| Nitrous acid(HONO)plays a crucial role in tropospheric photochemistry by directly influencing atmospheric oxidation capacity through its contribution to OH radicals.However,the precise mechanisms of HONO formation is still not well understood,and the new HONO formation mechanism proposed by laboratory studies(e.g.soil HONO emission)lack of the vertification of field observation.The relative importance of ground and aerosol surfaces for nocturnal NO2 heterogeneous reaction and unknown daytime HONO source remain controversial.The measurement of gradient and flux is of great significance to explain the formation mechanism and source/sink of HONO.The sensitivity and environmental adaptability of broadband cavity enhanced absorption spectroscopy(BBCEAS)system were optimized to meet the requirement of vertical gradient and flux measurement.Under the premise of ensuring the consistency and accuracy of multiple systems,the nocturnal high-resolution vertical profiles of HONO and NO2 in urban boundary layer and aerodynamic gradient HONO flux in typical cropland in the Huaihe River Basin were measured to investigate the vertical distribution characteristics of HONO and NO2 in urban boundary layer at night,the dominant surface for NO2 heterogeneous reaction,the characteristics and mechanisms of HONO emission in different seasons and its contribution to the unknown daytime HONO source.The parameters influencing the stability,signal-to-noise ratio and time resolution of BBCEAS system were optimized to meet the field measurement requirements of HONO vertical distribution and aerodynamic gradient HONO flux.The influence of ambient temperature fluctuation on spectral deformation was reduced by LED and overall system thermostatic design.The off-axis parabolic mirror was used to replace the near-ultroviolet achromatic lens to improve the beam coupling efficiency.The beam coupling efficiency of the off-axis parabolic mirror was improved by 33%,and the specteal signal-to-noise ratio was increased by-1.4 times.The detection limit(1σ)of HONO and NO2 are 36 pptv and 69 pptv,respectively,at the optimized system integration time of 30 s.In consideration of the influence of HONO reactivity,sampling loss and wall losses in close-path BBCEAS technology.The open-path broadband cavity enhanced absorption spectroscopy(OP-BBCEAS)technology based on iterative retrieval algorithm was proposed to simultaneously measure HONO and NO2,which was applied to atmospheric HONO and NO2 measurement for the first time.The intercomparison of OP-BBCEAS system and close-path BBCEAS system were performed with light(PM2.5<75 μg/m3)and moderate(PM2.5>75 μg/m3)aerosol loading.The concentration of HONO and NO2 measured by two BBCEAS systems were in good agreement(R2>0.99),and the measurement differences of HONO and NO2 concentrations were less than the systematic measurement errors,which verified the feasibility of iterative algorithm applied to OP-BBCEAS system for atmospheric HONO and NO2 measurement.To ensure the accuracy and reliability of HONO vertical distribution and aerodynamic gradient HONO flux measurement,the consistency and accuracy of BBCEAS system were intercompared and verified.The results showed that BBCEAS system had good consistency(R2=0.99,ICC was close to 1),and the comparison of different HONO measurement techniques confirms the accuracy of the BBCEAS system.Aiming at the reaction surface problem of heterogeneous formation of HONO at night,the BBCEAS technology was combined with pod mobile platform of meteorological tower to perform the measurement of high-resolution vertical profiles of HONO and NO2 in urban boundary layer at night.For the first time,the high-resolution vertical profiles of HONO and NO2 at night during different pollution periods(clean and haze)were obtained.The vertical distribution of HONO at night was consistence with small-scale stratification of the nocturnal atmosphere.The relative importance of ground and aerosol surfaces to heterogeneous formation of HONO was evaluated during clean and haze periods.The results showed that the ground surface dominated HONO formation by heterogeneous conversion of NO2 during clean period,while HONO formation by heterogeneous reaction of NO2 on aerosol surface dominated the HONO formation aloft during haze period.The heterogeneous HONO formation on aerosol surface(30～300 pptv)could explain the observed increase in HONO(15～368 pptv)in the residual layer during haze period,which is an important source of HONO at night that cannot be ignored.Aiming at the research of soil HONO emission and unknown daytime HONO source,the aerodynamic gradient method together with BBCEAS technology was established and applied to measure HONO flux over cropland.The influence of HONO chemical reactions overall HONO flux was evaluated(<10%),and the uncertantiy of HONO flux was 11%.The HONO flux measurements were performed in a typical rotation cropland(rice-wheat rotation)in the Huaihe River Basin.The characteristics of HONO emission were obtained after fertilization～8 weeks in winter and over a range of agricultural management activities(including rotary tillage,flooding irrigation,fertilization,transplant rice seedlings and top-dressing)in summer,and soil-atmosphere HONO exchange flux was quantified.Significant correlations of the daytime HONO flux with the photolysis frequency J(NO2)and the product of J(NO2)×NO2 were observed in winter,suggesting a light-driven HONO source responsible for the surface HONO flux during daytime,which accounted for～23%of the unknown daytime HONO source.However,the magnitudes of the observed daytime fluxes(0.25±0.13 nmol/(m2·s))were comparable to the nocturnal values(0.27±0.13 nmol/(m2·s))in summer,and there was a good correlation(R2=0.71)between HONO flux and NO flux,suggesting that HONO could be formed and released from microbiological processes in the soil.The continuous peaks of HONO flux and NO flux were observed over rotary tillage,indicating that rotary tillage increased soil HONO emission.The higher soil temperature(26.5±2.66℃)in summer was conducive to the improvement of microbial activity,and soil HONO emission could play a more important role in the source of HONO.The photolysis of HONO was an important source of OH radicals in both winter and summer,and the peak of net OH production from HONO photolysis conincided with the peak of HONO flux,revealing the important influence of HONO emission from cropland on atmospheric regional oxidation in the Huaihe River Basin.In this paper,the research revealed the relative importance of ground and aerosol surfaces in the production of nocturnal HONO by heterogeneous reaction of NO2 at different pollution periods,quatified the characteristics of HONO emission from the typical cropland.It is of great significance for research of HONO vertical distribution and soil-atmosphere HONO exchange flux,and provide effective data support for improvement and vertification of air quality model. |
PDF Full Text Request