HONO is a critical precursor of •OH, but its sources are controversial due to its complex formation mechanism. This study conducted comprehensive observations in Zhengzhou from April 26 to May 11, 2022. Low NOx concentrations were observed during the Covid epidemic period (EP) (10.4 ± 3.0 ppb), compared to the pre-epidemic period (PEP) (12.5 ± 3.8 ppb). The mean HONO concentration during EP (0.53 ± 0.34 ppb) was 0.09 ppb lower than that during PEP (0.62 ± 0.53 ppb). The decrease in HONO concentration during EP came mainly at night due to the reduction in the direct emission (Pemi) (0.03 ppb/hr), the homogeneous reaction between •OH and NO (POH+NO) (0.02 ppb/hr), and the heterogeneous conversion of NO2 on the ground (0.01 ppb/hr). Notably, there was no significant change in daytime HONO concentration. The daytime HONO budget indicated that the primary HONO sources during PEP were the nitrate photolysis (Pnitrate), followed by the POH+NO, Pemi, the photo-enhanced reaction of NO2 on the ground (Pground+hv) and aerosol surface (Paerosol+hv). The primary HONO sources were Pnitrate, POH+NO, Pemi, and Paerosol+hv during EP, respectively. The missing source has a high correlation with solar radiation, there might be other photo-related HONO sources or the contributions of photosensitized reactions were underestimated. In the extremely underestimated cases, HONO production rates from the Pnitrate, Pground+hv, and Paerosol+hv increased by 0.17, 0.10, and 0.10 ppb/hr during PEP, 0.23, 0.13, and 0.16 ppb/hr during EP, and Pnitrate was still the primary source during both PEP and EP.

HONO plays a crucial role as a precursor to OH radicals in the tropospheric atmosphere. The incongruity between HONO concentration and NOx emissions during the COVID-19 pandemic remains puzzling. Here, we show evidence from field observations of 10 sites in China where there was a noticeable increase in NH3 concentrations during the COVID-19 pandemic. In addition to the meteorological conditions, the significant decrease in sulfate and nitrate concentrations enhanced the conversion of NH4+ to NH3. Sensitivity analysis indicated that the decrease in anion concentrations (especially sulfate and nitrate) and the increase in cation concentrations during the COVID-19 pandemic led to an increase in particle pH. In other words, changes in the excess ammonia drove changes in particle pH that may consequently have impacted the rate of HONO formation. The calculation of reaction rates indicates that during the epidemic, the increase in pH may promote the generation of HONO by facilitating redox reactions, which highlights the importance of coordinating the control of SO2, NOx, and NH3 emissions.

Atmospheric hydrogen peroxide (H 2 O 2 ), as an important oxidant, plays a key role in atmospheric chemistry. To reveal its characteristics in polluted areas, comprehensive observations were conducted in Zhengzhou, China from February 22 to March 4, 2019, including heavy pollution days (HP) and light pollution days (LP). High NO concentrations (18 +/- 26 ppbv) were recorded in HP, preventing the recombination reaction of two HO 2 center dot radicals. Surprisingly, higher concentrations of H 2 O 2 were observed in HP (1.5 +/- 0.6 ppbv) than those in LP (1.2 +/- 0.6 ppbv). In addition to low wind speed and relative humidity, the elevated H 2 O 2 in HP could be mainly attributed to intensified particle-phase photoreactions and biomass burning. In terms of sulfate formation, transition-metal ions (TMI)-catalyzed oxidation emerged as the predominant oxidant pathway in both HP and LP. Note that the average H 2 O 2 oxidation rate increased from 3.6 x 10-2 in LP to 1.1 x 10-1 mu g m- 3 h-1 in HP. Moreover, the oxidation by H 2 O 2 might exceed that of TMI catalysis under specific conditions, emerging as the primary driver of sulfate formation.

We conducted a simultaneous field study of PM2.5 -bound particulate polycyclic aromatic hydrocarbons (PAHs) and aromatic acids (AAs) in a polluted city Zhengzhou to explore the concentration, sources and potential conversion pathways between PAHs and AAs in different seasons. The average concentrations of PM2.5 , 28PAHs and 8AAs during the sampling period were 77 mu g/m3 , 75 ng/m3 , and 283 ng/m3 , respectively. The concentration of both 28PAHs and 8AAs were highest in winter and lowest in summer with ratios of 6.3 and 2.3, respectively. PAHs with 5-7 rings were the main components of PAHs (52%), followed by 4 rings PAHs (30%) and 2-3 rings PAHs (18%). According to the source appointment results obtained by positive matrix factorization, the main sources of PAHs were combustion and vehicle emissions, which account for 37% and 34%, respectively. 8AAs were divided into three groups, including four benzene dicarboxylic acids (B2CAs), three benzene tricarboxylic acids (B3CAs) and one benzene tetracarboxylic acid (B4CA). And interspecies correlation analysis with PM2.5 source markers were used to investigate potential sources. Phthalic acid ( o -Ph) was the most abundant specie of 8AAs (157 ng/m3 , 55% of 8AAs), which was well correlated with sulfate. Meanwhile, B3CAs and B4CA were highly correlated with sulfate and weakly correlated with levoglucosan, suggesting that secondary formation was their main source. As logical oxidation products of PAHs, o -Ph and B3CAs showed good correlations with a number of PAHs, indicating possible photochemical oxidation pathway by PAHs. In addition, O3 , NO2 , temperature and relative humidity have positive effects on the secondary formation of B3CAs. (c) 2024 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.

Nitrous acid (HONO) is a vital precursor of the hydroxyl radicals, which plays a significant role in atmospheric chemistry. HONO can be produced by the redox reaction of NO2 with SO2. However, this reaction has often been neglected in previous studies on the HONO sources. To study this reaction on HONO production, continuous observation was carried out for one year in an inland city of China, and a sandy haze process was selected, divided into clean days (CD) and sand haze days (SHD). The mean HONO concentrations increased from 0.8 +/- 0.4 ppb (CD) to 1.4 +/- 1.0 ppb (SHD) during the nighttime. Since the rate of HONO sinks higher than that of sources, other elevation sources may contribute to the nighttime HONO concentration. In addition, the daytime HONO concentrations during the SHD periods also increased by 0.2 ppb, primarily due to the elevated photolysis of nitrate (0.20 ppb/h) and unknown sources (0.20 ppb/h) compared to the CD period. The positive correlations between HONO with NO2, SO2, particle pH, and SO42- indicate that HONO might be formed through the NO2 and SO2 redox reaction, and the calculated HONO production rates increased by two orders of magnitude under high pH conditions during the SHD period. In particular, on a heavily polluted SHD, this reaction explained 5% and 22% of HONO formation during the daytime and nighttime, respectively. In short, the NO2 and SO2 redox reaction may be more critical in high pH regions/periods and thus cannot be ignored when analyzing the HONO sources.

Peroxyacetyl nitrate (PAN) and ozone (O-3) are two typical indicators of photochemical pollution, which are harmful to human body and environment. In this study, PAN and O-3 were continuously monitored during static management (SM) of the COVID-19 epidemic in Zhengzhou. Briefly, SM refers to control policies in that all except for essential business are closed; work at home; and no people going out and no gathering, etc. During the SM, the temperature suddenly dropped from 26 degrees C to 14 degrees C, which provided a good experimental condition for the study of source transformation, accumulation, and consumption of PAN in the atmosphere. The concentrations of PAN were 0.6, 0.8, and 0.5 ppbv in three periods: before, during, and after the SM. During the SM, the lifetime of PAN was increased due to low temperature, and the accumulation time of PAN was longer, resulting in the increase of PAN concentration in daytime faster than the other two periods. During SM, especially after the temperature drops, the average atmospheric lifetime of PAN was 23.8 h, which was higher than 17.6 h before SM and 13.8 h after SM. This indicated that low temperature was more conducive to the accumulation of PAN. The high potential PAN source (CBPF>0.8) mainly came from the northeast of Zhengzhou, and the wind speed range was between 2.5 and 3 m/s. The backward trajectory analysis showed that 83.3% of the air mass from the northeast and northwest had a greater impact on the PAN concentration in Zhengzhou.