Increase In Secondary Organic Aerosol Research Articles

On-line study of the influence of seed particle acidity on ozonation reaction of pyrene

Acid-catalyzed heterogeneous reaction can lead to potentially multifold increase in secondary organic aerosol (SOA) generation, which may have a greater impact on climate forcing effects. In this study, single particle aerosol mass spectrometry (SPAMS) and scanning mobility particle sizer (SMPS) were applied to perform an online study of the chemical and physical characteristics of the pyrene-coated inorganic seed particle ozonation in a smog chamber to examine the influence of seed acidity on pyrene-SOA formation. Acidic seed particles are made of ammonium sulfate, ammonium bisulfate and sulfuric acid in different proportions. Fs, the inorganic seed composition number, is defined as a ratio of the molar concentration of SO42− to the total molar concentration of SO42− and NH4+ in this study. Suspended pyrene-coated acidic seed particles were generated by the vaporization-condensation method. Homogeneous and heterogeneous reaction products were identified and assigned. The results show that the seed particle acidity can enhance the mass yield, as well as the chemical diversity of pyrene-SOAs, and also dramatically increase the formation of new smaller particles. Kinetic analysis revealed that the first-order reaction rate constant (K) of acidic seed particles was much higher than that of neutral seed particles, with K values for Fs = 0.77 being 1.47 times that of Fs = 0.50 and 2.58 times that of Fs = 0.33. A mechanism for pyrene-SOA formation was also proposed based on real-time monitoring. For neutral seeds, the pyrene ozonation preferred ‘atom attack’ and mainly formed 1-hydroxypyrene; while for acidic seeds, the pyrene ozonation preferred ‘bond attack’ and formed a Criegee intermediate, followed by broken C–O and C–C bonds, thereby resulting in a variety of semi- and low-volatile products. This study presents a rapid analytical method for the combined use of smog chamber, SPAMS, and SMPS to immediately characterize the effect of seed particle acidity on the formation of polycyclic aromatic hydrocarbon SOA, which may deepen our understanding of new particle formation and particle growth in the atmosphere. Therefore, the present study provides a new insight for an in-depth understanding of haze pollution in China.

Increase in secondary organic aerosol in an urban environment

Abstract. The evolution of fine aerosol (PM1) species as well as the contribution of potential sources to the total organic aerosol (OA) at an urban background site in Barcelona, in the western Mediterranean basin (WMB) was investigated. For this purpose, a quadrupole aerosol chemical speciation monitor (Q-ACSM) was deployed to acquire real-time measurements for two 1-year periods: May 2014–May 2015 (period A) and September 2017–October 2018 (period B). Total PM1 concentrations showed a slight decrease (from 10.1 to 9.6 µg m−3 from A to B), although the relative contribution of inorganic and organic compounds varied significantly. Regarding inorganic compounds, SO42-, black carbon (BC) and NH4+ showed a significant decrease from period A to B (−21 %, −18 % and −9 %, respectively), whilst NO3- concentrations were higher in B (+8 %). Source apportionment revealed OA contained 46 % and 70 % secondary OA (SOA) in periods A and B, respectively. Two secondary oxygenated OA sources (OOA) were differentiated by their oxidation status (i.e. ageing): less oxidized (LO-OOA) and more oxidized (MO-OOA). Disregarding winter periods, when LO-OOA production was not favoured, LO-OOA transformation into MO-OOA was found to be more effective in period B. The lowest LO-OOA-to-MO-OOA ratio, excluding winter, was in September–October 2018 (0.65), implying an accumulation of aged OA after the high temperature and solar radiation conditions in the summer season. In addition to temperature, SOA (sum of OOA factors) was enhanced by exposure to NOx-polluted ambient and other pollutants, especially to O3 and during afternoon hours. The anthropogenic primary OA sources identified, cooking-related OA (COA), hydrocarbon-like OA (HOA), and biomass burning OA (BBOA), decreased from period A to B in both absolute concentrations and relative contribution (as a whole, 44 % and 30 %, respectively). However, their concentrations and proportion to OA grew rapidly during highly polluted episodes. The influence of certain atmospheric episodes on OA sources was also assessed. Both SOA factors were boosted with long- and medium-range circulations, especially those coming from inland Europe and the Mediterranean (triggering mainly MO-OOA) and summer breeze-driven regional circulation (mainly LO-OOA). In contrast, POA was enhanced either during air-renewal episodes or stagnation anticyclonic events.

Secondary inorganic aerosols in Europe: sources and the significant influence of biogenic VOC emissions, especially on ammonium nitrate

Abstract. Contributions of various anthropogenic sources to the secondary inorganic aerosol (SIA) in Europe as well as the role of biogenic emissions on SIA formation were investigated using the three-dimensional regional model CAMx (comprehensive air quality model with extensions). Simulations were carried out for two periods of EMEP field campaigns, February–March 2009 and June 2006, which are representative of cold and warm seasons, respectively. Biogenic volatile organic compounds (BVOCs) are known mainly as precursors of ozone and secondary organic aerosol (SOA), but their role on inorganic aerosol formation has not attracted much attention so far. In this study, we showed the importance of the chemical reactions of BVOCs and how they affect the oxidant concentrations, leading to significant changes, especially in the formation of ammonium nitrate. A sensitivity test with doubled BVOC emissions in Europe during the warm season showed a large increase in secondary organic aerosol (SOA) concentrations (by about a factor of two), while particulate inorganic nitrate concentrations decreased by up to 35 %, leading to a better agreement between the model results and measurements. Sulfate concentrations decreased as well; the change, however, was smaller. The changes in inorganic nitrate and sulfate concentrations occurred at different locations in Europe, indicating the importance of precursor gases and biogenic emission types for the negative correlation between BVOCs and SIA. Further analysis of the data suggested that reactions of the additional terpenes with nitrate radicals at night were responsible for the decline in inorganic nitrate formation, whereas oxidation of BVOCs with OH radicals led to a decrease in sulfate. Source apportionment results suggest that the main anthropogenic source of precursors leading to formation of particulate inorganic nitrate is road transport (SNAP7; see Table 1 for a description of the categories), whereas combustion in energy and transformation industries (SNAP1) was the most important contributor to sulfate particulate mass. Emissions from international shipping were also found to be very important for both nitrate and sulfate formation in Europe. In addition, we also examined contributions from the geographical source regions to SIA concentrations in the most densely populated region of Switzerland, the Swiss Plateau.

The influence of ocean halogen and sulfur emissions in the air quality of a coastal megacity: The case of Los Angeles

The oceans are the main source of natural halogen and sulfur compounds, which have a significant influence on the oxidizing capacity of the marine atmosphere; however, their impact on the air quality of coastal cities is currently unknown. We explore the effect of marine halogens (Cl, Br and I) and dimethyl sulfide (DMS) on the air quality of a large coastal city through a set of high-resolution (4-km) air quality simulations for the urban area of Los Angeles, US, using the Community Multiscale Air Quality (CMAQ model). The results indicate that marine halogen emissions decrease ozone and nitrogen dioxide levels up to 5ppbv and 2.5ppbv, respectively, in the city of Los Angeles. Previous studies suggested that the inclusion of chlorine in air quality models leads to the generation of ozone in urban areas through photolysis of nitryl chloride (ClNO2). However, we find that when considering the chemistry of Cl, Br and I together the net effect is a reduction of surface ozone concentrations. Furthermore, combined ocean emissions of halogens and DMS cause substantial changes in the levels of key urban atmospheric oxidants such as OH, HO2 and NO3, and in the composition and mass of fine particles. Although the levels of ozone, NO3 and HOx are reduced, we find a 10% increase in secondary organic aerosol (SOA) mean concentration, attributed to the increase in aerosol acidity and sulfate aerosol formation when combining DMS and bromine. Therefore, this new pathway for enhanced SOA formation may potentially help with current model under predictions of urban SOA. Although further observations and research are needed to establish these preliminary conclusions, this first city-scale investigation suggests that the inclusion of oceanic halogens and DMS in air quality models may improve regional air quality predictions over coastal cities around the world.