Chemical characterization of organic compounds involved in iodine-initiated new particle 1 formation from coastal macro-algal emission

13 Iodine-initiated new particle formation (I-NPF) has long been recognized in coastal hotspot 14 regions. However, no prior work has studied the exact chemical composition of organic compounds 15 and their role in the coastal I-NPF. Here we present an important complementary study to the 16 ongoing laboratory and field researches of iodine nucleation in coastal atmosphere. Oxidation and 17 NPF experiments with vapor emissions from real-world coastal macroalgae were simulated in a bag 18 reactor. On the basis of comprehensive mass spectrometry measurements, we reported for the first 19 time a variety of volatile precursors and their oxidation products in gas and particle phases in such a 20 highly complex system. Organic compounds overwhelmingly dominated over iodine in the new 21 particle growth initiated by iodine species. The identity and transformation mechanisms of organic 22 compounds were proposed in this study to provide a more complete story of coastal NPF from 23 low-tide macroalgal emission. 24


25
Coastal new particle formation (NPF) may be driven by daytime low-tide emission of iodine 26 species from macroalgae fully or partially exposed to the air. The phenomenon was reported in 27 hotspot locations of west Europe, Australia and polar regions (O'dowd et al., 2002;Heard et al., 2006;28 Mcfiggans et al., 2010;Whitehead et al., 2009;Sipilä et al., 2016;Allan et al., 2015;Baccarini et al., 29 2020;Beck et al., 2021). In the southeast coastline of China, we reported intense iodine-initiated 30 NPF based on particle number size distribution and iodine measurements (Yu et al., 2019). 31 addition were characterized by C 1,2,3,6 and O 2-3 formulas (Figure 3a). The top 7 compounds with 158 highest ion intensities were CH 2 O 2 , C 2 H 4 O 2 , C 3 H 6 O 3 , C 6 H 10 O 3 , C 2 H 6 O 2 , C 4 H 8 O 2 and C 6 H 12 O 3 , 159 which accounted for 82.5% of total ion intensity. They are C 1 -C 6 mono-carboxylic acids, hydroxyl 160 carboxylic acids or oxo-carboxylic acids with 2 to 3 oxygen atoms (Table 1). Their carbon atom 161 numbers are in general consistent with the VOCs detected by  Relatively high signals of NO 3 and HNO 3 Iwere observed before the addition of ozone to the bag 163 reactor. They were likely HNO 3 or nitrate vaporized from algal specimens or natural seawater. 164 Because NO 3 and HNO 3 Iwere also observed in the particle phase during the NPF (Figure 4), we 165 assume HNO 3 was also an important precursor of particle formation.

Gaseous organic products 221
After ozone addition, a gradual transformation from C 1 -C 3 precursors to C 5 -C 8 gaseous products 222 was observed during the NPF process ( Figure 2h). In the meanwhile, the oxygen atom number of the 223 compounds increased from 2-3 to 4-7 ( Figure 2g). The formation of compounds with more carbon 224 atoms than the parent VOCs is unlikely in the gas phase, except bimolecular reactions of stabilized  (Table 2).

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In the OH-enhanced experiment (dynamic mode), more particulate products were generated with 245 enhanced oxidation capacity: TI in the particles increased by a factor of 10.8; TOC increased by a 246 factor of 2.7; particle number concentration increased by a factor of 7.4. On the other hand, particle 247 size decreased to 73 nm and TOC/(TI+TOC) ratio decreased to 92.9% (Table 2). These differences 248 indicate that more iodine nuclei were produced with enhanced oxidation capacity, probably via

Particulate organic products 261
In the end of a typical ozonolysis experiment (dynamic mode), 100 and 364 new formulas were 262 observed in the gas and particle phases, respectively, including 73 semi-VOCs appeared in both gas and 364 organic formulas, respectively, in the particle phase. 167 organic formulas were commonly 274 observed by both methods, which accounted for 87% and 54% of total ion intensity of organic 275 formulas by the two methods, respectively ( Figure S1). As shown in Figure 3c and 3d,

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The measurement by ESI-orbitrap MS provided more insights about the formation mechanism of 280 particulate products. We compared the 336 formulas detected by ESI-orbitrap MS in our study with was found that 72% of the formulas in this study can also be found in isoprene SOA, but only 39% 284 can be found in alpha-pinene SOA. This seems to imply that some similar alkene ozonolysis 285 reactions occurred in our system and isoprene ozonolysis.  in which the C 6 , C 8 , C 14 and C 16 formulas in the equations are among the most abundant ones 300 observed in the particle phase by the iodide-CIMS.     . Oxygen and carbon atom number distributions of potential VOC precursors (a), gaseous products (b) and particulate products measured by iodide-CIMS (c), as well as the particulate products measured by ESI-orbitrap MS (d) in a typical ozonolysis experiment (dynamic mode). Hatched bars indicate the fractions of organic formulas observed in both gas and particle phases by iodide-CIMS. 你 Figure 4. Integrated ion intensities of inorganic molecules and radicals in the gas phase (static mode) and particle phase (dynamic mode) measured by iodide-CIMS in a typical ozonolysis experiment. The ions were coded in color according to their elemental composition. For each ion cluster, parent neutral molecule is on the left hand side of middle dot, while the clustering ion Ior NO 3 is on the right hand side.
Those without a clustering ion are shown as bare anions.