Transactions on Ecology and the Environment vol 29 © 1999 WIT Press, www.witpress.com, ISSN 1743-3541 The effect of rain on suspended participate matter and other pollutants in an urban area F. Polla Mattiot, E. Scafe EniTecnologie S.p.A, Via E. Ramarini 32 00016 Monterotondo, Italy E-Mail: fpolla@enitecnologie. eni. it Abstract A study on the effect of rain on atmospheric pollutants in the city of Rome from 1994 to 1996 has been conducted based on data collected by governmental monitoring stations located in different areas of the city. In the city of Rome, the average annual precipitation value is over 700 mm, referring to measurements collected from over a century. Almost every month exhibits at least one rainy day. Days have been divided into two categories: rainy days, if it has rained at least 2 mm in the day, and days with no rain, in the other cases. Particulate matter concentration has been averaged every month in both categories. The difference between the two monthly values has been divided by the days-with- no-rain average to calculate the monthly abatement. Abatement in the three years is around 10%, although there exist single events of heavily rainy days that reveal an up to 50 % decrease in hourly concentration. Concentrations of paniculate matter, CO and NO% have been correlated with precipitation, using working days and low wind speed conditions. Results have been compared with both the day preceding and succeeding the actual rainy day. 1 Introduction Atmospheric aerosols are produced by a variety of natural and anthropogenic sources and have a wide range of sizes and densities. Among anthropogenic sources, combustion originated paniculate matter (residential heating and mobile Transactions on Ecology and the Environment vol 29 © 1999 WIT Press, www.witpress.com, ISSN 1743-3541 944 Air Pollution sources) accounts for one third of the total Polycyclic Aromatic Hydrocarbons (PAH) emissions*. The major area of environmental concern with PAHs is their ability to produce cancer in exposed organisms. A large effort has been made to understand acid rain phenomena, which has stimulated many studies on the scavenging and solubility of gases during rainfall^ whereas, until now, fewer studies have been devoted to the removal of particulate matter by raindrops. Most of these papers are focused on the assessment of the physical-chemical process in simulated conditions. Nevertheless, positive effects from precipitation are generally experienced both in visibility analysis^ and in managing urban air quality; in fact many municipalities do not adopt programmed traffic restrictions in the event of rain. The aim of this paper is to analyze statistical effects of precipitation ~on particulate matter removal using real data, collected over three years by the Municipality of Rome. Our results allow a conservative estimate of the effect of rain, due to the dramatic reduction in the city of Rome of the circulating two- wheel/four-wheel ratio during rainy days. 2 Methods 2.1 Pollutants' concentration levels: site locations, sampling methods and data availability The Municipality of Rome has organized a network of monitoring stations in order to assess the air quality, to record the number of occasions that exceed the specified limits and to evaluate trends since 1992. The urban Air Quality Network is made up of nine measuring stations classified according to traffic density. Automated instruments are distributed among the stations depending on site characteristics. A NDIR (Non Dispersive InfraRed) CO analyzer and a chemiluminescence NOx instrument are present in almost every station. UV photometers for ozone are generally installed in areas reserved for pedestrians, and fluorescence SOj analyzer are located in densely populated areas. Total Suspended Particulate matter analyzers are p gauge instruments, now converted into PM,o analyzers. Ambient measurements are taken at ground level (1.5 m). Calibration is generally performed nightly. Hourly data are available for all pollutants except for particulate matter which displays an average value every two hours. Particulate matter is measured in four stations but our attention will focus on one station located in a heavily trafficked area. Corso Francia is a six-lane street, oriented with the direction of predominant wind in the city of Rome during fall- winter seasons. Transactions on Ecology and the Environment vol 29 © 1999 WIT Press, www.witpress.com, ISSN 1743-3541 Air Pollution 945 2.2 Meteorological variables Meteorological data are supplied by a governmental institute located in the historical center of the city at a height of 60 m. Temperature, pressure, solar radiation, relative humidity, rainfall (total precipitation in a day, beginning hour, maximum intensity) and wind speed and direction are recorded daily. Moreover, some of the monitoring network (at Via Arenula, for instance, but not at Corso Francia) are equipped for meteorological measurements at ground level. As reported in the annual bulletin*, the years from 1994 to 1996 appear similar on a macro scale through comparing monthly averages of meteorological variables. Winter 1994 was mild; wind speed presented anomalous data in fall 1994 and in winter 1995. Rainfall is spread all over the months although it is concentrated with a few days of stormy weather particularly during the fall. For instance, 1996 was a particularly rainy year (893 mm, 70% in the period August - December) exceeding the mean values (745 mm, 99 rainy days) calculated from 1862 to 1990. 2.3 Processing participate matter data As our aim is to examine how many days are effective for TSP removal, we studied the monthly behavior of TSP concentration with and without the presence of rain. A list of requirements were established and a detailed screening of the data was performed. First of all, codes given by the instrument were checked. Then, iterative procedures were run in order to find out concentration levels under urban background and if there is repetition of identical 10 digit values. Therefore we consider that 80% data reliability is the minimum required; that is to say, that when more than 20% of the data have been rejected in a month, it has to be excluded from the study. Anyway the number of months and rainy days are representative of every year and no insertion of calculated data - as suggested by Glen* - is necessary or convenient to our purposes. Moreover we realized that during a month in the city of Rome it can be observed that the mean of rainy days is less than six (< 20% in a month). Days have been divided into two categories: rainy days, with at least 2 mm of total precipitation^, and days with no rain. Particulate matter concentration has been averaged every month in both categories. The difference between the two monthly values has been divided by the days- with-no-rain average to calculate the abatement which is expressed as a percentage. Mean values are considered with 95 % confidence interval of the mean to compare data. Transactions on Ecology and the Environment vol 29 © 1999 WIT Press, www.witpress.com, ISSN 1743-3541 946 Air Pollution 3 Results and discussion 3.1 Annual trends of mayor pollutants The measurements from all the stations detecting the pollution level were averaged to obtain the mean urban values reported in Table 1. Averages from one station are also shown for comparison. Table 1 Annual pollutant levels 1994 1995 1996 NOxWnf) 254 (158) 230 (149) 219 (136) CO (mg/nf) 3.9 (3.6) 3.6 (3.4) 34 (3) TSP Wnf ) 73 (90) 80 (100) 72 (117) Discrepancies of single station data from the mean value are attributed to differences in traffic density and positioning with respect to wind direction. Decreasing trends are outlined for all the pollutants, except particulate matter. In particular Corso Francia TSP mean values show a noticeable increase from 1994 to 1996. Monthly behavior of particulate matter concentration is presented in Fig. 1 where an increase in the summer is outlined. Herein no distinction between rainy day and days with no rain has been accomplished. 1995 - Corso Francia- 160 150 -- 140 4* : 130 « 120 - §* 110 -~ , Q. [2 100 - 90 80 - 70 -- 60 1234567 9 10 11 12 months Fig. 1 Monthly average concentration of Total Suspended Particulate Matter Transactions on Ecology and the Environment vol 29 © 1999 WIT Press, www.witpress.com, ISSN 1743-3541 Air Pollution 947 3.2 Single events of heavily rainy days In order to examine the removal efficiency of rain on pollutants we observed some working days characterized by the presence of rain at peak TSP levels without noticeable increases in wind speed (VV). Nevertheless, TSP concentration and VV maxima in the city of Rome are nearly simultaneous and a slight increase in wind speed has been often observed at midday. Due to the complexity of the wind effect on paniculate matter, it is useful to analyze other pollutants and to correct the experimental data^, according to the box model approach* (1/VV). A significant example (Tuesday 5* September 1995) regarding particulate matter (Fig. 2), NOxand CO (Fig. 4, 5) is reported. In presence of rain, TSP concentration levels drop down from 50% of the initial value, as shown in Fig. 2, well below the average diurnal concentration levels? 5 Sep rain 12-14 6 Sep no rain 10 12 14 16 18 20 22 24 hour Fig. 2 TSP concentration data on September 1995. Comparison with the day preceding and succeeding the actual rainy day. Transactions on Ecology and the Environment vol 29 © 1999 WIT Press, www.witpress.com, ISSN 1743-3541 Air Pollution 948 1 average concentration + 4 Sep no rain _O_5 Sep rain 12-14 _•• 6 Sep no rain Fig. 3 NOx concentration data on September 1995. Comparison with the day preceding and succeeding the actual rainy day.