1.4 Non-linear partitioning and organic volatility distributions of urban aerosols

S. Madronich, A. J. Conley, J. Lee-Taylor, A. Hodzic (NCAR/ACOM)
L. I. Kleinman (Brookhaven National Lab)
B. Aumont (U. Paris)

Urban air typically contains large amounts of organic aerosols (OA) composed of emitted hydrocarbons and their atmospheric oxidation products – thousands of different multifunctional organic molecules (aldehydes, ketones, alcohols, and organic nitrates and peroxides) – that can condense to generate particles in a size range (0.1-1.0 μm) critical to both human health and radiative budgets. We used NCAR’s detailed chemical model (Generator of Explicit Chemistry and Kinetics of Organics in the atmosphere, GECKO-A) to quantify for the first time the scaling relationships between urban emissions and consequent OA particle mass.

Particle formation and growth is controlled by (i) the saturation vapor pressures of the molecules and (ii) the existing mass of particles into which the gases can dissolve. For pure compounds, the latter condition can induce highly non-linear growth of the particle phase as a function of the total (particle + gas) burden, e.g. supersaturation of water in fogs/clouds. For complex urban OA mixtures, the extent of such non-linearity is less clear because it depends on the vapor pressures of the many different molecules involved, and such data are generally not available from either measurements or models. The question arises whether such non-linearities could be exacerbating already high pollution levels in megacities like Mexico City or Beijing.

We used GECKO-A to simulate the chemical evolution of gases and particles in urban air parcels. Because GECKO-A retains the chemical identity of all molecules produced during hydrocarbon oxidation, it allows explicit computation of thermodynamic properties such as saturation vapor pressure, for every molecule and for the entire system. Figure 1 shows the resulting distribution of organic vapor pressures in Mexico City and its outflow plume, over the course of several days: The initial aerosol (day 1) is dominated by highly volatile material, with relatively small amounts in the particle phase. As oxidation proceeds (days 2-5) the abundance of low volatility molecules increases sharply, leading to condensational growth of particle mass. A sensitivity study using this volatility distribution showed that for a 1.0% increase in the total OA burden (e.g., a 1 % increase in the emission of precursor hydrocarbons), the particle mass will increase by 1.1-1.3%, only slightly more than the 1.0% expected from simple linear growth. Such near-linearity implies that there are no disproportionate benefits of emission reductions. Rather, megacity particulate pollution is not an inevitable convergence of non-linear effects, but can be addressed (much like in smaller urban areas) by rational and proportionate reductions in emissions.

Sample template image
Figure 1:  Volatility distribution of organic aerosols in Mexico City outflow, shown as the total amount available ti (particle + gas, μg/m3) in each saturation vapor pressure range ci (μg/m3). The distribution is shown for five days, the first of which represents the urban environment while the later ones represent multiday outflow. Dashed curves show particle phase.

Madronich, S., A. Conley, J. Lee-Taylor, L. Kleinman, A. Hodzic and B. Aumont, Non-linear partitioning and organic volatility distributions in urban environments, Royal Soc. Chem. Faraday Discussions, 189, 515-528, doi:10.1039/C5FD00209E, 2016.