Particles secondary organic aerosols

The yields of secondary organic aerosols from a series of aromatic hydrocarbon-NOx oxidations have been measured by Odum et al. (1997a, 1997b). They showed that the total secondary organic aerosol formed from a mixture of aromatic hydrocarbons can be approximated as the sum of the individual contributions. Based on their experiments, the yield of secondary organic aerosols expressed as the total organic particle mass concentrations formed, AM, (in fxg m 3), divided by the mass concentration of aromatic precursor reacted, A (aromatic), is given by... 

The yield of secondary organic aerosol depends on the organic particle mass concentration because of the gas-particle partioning of the semivolatile organic products (see later). Thus, Odum et al. (1996) showed that the yield of secondary organic aerosol, Y, is given by... 

In Eq. (LL), M is the concentration of the condensed-phase organic (in igm 3) available to absorb semivolatile organic products, ( is a constant that relates the concentration of the ith secondary organic aerosol component formed, C, to the amount of parent precursor organic reacted i.e., C, (ng m ) 1000a, A(parent organic in p,g m 3), and Kom i is the gas-particle partioning coefficient for the ith component. As discussed in more detail in Section D, Kim j is in effect an equilibrium constant between the condensed- and gas-phase concentrations. 

Thus, if a particle secondary oxidation product does not get partitioned efficiently into the condensed phase (i.e., KtmJ is small) or the available organic condensed phase for uptake of the semivolatile product is small, Eq. (LL) reduces to Y M(lE ,/fnlI1/ and the secondary organic aerosol yield is proportional to the amount of condensed phase available for uptake of the low-volatily gaseous products. On the other hand, if KomJ and M are large, Eq. (LL) becomes Y Ea, independent of the amount of condensed phase available for product uptake. 

In short, the same types of aerosol organic products have been identified both in model systems and in polluted urban ambient air and can generally be rationalized based on the oxidation of known constituents of air. The measured yields of organics in the particles can depend on the amount of particle phase available for uptake of the organic if it is semivolatile and partitions between the gas and condensed phases. This partitioning, and its dependence on the amount of condensed phase available, may be at least in part responsible for discrepancies in the yields of secondary organic aerosol reported in a number of studies. 

Figure 9.63, for example, plots log K against log pL for the partitioning of a series of PAH (see Chapter 10) between the gas phase and particles of either dioctyl phthalate (DOP) or secondary organic aerosol (SOA) generated from the photooxidation of gasoline vapor (Liang et al., 1997). The slope of the plot for uptake into DOP is 1.09 and that for uptake into SOA is... 

FIGURE 9.63 Plots of om-phase-normalized gas-particle partitioning constant log Kp iun vs logarithm of the subcooled liquid vapor pressure, log pL, for a series of semivolatile PAHs partitioning on ( ) dioctyl phthalate (DOP) or (a) secondary organic aerosol (SOA) from photooxidized gasoline vapor. PAHs are as follows naphthalene, A acenaphthalene, B fluorene, C and C phenanthrene, D and D anthracene, E and E fluoranthene, F and F pyrene, G and G chrysene, H (adapted from Liang el al., 1997). 

Bowman, F. M., J. R. Odum, J. H. Seinfeld, and S. N. Pandis, Mathematical Model for Gas-Particle Partitioning of Secondary Organic Aerosols, Atmos. Environ., 31, 3921-3931 (1997). 

Biogenic particles which comprise primary (fungal spores, bacteria, viruses, plant debris) and secondary organic aerosol (SOA) from biogenic non-methane VOCs are part of the commonly measured organic carbon fraction. Model results [51] indicate a... 

Also special care should be taken to reduce uncertainties on emission data and measurements. The validation of an aerosol model requires the analysis of the aerosol chemical composition for the main particulate species (ammonium, sulphate, nitrate and secondary organic aerosol). To find data to perform this kind of more complete evaluation is not always easy. The same applies to emissions data. The lack of detailed information regarding the chemical composition of aerosols obliges modellers to use previously defined aerosols components distributions, which are found in the literature. Present knowledge in emission processes is yet lacunal, especially concerning suspension and resuspension of deposited particles [37]. 

Certainly a number of aspects are not covered by this overview, such as ultrafine particle or secondary organic aerosol formation processes and their roles on air quality degradation, urban-scale dispersion models for air quality modelling or the... 

Limbeck, A., Kulmala, M., and Puxbaum, H. (2003). Secondary organic aerosol formation in the atmosphere via heterogeneous reaction of gaseous isoprene on acidic particles. Geophys. Res. Letters 30,1996, doi 10.1029/2003GL017738. 

Similarly, the relative humidity has a strong influence on the chemical composition of the secondary organic aerosol formed in the atmosphere by the reaction of ozone with 1-tetradecene <2000EST2116> thermal desorption particle beam mass spectrometric determinations found that the main products are a-hydroxytridecyl hydroperoxide and a peroxy-hemiaceta 1. 

Odum J. R., Hoffmann T., Bowman F., Collins D., Flagan R. C. and Seinfeld J. H. Gas/particle partitioning and secondary organic aerosol yields. Envir. Sci. Technol. 1996,30,2580-85. 

Qriffin, R. J., D. Dabdub, J. H. Seinfeld Development and initial evaluation of a dynamic species-resolved model for gas phase chemistry and size-resolved gas/particle partitioning associated with secondary organic aerosol formation, J. Geophys. Res. 110 (2005) doi 10.1029/2004JD005219... 

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