Rainfall scavenging coefficients for atmospheric nitric acid and nitrate in a subtropical coastal environment

Hourly-measured gas concentrations and 24-h integrated PM10 concentrations were used in conjunction with a below-cloud scavenging model to explain nitrate (NO3−) concentrations in rainwater samples collected at a bayside monitoring site in Tampa, Florida, USA. Mass particle concentrations were assumed to have a log-normal distribution as a function of particle diameter based on experimental data from the monitoring site. The effect of rain droplet diameter on modeled scavenging rates was studied using exponential, gamma, and log-normal droplet size distributions (DSD). For 11 summertime rain events and across these three DSDs, normalized mean scavenging coefficients (average ± standard deviation) for HNO3 and NO3− were 2.90 × 10−5 ± 1.80 × 10−5 (s × mm/h)−1 and 2.78 × 10−5 ± 0.56 × 10−5 (s mm/h)−1. Rainwater concentrations were modeled for two different cases: the first case assumed constant gas and particle concentrations and the second case assumed first-order removal of gases and particles. The below-cloud scavenging model explained 92.0 ± 40.2% of NO3− concentrations in the first case and 40.0 ± 24.6% in the second case. The model predicted that aerosol NO3− constituted the largest fractions of rainwater NO3−.