Additional HONO and OH Generation from Photoexcited Phenyl Organic Nitrates in the Photoreaction of Aromatics and NOx.

HONO acts as a major OH source, playing a vital role in secondary pollutant formation to deteriorate regional air quality. Strong unknown sources of daytime HONO have been widely reported, which significantly limit our understanding of radical cycling and atmospheric oxidation capacity. Here, we identify a potential daytime HONO and OH source originating from photoexcited phenyl organic nitrates formed during the photoreaction of aromatics and NOx. Significant HONO (1.56-4.52 ppb) and OH production is observed during the photoreaction of different kinds of aromatics with NOx (18.1-242.3 ppb). We propose an additional mechanism involving photoexcited phenyl organic nitrates (RONO2) reacting with water vapor to account for the higher levels of measured HONO and OH than the model prediction. The proposed HONO formation mechanism was evidenced directly by photolysis experiments using typical RONO2 under UV irradiation conditions, during which HONO formation was enhanced by relative humidity. The 0-D box model incorporated in this mechanism accurately reproduced the evolution of HONO and aromatic. The proposed mechanism contributes 5.9-36.6% of HONO formation as the NOx concentration increased in the photoreaction of aromatics and NOx. Our study implies that photoexcited phenyl organic nitrates are an important source of atmospheric HONO and OH that contributes significantly to atmospheric oxidation capacity.

Kinetic and reactivity of gas-phase reaction of acyclic dienes with hydroxyl radical in the 273-318 K temperature range.

As dienes contain two C[double bond, length as m-dash]C bonds, theoretically, they are much more chemically reactive with hydroxyl radical (˙OH) than alkenes and alkanes, and the reaction with ˙OH is one of the main atmospheric degradation routes of dienes during the daytime. In our work, rate coefficients of three types of acyclic dienes: conjugated as 3-methyl-1,3-pentadiene (3M13PD), isolated as 1,4-hexadiene (14HD), and cumulated as 1,2-pentadiene (12PD) reaction with ˙OH were measured in the temperature range of 273-318 K and 1 atm using the relative rate method. At 298 ± 3 K, the rate coefficients for those reactions were determined to be k3M13PD+OH = (15.09 ± 0.72) × 10-11, k14HD+OH = (9.13 ± 0.62) × 10-11, k12PD+OH = (3.34 ± 0.40) × 10-11 (as units of cm3 per molecule per s), in the excellent agreement with values of previously reported. The first measured temperature dependence for 3M13PD, 14HD and 12PD reaction with ˙OH can be expressed by the following Arrhenius expressions in units of cm3 per molecule per s: k3M13PD+OH = (8.10 ± 2.23) × 10-11 exp[(173 ± 71)/T]; k14HD+OH = (9.82 ± 5.10) × 10-12 exp[(666 ± 123)/T]; k12PD+OH = (1.13 ± 0.87) × 10-12 exp[(1038 ± 167)/T] (as units of cm3 per molecule per s). The kinetic discussion revealed that the relative position between these two C[double bond, length as m-dash]C could significantly affect the reactivity of acyclic dienes toward ˙OH. A simple structure-activity relationship (SAR) method was proposed to estimate the reaction rate coefficients of acyclic dienes with ˙OH.

Nitrogen and nitrous oxides emission characteristics of anoxic/oxic wastewater treatment process under different oxygen regulation strategies.

Nitrous oxide (N2O) and nitrogen oxides (NOx) (i.e., nitric oxide (NO) and nitrogen dioxide (NO2)), which could be produced in wastewater treatment process and result in greenhouse effect and atmospheric pollution, respectively, have been studied limitedly in their emission characteristics and transformation mechanisms. In this study, intelligent oxygen regulation was applied in anoxic/oxic wastewater treatment process (I-A/O), and its effects on regulating NOx and N2O transformations were extensively explored by comparing it with conventional A/O process (C-A/O). Results showed that the average emission amounts of N2O and NOx in I-A/O were 7.45 ± 0.66 mg and 1.88 ± 0.10 mg, respectively. Satisfactory reduction of N2O by 29.28 %-45.08 % was achieved in I-A/O compared to that of C-A/O, but together with increased NOx emission by 83.19 %-120.57 %. Pearson correlation and transcriptional analysis suggested that NO2--N reduction in the anoxic phase dominated N2O production, while no significant N2O production in the oxic phase was found. Hence, the reduced N2O production in I-A/O was mainly attributed to its efficient denitrification process. On the other hand, both the anoxic and oxic phases played important roles in NO production. More importantly, sufficient oxygen in I-A/O promoted the ammonia oxidation process, resulting in higher NO emission in I-A/O in the oxic phase. The imbalance in NO and N2O emissions was then amplified by the NOR enzyme, which mediates the conversion of NO to N2O in both the anoxic and oxic phases. Besides, carbon emission reduction by 31.32 %-36.50 % was obtained in I-A/O due to aeration consumption savings and greenhouse gas emissions reduction compared to C-A/O. Overall, intelligent oxygen regulation optimized the nitrogen transformation and achieved carbon emission reduction in A/O process, but special attention should be paid to the associated risk caused by increased NO emissions.