Molecular view modeling of atmospheric organic particulate matter: Incorporating molecular structure and co-condensation of water

Abstract

Most urban and regional models used to predict levels of organic particulate matter (OPM) are based on fundamental equations for gas/particle partitioning, but make the highly simplifying, anonymized-view (AV) assumptions that OPM levels are not affected by either: a) the molecular characteristics of the condensing organic compounds (other than simple volatility); or b) cocondensation of water as driven by non-zero relative humidity (RH) values. The simplifying assumptions have allowed parameterized chamber results for formation of secondary organic aerosol (SOA) (e.g., "two-product" (2p) coefficients) to be incorporated in chemical transport models. However, a return towards a less simplistic (and more computationally demanding) molecular view (MV) is needed that acknowledges that atmospheric OPM is a mixture of organic compounds with differing polarities, water, and in some cases dissolved salts. The higher computational cost of MV modeling results from a need for iterative calculations of the composition-dependent gas/particle partition coefficient values. MV modeling of OPM that considered water uptake (but not dissolved salts) was carried out for the southeast United States for the period August 29 through September 7, 2006. Three model variants were used at three universities: CMAQ-RH-2p (at PSU), UCD/CIT-RH-2p (at UCD), and CMAQ-RH-MCM (at TAMU). With the first two, MV structural characteristics (carbon number and numbers of functional groups) were assigned to each of the 2p products used in CMAQv.4.7.1 such that resulting predicted K p,i values matched those in CMAQv.4.7.1. When water uptake was allowed, most runs assumed that uptake occurred only into the SOA portion, and imposed immiscibility of SOA with primary organic aerosol (POA). (POA is often viewed as rather non-polar, while SOA is commonly viewed as moderately-to-rather polar. Some runs with UCD/CIT-RH-2p were used to investigate the effects of POA/SOA miscibility.) CMAQ-RH-MCM used MCM to generate oxidation products, and assumed miscibility of SOA and POA. In a ~500 km wide band from ACCEPTED MANUSCRIPT Louisiana through to at least North Carolina, CMAQ-RH-2p and UCD/CIT-RH-2p predicted that water uptake can increase SOA levels by as much as 50 to 100% (from a range of ~1 to 2 µg m-3 to a range of ~1 to 4 µg m-3). CMAQ-RH-MCM predicted much lower effects of water uptake on SOA levels (<10% increase). The results from CMAQ-RH-2p and UCD/CIT-RH-2p are considered more reflective of reality. In the Alabama/Georgia hotspot, both CMAQ-RH-2p and UCD/CIT-RH-2p predicted aerosol water levels that are up to nearly half the predicted SOA levels, namely ~0.5 to 2 µg m-3. Such water levels in SOA will affect particle optical properties, viscosity, gas/particle partitioning times, and rates of hydrolysis and water elimination reactions.

The effects of isoprene and NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; on secondary organic aerosols formed through reversible and irreversible uptake to aerosol water

“…Note that ALW can affect SOA formed through traditional absorptive partitioning by increasing the total concentration and decreasing the average molecular weight of the absorbing OM phase (Seinfeld and Pankow, 2003). Models predict that this phenomenon enhances SOA concentrations in the eastern USA (Pankow et al, 2015;Jathar et al, 2016) and that drying the particles will result in the evaporation of some semi-volatile SOA compounds in response to this perturbation (Pankow, 2010). However, the effect of ALW on gasparticle partitioning is more pronounced at low organic concentrations (1 to 2 µg m −3 ), and its sensitivity becomes less profound at higher OA levels (Pankow, 2010).…”

Water uptake by organic aerosol and its influence on gas/particle partitioning of secondary organic aerosol in the United States

“…UNIFAC calculates the mole fraction-based activity coefficient (z i ) for each interacting molecule using a group contribution approach. POA and the SOA model species (both semi-volatile and non-volatile) were assigned groups based on the work of Chang and Pankow (2010) and described in detail in Pankow et al (2015); for reference the groups are reproduced in Table S.1 in the supporting information. Briefly, the groups were determined using an iterative scheme in which the vapor pressure (calculated using the group contribution method SIMPOL (Pankow and Asher, 2008)) and molecular weight of the semi-volatile products were consistent with the two-product parameters in Carlton et al (2010).…”

“…This work used a simpler model (Wilson equation) to calculate activity coefficients for the OA-organic-phase water mixture and did not represent important processes such as "oligomerization" (aka accretion reactions). The second is the work by Pankow et al (2015) e a companion paper to this work e that simulates the influence of organic-phase water on SOA partitioning over a 10-day episode in the eastern US. We build on the latter study to explore the sensitivity of water uptake by OA and subsequent SOA gas/particle partitioning in different geographical domains (South Coast Air Basin (SoCAB) versus eastern US) to assumptions about the miscibility of POA and SOA and the mixing state of the aerosol (internally versus source-oriented externally mixed aerosol).…”

“…Most regional and global atmospheric models account for water uptake by inorganic salts but do not explicitly account for organic-phase water and its subsequent impact on gas/particle partitioning of semi-volatile OA. In this work, we incorporated the organic-phase water model described by Pankow et al (2015) into the UCD/CIT air quality model to simulate water uptake by OA and assessed its influence on total OA mass concentrations. The model was run for one summer month over two distinct regions: South Coast Air Basin (SoCAB) surrounding Los Angeles, California and the eastern United States (US).…”

Retention of Atmospheric Particles by Local Plant Leaves in the Mount Wutai Scenic Area, China

“…In addition, broad-leaved plants have flourishing crown canopies, and higher transpiration than needle-leaved plants. They can reduce atmospheric temperature, regulate the micro-region climate and effectively reduce the generation of secondary pollutants [24]. Broad-leaved plant leaves have characteristics that include deep grooves with large intervals on leaf surfaces, high stomatal density, abundant leaf hairs and good wettability, which are beneficial for leaves to retain particles.…”