Production of N ₂O ₅ and ClNO ₂ in summer in urban Beijing, China

The heterogeneous hydrolysis of dinitrogen pentoxide (inline-formulaN₂O₅) has a significant impact on both nocturnal particulate nitrate formation and photochemistry on the following day through the photolysis of nitryl chloride (inline-formulaClNO₂), yet these processes in highly polluted urban areas remain poorly understood. Here we present measurements of gas-phase inline-formulaN₂O₅ and inline-formulaClNO₂ by high-resolution time-of-flight chemical ionization mass spectrometer (ToF-CIMS) during summer in urban Beijing, China as part of the Air Pollution and Human Health (APHH) campaign. inline-formulaN₂O₅ and inline-formulaClNO₂ show large day-to-day variations with average (inline-formula±1σ) mixing ratios of inline-formula79.2±157.1 and inline-formula174.3±262.0 pptv, respectively. High reactivity of inline-formulaN₂O₅, with inline-formulaτ (inline-formulaN₂Oinline-formula₅)⁻¹ ranging from 0.20 inline-formula× 10inline-formula⁻² to 1.46 inline-formula× 10inline-formula⁻² sinline-formula⁻¹, suggests active nocturnal chemistry and a large nocturnal nitrate formation potential via inline-formulaN₂O₅ heterogeneous uptake. The lifetime of inline-formulaN₂O₅, inline-formulaτ (inline-formulaN₂O₅), decreases rapidly with the increase in aerosol surface area, yet it varies differently as a function of relative humidity with the highest value peaking at inline-formula∼ 40 %. The inline-formulaN₂O₅ uptake coefficients estimated from the product formation rates of inline-formulaClNO₂ and particulate nitrate are in the range of 0.017–0.19, corresponding to direct inline-formulaN₂O₅ loss rates of 0.00044–0.0034 sinline-formula⁻¹. Further analysis indicates that the fast inline-formulaN₂O₅ loss in the nocturnal boundary layer in urban Beijing is mainly attributed to its indirect loss via inline-formulaNO₃, for example through the reactions with volatile organic compounds and NO, while the contribution of the heterogeneous uptake of inline-formulaN₂O₅ is comparably small (7–33 %). High inline-formulaClNO₂ yields ranging from 0.10 to 0.35 were also observed, which might have important implications for air quality by affecting nitrate and ozone formation.