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HONO is an important precursor of OH radical and plays a key role in atmospheric chemistry, but its source and formation mechanism remain uncertain, especially during complex atmospheric pollution processes. In this study, HONO mixing ratios were measured by a custom-made instrument during a severe pollution event from 16 to 23 December 2016, at an urban area of Beijing. The measurement was divided into three periods: I (haze), II (severe haze) and III (clean), according to the levels of PM. This pollution episode was characterized by high levels of NO (75 ± 39 and 94 ± 40 ppbV during periods I and II, respectively) and HONO (up to 10.7 ppbV). During the nighttime, the average heterogeneous conversion frequency during the two haze periods were estimated to be 0.0058 and 0.0146 h, and it was not the important way to form HONO. Vehicle emissions contributed 52% (±16)% and 40% (±18)% to ambient HONO at nighttime during periods I and II. The contribution of homogeneous reaction of NO with OH should be reconsidered under high-NO conditions and could be noticeable to HONO sources during this pollution event. Furthermore, HONO was positively correlated with PM during periods I and II, suggesting a potential chemical link between HONO and haze particles.

Continuous measurements of nitrous acid (HONO) were performed from December 12 to December 22, 2015 in both urban and suburban areas of Beijing to study the formation mechanism of HONO. The measurement campaign in both sites included a cleanhaze-clean transformation process. HONO concentrations showed similar variations in the two sites, while they were always higher in the urban area. Moreover, correlations of HONO with NO x , NO 2 , NO, PM 2.5 and relative humidity (RH) were studied to explore possible HONO formation pathways, and the contributions of direct emissions, heterogeneous reactions, and homogeneous reactions were also calculated. This showed that HONO in urban and suburban areas underwent totally different formation procedures, which were affected by meteorological conditions, PM 2.5 concentrations, direct emissions, homogeneous reactions and heterogeneous reactions. PM 2.5 concentrations and RH would influence the NO 2 conversion efficiency. Heterogeneous reactions of NO 2 were more efficient in suburban areas and in clean periods while direct emissions and homogeneous reactions contributed more in urban areas and in polluted periods when the concentrations of NO x and NO were at a high level.

Despite increased awareness of the role played by heterogeneous reactions of formic acid on mineral aerosol, the experimental determination of how these atmospheric reaction rates vary with temperature remain a crucially important part of atmosphere science. Here we report the first measurement of heterogeneous uptake of formic acid on α-Al(2)O(3) as a function of temperature (T = 240-298 K) at ambient pressure using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). From the analysis of the spectral features, crystalline HCOOH was identified at low temperature besides common product (formate ions) on the surface. It was also interesting to find that crystalline HCOOH can continue to react with α-Al(2)O(3). The reaction mechanisms at both room and low temperature were discussed. Furthermore, the reactive uptake coefficients were acquired and found to increase with decreasing temperature. Finally, the atmospheric lifetime of formic acid because of heterogeneous loss on mineral aerosol was estimated at temperatures related to the upper troposphere.

To study the HONO formation mechanisms during a pollution period, a continuous measurement was performed in both urban and suburban aeras of Beijing. During this period, the PM 2.5 concentrations increased to 201 and 137 g/m 3 in urban and suburban areas, respectively. The concentrations of HONO, CO, SO 2 , O 3 , NO, NO 2 , NO x were 1.45 ppbv, 0.61 ppmv, 8.7 ppbv, 4.3 ppbv, 44.4 ppbv, 37.4 ppbv, 79.4 ppbv and 0.72 ppbv, 1.00 ppmv, 1.2 ppbv, 7.9 ppbv, 3.7 ppbv, 8.2 ppbv, 11.9 ppbv, in urban and suburban areas, respectively. To compare possible pathways of HONO formation in both sites, the contributions of direct emissions, heterogeneous formations, and homogeneous productions were studied. HONO/NO 2 ratios in the two sites indicated that heterogeneous reactions of NO 2 were more efficient in suburban areas. And in both urban and suburban areas, the increase of PM 2.5 concentrations and RH would promote the conversion efficiency in RH that ranged from 0% to 85%. However, when RH was above 85%, the HONO formation slowed down. Moreover, the study of direct emissions and homogeneous reactions showed that they contributed to a majority of HONO increase in urban areas than the 20% contributions in suburban areas. It implied that the high NO x concentrations and NO concentrations in urban areas or in pollution periods would make direct emissions and homogeneous reactions become dominant in HONO formations.

Improving the sensitivity of electrochemiluminescence (ECL) systems is highly desired for in vitro ECL diagnosis and bio-detections due to the often-low content of biomarkers in diseases. And dissolved O 2 (DO) as a co-reactant is considered superior to H 2 O 2 in the most commonly used luminol ECL systems due to better stability and low biotoxicity, but it still suffers from low ECL performance due to the low reactivity of DO. In this study, an efficient luminol-DO ECL system was developed through the complexing of Fe, Co dual single-atom catalysts (D-SACs) supported by N-doped graphene with the luminol-capped Ag nanoparticles (AgNPs). Benefiting from the electronic interaction between Fe and Co metal sites in the relevant D-SACs and plasmon enhancement of AgNPs, the performance of the corresponding ECL system could be significantly boosted up to ≈677-fold under optimal testing conditions, comparable to the classic luminol-O 2 system. Furthermore, the developed luminol-DO ECL system was successfully applied for the stable ultrasensitive detection of prostate-specific antigen (PSA) in a wide linear range of 1 fg/mL to 1 μg/mL, with a low limit of detection (0.98 fg/mL).

In this work, the heterogeneous reactions of NO 2 with CaCO 3 -(NH 4 ) 2 SO 4 mixtures with a series of weight percentage (wt %) of (NH 4 ) 2 SO 4 were investigated using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) at different relative humidity (RH) values. For comparison, the heterogeneous reactions of NO 2 with pure CaCO 3 particles and pure (NH 4 ) 2 SO 4 particles, as well as the reaction of CaCO 3 with (NH 4 ) 2 SO 4 particles, were also studied. The results indicated that NO 2 did not show any significant uptake on (NH 4 ) 2 SO 4 particles, and it reacted with CaCO 3 particles to form calcium nitrate under both dry and wet conditions. The heterogeneous reactions of NO 2 with CaCO 3 -(NH 4 ) 2 SO 4 mixtures were markedly dependent on RH. Calcium nitrate was formed from the heterogeneous reactions at all the RHs investigated, whereas CaSO 4 · 0.5H 2 O (bassanite), CaSO 4 · 2H 2 O (gypsum), and (NH 4 ) 2 Ca(SO 4 ) 2 · H 2 O (koktaite) were produced depending on RH. Under the dry condition, the heterogeneous uptake of NO 2 on the mixtures was similar to that on CaCO 3 particles with neglectable effects from (NH 4 ) 2 SO 4 ; the duration of initial stages and the NO − 3 mass concentrations had a negative linear relation with the mass fraction of (NH 4 ) 2 SO 4 in the mixtures. Under wet conditions, the chemical interaction of (NH 4 ) 2 SO 4 with Ca(NO 3 ) 2 enhances the nitrate formation, especially at medium RHs, while the coagulation of (NH 4 ) 2 SO 4 with CaCO 3 exhibits an increasing inhibiting effects with increasing RH at the same time. In addition, the heterogeneous uptake of NO 2 on the mixtures of CaCO 3 and (NH 4 ) 2 SO 4 was found to favor the formation of bassanite and gypsum due to the decomposition of CaCO 3 and the coagulation of Ca 2+ and SO 2− 4 . A possible reaction mechanism was proposed and the atmospheric implications were discussed.

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