Satellite observations of nitrogen dioxide (NO₂) provide valuable information on the location and strength of NO_x emissions, but spatial resolution is limited by horizontal transport and smearing of temporal averages due to changing wind fields. In this study, we map NO_x emissions on high spatial resolution from TROPOMI observations of NO₂ combined with wind fields based on the continuity equation. The divergence of horizontal fluxes proves to be highly sensitive for point sources like exhaust stacks. Thus, NO_x emissions from individual power plants can be resolved and quantified even on top of considerably high urban pollution from the Saudi Arabian capital city Riyadh. This allows us to catalog NO_x emissions from large point sources globally, as demonstrated for South Africa and Germany, with a detection limit of about 0.11 kg/s down to 0.03 kg/s for ideal conditions.

Gaseous nitrous acid (HONO) is an important source of OH radicals in the troposphere. However, its source, especially that during daytime hours remains unclear. We present an instrument for simultaneous unambiguous measurements of HONO and NO₂ with high time resolution based on incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS). To achieve robust performance and system stability under different environment conditions, the current IBBCEAS instrument has been developed with significant improvements in terms of efficient sampling as well as resistance against vibration and temperature change, and the IBBCEAS instrument also has low power consumption and a compact design that can be easily deployed on different platforms powered by a high-capacity lithium ion battery. The effective cavity length of the IBBCEAS was determined using the absorption of O₂-O₂ to account for the shortening effect caused by the mirror purge flows. The wall loss for HONO was estimated to be 2.0 % via a HONO standard generator. Measurement precisions (2σ) for HONO and NO₂ are about 180 and 340 ppt in 30 s, respectively. A field inter-comparison was carried out at a rural suburban site in Wangdu, Hebei Province, China. The concentrations of HONO and NO₂ measured by IBBCEAS were compared with a long optical path absorption photometer (LOPAP) and a NO_x analyzer (Thermo Fisher Electron Model 42i), and the results showed very good agreement, with correlation coefficients (R²) of HONO and NO₂ being ∼ 0.89 and ∼ 0.95, respectively; in addition, vehicle deployments were also tested to enable mobile measurements of HONO and NO₂, demonstrating the promising potential of using IBBCEAS for in situ, sensitive, accurate and fast simultaneous measurements of HONO and NO₂ in the future.

Formaldehyde (HCHO), a key aerosol precursor, plays a significant role in atmospheric photo-oxidation pathways. In this study, HCHO column densities were measured using a Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) instrument at the University of Chinese Academy of Science (UCAS) in Huairou District, Beijing, which is about 50 km away from the city center. Measurements were taken during the period of 1 October 2014 to 31 December 2014, and the Asia-Pacific Economic Cooperation (APEC) summit was organized on 5–11 November. Peak values of HCHO vertical column densities (VCDs) around noon and a good correlation coefficient R² of 0.73 between HCHO VCDs and surface O₃ concentration during noontime indicated that the secondary sources of HCHO through photochemical reactions of volatile organic compounds (VOCs) dominated the HCHO values in the area around UCAS. Dependences of HCHO VCDs on wind fields and backward trajectories were identified and indicated that the HCHO values in the area around UCAS were considerably affected by the transport of pollutants (VOCs) from polluted areas in the south. The effects of control measures on HCHO VCDs during the APEC period were evaluated. During the period of the APEC conference, the average HCHO VCDs were $\sim \mathrm{38}\mathit{\%}\pm \mathrm{20}\mathit{\%}$ and $\sim \mathrm{30}\mathit{\%}\pm \mathrm{24}\mathit{\%}$ lower than that during the pre-APEC and post-APEC periods calculated at the 95 % confidence limit, respectively. This phenomenon could be attributed to both the effects of prevailing northwest wind fields during APEC and strict control measures. We also compared the MAX-DOAS results with the Copernicus Atmosphere Monitoring Service (CAMS) model. The HCHO VCDs of the CAMS model and MAX-DOAS were generally consistent with a correlation coefficient R² greater than 0.68. The peak values were consistently captured by both data datasets, but the low values were systematically underestimated by the CAMS model. This finding may indicate that the CAMS model can adequately simulate the effects of the transport and the secondary sources of HCHO but underestimates the local primary sources.

NO_X (NO_X = NO + NO₂) emissions measurements in Beijing are of great significance because they can aid in understanding how NO_X pollution develops in mega-cities throughout China. However, NO_X emissions in mega-cities are difficult to measure due to changes in wind patterns and moving sources on roads during measurement. To obtain good spatial coverage on different ring roads in Beijing over a short amount of time, two mobile differential optical absorption spectroscopy (DOAS) instruments were used to measure NO_X emission flux from April 18th to 26th, 2018. In addition, a wind profile radar provided simultaneous wind field measurements for altitudes between 50 m and 1 km for each ring road measurement. We first determined NO_X emission flux of different ring roads using wind field averages from measured wind data. The results showed that the NO_X emission flux of Beijing’s fifth ring road, which represented the urban part, varied from (19.29 ± 5.26) × 10²⁴ molec./s to (36.46 ± 12.86) × 10²⁴ molec./s. On April 20th, NO_X emission flux for the third ring was slightly higher than the fourth ring because the two ring roads were measured at different time periods. We then analyzed the NO_X emission flux error budget and error sensitivity. The main error source was the wind field uncertainty. For some measurements, the main emission flux error source was either wind speed uncertainty or wind direction uncertainty, but not both. As Beijing’s NO_X emissions came from road vehicle exhaust, we found that emission flux error had a more diverse sensitivity to wind direction uncertainty, which improved our knowledge on this topic. The NO_X emission flux error sensitivity study indicated that more accurate measurements of the wind field are crucial for effective NO_X emission flux measurements in Chinese mega-cities. Obtaining actual time and high resolved wind measurements is an advantage for mega-cities’ NO_X emission flux measurements. The emission flux errors caused by wind direction and wind speed uncertainties were clearly distinguished. Other sensitivity studies indicated that NO_X/NO₂ ratio uncertainty dominated flux errors when the NO_X/NO₂ ratio uncertainty was >0.4. Using two mobile-DOAS and wind profile radars to measure NOx emission flux improved the quality of the emission flux measuring results. This approach could be applied to many other mega-cities in China and in others countries.

Mobile differential optical absorption spectroscopy (mobile DOAS) is an optical remote sensing method that can rapidly measure trace gas emission flux from air pollution sources (such as power plants, industrial areas, and cities) in real time. Generally, mobile DOAS is influenced by wind, drive velocity, and other factors, especially in the usage of wind field when the emission flux in a mobile DOAS system is observed. This paper presents a detailed error analysis and NOx emission with mobile DOAS system from a power plant in Shijiazhuang city, China. Comparison of the SO₂ emission flux from mobile DOAS observations with continuous emission monitoring system (CEMS) under different drive speeds and wind fields revealed that the optimal drive velocity is 30–40 km/h, and the wind field at plume height is selected when mobile DOAS observations are performed. In addition, the total errors of SO₂ and NO₂ emissions with mobile DOAS measurements are 32% and 30%, respectively, combined with the analysis of the uncertainties of column density, wind field, and drive velocity. Furthermore, the NO_x emission of 0.15 ± 0.06 kg/s from the power plant is estimated, which is in good agreement with that from CEMS observations of 0.17 ± 0.07 kg/s. This study has significantly contributed to the measurement of the mobile DOAS system on emission from air pollution sources, thus improving estimation accuracy.

This paper studied the method for converting the aerosol extinction to the mass concentration of particulate matter (PM) and obtained the spatio-temporal distribution and transportation of aerosol, nitrogen dioxide (NO₂), sulfur dioxide (SO₂), and formaldehyde (HCHO) based on multi-axis differential optical absorption spectroscopy (MAX-DOAS) observations in Dalian (38.85°N, 121.36°E), Qingdao (36.35°N, 120.69°E), and Shanghai (31.60°N, 121.80°E) from 2019 to 2020. The PM_2.5 measured by the in situ instrument and the PM_2.5 simulated by the conversion formula showed a good correlation. The correlation coefficients R were 0.93 (Dalian), 0.90 (Qingdao), and 0.88 (Shanghai). A regular seasonality of the three trace gases is found, but not for aerosols. Considerable amplitudes in the weekly cycles were determined for NO₂ and aerosols, but not for SO₂ and HCHO. The aerosol profiles were nearly Gaussian, and the shapes of the trace gas profiles were nearly exponential, except for SO₂ in Shanghai and HCHO in Qingdao. PM_2.5 presented the largest transport flux, followed by NO₂ and SO₂. The main transport flux was the output flux from inland to sea in spring and winter. The MAX-DOAS and the Copernicus Atmosphere Monitoring Service (CAMS) models’ results were compared. The overestimation of NO₂ and SO₂ by CAMS is due to its overestimation of near-surface gas volume mixing ratios.