Observational Study Of Atmospheric Nitric Acid Based On Differential Pyrolysis-optical Cavity Ring-down Spectroscopy

Nitric acid(HNO3),as the main sink of nitrogen oxides in the atmosphere,plays an important role in the atmospheric chemical cycles of reactive nitrogen oxides.It is an important link in processes between ozone(O3)photochemical pollution and the formation of secondary fine particles(PM2.5).It also plays an important role in the formation of acid rain.HNO3 is highly adsorbable to surface and the concentration is low in air,so it's a challenge to get high time-resolution HNO3 measurement data,which limits the in-depth study on the atmospheric cycle of active nitrogen compounds(NOy),especially for the gas-particles conversion processes,to a certain extent.This work systematically reviewed the measurement methods of HNO3 and their advantages and disadvantages.A technical approach of using differential pyrolysis to decompose NOy into NO2 and then measuring NO2 by optical cavity ring-down spectroscopy is selected to study online HNO3 measurement with high time resolution.Through the design of differential pyrolysis modules,combined with an advanced NO2 measurement technology,the HNO3 online measurement system was built independently.In the laboratory,the factors affecting the pyrolysis efficiency were tested and optimized,and a calibration procedure was established.Under optimized conditions,the pyrolysis conversion efficiency of HNO3 can be higher than 94%,the detection limit is 14 ppt,and the time resolution can reach the minute level.Field observations on HNO3 were conducted on the top of the Science Building of Peking University from January 2,2021 to January 25,2021 and on the top of the Science and Technology Building of Minzu University of China from January 25,2021 to March 20,2021.The obtained time-resolved HNO3 online measurement data,combined with meteorological and PM2.5 data were used to conduct preliminary analysis.The results show that the average concentration of atmospheric total HNO3(gaseous and particulate HNO3)and gaseous HNO3 are 4.24 ppb and 0.63 ppb,respectively.The higher the relative humidity,the higher the total atmospheric HNO3 concentration,and the gaseous HNO3 has no significant relationship with the relative humidity.During the measurement period,atmospheric ?HNO3 and gaseous HNO3 have a significant positive correlation with PM2.5 and a significant negative correlation with O3.Further,by adding pyrolysis channels and pipeline design,the classification of atmospheric active nitrogen such as alkyl nitrate,hydroxy nitrate,gaseous nitric acid and total nitric acid(including particulate nitrate)can be obtained by on-line measurement at the same time.