon number n-alkanes can be used to estimate airborne vegetative detritus concentrations, and certain resin acids can be used to track wood smoke aerosols. The source emission data have been used along with an atmospheric transport model to calculate primary source contributions to the concentrations of single particle-phase organic compounds in the Los Angeles atmosphere. The predicted and measured concentrations of stable primary organic compounds agree well. This indicates that a nearly complete knowledge of source/receptor relationships for many particle-phase primary organic compounds has been achieved for the first time. The model predictions indicate that aliphatic dicarboxylic and aromatic polycarboxylic acids measured in the urban atmosphere are indeed the products of atmospheric chemical reactions

Sources of fine organic aerosol. 4. Particulate abrasion products from leaf surfaces of urban plants

Green and dead leaves from 62 plant species characteristic of the Los Angeles area were harvested and composited according to the actual leaf mass distribution for that area. To simulate leaf surface abrasion by the wind, the leaf composites were agitated in clean Teflon bags while a purified airstream flowed through. Fine particles (d_p ≤ 2 µm) shed from the leaf surfaces were extracted and analyzed using gas chromatography/mass spectrometry. Organic constituents including n-alkanes, n-alkanoic and n-alkenoic acids, n-alkanols, n-alkanals, terpenoid compounds, and trace amounts of PAH were identified and quantified. n-Alkanes showed similar concentrations in both dead and green leaf surface matter; mono-, sesqui-, and triterpenoids were depleted in dead leaf material while n-alkanoic acids were enriched in dead leaf abrasion products. It is shown that the higher molecular weight n-alkanes (C_(27)-C_(33)), with their pronounced odd/even carbon number predominance, provide a suitable marker compound assemblage for tracing vegetative detritus in the urban atmosphere

Quantification of urban organic aerosols at a molecular level: Identification, abundance and seasonal variation

Organic aerosol samples collected systematically throughout a complete annual cycle at four urban sites in southern California are examined by high-resolution gas chromatography and gas chromatography/mass spectrometry. More than 80 organic compounds are quantified and their seasonal ambient concentration patterns are discussed. Primary organic aerosol constituents are readily identified, revealing an annual pattern, with high winter and low summer concentrations. In contrast, aliphatic dicarboxylic acids of possible secondary origin show a reverse pattern, with high concentrations in late spring/early summer. Concentration patterns similar to the secondary dicarboxylic acids also are found for aromatic polycarboxylic acids, certain lower molecular weight n-alkanoic acids, a nonanal and other compounds. Molecular markers characteristic of woodsmoke are identified, and their concentrations change by season in close agreement with prior estimates of the seasonal use of wood as a fuel. This data set can be used to evaluate the predictions of mathematical models for the atmospheric transport and reaction of organic aerosol constituents defined at a molecular level