Quantification of total peroxy nitrates (ΣPNs) and total alkyl nitrates (ΣANs) by a dual channel thermal dissociation broadband cavity-enhanced absorption spectroscopy (TD-BBCEAS)

Correspondence: Min Qin (mqin@aiofm.ac.cn)

Organic nitrates (ONs), including peroxy nitrates (PNs, RO₂NO₂) and alkyl nitrates (ANs, RONO₂), are significant nitrogen-containing organic compounds in the atmosphere. ONs are primarily produced by the reaction of volatile organic compounds (VOCs) and atmospheric oxidants (OH radical, NO₃ radical and O₃) in the presence of nitrogen oxides (NO_x = NO + NO₂). The generation and removal of ONs play a critical role in atmospheric nitrogen cycling, secondary organic aerosol (SOA) formation and climate change. The thermal dissociation (TD) method is widely employed for quantifying total PNs (ΣPNs) and total ANs (ΣANs). It indirectly measures ΣPNs and ΣANs by selectively converting them into nitrogen dioxide (NO₂) through TD inlets maintained at specific temperatures, followed by NO₂ detection. However, the accuracy of TD method can be compromised by secondary chemical reactions between TD-generated radicals (e.g., RO₂, RO) and other atmospheric components such as NO_x. This study presents a dual-channel thermal dissociation-broadband cavity-enhanced absorption spectroscopy (TD-BBCEAS) system designed for the measurement of ΣPNs and ΣANs. Two TD inlets set at 180°C (decomposing ΣPNs to NO₂ + RO₂ radicals) and 360°C (decomposing ΣANs to NO₂ + RO radicals), achieving > 99% TD efficiency while ensuring effective separation between ΣPNs and ΣANs. The NO₂ was measured by BBCEAS within the 435 - 455 nm. Potential interferences were systematically evaluated through laboratory experiments and numerical simulations. To suppress interference from NO oxidation, O₃ was introduced to convert all NO to NO₂, enabling precise measurement of total NO_x. The quantification of ΣPNs and ΣANs was then based on the differential NOₓ (ΔNO_x) between specific inlets. Additionally, the addition of quartz wool enhanced the effective collision and consumption of RO₂/RO radicals within the inlet, thereby preventing the recombination of TD products. Laboratory mixed-gas experiments confirmed stable ΣPNs/ΣANs responses under variable NOₓ levels, validating effective interference suppression. Field observations in Hefei showed excellent agreement (R² = 0.92) between the sum of ΣPNs, ΣANs, and NO_x measured by TD-BBCEAS and total reactive nitrogen (NO_y) from a commercial analyzer, accounting for 96% of NO_y. This result further validates the feasibility of measuring ΣPNs and ΣANs.

 

Acknowledgments: This work was supported by the National Natural Science Foundation of China (Grant No. 42175151), the National Key Research & Development Program of China (No.2022YFC3701100) and the Anhui Major Provincial Science & Technology Project (No.202203a07020003).