Size-resolved hygroscopicity of ambient submicron particles in a suburban atmosphere

Abstract

This work presents a comprehensive study of the size-resolved particle hygroscopicity measurements in a suburban ambient in Southern Europe (Madrid). Measurements were obtained by an H-TDMA instrument during a 16-month period for the size particles of 50, 80, 110, 190 and 265 nm. The annual, seasonal and daily patterns were investigated with the aim of characterizing the hygroscopic growth of ambient aerosol particles and their mixing state in the atmosphere. Simultaneously, the particle number size distributions, gaseous species (SO2, NO, NO2, and O3) and meteorological factors were measured and analyzed, complementing this study. The growth factor probability density functions (GF-PDFs) at the measurement site normally exhibit a multimodal pattern with two hygroscopic groups of particles clearly differentiated, indicating that the aerosol particles were externally mixed. Generally, particle hygroscopicity increased as particles were larger, indicating an increase less-to-more hygroscopic species. In cold seasons, aerosol particles were more externally mixed than in warm seasons. Thus, the growth factor of the more hygroscopic particle group (GFMH) was higher than in warm seasons varying from 1.41 (50 nm)-1.70 (265 nm). However, the number fraction of more hygroscopic particle group (NFMH) was lower. The persistence of anticyclonic stagnation conditions and the enhanced particulate nitrate formation from traffic emissions seemed to be the two main factors responsible for this result. A dominant hygroscopic particle group was often observed in warm seasons, due to frequent NPF bursts. In these seasons, and probably associated to the high atmospheric oxidative capacity, the highest average growth factors (GFmean) of the study period were obtained. This finding was also reflected in the diurnal cycle. Thus, GFmean and NFMH during rush-hour periods usually reached the minimum values of the day, especially in cold seasons. When comparing weekdays and weekends, the strong influence of traffic emissions on particle hygroscopicity was evidenced (WKs/WEs ratios for NFMH were<1.00). The more dispersive atmospheric conditions in the warm seasons hampered the impact of traffic emissions on particle hygroscopicity. Results from H-TDMA measurements obtained here are similar to those found in other suburban European sites and lower than in suburban Asian ones. With respect to urban sites, our values were lower than those found in urban European and Asian sites. Overall, the findings of this study highlight the strong dependence of the hygroscopic behavior of aerosol particles on the atmospheric conditions and particle sources, and thus, aerosol composition.