Measurements of higher alkanes using NO<sup>+</sup>PTR-ToF-MS: significant contributions of higher alkanes to secondary organic aerosols in China

Abstract

<p><strong>Abstract.</strong> Higher alkanes are a major class of intermediate-volatility organic compounds (IVOCs), which have been proposed to be important precursors of secondary organic aerosols (SOA) in the atmosphere. Accurate estimation of SOA from higher alkanes and their oxidation processes in the atmosphere are limited, partially due to difficulty in their measurements. High-time resolution (10 s) measurements of higher alkanes were performed using a novel online mass spectrometry method at an urban site of Guangzhou in Pearl River Delta (PRD) and at a rural site in North China Plain (NCP), respectively. High concentrations were observed in both environments, with significant diurnal variations. At both sites, SOA production from higher alkanes is estimated from their photochemical losses and SOA yields. Higher alkanes account for significant fractions of SOA formation at the two sites, with average contributions of 7.0 ± 8.0 % in Guangzhou and 7.1 ± 9.5 % in NCP, which are comparable or even higher than both single-ring aromatics and naphthalenes. The significant contributions of higher alkanes in SOA formation suggests that they should be explicitly included in current models for SOA formation. Our work also highlights the importance of NO<sup>+</sup>PTR-ToF-MS in measuring higher alkanes and quantifying their contributions to SOA formation.</p>

Measurement report: Important contributions of oxygenated compounds to emissions and chemistry of volatile organic compounds in urban air

“…The background level of formaldehyde was determined to be 0.16 ± 0.33 ppb, accounting for 24 % of its concentration. Acetone concentrations are dominated by anthropogenic primary emissions (53 %), consistent with previous studies (Yuan et al, 2012b;Wang et al, 2014a;Sahu and Saxena, 2015). The fitting results for other typical OVOCs are shown in Tables 2 and 3.…”

High Concentrations of Atmospheric Isocyanic Acid (HNCO) Produced from Secondary Sources in China

“…Accordingly, the average fractions in the particle phase ( F p ) were 0.004 ± 0.005 in PRD and 0.015 ± 0.021 in NCP. The higher F p in the NCP than in PRD is probably due to the higher concentrations of organic aerosol in the NCP (37.2 ± 26.3 μg/m 3 ) than in PRD (14.8 ± 8.7 μg/m 3 ) and the lower temperature in the NCP (0.5 ± 3.6 °C) than in PRD (24.3 ± 3.2 °C) . Since the lifetime of isocyanic acid against hydrolysis in the aqueous phase ranges from 5 h to over a month at atmospherically relevant water pH values and temperatures, the low F p in both sites suggests that only a very small portion of gaseous isocyanic acid was absorbed into the particle phase, which is consistent with the judgment that the partition into wet aerosol is not a significant sink for gaseous isocyanic acid due to the low aerosol liquid water content .…”

“…Then, assuming that Emission HNCO was 10% of the total production rate of isocyanic acid, the steady-state equation can be derived to where [HNCO], [Precursor], and [OH] are the average concentration of isocyanic acid, its precursors, and OH radicals at 14:00–16:00, respectively. The average concentration of the OH radical between 14:00 and 16:00 (3.0 ± 1.3 × 10 6 molecule cm –3 ) is determined from a simulation using a box model with MCM v3.3.1 as the chemical mechanism and constraints of observed VOCs, NO x , and other environmental parameters during the PRD campaign , (Section S12 in the Supporting Information). k loss is the first-order loss rate of isocyanic acid.…”

Intermediate‐Volatility Organic Compounds Observed in a Coastal Megacity: Importance of Non‐Road Source Emissions

“…Although SOA yields from b‐alkanes might be lower (Tkacik et al., 2012), their higher reactivity and more abundant ambient levels resulted in substantial contributions to SOA. The speciated gaseous PAHs contributed 0.06 ± 0.04 μg m −3 of the SOA production due to their lower ambient levels, while SOA production from speciated alkanes reached 0.67 ± 0.38 μg m −3 , slightly higher than that from single‐ring aromatic hydrocarbons and quite near SOA production of 0.6 ± 0.8 μg m −3 by C 8 ‐C 21 alkanes reported previously in the PRD region (Wang et al., 2020). The higher‐carbon‐number alkanes measured in their study could explain 7.0% ± 8.0% of SOA formation, which was also slightly higher than that from single‐ring aromatics (6.2% ± 7.7%).…”