Observation Research On Soil HONO Emission Flux By Dynamic Chamber Method Based On BBCEAS

Gaseous nitrous acid(HONO)is an important reactive trace gas in the earth's atmosphere,which plays a key role in tropospheric atmospheric chemistry.The photolysis of HONO in the daytime contributes to the production of OH radical,which is the most important oxidation group in the atmosphere and reacts with volatile organic compounds(VOCs)to form secondary pollutants such as ozone(O3)and secondary organic aerosol(SOA).Recent studies have shown that the photolysis of HONO contributes as much as 80%to the formation of OH radical during the day.However,the mechanism of HONO formation is still unclear,especially in the daytime.As a hot issue of HONO source,soil interface emission has attracted extensive attention in recent years.However,the relevant research is still in the stage of laboratory mechanism research,lacking direct evidence of field soil emission.In order to clarify the contribution of soil interface emission to the formation of HONO,a dynamic chamber method based on broadband cavity enhanced absorption spectroscopy(BBCEAS)was carried out to observe the HONO exchange flux at the soil-atmosphere interface.The research was mainly carried out from three aspects:the optimization of BBCEAS system for flux measurement,the construction of HONO flux measurement system and field observation.The detailed research is as follows:1)The sensitivity and environmental adaptability of BBCEAS system are optimized to meet the requirements of flux measurement.An iterative algorithm for spectral retrieval of BBCEAS system is proposed to simultaneously detect atmospheric HONO and NO2.Through the application of the new retrieval algorithm,the influence of light source instability and mechanical vibration on the retrieval results is reduced.Kalman filtering technology is used to improve the measurement accuracy of BBCEAS system,and the real-time on-line high-sensitivity detection in the field flux observation is realized.The detection limits(2?)of HONO and NO2 were 0.08 ppbv and 0.14 ppbv respectively in 60 s integration time.At the same time,the system control software is used to realize the automatic operation of the system in the field observation.Aiming at the problem of missing values in field observation,a missing value processing method based on model method is proposed The missing value processing is equivalent to prediction modeling problem.A stacking ensemble learning model based on Extreme Gradient Boost(XGBoost),Light Gradient Boosting Machine(LightGBM)and Random Forest(RF)algorithm is constructed.The performance of the model is evaluated,and the reliability of the model-based method to complete missing values is verified.2)A dynamic chamber system based on BBCEAS technology was established and applied to the measurement of HONO exchange flux between atmospheric and soil interface in typical areas of North China Plain and Huaihe River Basin.In order to determine the background concentration of the HONO in the dynamic chamber and to reduce the loss and secondary generation of the HONO in the chamber,the flux measurement system adopts the dual dynamic chamber design of the reference(blank)chamber-measurement chamber.To ensure the accuracy and reliability of the flux measurements,the consistency and accuracy of the two sets of BBCEAS instruments were tested before the external field observations,and the mixing uniformity of the gas in the dynamic chamber is verified.In order to further ensure the timely exchange of material energy inside and outside the chamber,avoid the large temperature increase in the dynamic chamber,and obtain the accurate flux measurement results,the dual dynamic chamber system is optimized,and the intermittent freely closed dynamic chamber lid is designed.In order to realize the measurement of gas concentration in different dynamic chambers by a single BBCEAS instrument,a multi-channel switching device is designed.The influence of the chemical reaction and temperature and humidity in the dynamic chamber is investigated on the flux measurement results.Under the condition of closing the chamber lid for a short time,the photolysis of HONO and the change of temperature and humidity have no significant effect on the flux measurement.There is a good correlation between the given and measured flux in the dynamic chamber,and the difference is about 7%.3)The characteristics of HONO emission flux in typical areas of the North China Plain and Huaihe River Basin were obtained.The observation results of Wangdu in North China Plain showed that the HONO flux showed obvious diurnal variation within one week after fertilization,reaching the maximum value of 20.25 ng N m-2 s-1 around noon and the minimum value of 0.86 ng N m-2 s-1 in the early morning,which was one order of magnitude higher than the flux field observed in the literatures.The results of field observation showed that the high HONO flux mainly came from the direct emission of soil,rather than the heterogeneous reaction of NO2.Agricultural fertilization will affect the balance of HONO budget,and then affect the atmospheric oxidation in North China Plain.The field observation of Shouxian in Huaihe River Basin showed that the diurnal variation of HONO flux and NO flux before and after rainfall showed two different trends.Before the rainfall and in the latter stage after the rainfall,the maximum of HONO flux and NO flux appeared around 10:00,and then decreased gradually.However,in the early stage after rainfall,HONO flux and NO flux increased gradually in the morning and reached the maximum at about 13:00.During the measurement period,the maximum HONO flux was 7.69 ng N m-2 s-1,and the maximum NO flux was 34.52 ng N m-2 s-1.There was a good correlation between HONO flux and NO flux before and after rainfall(R2>0.6).The correlation between HONO flux and temperature before the rainfall and in the latter stage after the rainfall is very weak.However,there was a significant correlation between HONO flux and temperature at the early stage after rainfall(R2=0.78).The HONO fluxes of fresh soil samples were in the same order of magnitude as those observed in the field.The results of field observation show that the soil interface emission during crop growth in the region is an important source of atmospheric HONO.4)According to the results of field observation,the effects of soil temperature,soil water content,atmospheric relative humidity and soil pH on soil HONO emission were studied The relationship between soil HONO flux and pH in different literatures was summarized.The results showed that there was a positive correlation between soil pH and the logarithm of HONO flux(R2=0.33).The results of HONO emission at different temperatures show that the increase of temperature can promote HONO emission in the temperature range of 10? to 40?.With the decrease of microbial activity,soil HONO emission decreased significantly.Under the condition of low soil water content(dry soils),high humidity atmosphere can promote soil emission of HONO.Nitrate content in soil is an important factor affecting the emission of HONO,and a certain high soil water content level seems to be conducive to the emission of HONO.