Observational Study Of Atmospheric Hydrogen Peroxide At Urban And Rural Sites In North China

Measurements of gaseous H₂O₂ were made from 13 Nov to 24 Nov,2016 at Gucheng,a rural site in Huabei province and from 27 Dec,2016 to 28 Apr,2017 at CMA,an urban site in the northwest of mega-city Beijing,using a H₂O₂ analyzer.The concentration level,variation and influence factors under different conditions were analyzed with several simultaneously observed pollutants(O₃,PAN,NO_X,PM_2.5,etc.)and meteorological parameters.The mean mixing ratio of Beijing for the entire period was（0.65±0.59）ppbv,with a higher mean of（0.83±0.67）ppbv in spring and a lower mean of（0.51±0.47）ppbv in winter.The concentration of H₂O₂ showed pronounced diurnal cycles with peaks in the period of18:00-21:00,occuring later than those reported for other sites in China or other countries and shows a delay of about 4 to 7 hours compared with the peaking time of O₃.H₂O₂,O₃ and PAN showed different diurnal patterns and levels under different pollution conditions.H₂O₂showed smaller average levels for clean and hazy days,with a higher peak but a lower level in11:00-15:00 under the clean condition.O₃ showed a higher mixing ratio under clean condition than under hazy condition,while PAN revealed an opposite trend.Results also indicated that dynamical transport could be an important influencing factor of the variations and levels of H₂O₂ and O₃.The H₂O₂ level was found to be negatively correlated with relative humidity（RH）,especially when only considering the maximum H₂O₂ level under RH over 55%.This is consistent with the uptake of gaseous H₂O₂ by water-containing aerosol particles under higher RH conditions.The H₂O₂ peaking time and peak level were closely related with RH as well as other factors,such as the NO_X level.Under conditions of daily RH lower than 55%the mean H₂O₂ level could reach a peak value of 1.52 ppbv with peaking times in 18:00-24:00,while it peaked lower（1 ppbv）and earlier（before 17:00）with the daily mean RH higher than65%.The impact of photochemistry on haze formation in colder months in the urban environment of Beijing and its feedback warrant further studies,particularly the role of H₂O₂in the formation of sulfate aerosol.The concentration of H₂O₂ at Gucheng averaged（0.55±0.41）ppbv with a maximum of2.22 ppbv,which is close to the level of Beijing in Winter.The diurnal circle of H₂O₂ in Gucheng reached a peak at 13:00,1 hour ealier than O₃ in Gucheng,which was 5 to 8 hours ealier than the H₂O₂ peak showed in Beijing.The differences of the meteorological and pollution situations between mega-city and rural areas in Huabei leaded to different H₂O₂levels and trends.The concentrations of H₂O₂ and O₃ at both sites were positively correlated and the nighttime correlations were more significant which implicated that they shared similar source during night.The negative correlation between H₂O₂ and NO proven that NO at the sites was abundant enough to suppress the production of H₂O₂.The levels of H₂O₂ and O₃ significantly dropped with the PM_2.5 level higher than 250μg/m³,while H₂O₂ could show high levels with the concentration of PM_2.5 between 100 and 250μg/m³.O₃ was more influenced by transport than H₂O₂ and was well correlated to surface wind speed,while H₂O₂ maintained a higher level but still showed pronounced diurnal cycles.The patterns of pollutants of Beijing and Gucheng were totally different under 16 wind directions.H₂O₂ and O₃ showed similar trends with higher levels under high northwest wind,which rised the nighttime level of H₂O₂ and O₃ to 0.76 ppbv and 22 ppbv,respectively,two times higher than those under the southwesterly,while PAN stayed at low level as the wind speed was high.It reveals the different pollution conditions between the north and south of Beijing and the characteristic of the cold front from northwest which brought high H₂O₂,O₃ and low PAN to the ground.It is found that the levels of H₂O₂,O₃ and PAN all went up under the south wind in spring presumably because of intensified chemical reactions in spring.As the northwest wind caused the increase of H₂O₂ in Beijing,the concentration of H₂O₂ under the same wind condition decreased in Gucheng,where the highest H₂O₂ level was observed under west to southwest winds.The traffic emission of the highway in the northwest of site could be the influencing factor.