Study On Heterogeneous Reaction Of NO₂ And Air Conditioning Filter Dust

Nitrous acid（HONO）,which can cause adverse effects on human health,is an important gaseous pollutant and the main source of hydroxyl radicals（OH）in both atmosphere and indoor air.Air Conditioners are widely used in ordinary households and offices.While serving various purposes such as refrigeration,dehumidification,heating and purification,the dust attached on the air conditioner filter is also an important source of indoor pollution.With a very large specific surface area and complex composition,various gases can be adsorbed and reacted on the air conditioner filter dust,thus causing more secondary pollution.To the best of our knowledge,no researchers have studied the reaction of gases on the surface of air conditioner filter dust.We suspect that when the air conditioner is working,NO₂ will react heterogeneously on the surface of the air conditioner filter dust,which may be an important source of indoor HONO.To this end,we systematically studied the effects of gas flow rate,relative humidity（RH）and NO₂ concentration,and on the NO₂ uptake coefficient by taking the dust on the air-conditioning filters in chemistry laboratory as the research object,and evaluated the contribution of this reaction to HONO pollution in typical indoor environments with a dynamic mass balance model.The study further tested the effect of NH₃ and SO₂ on the heterogeneous reaction of NO₂ on the surface of the air conditioner filter dust.The main conclusions of this study are as follows:（1）In the gas flow rate range of 1.4 L/min–16 L/min,the uptake coefficientγof NO₂ increases with the rising of gas flow rate,from 5.7×10^-8（at 1.4 L/min gas flow rate）to 1.3×10^-7（at 16 L/min gas flow rate）.When the gas flow rate is greater than16 L/min,the NO₂ uptake coefficientγreaches saturation;（2）The uptake coefficientγof NO₂ reacting with air conditioner filter dust increases significantly from 8.0×10^-8 at 15%RH to 1.7×10^-7 at 90%RH,and is linearly related to RH as follows:γ=（1.2±0.1）×10^-7%RH+（7.5±0.9）×10^-8.The surface flux of HONO of the reaction of NO₂ with air conditioning filter dust increases from 2.9×10¹¹ molecular cm^-2 s^-1 at 15%RH to 5.1×10¹¹ molecular cm^-2s^-1 at 90%RH.（3）The uptake coefficient of NO₂γand the initial concentration of NO₂ showed a nonlinear decrease,from 2.1×10^-7 at 40 ppbv NO₂ to 1.2×10^-8 at 110 ppbv NO₂.1/γis very linearly related to the initial concentration of NO₂:1/γ=（4.0±0.6）×10^-6[NO₂](molecules cm^-3)+（9.5±12）×10⁵.（4）The coexistence of NH₃ and NO₂ shows that NH₃ does not promote the uptake of NO₂ and the generation of HONO;while the coexistence of SO₂ and NO₂ will significantly promote the uptake of NO₂ and the generation of HONO.And the coexistence of SO₂,NH₃,and NO₂ will further promote the uptake of NO₂ and the generation of HONO.（5）Dynamic mass balance model was used to simulate the heterogeneous reaction of NO₂ and air-conditioning filter dust to form HONO.At 60%RH air-conditioning secondary wind speed（about 16 L/min）,the reaction of 60 ppbv NO₂ and air-conditioning filter dust can lead to about 3.8 ppbv HONO indoor.Considering that HONO is the main source of OH in indoor air,high concentrations of HONO released through the heterogeneous reaction of NO₂ and air conditioning filter dust may have serious effects on indoor air and human health.