The potential health impact of low-level ambient formaldehyde has been historically overlooked. We conducted a two-stage time series analysis to investigate associations between ambient formaldehyde and daily nonaccidental, circulatory, and respiratory mortality and six subtypes based on 5,325,585 deaths in 275 Chinese counties between 2013 and 2018 and estimated a concentration-response curve to identify overall associations. After controlling for confounders from meteorological factors, air pollutants, time trend, and day of the week effect, with a 1–part per billion (ppb) increase in the daily concentration of formaldehyde on lag0 day, we found that mortality risks in nonaccidental, circulatory, and respiratory diseases increased by 0.36%, 0.36% and 0.41%, respectively. The curve indicated a possible threshold concentration at approximately 5 ppb for significant impact on nonaccidental and circulatory diseases. We suggest that ambient formaldehyde may represent a potential threat to public health and needs further investigation to support timely pollution regulation and health protection.

Air quality is strongly influenced by both local emissions and regional transport. Atmospheric chemical transport models can distinguish between emissions and regional transport sources in air pollutant concentrations. However, quantifying model inventories is challenging due to emission changes caused by the recent strict control measures taken by the Chinese government. In this study, we use NO₂ column observations from the Tropospheric Monitoring Instrument to retrieve top-down nitrogen oxide (NO_X) emissions and quantify the contributions of local emissions and regional transport to NO_x in Beijing (BJ), from 1 November 2018 to 28 February 2019 (W_2018) and 1 November 2019 to 29 February 2020 (W_2019). In W_2018 and W_2019, the BJ bottom-up NO_X emissions from the multi-resolution emission inventory for China in 2017 were overestimated by 11.8% and 40.5%, respectively, and the input of NO_X from other cities to BJ was overestimated by 10.9% and 51.6%, respectively. The simulation using our adjusted inventory exhibited a much higher spatial agreement (slope = 1.0, R² = 0.79) and reduced a mean relative error by 45% compared to those of bottom-up NO_X emissions. The top-down inventory indicated that (1) city boundary transport contributes approximately 40% of the NO_X concentration in BJ; (2) in W_2019, NO_X emissions and transport in BJ decreased by 20.4% and 17.2%, respectively, compared to those of W_2018; (3) in W_2019, NO_X influx substantially decreased (−699 g/s) in BJ compared to that of W_2018 despite negative meteorological conditions that should have increased NOx influx by +503 g/s. Overall, the contribution of intercity input to NOx in BJ has declined with decreasing emissions in the surrounding cities due to regional cooperative control measures, and the role of local emissions in BJ NOx levels was more prominent. Our findings indicate that local emissions may play vital roles in regional center city air quality.

In this paper, we present the Hefei EMI Cloud Retrieval Algorithm (HECORA), which uses information from the O₂-O₂ absorption band around 477 nm to retrieve effective cloud fraction and effective cloud pressure from satellite observations. The retrieved cloud information intends to improve the atmospheric trace gas products based on the Environment Monitoring Instrument (EMI) spectrometer. The HECORA method builds on OMCLDO2 and presents some evolutions. The Vector Linearized Discrete Ordinate Radiative Transfer (VLIDORT) model has been used to produce the Top of the Atmosphere (TOA) reflectance Look-up Tables (LUT) as a function of the cloud fraction and cloud pressure. Applying the Differential Optical Absorption Spectroscopy (DOAS) technique to the synthetic reflectance LUT, the reflectance spectra can be associated with O₂-O₂ geometrical vertical column densities (VCD_geo) and continuum reflectance. This is the core of the retrieval method, since there is a one-to-one relationship between O₂-O₂ VCD_geo and continuum reflectance, on the one hand, and effective cloud fraction and effective cloud pressure, on the other hand, for a given illumination and observing geometry and given surface height and surface albedo. We first used the VLIDORT synthetic spectra to verify the HECORA algorithm and obtained good results in both the Lambertian cloud model and the scattering cloud model. Secondly, HECORA is applied to OMI and TROPOMI and compared with OMCLDO2, FRESCO+, and OCRA/ROCINN cloud products. Later, the cloud pressure results from TROPOMI observations obtained using HECORA and FRESCO+ are compared with the CALIOP Cloud Layer product. HECORA is closer to the CALIOP results under low cloud conditions, while FRESCO+ is closer to high clouds due to the higher sensitivity of the O₂ A-band to cloud vertical information. Finally, HECORA is applied to the TROPOMI NO₂ retrieval. Validation of the tropospheric NO₂ VCD with ground-based MAX-DOAS measurements shows that choosing HECORA cloud products to correct for photon path variations on the TROPOMI tropospheric NO₂ VCD retrievals has better performance than using FRESCO+ under low cloud conditions. In conclusion, this paper shows that the HECORA cloud products are in good agreement with the well-established cloud products and that they are suitable for correcting the effect of cloud in trace gas retrievals. Therefore, HECORA has the potential to be applied to EMI.

Analyzing vertical distribution characters of air pollutants is conducive to study the mechanisms under polluted atmospheric conditions. Nitrous acid (HONO) is a kind of crucial species in photochemical cycles. Exploring the influence and sources of HONO in air pollution at different altitudes offers some insights into the research of tropospheric oxidation chemistry processes. Ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements were conducted in Shanghai, China, from December 2017 to March 2018 to investigate vertical distributions and diurnal variations of trace gases (NO₂, HONO, HCHO, SO₂, and water vapor) and aerosol extinction coefficient in the boundary layer. Aerosol and NO₂ showed decreasing profile exponentially, SO₂ and HCHO concentrations were observed relatively high values in the middle layer. SO₂ was caused by industrial emissions, while HCHO was from secondary sources. As for HONO, below 0.82 km, the heterogeneous reactions of NO₂ impacted on forming HONO, while in the upper layers, vertical diffusion might be the dominant source. The contribution of OH production from HONO photolysis at different altitudes was mainly controlled by the concentration of HONO. MAX-DOAS measurements characterize the vertical structure of air pollutants in Shanghai and provide further understanding for HONO formation, which can help deploy advanced measurement platforms of regional air pollution over eastern China.

Formaldehyde (HCHO) is a serious hazardous air pollutant and crucial precursor of PM_2.5 and ozone compound pollution. There has been a dearth of HCHO research in Tibet where pressing need to protect the unique ecosystem. Therefore, this study aims to investigate the spatial-temporal distribution of HCHO from 2013 to 2021 and identify its influencing factors using satellite observations. Our findings reveal a noteworthy annual growth rate of 2.25% yr⁻¹ in HCHO vertical column density (VCD) in Tibet. This rate is comparable to that in India and even surpasses levels observed in many other regions worldwide, including eastern China. Furthermore, unlike other areas, the eastern region of Tibet exhibits no discernible seasonal pattern in HCHO VCD. The anomalous variation in HCHO concentrations in Tibet can primarily be attributed to long-distance transnational transport originating from incomplete combustion in India Assam. Our research underscores the urgent need for enhanced atmospheric environmental management in Tibet.