Real-Time Characterization of Aerosol Particle Composition above the Urban Canopy in Beijing: Insights into the Interactions between the Atmospheric Boundary Layer and Aerosol Chemistry

Abstract

Despite extensive efforts into the characterization of air pollution during the past decade, real-time characterization of aerosol particle composition above the urban canopy in the megacity Beijing has never been performed to date. Here we conducted the first simultaneous real-time measurements of aerosol composition at two different heights at the same location in urban Beijing from December 19, 2013 to January 2, 2014. The nonrefractory submicron aerosol (NR-PM1) species were measured in situ by a high-resolution aerosol mass spectrometer at near-ground level and an aerosol chemical speciation monitor at 260 m on a 325 m meteorological tower in Beijing. Secondary aerosol showed similar temporal variations between ground level and 260 m, whereas much weaker correlations were found for the primary aerosol. The diurnal evolution of the ratios and correlations of aerosol species between 260 m and the ground level further illustrated a complex interaction between vertical mixing processes and local source emissions on aerosol chemistry in the atmospheric boundary layer. As a result, the aerosol compositions at the two heights were substantially different. Organic aerosol (OA), mainly composed of primary OA (62%), at the ground level showed a higher contribution to NR-PM1 (65%) than at 260 m (54%), whereas a higher concentration and contribution (15%) of nitrate was observed at 260 m, probably due to the favorable gas-particle partitioning under lower temperature conditions. In addition, two different boundary layer structures were observed, each interacting differently with the evolution processes of aerosol chemistry.

Fast particulate nitrate formation via N<sub>2</sub>O<sub>5</sub> uptake aloft in winter in Beijing

“…During the nighttime, the pNO − 3 formed aloft via N 2 O 5 uptake led to a much higher particle nitrate concentration than in the surface layer, which has been reported in many field observations (Watson and Chow, 2002;Ferrero et al, 2012;Sun et al, 2015b). The elevated pNO …”

“…(3) and (5), high S a , high NO x , low k NO 3 , or low temperature allow the N 2 O 5 uptake to more easily be located in the γ N 2 O 5 -insensitive region. Here, the critical value of the N 2 O 5 uptake coefficient (2 × 10 −3 ) was relatively low com- pared with that recommended for the surface of mineral dust (0.013, 290-300 K; Crowley et al, 2010b;Tang et al, 2017) or determined in many field experiments (e.g., S. S. Brown et al, 2006bBrown et al, , 2009Wagner et al, 2013;Morgan et al, 2015;Phillips et al, 2016;Brown et al, 2016;Wang et al, 2017b;. This suggests that the NO x loss and pNO − 3 formation by N 2 O 5 uptake were easily maximized in the pollution episode and further worsened the PM pollution.…”

“…Model studies showed that pNO − 3 varied at different heights and stressed the importance of the heterogeneous formation mechanism (Kim et al, 2014;Ying, 2011;Su et al, 2017). The mass fraction and concentration of pNO − 3 in Beijing was reported higher aloft (260 m) than at the ground level (Chan et al, 2005;Sun et al, 2015b), which was explained by favorable gas-particle partitioning aloft under lower temperature conditions. Overall, the active nighttime chemistry in the upper level plays an important role in surface PM pollution through mixing and dispersing within the planet boundary layer (PBL; Prabhakar et al, 2017), and the pollution was even worse in valley terrain regions coupled with adverse meteorological processes (Baasandorj et al, 2017;Green et al, 2015).…”

Simultaneous measurements of particle number size distributions at ground level and 260 m on a meteorological tower in urban Beijing, China

“…The meteorological variables, including wind speed (WS), wind direction (WD), relative humidity (RH), and temperature (T ), were obtained from the measurements on the meteorological tower. The operations of the HR-AMS, ACSM, and Aethalometers and subsequent data analysis are detailed in Sun et al (2015b) and Zhao et al (2017). All the data in this study are reported in Beijing local time (UTC+8 h).…”

Self-organized classification of boundary layer meteorology and associated characteristics of air quality in Beijing

“…Several studies investigated the interactions between ABL meteorology and air quality in Beijing using tower-based observations (Sun et al, , 2015Guinot et al, 2006;Guo et al, 2014). However, the results are not ideal because the tower-based observations have a low observational height (325 m).…”