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Nitrous acid (HONO), an important precursor of the hydroxyl radical (OH), plays a key role in atmospheric chemistry, but its sources are still debated. The production of HONO on aerosol surfaces or on ground surfaces in nocturnal atmospheres remains controversial. The vertical profile provides vertical information on HONO and NO2 to understand the nocturnal HONO production and loss. In this study, we report the first high-resolution (<2.5 m) nocturnal vertical profiles of HONO and NO2 measured from in situ instruments on a movable container that was lifted on the side wiring of a 325 m meteorological tower in Beijing, China. High-resolution vertical profiles revealed the negative gradients of HONO and NO2 in nocturnal boundary layers, and a shallow inversion layer affected the vertical distribution of HONO. The vertical distribution of HONO was consistent with stratification and layering in the nocturnal urban atmosphere below 250 m. The increase in the HONO / NO2 ratio was observed throughout the column from the clean episode to the haze episode, and relatively constant HONO/NO2 ratios in the residual layer were observed during the haze episode. Direct HONO emissions from traffic contributed 29.3 % ± 12.4 % to the ambient HONO concentrations at night. The ground surface dominates HONO production by heterogeneous uptake of NO2 during clean episodes. In contrast, the HONO production on aerosol surfaces (30–300 ppt) explained the observed HONO increases (15–368 ppt) in the residual layer, suggesting that the aerosol surface dominates HONO production aloft during haze episodes, while the surface production of HONO and direct emissions into the overlying air are minor contributors. Average dry deposition rates of 0.74±0.31 and 1.55±0.32 ppb h -1 were estimated during the clean and haze episodes, respectively, implying that significant quantities of HONO could be deposited to the ground surface at night. Our results highlight the ever-changing contributions of aerosol and ground surfaces in nocturnal HONO production at different pollution levels and encourage more vertical gradient observations to evaluate the contributions from varied HONO sources. [ABSTRACT FROM AUTHOR]

Nitrous acid (HONO) is a key determinant of the daytime radical budget in the daytime boundary layer, with quantitative measurement required to understand OH radical abundance. Accurate and precise measurements of HONO are therefore needed; however HONO is a challenging compound to measure in the field, in particular in a chemically complex and highly polluted environment. Here we report an intercomparison exercise between HONO measurements performed by two wet chemical techniques (the commercially available a long-path absorption photometer (LOPAP) and a custom-built instrument) and two broadband cavity-enhanced absorption spectrophotometer (BBCEAS) instruments at an urban location in Beijing. In addition, we report a comparison of HONO measurements performed by a time-of-flight chemical ionization mass spectrometer (ToF-CIMS) and a selected ion flow tube mass spectrometer (SIFT-MS) to the more established techniques (wet chemical and BBCEAS). The key finding from the current work was that all instruments agree on the temporal trends and variability in HONO (r2 > 0.97), yet they displayed some divergence in absolute concentrations, with the wet chemical methods consistently higher overall than the BBCEAS systems by between 12 % and 39 %. We found no evidence for any systematic bias in any of the instruments, with the exception of measurements near instrument detection limits. The causes of the divergence in absolute HONO concentrations were unclear, and may in part have been due to spatial variability, i.e. differences in instrument location and/or inlet position, but this observation may have been more associative than casual. [ABSTRACT FROM AUTHOR]

We report the development of an instrument for simultaneous fast measurements of glyoxal (CHOCHO) and NO2 based on incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) in the 438–465 nm wavelength region. The highly reflective cavity mirrors were protected from contamination by N2 purge gas. The reduction of the effective cavity length was calibrated by measuring collision-induced oxygen absorption at ∼477 nm of pure oxygen gas input with and without the N2 mirror purge gas. The detection limits of the developed system were evaluated to be 23 parts per trillion by volume (pptv, 2 σ) for CHOCHO and 29 pptv (2 σ) for NO2 with a 30 s acquisition time. A potential cross-interference of NO2 absorption on accurate CHOCHO measurements has been investigated in this study, as the absorption of NO2 in the atmosphere could often be several hundred-fold higher than that of glyoxal, especially in contaminated areas. Due to non-linear spectrometer dispersion, simulation spectra of NO2 based on traditional convolution simulation did not match the measurement spectra well enough. In this work, we applied actual NO2 spectral profile measured by the same spectrometer as a reference spectral profile in subsequent atmospheric spectral analysis and retrieval of NO2 and CHOCHO concentrations. This effectively reduced the spectral fitting residuals. The instrument was successfully deployed for 24 d of continuous measurements of CHOCHO and NO2 in the atmosphere in a comprehensive field campaign in Beijing in June 2017. [ABSTRACT FROM AUTHOR]

Nitrous acid (HONO), as a major precursor of the hydroxyl radical (OH), plays an importantrole in tropospheric photochemistry. Due to the low OH concentration, the heterogeneousconversion of NO2 is known as the primary source of HONO at night. Nevertheless, theconversion process primarily occurs on ground surface or aerosol surface is still controversial.Therefore, we measured simultaneously vertical profiles of HONO and NO2 using theincoherent broadband cavity enhanced absorption spectroscopy (IBBCEAS) which wasmounted on a mobile carriage of the 325-m meteorological observation tower in winterin Beijing [116˚ 23&#x2019;05"N, 39˚ 58&#x2019;55"E]. The carriage ascended and descendedat a rate of 0.16 ms−1 with a height limit of 240m at night, and the full verticalprofiles were measured within 1h. The time resolution of IBBCEAS was 15s (verticalresolution of 2.4m for this work) and measured the mean concentration within 2.4m.For comparison with HONO at the ground, another IBBCEAS with measurementtime resolution of 30s was mounted in a temperature-stabilized container on theground. During the entire measurement periods, based on the level of PM1.0, four differentmeteorological conditions were characterized including the clean period (P2), the transitionalperiod (P3) and the haze periods (P1 and P4). The vertical distribution of HONO is consistentwith stratification in several hundred meters of atmosphere at night. The analysis of verticalprofiles of HONO and the contribution of aerosol surface to the production of nocturnalHONO revealed that the heterogeneous conversion of NO2 on ground surface is primarysource of nocturnal HONO, and the significant HONO production on aerosol surfaces couldbe excluded at the measurement site. Acknowledgements. This work was supported by National Nature of Science Foundationof China (Grant Nos. 911544104, 41571130023). We are grateful to the team ofInstitute of Atmospheric Physics for their support, which enabled the vertical profileobservation. We also thank to researcher Pingqing Fu, researcher Yele Sun and theirgroups for providing the meteorological parameters, PM1.0 and other parameters. [ABSTRACT FROM AUTHOR]