ISR(TIMER2_OVF_vect) compiling error for grove sensor

Hello I am using Grove - Air Quality Sensor v1.3 with Seedunio LoRaWAN. I have uploaded my grove sensor code in arduino but I got this error:

Arduino: 1.8.15 Hourly Build 2021/05/19 12:33 (Windows 10), Board: "Seeeduino LoRaWAN, Arduino, Off" 

sketch_jun24b:25:5: error: expected constructor, destructor, or type conversion before ‘(’ token

ISR(TIMER2_OVF_vect)

 ^

sketch_jun24b:25:4: error: expected constructor, destructor, or type conversion before ‘(’ token

ISR(TIMER2_OVF_vect)

^

exit status 1

expected constructor, destructor, or type conversion before ‘(’ token

My sensor code is here:

#include"AirQuality.h"

#include"Arduino.h"
AirQuality airqualitysensor;
int current_quality =-1;
void setup()
{
Serial.begin(9600);
airqualitysensor.init(A0);
}
void loop()
{
current_quality=airqualitysensor.slope();
if (current_quality >= 0)// if a valid data returned.
{
if (current_quality==0)
Serial.println(“High pollution! Force signal active”);
else if (current_quality==1)
Serial.println(“High pollution!”);
else if (current_quality==2)
Serial.println(“Low pollution!”);
else if (current_quality ==3)
Serial.println(“Fresh air”);
}
}
ISR(TIMER2_OVF_vect)
{
if(airqualitysensor.counter==122)//set 2 seconds as a detected duty
{
airqualitysensor.last_vol=airqualitysensor.first_vol;
airqualitysensor.first_vol=analogRead(A0);
airqualitysensor.counter=0;
airqualitysensor.timer_index=1;
PORTB=PORTB^0x20;
}
else
{
airqualitysensor.counter++;
}
}

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