Nitrous acid (HONO) is one of the most important photochemical precursors of the hydroxyl radical in the sunlit urban atmosphere. The sources of HONO, however, are still poorly characterized, yet there is a disagreement between the field observations and the model results. Here, we show that light-induced NO2 heterogeneous chemistry on authentic urban grime can make an important contribution to the total HONO levels in the urban atmosphere. The obtained results indicate that the effective uptake coefficients of NO2 on urban grime in the presence of ultraviolet light [2.6 × 1015 photons cm–2 s–1 (300 nm < λ < 400 nm)] increased markedly from (1.1 ± 0.2) × 10–6 at 0% relative humidity (RH) to (5.8 ± 0.7) × 10–6 at 90% RH, exhibiting the following linear correlation with RH: γ­(NO2) = (7.4 ± 3.3) × 10–7 + (5.5 ± 0.6) × 10–8 × RH%. The flux densities of HONO mediated by light-induced heterogeneous conversion of NO2 (46 ppb) on urban grime were enhanced by ∼1 order of magnitude from (2.3 ± 0.2) × 109 molecules cm–2 s–1 at 0% RH to (1.5 ± 0.01) × 1010 molecules cm–2 s–1 at 90% RH. This study promotes light-induced NO2 chemistry on urban grime being an important source of HONO and suggests that further experiments be performed in the future.

Nitrous acid (HONO) is an important gaseous pollutant contributing to indoor air pollution because it causes adverse health effects and is the main source of hydroxyl radicals (OH). Here, we present direct measurements of HONO produced through light-induced heterogeneous reactions of NO2 with grime adsorbed on glass window. The uptake coefficients of NO2 [γ­(NO2)] on the glass plates from the kitchen increased markedly from (2.3 ± 0.1) × 10–6 at 0% RH to (4.1 ± 0.5) × 10–6 at 90% RH. We report a significant quantity of daytime HONO produced in the kitchen, compared to the living room and bedroom. Kinetic modeling suggests that phase state and bulk diffusivity play important roles in the NO2 uptake; the best fit to the measured uptake coefficients is obtained with fixed diffusion coefficients. Photon scattering may be occurring at the surface of the films, leading to enhanced photon-excitation rates of polycyclic aromatic hydrocarbons. By taking these effects into account, the results from this study indicate that the HONO yields obtained in this study can explain the missing HONO in the photochemical models describing the indoor air chemistry.

Humic-like substances (HULIS) are a class of high molecular weight, light-absorbing compounds that are highly related to brown carbon (BrC). In this study, the sources and compositions of HULIS isolated from fine particles collected in Beijing, China during the 2014 Asia-Pacific Economic Cooperation (APEC) summit were characterized based on carbon isotope (C and C) and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analyses, respectively. HULIS were the main light-absorbing components of water-soluble organic carbon (WSOC), accounting for 80.2 ± 6.1% of the WSOC absorption capacity at 365 nm. The carbon isotope data showed that HULIS had a lower non-fossil contribution (53 ± 4%) and were less enriched with C (-24.2 ± 0.6‰) relative to non-HULIS (62 ± 8% and -20.8 ± 0.3‰, respectively). The higher relative intensity fraction of sulfur-containing compounds in HULIS before and after APEC was attributed to higher sulfur dioxide levels emitted from fossil fuel combustion, whereas the higher fraction of nitrogen-containing compounds during APEC may have been due to the relatively greater contribution of non-fossil compounds or the influence of nitrate radical chemistry. The results of investigating the relationships among the sources, elemental compositions, and optical properties of HULIS demonstrated that the light absorption of HULIS appeared to increase with increasing unsaturation degree, but decrease with increasing oxidation level. The unsaturation of HULIS was affected by both sources and aging level.