A Study on Three Factors Influencing Uptake Rates of Nitric Acid onto Dust Particles

Recent studies have indicated that the observed nitric acid ($HNO_3$) uptake rates ($R_{HNO_3}$) onto dust particles are much slower than $R_{HNO_3}$ used in the previous modeling studies. Three factors that possibly affect $R_{HNO_3}$ onto dust particles are discussed in this study: (1) the magnitude of reaction probability of $HNO_3$ (${\gamma}_{HNO_3}$), (2) aerosol surface areas, and (3) gas-phase $HNO_3$ mixing ratio. Through the discussion presented here, it is shown that the use of accurate ${\gamma}_{HNO_3}$ is of primary importance. We suggest that the use of ${\gamma}_{HNO_3}$ values between $\sim10^{-3}$ and $\sim10^{-5}$ produces more realistic results than the use of ${\gamma}_{HNO_3}$ values between $\sim10^{-1}$ and $\sim10^{-2}$ does, more accurately modeling the nitrate formation characteristics on/in dust particles. We also discuss two different types of aerosol surface area, active and geometric, since the use of different aerosol surface areas often leads to an erroneous result in $R_{HNO_3}$. In addition, the levels of the gas-phase $HNO_3$ are investigated with the example cases of TRACE-P DC-8 flights in East Asia. The $HNO_3$ levels were found to be relatively high, indicating that they can not limit nitrate formation in dust particles.