Primary sources of HONO vary during the daytime: Insights based on a field campaign.

Nitrous acid (HONO) is an established precursor of hydroxyl (OH) radical and has significant impacts on the formation of PM2.5 and O3. Despite extensive research on HONO observation in recent years, knowledge regarding its sources and sinks in urban areas remains inadequate. In this study, we monitored the atmospheric concentrations of HONO and related pollutants, including gaseous nitric acid and particulate nitrate, simultaneously at a supersite in downtown Chengdu, a megacity in southwestern China during spring, when was chosen due to its tolerance for both PM2.5 and O3 pollution. Furthermore, we employed the random forest model to fill the missing data of HONO, which exhibited good predictive performance (R2 = 0.96, RMSE = 0.36 ppbv). During this campaign, the average mixing ratio of HONO was measured to be 1.0 ± 0.7 ppbv. Notably, during periods of high O3 and PM2.5 concentrations, the mixing ratio of HONO was >50 % higher compared to the clean period. We developed a comprehensive parameterization scheme for the HONO budget, and it performed well in simulating diurnal variations of HONO. Based on the HONO budget analysis, we identified different mechanisms that dominate HONO formation at different times of the day. Vehicle emissions and NO2 heterogeneous conversions were found to be the primary sources of HONO during nighttime (21.0 %, 30.2 %, respectively, from 18:00 to 7:00 the next day). In the morning (7:00-12:00), NO2 heterogeneous conversions and the reaction of NO with OH became the main sources (35.0 %, 32.2 %, respectively). However, in the afternoon (12:00-18:00), the heterogeneous photolysis of HNO3 on PM2.5 was identified as the most substantial source of HONO (contributing 52.5 %). This study highlights the significant variations in primary HONO sources throughout the day.

High fraction of soluble trace metals in fine particles under heavy haze in central China.

Atmospheric trace metals are a key component of particulate matter and significantly influence the atmospheric process and human health. The dissolved fraction of trace metals represents their bioavailability and exhibits high chemical activity. However, the optimum measurement method for detecting the soluble fraction of trace metals is still undetermined. The impact of variations in pollution on the soluble fraction is largely unrevealed. Therefore, in this work, a one-month field observation was conducted in Central China and different extraction solvents were used to determine the proper measurement method for the soluble fraction of trace metals and investigate the variation pattern under different pollution conditions. The findings show that solvents with acidity near that of aerosol water can better reflect the actual soluble fraction of trace metals in fine particulate matter. The soluble fraction of trace metals tends to increase with pollution level increased, demonstrating unexpectedly high health risks and chemical activity under heavy haze conditions. Our results indicate that remediation and trace metal pollution control are urgently needed.

Long-term winter observation of nitrous acid in the urban area of Beijing.

The particulate matter (PM) pollution has been significantly improved by carrying out various valid emission control strategies since 2013 in China. Meanwhile the variation trend of nitrous acid (HONO) is worthy to investigate due to its vital role in the atmospheric oxidation process. In this study, field observation in the winter is conducted to investigate the concentration of HONO in an urban area of Beijing. In the winter of 2019, the mean HONO concentration is 1.38 ppbV during the whole winter. Photo-enhanced NO2 heterogeneous reactions on the ground and aerosol surfaces were found as the possible daytime sources of HONO. Compared to O3, photolysis of HONO dominates the primary OH sources during the winter time. To understand the HONO pollution patterns by years variation, multi-year data is summarized and finds that primary pollutants including CO and NO decreased, but secondary pollutants i.e., HONO (mostly generated via secondary process) increased. Our study highlights the requirement to mitigate secondary pollution by control HONO concentration.

Reactions of C12-C14 n-Alkylcyclohexanes with Cl Atoms: Kinetics and Secondary Organic Aerosol Formation.

Long-chain alkanes are a type of intermediate volatility organic compound (IVOC) in the atmosphere and a potential source of secondary organic aerosols (SOAs). C12-C14 n-alkylcyclohexanes are important compositions of IVOCs, with considerable concentrations and emission rates. The reaction rate constants and SOA formation of the reactions of C12-C14 n-alkylcyclohexanes with Cl atoms were investigated in the present study. The reaction rate constants of the long-chain alkanes obtained via the relative-rate method at 298 ± 0.2 K (in units of ×10-10 cm3 molecule-1 s-1) were as follows: khexylcyclohexane = 5.11 ± 0.28, kheptylcyclohexane = 5.56 ± 0.30, and koctylcyclohexane = 5.74 ± 0.31. The gas-phase products of the reactions were identified as mainly small molecules of aldehydes, ketones, and acids. The particle-phase products were mostly monomers and oligomers, but there were still trimers even under high-NOx conditions. Moreover, under high-NOx conditions (urban atmosphere), the SOA yields of hexylcyclohexane are higher than that under low-NOx conditions (remote atmosphere), indicating that more attention should be given to the SOA formation of Cl-initiated n-alkylcyclohexane oxidations in polluted regions. This research can further clarify the oxidation processes and SOA formation of n-alkylcyclohexanes in the atmosphere.

Sulfate formation is dominated by manganese-catalyzed oxidation of SO2 on aerosol surfaces during haze events.

The formation mechanism of aerosol sulfate during wintertime haze events in China is still largely unknown. As companions, SO2 and transition metals are mainly emitted from coal combustion. Here, we argue that the transition metal-catalyzed oxidation of SO2 on aerosol surfaces could be the dominant sulfate formation pathway and investigate this hypothesis by integrating chamber experiments, numerical simulations and in-field observations. Our analysis shows that the contribution of the manganese-catalyzed oxidation of SO2 on aerosol surfaces is approximately one to two orders of magnitude larger than previously known routes, and contributes 69.2% ± 5.0% of the particulate sulfur production during haze events. This formation pathway could explain the missing source of sulfate and improve the understanding of atmospheric chemistry and climate change.

Light absorption properties and potential sources of brown carbon in Fenwei Plain during 2018-2019.

A distinctive kind of organic carbon aerosol that could absorb ultraviolet-visible radiation is called brown carbon (BrC), which has an important positive influence on radiative budget and climate change. In this work, we reported the absorption properties and potential source of BrC based on a seven-wavelength aethalometer in the winter of 2018-2019 at an urban site of Sanmenxia in Fenwei Plain in central China. Specifically, the mean value of BrC absorption coefficient was 59.6 ± 36.0 Mm-1 at 370 nm and contributed 37.7% to total absorption, which made a significant impact on visibility and regional environment. Absorption coefficients of BrC showed double-peak pattern, and BrC had shown small fluctuations under haze days compared with clean days. As for the sources of BrC, BrC absorption coefficients expressed strong correlations with element carbon aerosols and primary organic carbon aerosols, indicating that most of BrC originated from primary emissions. The linear correlations between trace metal elements (K, As, Fe, Mn, Zn, and Pb) and BrC absorption coefficients further referred that the major sources of BrC were primary emissions, like coal burning, biomass burning, and vehicle emissions. The moderate relationship between BrC absorption coefficients and secondary organic aerosols suggested that secondary production of BrC also played an important role. The 120 hr backward air mass trajectories analysis and concentration-weighted trajectories analysis were also used to investigate potential sources of BrC in and around this area, which inferred most parts of BrC were derived from local emissions.

Elucidating the effect of HONO on O3 pollution by a case study in southwest China.

Photolysis of nitrous acid (HONO) is one of the major sources for atmospheric hydroxyl radicals (OH), playing significant role in initiating tropospheric photochemical reactions for ozone (O3) production. However, scarce field investigations were conducted to elucidate this effect. In this study, a field campaign was conducted at a suburban site in southwest China. The whole observation was classified into three periods based on O3 levels and data coverage: the serious O3 pollution period (Aug 13-18 as P1), the O3 pollution period (Aug 22-28 as P2) and the clean period (Sep 3-12 as P3), with average O3 peak values of 96 ppb, 82 ppb and 44 ppb, respectively. There was no significant difference of the levels of O3 precursors (VOCs and NOx) between P1 and P2, and thus the evident elevation of OH peak values in P1 was suspected to be the most possible explanation for the higher O3 peak values. Considering the larger contribution of HONO photolysis to HOX primary production than photolysis of HCHO, O3 and ozonolysis of Alkenes, sensitivity tests of HONO reduction on O3 production rate in P1 are conducted by a 0-dimension model. Reduced HONO concentration effectively slows the O3 production in the morning, and such effect correlates with the calculated production rate of OH radicals from HONO photolysis. Higher HONO level supplying for OH radical initiation in the early morning might be the main reason for the higher O3 peak values in P1, which explained the correlation (R2 = 0.51) between average O3 value during daytime (10:00-19:00 LT) and average HONO value during early morning (00:00-05:00 LT). For nighttime accumulation, a suitable range of relative humidity that favored NO2 conversion within P1 was assumed to be the reason for the higher HONO concentration in the following early morning which promoted O3 peak values.

Evaluation and impact factors of indoor and outdoor gas-phase nitrous acid under different environmental conditions.

As an important indoor pollutant, nitrous acid (HONO) can contribute to the concentration of indoor OH radicals by photolysis via sunlight penetrating into indoor environments, thus affecting the indoor oxidizing capability. In order to investigate the concentration of indoor HONO and its impact factors, three different indoor environments and two different locations in urban and suburban areas were selected to monitor indoor and outdoor pollutants simultaneously, including HONO, NO, NO2, nitrogen oxides (NOx), O3, and particle mass concentration. In general, the concentration of indoor HONO was higher than that outdoors. In the urban area, indoor HONO with high average concentration (7.10 ppbV) was well-correlated with the temperature. In the suburban area, the concentration of indoor HONO was only about 1-2 ppbV, and had a good correlation with indoor relative humidity. It was mainly attributed to the heterogeneous reaction of NO2 on indoor surfaces. The sunlight penetrating into indoor environments from outside had a great influence on the concentration of indoor HONO, leading to a concentration of indoor HONO close to that outdoors.