Evaluation of gas-particle partitioning in a regional air quality model for organic pollutants

Abstract

Abstract. Persistent organic pollutants (POPs) are of considerable concern due to their well-recognized toxicity and their potential to bioaccumulate and engage in long-range transport. These compounds are semi-volatile and, therefore, create a partition between vapour and condensed phases in the atmosphere, while both phases can undergo chemical reactions. This work describes the extension of the Community Multiscale Air Quality (CMAQ) modelling system to POPs with a focus on establishing an adaptable framework that accounts for gaseous chemistry, heterogeneous reactions, and gas-particle partitioning (GPP). The effect of GPP is assessed by implementing a set of independent parameterizations within the CMAQ aerosol module, including the Junge–Pankow (JP) adsorption model, the Harner–Bidleman (HB) organic matter (OM) absorption model, and the dual Dachs–Eisenreich (DE) black carbon (BC) adsorption and OM absorption model. Use of these descriptors in a modified version of CMAQ for benzo[a]pyrene (BaP) results in different fate and transport patterns as demonstrated by regional-scale simulations performed for a European domain during 2006. The dual DE model predicted 24.1 % higher average domain concentrations compared to the HB model, which was in turn predicting 119.2 % higher levels compared to the baseline JP model. Evaluation with measurements from the European Monitoring and Evaluation Programme (EMEP) reveals the capability of the more extensive DE model to better capture the ambient levels and seasonal behaviour of BaP. It is found that the heterogeneous reaction of BaP with O3 may decrease its atmospheric lifetime by 25.2 % (domain and annual average) and near-ground concentrations by 18.8 %. Marginally better model performance was found for one of the six EMEP stations (Košetice) when heterogeneous BaP reactivity was included. Further analysis shows that, for the rest of the EMEP locations, the model continues to underestimate BaP levels, an observation that can be attributed to low emission estimates for such remote areas. These findings suggest that, when modelling the fate and transport of organic pollutants on large spatio-temporal scales, the selection and parameterization of GPP can be as important as degradation (reactivity).

Simultaneous Measurements of PM2.5- and PM10-bound Benzo(a)pyrene in a Coastal Urban Atmosphere in Poland: Seasonality of Dry Deposition Fluxes and Influence of Atmospheric Transport

“…In the coastal regions, the contribution of low-and high-molecular PAH congeners to the total PAH budget may vary largely between seasons, due to impact of different emission sources and meteorology (Staniszewska et al, Siudek and Frankowski, 2018a). Recent modelling work by Efstathiou et al (2016) pointed out that ground-level concentrations of BaP within the southern Baltic Sea domain are higher than in other coastal areas (e.g. coasts of the Mediterranean Sea).…”

Diffusivity measurements of volatile organics in levitated viscous aerosol particles

“…All data points correspond to values of x suc between 0.38 and 0.41. Hydrodynamic radius ranges were taken from the literature (Robinson and Stokes, 2012;Mustafa et al, 1993;Ribeiro et al, 2006;Zarnitsyn et al, 2008;Müller et al, 2008;Tamba et al, 2010;Majer and Zick, 2015; Table 4 with measurement stations in the cold winter season, whereas there is good agreement in summer (Efstathiou et al, 2016). Furthermore, model-measurement discrepancies are larger for Arctic sites than for mid-latitudes (Friedman et al, 2014).…”

Reply to comments of reviewer 1.

“…Zelenyuk et al (2012) and Shrivastava et al (2017) suggested that long-range transport of PAHs trapped in highly viscous SOA may contribute to unexpectedly high particle-bound PAH concentrations in remote regions. In addition, there have been suggestions that PAHs may be trapped irreversibly within the soot matrix (Fernández et al, 2002), but we do not further discuss this mechanism here.…”